From Wikipedia, the free encyclopedia
Evidence of common descent of
living organisms has been found by scientists working in a variety of fields over many decades and has demonstrated
common descent and that
life on earth developed from a
last universal ancestor, that
evolution does occur, and is able to show the natural processes by which the
biodiversity of
life on Earth developed. This evidence supports the
modern evolutionary synthesis, the current
scientific theory
that explains how and why life changes over time. Evolutionary
biologists document evidence of common descent through making testable
predictions, testing hypotheses, and developing theories that illustrate
and describe its causes.
Comparison of the
DNA genetic sequences of organisms has revealed that organisms that are
phylogenetically
close have a higher degree of DNA sequence similarity than organisms
that are phylogenetically distant. Further evidence for common descent
comes from genetic detritus such as
pseudogenes, regions of
DNA that are
orthologous to a gene in a related
organism, but are no longer active and appear to be undergoing a steady process of degeneration from cumulative mutations.
Fossils are important for estimating when various lineages developed in
geologic time. As fossilization is an uncommon occurrence, usually requiring hard body parts and death near a site where
sediments are being deposited, the fossil record only provides sparse and intermittent information about the evolution of life.
Scientific evidence
of organisms prior to the development of hard body parts such as
shells, bones and teeth is especially scarce, but exists in the form of
ancient
microfossils, as well as impressions of various soft-bodied organisms. The comparative study of the
anatomy
of groups of animals shows structural features that are fundamentally
similar or homologous, demonstrating phylogenetic and ancestral
relationships with other organisms, most especially when compared with
fossils of ancient
extinct organisms.
Vestigial structures and comparisons in
embryonic development are largely a contributing factor in anatomical resemblance in concordance with common descent. Since
metabolic
processes do not leave fossils, research into the evolution of the
basic cellular processes is done largely by comparison of existing
organisms’
physiology and
biochemistry.
Many lineages diverged at different stages of development, so it is
possible to determine when certain metabolic processes appeared by
comparing the traits of the descendants of a common ancestor. Universal
biochemical organization and molecular variance patterns in all
organisms also show a direct correlation with common descent.
Further evidence comes from the field of
biogeography
because evolution with common descent provides the best and most
thorough explanation for a variety of facts concerning the geographical
distribution of plants and animals across the world. This is especially
obvious in the field of
insular biogeography. Combined with the theory of
plate tectonics
common descent provides a way to combine facts about the current
distribution of species with evidence from the fossil record to provide a
logically consistent explanation of how the distribution of living
organisms has changed over time.
The development and spread of
antibiotic resistant bacteria, like the spread of pesticide resistant forms of plants and insects provides evidence that evolution due to
natural selection
is an ongoing process in the natural world. Alongside this, are
observed instances of the separation of populations of species into sets
of new species (
speciation).
Speciation has been observed directly and indirectly in the lab and in
nature. Multiple forms of such have been described and documented as
examples for individual modes of speciation. Furthermore,
evidence of common descent extends from direct laboratory experimentation with the
selective breeding
of organisms—historically and currently—and other controlled
experiments involving many of the topics in the article. This article
explains the different types of evidence for evolution with common
descent along with many specialized examples of each.
Evidence from comparative physiology and biochemistry
Genetics
One of the strongest evidences for common descent comes from the study of gene sequences.
Comparative sequence analysis examines the relationship between the DNA sequences of different species,
[1]
producing several lines of evidence that confirm Darwin's original
hypothesis of common descent. If the hypothesis of common descent is
true, then species that share a common ancestor inherited that
ancestor's DNA sequence, as well as mutations unique to that ancestor.
More closely related species have a greater fraction of identical
sequence and shared substitutions compared to more distantly related
species.
The simplest and most powerful evidence is provided by
phylogenetic reconstruction.
Such reconstructions, especially when done using slowly evolving
protein sequences, are often quite robust and can be used to reconstruct
a great deal of the evolutionary history of modern organisms (and even
in some instances of the evolutionary history of extinct organisms, such
as the recovered gene sequences of
mammoths or
Neanderthals).
These reconstructed phylogenies recapitulate the relationships
established through morphological and biochemical studies. The most
detailed reconstructions have been performed on the basis of the
mitochondrial genomes shared by all
eukaryotic
organisms, which are short and easy to sequence; the broadest
reconstructions have been performed either using the sequences of a few
very ancient proteins or by using
ribosomal RNA sequence
[citation needed].
Phylogenetic relationships also extend to a wide variety of
nonfunctional sequence elements, including repeats, transposons,
pseudogenes, and mutations in protein-coding sequences that do not
result in changes in amino-acid sequence. While a minority of these
elements might later be found to harbor function, in aggregate they
demonstrate that identity must be the product of common descent rather
than common function
[citation needed].
Universal biochemical organisation and molecular variance patterns
All known
extant (surviving) organisms are based on the same biochemical processes: genetic information encoded as nucleic acid (
DNA, or
RNA for many viruses), transcribed into
RNA, then translated into
proteins (that is, polymers of
amino acids) by highly conserved
ribosomes. Perhaps most tellingly, the
Genetic Code (the "translation table" between DNA and amino acids) is the same for almost every organism, meaning that a piece of
DNA in a
bacterium codes for the same amino acid as in a human
cell.
ATP is used as energy currency by all extant life. A deeper understanding of
developmental biology shows that common morphology is, in fact, the product of shared genetic elements.
[2] For example, although camera-like eyes are believed to have evolved independently on many separate occasions,
[3] they share a common set of light-sensing proteins (
opsins), suggesting a common point of origin for all sighted creatures.
[4][5] Another noteworthy example is the familiar vertebrate body plan, whose structure is controlled by the
homeobox (Hox) family of genes.
DNA sequencing
Comparison of the DNA sequences allows organisms to be grouped by sequence similarity, and the resulting
phylogenetic trees are typically congruent with traditional
taxonomy,
and are often used to strengthen or correct taxonomic classifications.
Sequence comparison is considered a measure robust enough to correct
erroneous assumptions in the phylogenetic tree in instances where other
evidence is scarce. For example, neutral human DNA sequences are
approximately 1.2% divergent (based on substitutions) from those of
their nearest genetic relative, the
chimpanzee, 1.6% from
gorillas, and 6.6% from
baboons.
[6][7] Genetic sequence evidence thus allows inference and quantification of genetic relatedness between humans and other
apes.
[8][9] The sequence of the
16S ribosomal RNA gene, a vital gene encoding a part of the
ribosome, was used to find the broad phylogenetic relationships between all extant life. The analysis, originally done by
Carl Woese, resulted in the
three-domain system, arguing for two major splits in the early evolution of life. The first split led to modern
Bacteria and the subsequent split led to modern
Archaea and
Eukaryotes.
Some DNA sequences are shared by very different organisms. It has
been predicted by the theory of evolution that the differences in such
DNA sequences between two organisms should roughly resemble both the
biological difference between them according to their
anatomy and the time that had passed since these two organisms have separated in the course of evolution, as seen in
fossil evidence. The rate of accumulating such changes should be low for some sequences, namely those that code for critical
RNA or
proteins,
and high for others that code for less critical RNA or proteins; but
for every specific sequence, the rate of change should be roughly
constant over time. These results have been experimentally confirmed.
Two examples are DNA sequences coding for
rRNA, which is highly conserved, and DNA sequences coding for
fibrinopeptides (
amino acid chains that are discarded during the formation of
fibrin), which are highly non-conserved.
[10]
Endogenous retroviruses
Endogenous retroviruses
(or ERVs) are remnant sequences in the genome left from ancient viral
infections in an organism. The retroviruses (or virogenes) are always
passed on
to the next generation of that organism that received the infection.
This leaves the virogene left in the genome. Because this event is rare
and random, finding identical chromosomal positions of a virogene in two
different species suggests common ancestry.
[11] Cats (
Felidae)
present an notable instance of virogene sequences demonstrating common
descent. The standard phylogenetic tree for Felidae have smaller cats (
Felis chaus,
Felis silvestris,
Felis nigripes, and
Felis catus) diverging from larger cats such as the subfamily
Pantherinae and other
carnivores.
The fact that small cats have an ERV where the larger cats do not
suggests that the gene was inserted into the ancestor of the small cats
after the larger cats had diverged.
[12]
Another example of this is with humans and chimps. Humans contain
numerous ERVs that comprise a considerable percentage of the genome.
Sources vary, however, 1%
[13] to 8%
[14]
has been proposed.
Humans and chimps share seven different occurrences
of virogenes while all primates share similar retroviruses congruent
with phylogeny.
[15]
Proteins
The
proteomic evidence also supports the universal ancestry of life. Vital
proteins, such as the
ribosome,
DNA polymerase, and
RNA polymerase,
are found in everything from the most primitive bacteria to the most
complex mammals. The core part of the protein is conserved across all
lineages of life, serving similar functions. Higher organisms have
evolved additional
protein subunits, largely affecting the regulation and
protein-protein interaction of the core. Other overarching similarities between all lineages of extant organisms, such as
DNA,
RNA, amino acids, and the
lipid bilayer,
give support to the theory of common descent. Phylogenetic analyses of
protein sequences from various organisms produce similar trees of
relationship between all organisms.
[16] The
chirality
of DNA, RNA, and amino acids is conserved across all known life. As
there is no functional advantage to right- or left-handed molecular
chirality, the simplest hypothesis is that the choice was made randomly
by early organisms and passed on to all extant life through common
descent. Further evidence for reconstructing ancestral lineages comes
from
junk DNA such as
pseudogenes, "dead" genes that steadily accumulate mutations.
[17]
Pseudogenes
Pseudogenes, also known as
noncoding DNA,
are extra DNA in a genome that do not get transcribed into RNA to
synthesize proteins. Some of this noncoding DNA has known functions, but
much of it has no known function and is called "Junk DNA". This is an
example of a vestige since replicating these genes uses energy, making
it a waste in many cases. A pseudogene can be produced when a coding
gene accumulates mutations that prevent it from being transcribed,
making it non-functional. But since it is not transcribed, it may
disappear without affecting fitness, unless it has provided some
beneficial function as non-coding DNA. Non-functional pseudogenes may be
passed on to later species, thereby labeling the later species as
descended from the earlier species.
Other mechanisms
There is also a large body of molecular evidence for a number of
different mechanisms for large evolutionary changes, among them:
genome and
gene duplication,
which facilitates rapid evolution by providing substantial quantities
of genetic material under weak or no selective constraints;
horizontal gene transfer,
the process of transferring genetic material to another cell that is
not an organism's offspring, allowing for species to acquire beneficial
genes from each other; and
recombination, capable of reassorting large numbers of different
alleles and of establishing
reproductive isolation. The
Endosymbiotic theory explains the origin of
mitochondria and
plastids (
e.g. chloroplasts), which are
organelles of eukaryotic cells, as the incorporation of an ancient
prokaryotic cell into ancient
eukaryotic cell. Rather than evolving
eukaryotic organelles
slowly, this theory offers a mechanism for a sudden evolutionary leap
by incorporating the genetic material and biochemical composition of a
separate species. Evidence supporting this mechanism has been found in
the
protist Hatena: as a predator it engulfs a
green algae cell, which subsequently behaves as an
endosymbiont, nourishing
Hatena, which in turn loses its feeding apparatus and behaves as an
autotroph.
[18][19]
Since
metabolic
processes do not leave fossils, research into the evolution of the
basic cellular processes is done largely by comparison of existing
organisms. Many lineages diverged when new metabolic processes appeared,
and it is theoretically possible to determine when certain metabolic
processes appeared by comparing the traits of the descendants of a
common ancestor or by detecting their physical manifestations. As an
example, the appearance of
oxygen in the
earth's atmosphere is linked to the evolution of
photosynthesis.
Specific examples
Chromosome 2 in humans
Fusion of ancestral chromosomes left distinctive remnants of telomeres, and a vestigial centromere
Evidence for the evolution of
Homo sapiens from a common ancestor with chimpanzees is found in the number of chromosomes in humans as compared to all other members of
Hominidae.
All hominidae have 24 pairs of chromosomes, except humans, who have
only 23 pairs. Human chromosome 2 is a result of an end-to-end fusion of
two ancestral chromosomes.
[20][21]
The evidence for this includes:
- The correspondence of chromosome 2 to two ape chromosomes. The closest human relative, the common chimpanzee,
has near-identical DNA sequences to human chromosome 2, but they are
found in two separate chromosomes. The same is true of the more distant gorilla and orangutan.[22][23]
- The presence of a vestigial centromere. Normally a chromosome has just one centromere, but in chromosome 2 there are remnants of a second centromere.[24]
- The presence of vestigial telomeres.
These are normally found only at the ends of a chromosome, but in
chromosome 2 there are additional telomere sequences in the middle.[25]
Chromosome 2 thus presents very strong evidence in favour of the common descent of humans and other
apes.
According to J. W. IJdo, "We conclude that the locus cloned in cosmids
c8.1 and c29B is the relic of an ancient telomere-telomere fusion and
marks the point at which two ancestral ape chromosomes fused to give
rise to human chromosome 2."
[25]
Cytochrome c and b
A classic example of biochemical evidence for evolution is the
variance of the ubiquitous (i.e. all living organisms have it, because
it performs very basic life functions)
protein Cytochrome c
in living cells. The variance of cytochrome c of different organisms is
measured in the number of differing amino acids, each differing amino
acid being a result of a
base pair substitution, a
mutation. If each differing amino acid is assumed the result of
one
base pair substitution, it can be calculated how long ago the two
species diverged by multiplying the number of base pair substitutions by
the estimated time it takes for a substituted base pair of the
cytochrome c gene to be successfully passed on. For example, if the
average time it takes for a base pair of the cytochrome c gene to mutate
is N years, the number of amino acids making up the cytochrome c
protein in monkeys differ by one from that of humans, this leads to the
conclusion that the two species diverged N years ago.
The primary structure of cytochrome c consists of a chain of about 100
amino acids. Many higher order organisms possess a chain of 104 amino acids.
[26]
The cytochrome c molecule has been extensively studied for the glimpse it gives into evolutionary biology. Both
chicken and
turkeys have identical sequence homology (amino acid for amino acid), as do
pigs,
cows and
sheep. Both
humans and
chimpanzees share the identical molecule, while
rhesus monkeys share all but one of the amino acids:
[27] the 66th amino acid is
isoleucine in the former and
threonine in the latter.
[26]
What makes these homologous similarities particularly suggestive of
common ancestry in the case of cytochrome c, in addition to the fact
that the phylogenies derived from them match other phylogenies very
well, is the high degree of functional redundancy of the cytochrome c
molecule. The different existing configurations of amino acids do not
significantly affect the functionality of the protein, which indicates
that the base pair substitutions are not part of a directed design, but
the result of random mutations that aren't subject to selection.
[28]
In addition, Cytochrome b is commonly used as a region of
mitochondrial DNA to determine
phylogenetic
relationships between organisms due to its sequence variability. It is
considered most useful in determining relationships within
families and
genera.
Comparative studies involving cytochrome b have resulted in new
classification schemes and have been used to assign newly described
species to a genus, as well as deepen the understanding of evolutionary
relationships.
[29]
Recent African origin of modern humans
Mathematical models of evolution, pioneered by the likes of
Sewall Wright,
Ronald Fisher and
J. B. S. Haldane and extended via
diffusion theory by
Motoo Kimura,
allow predictions about the genetic structure of evolving populations.
Direct examination of the genetic structure of modern populations via
DNA sequencing has allowed verification of many of these predictions.
For example, the
Out of Africa
theory of human origins, which states that modern humans developed in
Africa and a small sub-population migrated out (undergoing a
population bottleneck),
implies that modern populations should show the signatures of this
migration pattern. Specifically, post-bottleneck populations (Europeans
and Asians) should show lower overall genetic diversity and a more
uniform distribution of allele frequencies compared to the African
population. Both of these predictions are borne out by actual data from a
number of studies.
[30]
Evidence from comparative anatomy
Comparative study of the anatomy
of groups of animals or plants reveals that certain structural features
are basically similar. For example, the basic structure of all
flowers consists of
sepals,
petals,
stigma, style and ovary; yet the size,
colour,
number
of parts and specific structure are different for each individual
species. The neural anatomy of fossilized remains may also be compared
using advanced imaging techniques.
[31]
Atavisms
Hindlegs of a humpback whale reported in 1921 by the American Museum
An atavism is an evolutionary throwback, such as traits reappearing that had disappeared generations ago.
[32]
Atavisms occur because genes for previously existing phenotypical
features are often preserved in DNA, even though the genes are not
expressed in some or most of the organisms possessing them.
[33] Some examples of this are hind-legged snakes
[34] or whales
[35] (In July 1919 a humpback whale was caught by a ship operating out of Vancouver that had legs 4 ft 2 in (1.27 m) long.
[36]); the extra toes of
ungulates that do not even reach the ground,
[37] chicken's teeth,
[38] reemergence of
sexual reproduction in
Hieracium pilosella and
Crotoniidae;
[39] and humans with tails,
[32] extra
nipples[citation needed], and large
canine teeth[citation needed].
Evolutionary developmental biology and embryonic development
Evolutionary developmental biology is the biological field that
compares the developmental process of different organisms to determine
ancestral relationships between species. A large variety of organism’s
genomes contain a
small fraction of genes that control the organisms development.
Hox genes
are an example of these types of nearly universal genes in organisms
pointing to an origin of common ancestry.
Embryological evidence comes
from the development of organisms at the embryological level with the
comparison of different organisms embryos similarity. Remains of
ancestral traits often appear and disappear in different stages of the
embryological development process. Examples include such as hair growth
and loss (
lanugo) during human development;
[40] development and degeneration of a
yolk sac;
terrestrial frogs and salamanders passing through the larval stage
within the egg—with features of typically aquatic larvae—but hatch ready
for life on land;
[41] and the appearance of gill-like structures (
pharyngeal arch) in vertebrate embryo development. Note that in fish, the arches continue to develop as
branchial arches while in humans, for example, they give rise to a
variety of structures within the head and neck.
Homologous structures and divergent (adaptive) evolution
If widely separated groups of organisms are originated from a common
ancestry, they are expected to have certain basic features in common.
The degree of resemblance between two organisms should indicate how
closely related they are in evolution:
- Groups with little in common are assumed to have diverged from a common ancestor much earlier in geological history than groups with a lot in common;
- In deciding how closely related two animals are, a comparative anatomist looks for structures that are fundamentally similar, even though they may serve different functions in the adult. Such structures are described as homologous and suggest a common origin.
- In cases where the similar structures serve different functions in adults, it may be necessary to trace their origin and embryonic development. A similar developmental origin suggests they are the same structure, and thus likely derived from a common ancestor.
When a group of organisms share a homologous structure that is
specialized to perform a variety of functions to adapt different
environmental conditions and modes of life, it is called
adaptive radiation. The gradual spreading of organisms with adaptive radiation is known as
divergent evolution.
Nested hierarchies and classification
Taxonomy
is based on the fact that all organisms are related to each other in
nested hierarchies based on shared characteristics. Most existing
species can be organized rather easily in a nested hierarchical
classification. This is evident from the Linnaean classification scheme.
Based on shared derived characters, closely related organisms can be
placed in one group (such as a genus), several genera can be grouped
together into one family, several families can be grouped together into
an order, etc.
[42]
The existence of these nested hierarchies was recognized by many
biologists before Darwin, but he showed that his theory of evolution
with its branching pattern of common descent could explain them.
[42][43] Darwin described how common descent could provide a logical basis for classification:
[44]
| “ |
All
the foregoing rules and aids and difficulties in classification are
explained, if I do not greatly deceive myself, on the view that the
natural system is founded on descent with modification; that the
characters which naturalists consider as showing true affinity between
any two or more species, are those which have been inherited from a
common parent, and, in so far, all true classification is genealogical;
that community of descent is the hidden bond which naturalists have been
unconsciously seeking, ... |
” |
|
|
Evolutionary trees
An
evolutionary tree
(of Amniota, for example, the last common ancestor of mammals and
reptiles, and all its descendants) illustrates the initial conditions
causing evolutionary patterns of similarity (e.g., all Amniotes produce
an egg that possesses the
amnios)
and the patterns of divergence amongst lineages (e.g., mammals and
reptiles branching from the common ancestry in Amniota). Evolutionary
trees provide conceptual models of evolving systems once thought limited
in the domain of making predictions out of the theory.
[45] However, the method of
phylogenetic bracketing
is used to infer predictions with far greater probability than raw
speculation. For example, paleontologists use this technique to make
predictions about nonpreservable traits in fossil organisms, such as
feathered dinosaurs, and molecular biologists use the technique to posit
predictions about RNA metabolism and protein functions.
[46][47]
Thus evolutionary trees are evolutionary hypotheses that refer to
specific facts, such as the characteristics of organisms (e.g., scales,
feathers, fur), providing evidence for the patterns of descent, and a
causal explanation for modification (i.e., natural selection or neutral
drift) in any given lineage (e.g., Amniota). Evolutionary biologists
test evolutionary theory using phylogenetic systematic methods that
measure how much the hypothesis (a particular branching pattern in an
evolutionary tree) increases the likelihood of the evidence (the
distribution of characters among lineages).
[48][49][50]
The severity of tests for a theory increases if the predictions "are
the least probable of being observed if the causal event did not occur."
[51] "Testability is a measure of how much the hypothesis increases the likelihood of the evidence."
[52]
Vestigial structures
A strong and direct evidence for common descent comes from vestigial structures.
[53]
Rudimentary body parts, those that are smaller and simpler in structure
than corresponding parts in the ancestral species, are called vestigial
organs. They are usually degenerated or underdeveloped. The existence
of vestigial organs can be explained in terms of changes in the
environment or modes of life of the species.
Those organs are typically
functional in the ancestral species but are now either nonfunctional or
re-purposed. Examples are the
pelvic girdles of whales,
haltere (hind
wings) of
flies and mosquitos, wings of flightless birds such as
ostriches, and the
leaves of some
xerophytes (
e.g. cactus) and
parasitic plants (
e.g. dodder). However, vestigial structures may have their original function replaced with another. For example, the
halteres in
dipterists help balance the insect while in flight and the wings of ostriches are used in
mating rituals.
Specific examples
Figure 5a: Skeleton of a
Baleen whale
with the hind limb and pelvic bone structure circled in red. This bone
structure stays internal during the entire life of the species.
Figure 5b: Adaptation of insect mouthparts: a,
antennae; c,
compound eye; lb, labrium; lr, labrum; md, mandibles; mx, maxillae.
(A) Primitive state — biting and chewing:
e.g. grasshopper. Strong mandibles and maxillae for manipulating food.
(B) Ticking and biting:
e.g. honey bee. Labium long to lap up
nectar; mandibles chew
pollen and mould
wax.
(C) Sucking:
e.g. butterfly. Labrum reduced; mandibles lost; maxillae long forming sucking tube.
(D) Piercing and sucking,
e.g..
female mosquito. Labrum and maxillae form tube; mandibles form piercing stylets; labrum grooved to hold other parts.
Figure 5c: Illustration of the
Eoraptor lunensis pelvis of the
saurischian order and the
Lesothosaurus diagnosticus pelvis of the
ornithischian order in the
Dinosauria superorder. The parts of the pelvis show modification over time. The
cladogram is shown to illustrate the distance of divergence between the two species.
Figure 5d: The principle of
homology
illustrated by the adaptive radiation of the forelimb of mammals. All
conform to the basic pentadactyl pattern but are modified for different
usages. The third metacarpal is shaded throughout; the shoulder is
crossed-hatched.
Figure 5e: The path
of the recurrent laryngeal nerve in giraffes. The laryngeal nerve is
compensated for by subsequent tinkering from natural selection.
Hind structures in whales
Whales possess internally reduced hind parts such as the pelvis and hind legs (Fig. 5a).
[54][55]
Occasionally, the genes that code for longer extremities cause a modern
whale to develop legs. On October 28, 2006, a four-finned bottlenose
dolphin was caught and studied due to its extra set of hind limbs.
[56] These legged
Cetacea display an example of an atavism predicted from their common ancestry.
Insect mouthparts
Many different species of insects have mouthparts derived from the
same embryonic structures, indicating that the mouthparts are
modifications of a common ancestor's original features. These include a
labrum (upper lip), a pair of
mandibles, a
hypopharynx (floor of mouth), a pair of
maxillae, and a
labium.
(Fig. 5b) Evolution has caused enlargement and modification of these
structures in some species, while it has caused the reduction and loss
of them in other species. The modifications enable the insects to
exploit a variety of food materials.
Other arthropod appendages
Insect mouthparts and antennae are considered homologues of insect legs. Parallel developments are seen in some
arachnids: The anterior pair of legs may be modified as analogues of antennae, particularly in
whip scorpions,
which walk on six legs. These developments provide support for the
theory that complex modifications often arise by duplication of
components, with the duplicates modified in different directions.
Pelvic structure of dinosaurs
Similar to the pentadactyl limb in mammals, the earliest
dinosaurs split into two distinct orders—the
saurischia and
ornithischia. They are classified as one or the other in accordance with what the fossils demonstrate. Figure 5c, shows that early
saurischians resembled early
ornithischians. The pattern of the
pelvis
in all species of dinosaurs is an example of homologous structures.
Each order of dinosaur has slightly differing pelvis bones providing
evidence of common descent. Additionally, modern
birds show a similarity to ancient
saurischian pelvic structures indicating the
evolution of birds from dinosaurs. This can also be seen in Figure 5c as the
Aves branch off the
Theropoda suborder.
Pentadactyl limb
The pattern of limb bones called
pentadactyl limb is an example of homologous structures (Fig. 5d). It is found in all classes of
tetrapods (
i.e. from
amphibians to
mammals). It can even be traced back to the
fins of certain
fossil fishes from which the first amphibians evolved such as
tiktaalik. The limb has a single proximal bone (
humerus), two distal bones (
radius and
ulna), a series of
carpals (
wrist bones), followed by five series of metacarpals (
palm bones) and
phalanges
(digits). Throughout the tetrapods, the fundamental structures of
pentadactyl limbs are the same, indicating that they originated from a
common ancestor. But in the course of evolution, these fundamental
structures have been modified. They have become superficially different
and unrelated structures to serve different functions in adaptation to
different environments and modes of life. This phenomenon is shown in
the forelimbs of mammals. For example:
- In the monkey, the forelimbs are much elongated to form a grasping hand for climbing and swinging among trees.
- In the pig,
the first digit is lost, and the second and fifth digits are reduced.
The remaining two digits are longer and stouter than the rest and bear a
hoof for supporting the body.
- In the horse, the forelimbs are adapted for support and running by great elongation of the third digit bearing a hoof.
- The mole has a pair of short, spade-like forelimbs for burrowing.
- The anteater uses its enlarged third digit for tearing down ant hills and termite nests.
- In the whale, the forelimbs become flippers for steering and maintaining equilibrium during swimming.
- In the bat, the forelimbs have turned into wings for flying by great elongation of four digits, while the hook-like first digit remains free for hanging from trees.
Recurrent laryngeal nerve in giraffes
The
recurrent laryngeal nerve is a fourth branch of the
vagus nerve, which is a
cranial nerve.
In mammals, its path is unusually long. As a part of the vagus nerve,
it comes from the brain, passes through the neck down to heart, rounds
the
dorsal aorta and returns up to the
larynx, again through the neck. (Fig. 5e)
This path is suboptimal even for humans, but for
giraffes
it becomes even more suboptimal. Due to the lengths of their necks, the
recurrent laryngeal nerve may be up to 4m long (13 ft), despite its
optimal route being a distance of just several inches.
The indirect route of this nerve is the result of evolution of
mammals from fish, which had no neck and had a relatively short nerve
that innervated one gill slit and passed near the gill arch. Since then,
the gill it innervated has become the larynx and the gill arch has
become the dorsal aorta in mammals.
[57][58]
Route of the vas deferens
Route of the vas deferens from the testis to the penis.
Similar to the laryngeal nerve in giraffes, the
vas deferens is part of the
male anatomy of many
vertebrates; it transports sperm from the
epididymis in anticipation of
ejaculation. In humans, the vas deferens routes up from the
testicle, looping over the
ureter, and back down to the
urethra and
penis.
It has been suggested that this is due to the descent of the testicles
during the course of human evolution—likely associated with temperature.
As the testicles descended, the vas deferens lengthened to accommodate
the accidental "hook" over the ureter.
[58][59]
Evidence from paleontology
An insect trapped in
amber.
When organisms die, they often
decompose rapidly or are consumed by
scavengers, leaving no permanent evidences of their existence. However, occasionally, some organisms are preserved. The remains or
traces of organisms from a past
geologic age embedded in
rocks by natural processes are called
fossils. They are extremely important for understanding the
evolutionary history of life on Earth, as they provide direct evidence of evolution and detailed information on the ancestry of organisms.
Paleontology is the study of past life based on fossil records and their relations to different geologic time periods.
For fossilization to take place, the traces and remains of organisms must be quickly buried so that
weathering
and decomposition do not occur. Skeletal structures or other hard parts
of the organisms are the most commonly occurring form of fossilized
remains (Paul, 1998), (Behrensmeyer, 1980) and (Martin, 1999). There are
also some trace "fossils" showing
moulds, cast or imprints of some previous organisms.
As an animal dies, the organic materials gradually decay, such that the
bones become porous. If the animal is subsequently buried in
mud,
mineral
salts infiltrate into the bones and gradually fill up the pores. The
bones harden into stones and are preserved as fossils. This process is
known as
petrification. If dead animals are covered by wind-blown
sand, and if the sand is subsequently turned into mud by heavy
rain or
floods,
the same process of mineral infiltration may occur. Apart from
petrification, the dead bodies of organisms may be well preserved in
ice, in hardened
resin of
coniferous trees (
amber), in tar, or in anaerobic,
acidic peat.
Fossilization can sometimes be a trace, an impression of a form.
Examples include leaves and footprints, the fossils of which are made in
layers that then harden.
Fossil record
It is possible to find out how a particular group of organisms
evolved by arranging its fossil records in a chronological sequence.
Such a sequence can be determined because fossils are mainly found in
sedimentary rock. Sedimentary rock is formed by layers of
silt or mud on top of each other; thus, the resulting rock contains a series of horizontal layers, or
strata. Each layer contains fossils typical for a specific
time period
when they formed. The lowest strata contain the oldest rock and the
earliest fossils, while the highest strata contain the youngest rock and
more recent fossils.
A succession of animals and plants can also be seen from fossil
discoveries. By studying the number and complexity of different fossils
at different
stratigraphic
levels, it has been shown that older fossil-bearing rocks contain fewer
types of fossilized organisms, and they all have a simpler structure,
whereas younger rocks contain a greater variety of fossils, often with
increasingly complex structures.
[60]
For many years, geologists could only roughly estimate the ages of
various strata and the fossils found. They did so, for instance, by
estimating the time for the formation of sedimentary rock layer by
layer. Today, by measuring the proportions of
radioactive and stable
elements in a given rock, the ages of fossils can be more precisely dated by scientists. This technique is known as
radiometric dating.
Throughout the fossil record, many species that appear at an early
stratigraphic level disappear at a later level. This is interpreted in
evolutionary terms as indicating the times when species originated and
became extinct. Geographical regions and climatic conditions have varied
throughout the
Earth's history.
Since organisms are adapted to particular environments, the constantly
changing conditions favoured species that adapted to new environments
through the mechanism of
natural selection.
Extent of the fossil record
Charles
Darwin collected fossils in South America, and found fragments of armor
he thought were like giant versions of the scales on the modern
armadillos living nearby. The anatomist
Richard Owen showed him that the fragments were from gigantic extinct
glyptodons, related to the armadillos. This was one of the patterns of distribution that helped Darwin to develop his theory.
[61]
Despite the relative rarity of suitable conditions for fossilization, approximately 250,000 fossil species are known.
[62]
The number of individual fossils this represents varies greatly from
species to species, but many millions of fossils have been recovered:
for instance, more than three million fossils from the last
Ice Age have been recovered from the
La Brea Tar Pits in Los Angeles.
[63]
Many more fossils are still in the ground, in various geological
formations known to contain a high fossil density, allowing estimates of
the total fossil content of the formation to be made. An example of
this occurs in South Africa's
Beaufort Formation (part of the
Karoo Supergroup, which covers most of South Africa), which is rich in vertebrate fossils, including
therapsids (reptile/mammal
transitional forms).
[64] It has been estimated that this formation contains 800 billion vertebrate fossils.
[65]
Limitations
The fossil record is an important source for scientists when tracing
the evolutionary history of organisms. However, because of limitations
inherent in the record, there are not fine scales of intermediate forms
between related groups of species. This lack of continuous fossils in
the record is a major limitation in tracing the descent of biological
groups. When
transitional fossils
are found that show intermediate forms in what had previously been a
gap in knowledge, they are often popularly referred to as "missing
links".
There is a gap of about 100 million years between the beginning of the
Cambrian period and the end of the
Ordovician period. The early Cambrian period was the period from which numerous fossils of
sponges,
cnidarians (
e.g.,
jellyfish),
echinoderms (
e.g.,
eocrinoids),
molluscs (
e.g.,
snails) and
arthropods (
e.g.,
trilobites) are found. The first animal that possessed the typical features of
vertebrates, the
Arandaspis, was dated to have existed in the later Ordovician period. Thus few, if any, fossils of an intermediate type between
invertebrates and vertebrates have been found, although likely candidates
include the
Burgess Shale animal,
Pikaia gracilens,
[66] and its
Maotianshan shales relatives,
Myllokunmingia,
Yunnanozoon,
Haikouella lanceolata,
[67] and
Haikouichthys.
[68]
Some of the reasons for the incompleteness of fossil records are:
- In general, the probability that an organism becomes fossilized is very low;
- Some species or groups are less likely to become fossils because they are soft-bodied;
- Some species or groups are less likely to become fossils because
they live (and die) in conditions that are not favourable for
fossilization;
- Many fossils have been destroyed through erosion and tectonic movements;
- Most fossils are fragmentary;
- Some evolutionary change occurs in populations at the limits of a
species' ecological range, and as these populations are likely small,
the probability of fossilization is lower (see punctuated equilibrium);
- Similarly, when environmental conditions change, the population of a
species is likely to be greatly reduced, such that any evolutionary
change induced by these new conditions is less likely to be fossilized;
- Most fossils convey information about external form, but little about how the organism functioned;
- Using present-day biodiversity
as a guide, this suggests that the fossils unearthed represent only a
small fraction of the large number of species of organisms that lived in
the past.
Specific examples
Evolution of the horse
Evolution of the horse
showing reconstruction of the fossil species obtained from successive
rock strata. The foot diagrams are all front views of the left forefoot.
The third
metacarpal is shaded throughout. The teeth are shown in longitudinal section.
Due to an almost-complete fossil record found in
North American sedimentary deposits from the early
Eocene to the present, the
horse provides one of the best examples of evolutionary history (
phylogeny).
This evolutionary sequence starts with a small animal called
Hyracotherium (commonly referred to as
Eohippus), which lived in North America about 54 million years ago then spread across to
Europe and
Asia. Fossil remains of
Hyracotherium show it to have differed from the modern horse in three important respects: it was a small animal (the size of a
fox),
lightly built and adapted for running; the limbs were short and
slender, and the feet elongated so that the digits were almost vertical,
with four digits in the
forelimbs and three digits in the
hindlimbs; and the
incisors were small, the
molars having low crowns with rounded
cusps covered in
enamel.
[69]
The probable course of development of horses from
Hyracotherium to
Equus (the modern horse) involved at least 12
genera and several hundred
species. The major trends seen in the development of the horse to changing environmental conditions may be summarized as follows:
- Increase in size (from 0.4 m to 1.5 m — from 15in to 60in);
- Lengthening of limbs and feet;
- Reduction of lateral digits;
- Increase in length and thickness of the third digit;
- Increase in width of incisors;
- Replacement of premolars by molars; and
- Increases in tooth length, crown height of molars.
Fossilized plants found in different strata show that the
marshy, wooded country in which
Hyracotherium
lived became gradually drier. Survival now depended on the head being
in an elevated position for gaining a good view of the surrounding
countryside, and on a high turn of speed for escape from
predators,
hence the increase in size and the replacement of the splayed-out foot
by the hoofed foot. The drier, harder ground would make the original
splayed-out foot unnecessary for support. The changes in the teeth can
be explained by assuming that the diet changed from soft
vegetation to
grass.
A dominant genus from each geological period has been selected to show
the slow alteration of the horse lineage from its ancestral to its
modern form.
[70]
Transition from fish to amphibians
Prior to 2004, paleontologists had found fossils of amphibians with
necks, ears, and four legs, in rock no older than 365 million years old.
In rocks more than 385 million years old they could only find fish,
without these amphibian characteristics. Evolutionary theory predicted
that since amphibians evolved from fish, an intermediate form should be
found in rock dated between 365 and 385 million years ago. Such an
intermediate form should have many fish-like characteristics, conserved
from 385 million years ago or more, but also have many amphibian
characteristics as well. In 2004, an expedition to islands in the
Canadian arctic searching specifically for this fossil form in rocks
that were 375 million years old discovered fossils of
Tiktaalik.
[71] Some years later, however, scientists in
Poland found evidence of fossilised
tetrapod tracks predating
Tiktaalik.
[72]
Evidence from geographical distribution
Data about the presence or absence of species on various
continents and
islands (
biogeography) can provide evidence of common descent and shed light on patterns of
speciation.
Continental distribution
All organisms are adapted to their environment to a greater or lesser extent. If the abiotic and biotic factors within a
habitat
are capable of supporting a particular species in one geographic area,
then one might assume that the same species would be found in a similar
habitat in a similar geographic area, e.g. in
Africa and
South America. This is not the case. Plant and animal species are discontinuously distributed throughout the world:
- Africa has Old World monkeys, apes, elephants, leopards, giraffes, and hornbills.
- South America has New World monkeys, cougars, jaguars, sloths, llamas, and toucans.
- Deserts in North and South America have native cacti, but deserts in Africa, Asia, and Australia have succulent (apart from Rhipsalis baccifera)[73] which are native euphorbs that resemble cacti but are very different.
Even greater differences can be found if
Australia is taken into consideration, though it occupies the same
latitude as much of South America and Africa.
Marsupials like
kangaroos,
bandicoots, and
quolls make up about half of Australia's indigenous mammal species.
[74]
By contrast, marsupials are today totally absent from Africa and form a
smaller portion of the mammalian fauna of South America, where
opossums,
shrew opossums, and the
monito del monte occur. The only living representatives of primitive egg-laying mammals (
monotremes) are the
echidnas and the
platypus. The short-beaked echidna (Tachyglossus aculeatus) and its subspecies populate Australia,
Tasmania,
New Guinea, and
Kangaroo Island
while the long-beaked echidna (Zaglossus bruijni) lives only in New
Guinea. The platypus lives in the waters of eastern Australia. They have
been introduced to Tasmania,
King Island, and Kangaroo Island. These Monotremes are totally absent in the rest of the world.
[75] On the other hand, Australia is missing many groups of
placental mammals that are common on other continents (
carnivorans,
artiodactyls,
shrews,
squirrels,
lagomorphs), although it does have indigenous
bats and
murine rodents; many other placentals, such as
rabbits and
foxes, have been introduced there by humans.
Other animal distribution examples include
bears,
located on all continents excluding Africa, Australia and Antarctica,
and the polar bear only located solely in the Arctic Circle and adjacent
land masses.
[76] Penguins are located only around the South Pole despite similar weather conditions at the North Pole. Families of
sirenians are distributed exclusively around the earth’s waters, where
manatees
are located in western Africa waters, northern South American waters,
and West Indian waters only while the related family, the
Dugongs, are located only in
Oceanic waters north of Australia, and the coasts surrounding the
Indian Ocean Additionally, the now extinct
Steller's Sea Cow resided in the
Bering Sea.
[77]
The same kinds of fossils are found from areas known to be adjacent to one another in the past but that, through the process of
continental drift,
are now in widely divergent geographic locations. For example, fossils
of the same types of ancient amphibians, arthropods and ferns are found
in South America, Africa, India, Australia and Antarctica, which can be
dated to the
Paleozoic Era, when these regions were united as a single landmass called
Gondwana.
[78]
Sometimes the descendants of these organisms can be identified and show
unmistakable similarity to each other, even though they now inhabit
very different regions and climates.
Island biogeography
Types of species found on islands
Evidence from
island biogeography has played an important and historic role in the development of
evolutionary biology. For purposes of
biogeography, islands are divided into two classes. Continental islands are islands like
Great Britain, and
Japan that have at one time or another been part of a continent. Oceanic islands, like the
Hawaiian islands, the
Galapagos islands and
St. Helena,
on the other hand are islands that have formed in the ocean and never
been part of any continent. Oceanic islands have distributions of native
plants and animals that are unbalanced in ways that make them distinct
from the
biotas
found on continents or continental islands. Oceanic islands do not have
native terrestrial mammals (they do sometimes have bats and seals),
amphibians, or fresh water fish. In some cases they have terrestrial
reptiles (such as the iguanas and giant tortoises of the Galapagos
islands) but often (for example Hawaii) they do not. This despite the
fact that when species such as rats, goats, pigs, cats, mice, and
cane toads, are introduced to such islands by humans they often thrive. Starting with
Charles Darwin,
many scientists have conducted experiments and made observations that
have shown that the types of animals and plants found, and not found, on
such islands are consistent with the theory that these islands were
colonized accidentally by plants and animals that were able to reach
them. Such accidental colonization could occur by air, such as plant
seeds carried by migratory birds, or bats and insects being blown out
over the sea by the wind, or by floating from a continent or other
island by sea, as for example by some kinds of plant seeds like coconuts
that can survive immersion in salt water, and reptiles that can survive
for extended periods on rafts of vegetation carried to sea by storms.
[79]
Endemism
Many of the species found on remote islands are
endemic
to a particular island or group of islands, meaning they are found
nowhere else on earth. Examples of species endemic to islands include
many flightless birds of
New Zealand,
lemurs of
Madagascar, the
Komodo dragon of
Komodo,
[80] the Dragon’s blood tree of
Socotra,
[81] Tuatara of New Zealand,
[82][83]
and others. However many such endemic species are related to species
found on other nearby islands or continents; the relationship of the
animals found on the Galapagos Islands to those found in South America
is a well-known example.
[79]
All of these facts, the types of plants and animals found on oceanic
islands, the large number of endemic species found on oceanic islands,
and the relationship of such species to those living on the nearest
continents, are most easily explained if the islands were colonized by
species from nearby continents that evolved into the endemic species now
found there.
[79]
Other types of endemism do not have to include, in the strict sense,
islands. Islands can mean isolated lakes or remote and isolated areas.
Examples of these would include the highlands of
Ethiopia,
Lake Baikal,
Fynbos of
South Africa, forests of
New Caledonia, and others. Examples of endemic organisms living in isolated areas include the
Kagu of New Caledonia,
[84] cloud rats of the
Luzon tropical pine forests of the
Philippines,
[85][86] the boojum tree (
Fouquieria columnaris) of the
Baja California peninsula,
[87] the
Baikal Seal[88] and the
omul of Lake Baikal.
Adaptive radiations
Oceanic islands are frequently inhabited by clusters of closely related species that fill a variety of
ecological niches, often niches that are filled by very different species on continents. Such clusters, like the Finches of the Galapagos,
Hawaiian honeycreepers, members of the sunflower family on the
Juan Fernandez Archipelago and wood weevils on St. Helena are called
adaptive radiations
because they are best explained by a single species colonizing an
island (or group of islands) and then diversifying to fill available
ecological niches. Such radiations can be spectacular; 800 species of
the fruit fly family
Drosophila, nearly half the world's total, are endemic to the Hawaiian islands. Another illustrative example from Hawaii is the
Silversword alliance, which is a group of thirty species found only on those islands. Members range from the
Silverswords
that flower spectacularly on high volcanic slopes to trees, shrubs,
vines and mats that occur at various elevations from mountain top to sea
level, and in Hawaiian habitats that vary from deserts to rainforests.
Their closest relatives outside Hawaii, based on molecular studies, are
tarweeds found on the west coast of North America. These tarweeds have sticky seeds that facilitate distribution by migrant birds.
[89] Additionally, nearly all of the species on the island can be crossed and the hybrids are often fertile,
[41] and they have been hybridized experimentally with two of the west coast tarweed species as well.
[90]
Continental islands have less distinct biota, but those that have been
long separated from any continent also have endemic species and adaptive
radiations, such as the 75
lemur species of
Madagascar, and the eleven extinct
moa species of
New Zealand.
[79][91]
Ring species
In biology, a ring species is a connected series of neighboring
populations that can interbreed with relatively closely related
populations, but for which there exist at least two "end" populations in
the series that are too distantly related to interbreed. Often such
non-breeding-though-genetically-connected populations co-exist in the
same region thus creating a "ring". Ring species provide important
evidence of evolution in that they illustrate what happens over time as
populations genetically diverge, and are special because they represent
in living populations what normally happens over time between long
deceased ancestor populations and living populations. If any of the
populations intermediate between the two ends of the ring were gone they
would not be a continuous line of reproduction and each side would be a
different species.
[92][93]
Specific examples
Figure 6a: Current distribution of Glossopteris
placed on a Permian map showing the connection of the continents. (1,
South America; 2, Africa; 3, Madagascar; 4, India; 5, Antarctica; and 6,
Australia)
Figure 6b: Present day distribution of marsupials. (Distribution shown in blue. Introduced areas shown in green.)
Figure 6c: A
dymaxion map
of the world showing the distribution of present species of camelid.
The solid black lines indicate migration routes and the blue represents
current camel locations.
Distribution of Glossopteris
The combination of continental drift and evolution can sometimes be used to predict what will be found in the fossil record.
Glossopteris is an extinct species of
seed fern plants from the
Permian.
Glossopteris appears in the fossil record around the beginning of the Permian on the ancient continent of
Gondwana.
[94] Continental drift explains the current biogeography of the tree. Present day
Glossopteris
fossils are found in Permian strata in southeast South America,
southeast Africa, all of Madagascar, northern India, all of Australia,
all of New Zealand, and scattered on the southern and northern edges of
Antarctica. During the Permian, these continents were connected as
Gondwana (see figure 6a) in agreement with magnetic striping, other
fossil distributions, and glacial scratches pointing away from the
temperate climate of the South Pole during the Permian.
[79][95]
Distribution of marsupials
The history of
marsupials
also provides an example of how the theories of evolution and
continental drift can be combined to make predictions about what will be
found in the fossil record. The
oldest metatherian fossils (
Metatheria being a larger clade that groups marsupials with some of their extinct relatives) are found in present-day
China.
[96]
Metatherians spread westward into modern North America (still attached
to Eurasia) and then to South America, which was connected to North
America until around 65 mya. Marsupials reached Australia via Antarctica
about 50 mya, shortly after Australia had split off suggesting a single
dispersion event of just one species.
[97]
The theory of evolution suggests that the Australian marsupials
descended from the older ones found in the Americas. The theory of
continental drift says that between 30 and 40 million years ago South
America and Australia were still part of the Southern hemisphere super
continent of
Gondwana
and that they were connected by land that is now part of Antarctica.
Therefore combining the two theories scientists predicted that
marsupials migrated from what is now South America across what is now
Antarctica to what is now Australia between 40 and 30 million years ago.
A first
marsupial fossil of the extinct family
Polydolopidae was found on
Seymour Island on the
Antarctic Peninsula in 1982.
[98] Further fossils have subsequently been found, including members of the marsupial orders
Didelphimorphia (opossum) and
Microbiotheria,
[99] as well as
ungulates and a member of the enigmatic extinct order
Gondwanatheria, possibly
Sudamerica ameghinoi.
[100][101][102]
Migration, isolation, and distribution of the Camel
The history of the
camel
provides an example of how fossil evidence can be used to reconstruct
migration and subsequent evolution. The fossil record indicates that the
evolution of
camelids
started in North America (see figure 6c), from which, six million years
ago, they migrated across the Bering Strait into Asia and then to
Africa, and 3.5 million years ago through the Isthmus of Panama into
South America. Once isolated, they evolved along their own lines, giving
rise to the
Bactrian camel and
Dromedary in Asia and Africa and the
llama and its relatives in South America. Camelids then went extinct in North America at the end of the last
ice age.
[103]
Evidence from observed natural selection
Examples for the evidence for evolution often stem from direct observation of
natural selection
in the field and the laboratory. Scientists have observed and
documented a multitude of events where natural selection is in action.
The most well known examples are antibiotic resistance in the medical
field along with better-known laboratory experiments documenting
evolution's occurrence. Natural selection is tantamount to common
descent in that long-term occurrence and selection pressures can lead to
the diversity of life on earth as found today. All
adaptations—documented and undocumented changes concerned—are caused by
natural selection (and a few other minor processes). The examples below
are only a small fraction of the actual experiments and observations.
Specific examples of natural selection in the lab and in the field
Antibiotic and pesticide resistance
The development and spread of antibiotic-resistant
bacteria, like the spread of
pesticide-resistant
forms of plants and insects, is evidence for evolution of species, and
of change within species. Thus the appearance of
vancomycin-resistant
Staphylococcus aureus, and the danger it poses to hospital patients, is a direct result of evolution through natural selection. The rise of
Shigella strains resistant to the synthetic antibiotic class of
sulfonamides also demonstrates the generation of new information as an evolutionary process.
[104] Similarly, the appearance of
DDT resistance in various forms of
Anopheles mosquitoes, and the appearance of
myxomatosis
resistance in breeding rabbit populations in Australia, are both
evidence of the existence of evolution in situations of evolutionary
selection pressure in species in which generations occur rapidly.
E. coli long-term evolution experiment
Experimental evolution uses controlled experiments to test hypotheses and theories of evolution. In one early example,
William Dallinger
set up an experiment shortly before 1880, subjecting microbes to heat
with the aim of forcing adaptive changes. His experiment ran for around
seven years, and his published results were acclaimed, but he did not
resume the experiment after the apparatus failed.
[105]
Richard Lenski observed that some strains of
E. coli evolved a complex new ability, the ability to metabolize
citrate, after tens of thousands of generations.
[106][107]
The evolutionary biologist Jerry Coyne commented, saying, "the thing I
like most is it says you can get these complex traits evolving by a
combination of unlikely events. That's just what creationists say can't
happen."
[106][108] The
E. coli
long-term evolution experiment that began in 1988 is still in progress,
and has shown adaptations including the evolution of a strain of
E. coli that was able to grow on citric acid in the growth media—a trait absent in all other known forms of
E. coli, including the initial strain.
Humans
Natural selection is observed in contemporary human populations, with
recent findings demonstrating that the population at risk of the severe
debilitating disease
kuru has significant over-representation of an immune variant of the
prion protein gene G127V versus non-immune alleles. Scientists postulate one of the reasons for the rapid selection of this
genetic variant is the lethality of the disease in non-immune persons.
[109][110]
Other reported evolutionary trends in other populations include a
lengthening of the reproductive period, reduction in cholesterol levels,
blood glucose and blood pressure.
[111]
Lactose tolerance in humans
Lactose intolerance is the inability to
metabolize lactose, because of a lack of the required enzyme
lactase
in the digestive system. The normal mammalian condition is for the
young of a species to experience reduced lactase production at the end
of the
weaning
period (a species-specific length of time). In humans, in non-dairy
consuming societies, lactase production usually drops about 90% during
the first four years of life, although the exact drop over time varies
widely.
[112]
However, certain human populations have a mutation on chromosome 2 that
eliminates the shutdown in lactase production, making it possible for
members of these populations to continue consumption of raw milk and
other fresh and fermented dairy products throughout their lives without
difficulty. This appears to be an evolutionarily recent (around 7,000
years) adaptation to dairy consumption, and has occurred independently
in both northern Europe and east Africa in populations with a
historically pastoral lifestyle.
[113][114]
Nylon-eating bacteria
Nylon-eating bacteria are a strain of
Flavobacterium that are capable of digesting certain byproducts of
nylon 6
manufacture. There is scientific consensus that the capacity to
synthesize nylonase most probably developed as a single-step mutation
that survived because it improved the fitness of the bacteria possessing
the mutation. This is seen as a good example of evolution through
mutation and natural selection that has been observed as it occurs.
[115][116][117][118]
PCB tolerance
After
General Electric dumped
polychlorinated biphenyls (PCBs) in the
Hudson River from 1947 through 1976,
tomcods living in the river were found to have evolved an increased resistance to the compound's toxic effects.
[119]
At first, the tomcod population was devastated, but it recovered.
Scientists identified the genetic mutation that conferred the
resistance. The mutated form was present in 99 per cent of the surviving
tomcods in the river, compared to fewer than 10 percent of the tomcods
from other waters.
[119]
Peppered moth
One classic example of adaptation in response to selection pressure
is the case of the peppered moth. The color of the moth has gone from
light to dark to light again over the course of a few hundred years due
to the appearance and later disappearance of pollution from the
Industrial Revolution in England.
Radiotrophic fungus
Radiotrophic fungi are
fungi that appear to use the pigment
melanin to convert
gamma radiation into chemical energy for growth
[120][121] and were first discovered in 2007 as black
molds growing inside and around the
Chernobyl Nuclear Power Plant.
[120] Research at the
Albert Einstein College of Medicine showed that three melanin-containing fungi,
Cladosporium sphaerospermum,
Wangiella dermatitidis, and
Cryptococcus neoformans, increased in
biomass and accumulated
acetate faster in an environment in which the
radiation level was 500 times higher than in the normal environment.
Urban wildlife
Urban wildlife is
wildlife that is able to live or thrive in
urban
environments. These types of environments can exert selection pressures
on organisms, often leading to new adaptations. For example, the weed
Crepis sancta,
found in France, has two types of seed, heavy and fluffy. The heavy
ones land nearby to the parent plant, whereas fluffy seeds float further
away on the wind. In urban environments, seeds that float far often
land on infertile concrete. Within about 5–12 generations, the weed
evolves to produce significantly heavier seeds than its rural relatives.
[122][123] Other examples of urban wildlife are
rock pigeons and species of crows adapting to city environments around the world; African penguins in
Simon's Town;
baboons in
South Africa; and a variety of insects living in human habitations.
Evidence from speciation
Speciation
is the evolutionary process by which new biological species arise.
Speciation can occur from a variety of different causes and are
classified in various forms (e.g. allopatric, sympatric,
polyploidization, etc.). Scientists have observed numerous examples of
speciation in the laboratory and in nature, however, evolution has
produced far more species than an observer would consider necessary.
For
example, there are well over 350,000 described species of beetles.
[124]
Great examples of observed speciation come from the observations of
island biogeography and the process of adaptive radiation, both
explained in an earlier section. The examples shown below provide strong
evidence for common descent and are only a small fraction of the
instances observed.
Specific examples
Blackcap
The bird species,
Sylvia atricapilla,
commonly referred to as Blackcaps, lives in Germany and flies southwest
to Spain while a smaller group flies northwest to Great Britain during
the winter. Gregor Rolshausen from the
University of Freiburg
found that the genetic separation of the two populations is already in
progress. The differences found have arisen in about 30 generations.
With DNA sequencing, the individuals can be assigned to a correct group
with an 85% accuracy. Stuart Bearhop from the
University of Exeter
reported that birds wintering in England tend to mate only among
themselves, and not usually with those wintering in the Mediterranean.
[125]
It is still inference to say that the populations will become two
different species, but researchers expect it due to the continued
genetic and geographic separation.
[126]
Drosophila melanogaster
A common fruit fly (Drosophila melanogaster).
William R. Rice and George W. Salt found experimental evidence of
sympatric speciation in the
common fruit fly. They collected a population of
Drosophila melanogaster from
Davis, California
and placed the pupae into a habitat maze. Newborn flies had to
investigate the maze to find food. The flies had three choices to take
in finding food. Light and dark (
phototaxis), up and down (
geotaxis), and the scent of
acetaldehyde and the scent of ethanol (
chemotaxis) were the three options. This eventually divided the flies into 42 spatio-temporal habitats.
They then cultured two strains that chose opposite habitats. One of
the strains emerged early, immediately flying upward in the dark
attracted to the
acetaldehyde.
The other strain emerged late and immediately flew downward, attracted
to light and ethanol. Pupae from the two strains were then placed
together in the maze and allowed to mate at the food site. They then
were collected. A selective penalty was imposed on the female flies that
switched habitats. This entailed that none of their
gametes
would pass on to the next generation. After 25 generations of this
mating test, it showed reproductive isolation between the two strains.
They repeated the experiment again without creating the penalty against
habitat switching and the result was the same; reproductive isolation
was produced.
[127][128][129]
Hawthorn fly
One example of evolution at work is the case of the hawthorn fly,
Rhagoletis pomonella, also known as the apple maggot fly, which appears to be undergoing
sympatric speciation.
[130]
Different populations of hawthorn fly feed on different fruits. A
distinct population emerged in North America in the 19th century some
time after
apples,
a non-native species, were introduced. This apple-feeding population
normally feeds only on apples and not on the historically preferred
fruit of
hawthorns.
The current hawthorn feeding population does not normally feed on
apples. Some evidence, such as the fact that six out of thirteen
allozyme
loci are different, that hawthorn flies mature later in the season and
take longer to mature than apple flies; and that there is little
evidence of interbreeding (researchers have documented a 4–6%
hybridization rate) suggests that speciation is occurring.
[131][132][133][134][135]
London Underground mosquito
The
London Underground mosquito is a species of
mosquito in the genus
Culex found in the
London Underground. It evolved from the overground species
Culex pipiens.
This mosquito, although first discovered in the London Underground
system, has been found in underground systems around the world. It is
suggested that it may have adapted to human-made underground systems
since the last century from local above-ground
Culex pipiens,
[136]
although more recent evidence suggests that it is a southern mosquito
variety related to Culex pipiens that has adapted to the warm
underground spaces of northern cities.
[137]
The species have very different behaviours,
[138] are extremely difficult to mate,
[136] and with different allele frequency, consistent with genetic drift during a
founder event.
[139] More specifically, this mosquito,
Culex pipiens molestus, breeds all-year round, is cold intolerant, and bites rats, mice, and humans, in contrast to the above ground species
Culex pipiens
that is cold tolerant, hibernates in the winter, and bites only birds.
When the two varieties were cross-bred the eggs were infertile
suggesting reproductive isolation.
[136][138]
The genetic data indicates that the
molestus form in the
London Underground mosquito appears to have a common ancestry, rather
than the population at each station being related to the nearest
aboveground population (i.e. the
pipiens form). Byrne and
Nichols' working hypothesis was that adaptation to the underground
environment had occurred locally in London only once.
These widely separated populations are distinguished by very minor
genetic differences, which suggest that the molestus form developed: a
single
mtDNA difference shared among the underground populations of ten Russian cities;
[140] a single fixed
microsatellite difference in populations spanning Europe, Japan, Australia, the middle East and Atlantic islands.
[137]
Mollies
The Shortfin Molly (
Poecilia mexicana) is a small fish that lives in the
Sulfur Caves
of Mexico. Years of study on the species have found that two distinct
populations of mollies—the dark interior fish and the bright surface
water fish—are becoming more genetically divergent.
[141]
The populations have no obvious barrier separating the two; however, it
was found that the mollies are hunted by a large water bug (
Belostoma spp).
Tobler collected the bug and both types of mollies, placed them in
large plastic bottles, and put them back in the cave. After a day, it
was found that, in the light, the cave-adapted fish endured the most
damage, with four out of every five stab-wounds from the water bugs
sharp mouthparts. In the dark, the situation was the opposite. The
mollies’ senses can detect a predator’s threat in their own habitats,
but not in the other ones. Moving from one habitat to the other
significantly increases the risk of dying. Tobler plans on further
experiments, but believes that it is a good example of the rise of a new
species.
[142]\
Polar bear
A remarkable example of natural selection, geographic isolation, and speciation in progress is the relationship of the
polar bear (
Ursus maritimus) and the
brown bear (
Ursus arctos).
Once thought to be two entirely different species, recent evidence
suggests that both bears can interbreed and produce fertile offspring.
Molecular data gives estimates of a divergence time ranging from 70,000
to 1.5 million years ago. The oldest known fossil evidence of polar
bears dates around 100,000 years ago. Scientists hypothesize that around
200,000 years ago (when the Arctic Ocean was entirely covered with ice
and the earth was at its near-glacial maximum), glaciers isolated a
population of brown bears (approximately 125,000 years ago) of which
evolved over time adapting to their environment.
[143] This process is known as
allopatric speciation.
The bears acquired significant physiological differences from the brown
bear allowing the polar bear to comfortably survive in conditions that
the brown bear could not. The ability to swim sixty miles or more at a
time in freezing waters, to blend in with the snow, and to stay warm in
the arctic environment are some of the adaptations of the polar bear.
Additionally, the elongation of the neck makes it easier to keep their
heads above water while swimming alongside the oversized webbed feet
that act as paddles when swimming. The polar bear has also evolved small
papillae and vacuole-like suction cups on the soles to make them less
likely to slip on the ice alongside the fact that their feet have become
covered with heavy matting to protect the bottoms from intense cold and
to provide traction. They also have smaller ears for a reduction of
heat loss, eyelids that act like sunglasses, accommodations for their
all-meat diet, a large stomach capacity to enable opportunistic feeding,
and the ability to fast for up to nine months while recycling their
urea.
[144][145]
Despite all these differing traits, the two bear species have now been
reunited due to the warming of the Arctic and the receding glaciers.
Surprisingly, the bears can interbreed but
Ursus maritimus is considered a subspecies of
Ursus arctos.
This example presents a macro-evolutionary change involving an
amalgamation of several fields of evolutionary biology, e.g. adaptation
through natural selection, geographic isolation, speciation, and
hybridization.
Thale cress
Arabidopsis thaliana (colloquially known as thale cress, mouse-ear cress or Arabidopsis).
Kirsten Bomblies et al. from the
Max Planck Institute for Developmental Biology discovered that two genes passed down by each parent of the thale cress plant,
Arabidopsis thaliana.
When the genes are passed down, it ignites a reaction in the hybrid
plant that turns its own immune system against it. In the parents, the
genes were not detrimental, but they evolved separately to react
defectively when combined.
[146]
To test this, Bomblies crossed 280 genetically different strains of
Arabidopsis
in 861 distinct ways and found that 2 per cent of the resulting hybrids
were necrotic. Along with allocating the same indicators, the 20 plants
also shared a comparable collection of genetic activity in a group of
1,080 genes. In almost all of the cases, Bomblies discovered that only
two genes were required to cause the autoimmune response. Bomblies
looked at one hybrid in detail and found that one of the two genes
belonged to the
NB-LRR class,
a common group of disease resistance genes involved in recognizing new
infections. When Bomblies removed the problematic gene, the hybrids
developed normally.
[146]
Over successive generations, these incompatibilities could create
divisions between different plant strains, reducing their chances of
successful mating and turning distinct strains into separate species.
[147]
Interspecies fertility or hybridization
Understood from laboratory studies and observed instances of
speciation in nature, finding species that are able reproduce
successfully or create
hybrids
between two different species infers that their relationship is close.
In conjunction with this, hybridization has been found to be a precursor
to the creation of new species by being a source of new genes for a
species. The examples provided are only a small fraction of the observed
instances of speciation through hybridization. Plants are often subject
to the creation of a new species though hybridization.
Mimulus peregrinus
The creation of a new allopolyploid species (
Mimulus peregrinus)
was observed on the banks of the Shortcleuch Water—a river in
Leadhills, South Lanarkshire, Scotland. Parented from the cross of the
two species
Mimulus guttatus (containing 14 pairs of chromosomes) and
Mimulus luteus (containing 30-31 pairs from a chromosome duplication),
M. peregrinus
has six copies of its chromosomes (caused by the duplication of the
sterile hybrid triploid). Due to the nature of these species, they have
the ability to self-fertilize. Because of its number of chromosomes it
is not able to pair with
M. guttatus,
M. luteus, or their sterile triploid offspring.
M. peregrinus will either die producing no offspring or reproduce with itself making a new species.
[148]
Raphanobrassica
Raphanobrassica includes all
intergeneric hybrids between the genera
Raphanus (radish) and
Brassica (cabbages, etc.).
[149][150]
The
Raphanobrassica is an
allopolyploid cross between the
radish (
Raphanus sativus) and
cabbage (
Brassica oleracea). Plants of this parentage are now known as radicole. Two other fertile forms of
Raphanobrassica are known. Raparadish, an allopolyploid hybrid between
Raphanus sativus and
Brassica rapa is grown as a fodder crop. "Raphanofortii" is the allopolyploid hybrid between
Brassica tournefortii and
Raphanus caudatus.
The
Raphanobrassica is a fascinating plant, because (in spite
of its hybrid nature), it is not sterile. This has led some botanists to
propose that the accidental hybridization of a flower by pollen of
another species in nature could be a mechanism of speciation common in
higher plants.
Salsify
Purple Salsify, Tragopogon porrifolius
Tragopogon is one example where
hybrid speciation
has been observed. In the early 20th century, humans introduced three
species of salsify into North America. These species, the western
salsify (
Tragopogon dubius), the meadow salsify (
Tragopogon pratensis), and the
oyster plant (
Tragopogon porrifolius), are now common weeds in urban wastelands. In the 1950s, botanists found two new species in the regions of
Idaho and
Washington, where the three already known species overlapped. One new species,
Tragopogon miscellus, is a
tetraploid hybrid of
T. dubius and
T. pratensis. The other new species,
Tragopogon mirus, is also an allopolyploid, but its ancestors were
T. dubius and
T. porrifolius. These new species are usually referred to as "the Ownbey hybrids" after the botanist who first described them. The
T. mirus population grows mainly by reproduction of its own members, but additional episodes of hybridization continue to add to the
T. mirus population.
[151]
T. dubius and
T. pratensis mated in Europe but were
never able to hybridize. A study published in March 2011 found that when
these two plants were introduced to North America in the 1920s, they
mated and doubled the number of chromosomes in there hybrid
Tragopogon miscellus allowing for a "reset" of its genes, which in turn, allows for greater genetic variation. Professor Doug Soltis of the
University of Florida
said, "We caught evolution in the act…New and diverse patterns of gene
expression may allow the new species to rapidly adapt in new
environments".
[152][153]
This observable event of speciation through hybridization further
advances the evidence for the common descent of organisms and the time
frame in which the new species arose in its new environment. The
hybridizations have been reproduced artificially in laboratories from
2004 to present day.
Welsh groundsel
Welsh groundsel is an allopolyploid, a plant that contains sets of
chromosomes originating from two different species. Its ancestor was
Senecio × baxteri, an infertile hybrid that can arise spontaneously when the closely related groundsel (
Senecio vulgaris) and Oxford ragwort (
Senecio squalidus) grow alongside each other. Sometime in the early 20th century, an accidental doubling of the number of chromosomes in an
S. × baxteri plant led to the formation of a new fertile species.
[154][155]
York groundsel
The
York groundsel (
Senecio eboracensis) is a hybrid species of the
self-incompatible Senecio squalidus (also known as Oxford ragwort) and the self-compatible
Senecio vulgaris (also known as Common groundsel). Like
S. vulgaris,
S. eboracensis
is self-compatible, however, it shows little or no natural crossing
with its parent species, and is therefore reproductively isolated,
indicating that strong breed barriers exist between this new hybrid and
its parents.
It resulted from a
backcrossing of the
F1 hybrid of its parents to
S. vulgaris.
S. vulgaris is native to Britain, while
S. squalidus was introduced from Sicily in the early 18th century; therefore,
S. eboracensis has speciated from those two species within the last 300 years.
Other hybrids descended from the same two parents are known. Some are infertile, such as
S. x
baxteri. Other fertile hybrids are also known, including
S. vulgaris var. hibernicus, now common in Britain, and the
allohexaploid S. cambrensis,
which according to molecular evidence probably originated independently
at least three times in different locations. Morphological and genetic
evidence support the status of
S. eboracensis as separate from other known hybrids.
[156]
Evidence from artificial selection
Artificial selection
demonstrates the diversity that can exist among organisms that share a
relatively recent common ancestor. In artificial selection, one species
is bred selectively at each generation, allowing only those organisms
that exhibit desired characteristics to reproduce. These characteristics
become increasingly well developed in successive generations.
Artificial selection was successful long before science discovered the
genetic basis. Examples of artificial selection would be
dog breeding,
genetically modified food, flower breeding, cultivation of foods such as
wild cabbage,
[157] and others.
Evidence from computation and mathematical iteration
Computer science allows the
iteration of self-changing
complex systems
to be studied, allowing a mathematical understanding of the nature of
the processes behind evolution; providing evidence for the hidden causes
of known evolutionary events. The evolution of specific cellular
mechanisms like
spliceosomes
that can turn the cell's genome into a vast workshop of billions of
interchangeable parts that can create tools that create tools that
create tools that create us can be studied for the first time in an
exact way.
"It has taken more than five decades, but the electronic computer is now powerful enough to simulate evolution,"
[158] assisting
bioinformatics in its attempt to solve biological problems.
Computational evolutionary biology has enabled researchers to trace
the evolution of a large number of organisms by measuring changes in
their DNA, rather than through physical taxonomy or physiological
observations alone. It has compared entire genomes permitting the study
of more complex evolutionary events, such as
gene duplication,
horizontal gene transfer,
and the prediction of factors important in speciation. It has also
helped build complex computational models of populations to predict the
outcome of the system over time and track and share information on an
increasingly large number of species and organisms.
Future endeavors are to reconstruct a now more complex tree of life.
Christoph Adami, a professor at the
Keck Graduate Institute made this point in
Evolution of biological complexity:
- To make a case for or against a trend in the evolution of complexity
in biological evolution, complexity must be both rigorously defined and
measurable. A recent information-theoretic (but intuitively evident)
definition identifies genomic complexity with the amount of information a
sequence stores about its environment. We investigate the evolution of
genomic complexity in populations of digital organisms and monitor in
detail the evolutionary transitions that increase complexity. We show
that, because natural selection forces genomes to behave as a natural "Maxwell Demon", within a fixed environment, genomic complexity is forced to increase.[159]
David J. Earl and Michael W. Deem—professors at
Rice University made this point in
Evolvability is a selectable trait:
- Not only has life evolved, but life has evolved to evolve. That is,
correlations within protein structure have evolved, and mechanisms to
manipulate these correlations have evolved in tandem. The rates at which
the various events within the hierarchy of evolutionary moves occur are
not random or arbitrary but are selected by Darwinian evolution.
Sensibly, rapid or extreme environmental change leads to selection for
greater evolvability. This selection is not forbidden by causality and
is strongest on the largest-scale moves within the mutational hierarchy.
Many observations within evolutionary biology, heretofore considered
evolutionary happenstance or accidents, are explained by selection for
evolvability. For example, the vertebrate immune system shows that the
variable environment of antigens has provided selective pressure for the
use of adaptable codons and low-fidelity polymerases during somatic hypermutation.
A similar driving force for biased codon usage as a result of
productively high mutation rates is observed in the hemagglutinin
protein of influenza A.[160]
"Computer simulations of the evolution of linear sequences have
demonstrated the importance of recombination of blocks of sequence
rather than point mutagenesis alone. Repeated cycles of point
mutagenesis, recombination, and selection should allow
in vitro molecular evolution of complex sequences, such as proteins."
[161] Evolutionary molecular engineering, also called directed evolution or
in vitro
molecular evolution involves the iterated cycle of mutation,
multiplication with recombination, and selection of the fittest of
individual molecules (proteins, DNA, and RNA). Natural evolution can be
relived showing us possible paths from catalytic cycles based on
proteins to based on RNA to based on DNA.
[161][162][163][164]
Specific examples
Avida simulation
Richard Lenski, Charles Ofria, et al. at
Michigan State University developed an
artificial life
computer program with the ability to detail the evolution of complex
systems. The system uses values set to determine random mutations and
allows for the effect of natural selection to conserve beneficial
traits. The program was dubbed Avida and starts with an artificial petri
dish where organisms reproduce and perform mathematical calculations to
acquire rewards of more computer time for replication. The program
randomly adds mutations to copies of the artificial organisms to allow
for natural selection. As the artificial life reproduced, different
lines adapted and evolved depending on their set environments. The
beneficial side to the program is that it parallels that of real life at
rapid speeds.
[165][166][167]
