Convergent and Divergent Evolution
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Convergent Evolution
Example: Two succulent plant genera, Euphorbia and Astrophytum, are only distantly related, but have independently converged on a similar body form.
Convergent evolution describes the independent evolution of similar features in species of different lineages. Convergent evolution creates analogous structures that have similar form or function, but that were not present in the last common ancestor of those groups.[1] The cladistic term for the same phenomenon is homoplasy, from Greek for same form.[2] The recurrent evolution of flight is a classic example of convergent evolution. Flying insects, birds, and bats have all evolved the capacity of flight independently. They have "converged" on this useful trait.
Functionally similar features arising through convergent evolution are termed analogous, in contrast to homologous structures or traits, which have a common origin, but not necessarily similar function.[1]
The British anatomist Richard Owen was the first scientist to recognise the fundamental difference between analogies and homologies.[3] Bat and pterosaur wings constitute an example of analogous structures, while the bat wing is homologous to human and other mammal forearms, sharing an ancestral state despite serving different functions. The opposite of convergent evolution is divergent evolution, whereby related species evolve different traits. On a molecular level, this can happen due to random mutation unrelated to adaptive changes; see long branch attraction.
Convergent evolution is similar to, but distinguishable from, the phenomena of parallel evolution. Parallel evolution occurs when two independent but similar species evolve in the same direction and thus independently acquire similar characteristics—for instance gliding frogs have evolved in parallel from multiple types of tree frog.
Causes
In morphology, analogous traits will often arise where different species live in similar ways and/or similar environment, and so face the same environmental factors. When occupying similar ecological niches (that is, a distinctive way of life) similar problems lead to similar solutions.[4]
In biochemistry, physical and chemical constraints on mechanisms cause some active site arrangements to independently evolve multiple times in separate enzyme superfamilies (for example, see also catalytic triad).[5]
Significance
Convergence has been associated with Darwinian evolution in the popular imagination since at least the 1940s. For example, Elbert A. Rogers argued: "If we lean toward the theories of Darwin might we not assume that man was [just as] apt to have developed in one continent as another?"[6]
In his book Wonderful Life, Stephen Jay Gould argues that if the tape of life were re-wound and played back, life would have taken a very different course.[7] Simon Conway Morris disputes this conclusion, arguing that convergence is a dominant force in evolution, and given that the same environmental and physical constraints are at work, life will inevitably evolve toward an "optimum" body plan, and at some point, evolution is bound to stumble upon intelligence, a trait presently identified with at least primates, corvids, and cetaceans.[8]
Convergence is difficult to quantify, so progress on this issue may require exploitation of engineering specifications (as of wing aerodynamics) and comparably rigorous measures of "very different course" in terms of phylogenetic (molecular) distances.[citation needed]
Distinctions
Convergent evolution is a topic touched by many different fields of biology, many of which use slightly different nomenclature. This section attempts to clarify some of those terms.
Cladistic definition
In cladistics, a homoplasy or a homoplastic character state is a trait (genetic, morphological etc.) that is shared by two or more taxa because of convergence, parallelism or reversal.[9] Homoplastic character states require extra steps to explain their distribution on a most parsimonious cladogram.
Homoplasy is only recognizable when other characters imply an alternative hypothesis of grouping, because in the absence of such evidence, shared features are always interpreted as similarity due to common descent.[10] Homoplasious traits or changes (derived trait values acquired in unrelated organisms in parallel) can be compared with synapomorphy (a derived trait present in all members of a monophyletic clade), autapomorphy (derived trait present in only one member of a clade), or apomorphies, derived traits acquired in all members of a monophyletic clade following divergence where the most recent common ancestor had the ancestral trait (the ancestral trait manifesting in paraphyletic species as a plesiomorphy).
Re-evolution vs. convergent evolution
In some cases, it is difficult to tell whether a trait has been lost then re-evolved convergently, or whether a gene has simply been 'switched off' and then re-enabled later. Such a re-emerged trait is called an atavism. From a mathematical standpoint, an unused gene (selectively neutral) has a steadily decreasing probability of retaining potential functionality over time. The time scale of this process varies greatly in different phylogenies; in mammals and birds, there is a reasonable probability of remaining in the genome in a potentially functional state for around 6 million years.[11]
Parallel vs. convergent evolution
Evolution at an amino acid position. In each case, the left-hand species changes from incorporating alanine (A) at a specific position within a protein in a hypothetical common ancestor deduced from comparison of sequences of several species, and now incorporates serine (S) in its present-day form. The right-hand species may undergo divergent, parallel, or convergent evolution at this amino acid position relative to that of the first species.
For a particular trait, proceeding in each of two lineages from a specified ancestor to a later descendant, parallel and convergent evolutionary trends can be strictly defined and clearly distinguished from one another.[12] However the cutoff point for what is considered convergent and what is considered parallel evolution is assigned somewhat arbitrarily. When two species are similar in a particular character, evolution is defined as parallel if the ancestors were also similar and convergent if they were not. However, this definition is somewhat murky. All organisms share a common ancestor more or less recently, so the question of how far back to look in evolutionary time and how similar the ancestors need to be for one to consider parallel evolution to have taken place is not entirely resolved within evolutionary biology. Some scientists have argued parallel evolution and convergent evolution are more or less indistinguishable from one another.[13] Others have argued that we should not shy away from the gray area and that there are still important distinctions between parallel and convergent evolution.[14]
When the ancestral forms are unspecified or unknown, or the range of traits considered is not clearly specified, the distinction between parallel and convergent evolution becomes more subjective. For instance, the striking example of similar placental and marsupial forms is described by Richard Dawkins in The Blind Watchmaker as a case of convergent evolution,[15] because mammals on each continent had a long evolutionary history prior to the extinction of the dinosaurs under which to accumulate relevant differences. Stephen Jay Gould describes many of the same examples as parallel evolution starting from the common ancestor of all marsupials and placentals. Many evolved similarities can be described in concept as parallel evolution from a remote ancestor, with the exception of those where quite different structures are co-opted to a similar function. For example, consider Mixotricha paradoxa, a microbe that has assembled a system of rows of apparent cilia and basal bodies closely resembling that of ciliates but that are actually smaller symbiont micro-organisms, or the differently oriented tails of fish and whales. On the converse, any case in which lineages do not evolve together at the same time in the same ecospace might be described as convergent evolution at some point in time.
The definition of a trait is crucial in deciding whether a change is seen as divergent, or as parallel or convergent. In the image above, note that, since serine and threonine possess similar structures with an alcohol side-chain, the example marked "divergent" would be termed "parallel" if the amino acids were grouped by similarity instead of being considered individually. As another example, if genes in two species independently become restricted to the same region of the animals through regulation by a certain transcription factor, this may be described as a case of parallel evolution — but examination of the actual DNA sequence will probably show only divergent changes in individual base-pair positions, since a new transcription factor binding site can be added in a wide range of places within the gene with similar effect.
A similar situation occurs considering the homology of morphological structures. For example, many insects possess two pairs of flying wings. In beetles, the first pair of wings is hardened into wing covers with little role in flight, while in flies the second pair of wings is condensed into small halteres used for balance. If the two pairs of wings are considered as interchangeable, homologous structures, this may be described as a parallel reduction in the number of wings, but otherwise the two changes are each divergent changes in one pair of wings.
Similar to convergent evolution, evolutionary relay describes how independent species acquire similar characteristics through their evolution in similar ecosystems, but not at the same time (dorsal fins of sharks and ichthyosaurs).
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Divergent Evolution
Darwin's finches are a clear and famous example of divergent evolution, in which an ancestral species radiates into a number of descendant species with both similar and different traits.
Divergent evolution is the accumulation of differences between groups which can lead to the formation of new species, usually a result of diffusion of the same species to different and isolated environments which blocks the gene flow among the distinct populations allowing differentiated fixation of characteristics through genetic drift and natural selection. Primarily diffusion is the basis of molecular division can be seen in some higher-level characters of structure and function that are readily observable in organisms. For example, the vertebrate limb is one example of divergent evolution. The limb in many different species has a common origin, but has diverged somewhat in overall structure and function.[citation needed]
Alternatively, "divergent evolution" can be applied to molecular biology characteristics. This could apply to a pathway in two or more organisms or cell types, for example. This can apply to genes and proteins, such as nucleotide sequences or protein sequences that derive from two or more homologous genes. Both orthologous genes (resulting from a speciation event) and paralogous genes (resulting from gene duplication within a population) can be said to display divergent evolution. Because of the latter, it is possible for divergent evolution to occur between two genes within a species.
In the case of divergent evolution, similarity is due to the common origin, such as divergence from a common ancestral structure or function has not yet completely obscured the underlying similarity. In contrast, convergent evolution arises when there are some sort of ecological or physical drivers toward a similar solution, even though the structure or function has arisen independently, such as different characters converging on a common, similar solution from different points of origin. This includes analogous structures.
Usage
J. T. Gulick founded the usage of this term[1] and other related terms, which can vary slightly from one researcher to the next. Furthermore, the actual relationships might be more complex than the simple definitions of these terms allow. "Divergent evolution" is most commonly meant when someone invokes evolutionary relationships and "convergent evolution" is applied when similarity is created by evolution independently creating similar structures and functions. The term parallel evolution is also sometimes used to describe the appearance of a similar structure in closely related species, whereas convergent evolution is used primarily to refer to similar structures in much more distantly related clades. For example, some might call the modification of the vertebrate limb to become a wing in bats and birds to be an example of parallel evolution. Vertebrate forelimbs have a common origin and thus, in general, show divergent evolution. However, the modification to the specific structure and function of a wing evolved independently and in parallel within several different vertebrate clades. Also, it has much to do with humans and the way they function from day to day.In complex structures, there may be other cases where some aspects of the structures are due to divergence and some aspects that might be due to convergence or parallelism. In the case of the eye, it was initially thought that different clades had different origins of the eye, but this is no longer thought by some researchers. It is possible that induction of the light-sensing eye during development might be diverging from a common ancestor across many clades, but the details of how the eye is constructed—and in particular the structures that focus light in cephalopods and vertebrates, for example—might have some convergent or parallel aspects to it, as well.[2]
A good example of a divergent evolution is Darwin's finches, which now have over 80 varieties which all diverged from one original species of finch. (John Barnes)Another example of divergent evolution are the organisms having the 5 digit pentadactyle limbs like the humans, bats, and whales.
They have evolved from a common ancestor but, today they are different due to environmental pressures
Divergent species
Divergent species are a consequence of divergent evolution. The divergence of one species into two or more descendant species can occur in four major ways:[3]
- Allopatric speciation occurs when a population becomes separated into two entirely isolated subpopulations. Once the separation occurs, natural selection and genetic drift operate on each subpopulation independently, producing different evolutionary outcomes.
- Peripatric speciation is somewhat similar to allopatric speciation, but specifically occurs when a very small subpopulation becomes isolated from a much larger majority. Because the isolated subpopulation is so small, divergence can happen relatively rapidly due to the founder effect, in which small populations are more sensitive to genetic drift and natural selection acts on a small gene pool.
- Parapatric speciation occurs when a small subpopulation remains within the habitat of an original population but enters a different niche. Effects other than physical separation prevent interbreeding between the two separated populations. Because one of the genetically isolated populations is so small, however, the founder effect can still play a role in speciation.
- Sympatric speciation, the rarest and most controversial form of speciation, occurs with no form of isolation (physical or otherwise) between two populations.
An example of divergent species is the apple maggot fly. The apple maggot fly once infested the fruit of a native Australian hawthorn. In the 1860s some maggot flies began to infest apples. They multiplied rapidly because they were able to make use of an abundant food supply. Now there are two distinct species, one that reproduces when the apples are ripe, and another that continues to infest the native hawthorn. Furthermore, they have not only evolved different reproductive timing, but also now have distinctive physical characteristics.
