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Tuesday, October 15, 2024

Coloration evidence for natural selection

Natural selection has driven the ptarmigan to change from snow camouflage in winter to disruptive coloration suiting moorland in summer.
Selective breeding transformed teosinte's small spikes (left) into modern maize (right). Darwin argued that evolution worked in a similar way.

Animal coloration provided important early evidence for evolution by natural selection, at a time when little direct evidence was available. Three major functions of coloration were discovered in the second half of the 19th century, and subsequently used as evidence of selection: camouflage (protective coloration); mimicry, both Batesian and Müllerian; and aposematism.

Charles Darwin's On the Origin of Species was published in 1859, arguing from circumstantial evidence that selection by human breeders could produce change, and that since there was clearly a struggle for existence, that natural selection must be taking place. But he lacked an explanation either for genetic variation or for heredity, both essential to the theory. Many alternative theories were accordingly considered by biologists, threatening to undermine Darwinian evolution.

Some of the first evidence was provided by Darwin's contemporaries, the naturalists Henry Walter Bates and Fritz Müller. They described forms of mimicry that now carry their names, based on their observations of tropical butterflies. These highly specific patterns of coloration are readily explained by natural selection, since predators such as birds which hunt by sight will more often catch and kill insects that are less good mimics of distasteful models than those that are better mimics, but the patterns are otherwise hard to explain.

Darwinists such as Alfred Russel Wallace and Edward Bagnall Poulton, and in the 20th century Hugh Cott and Bernard Kettlewell, sought evidence that natural selection was taking place. Wallace noted that snow camouflage, especially plumage and pelage that changed with the seasons, suggested an obvious explanation as an adaptation for concealment. Poulton's 1890 book, The Colours of Animals, written during Darwinism's lowest ebb, used all the forms of coloration to argue the case for natural selection. Cott described many kinds of camouflage, and in particular his drawings of coincident disruptive coloration in frogs convinced other biologists that these deceptive markings were products of natural selection. Kettlewell experimented on peppered moth evolution, showing that the species had adapted as pollution changed the environment; this provided compelling evidence of Darwinian evolution.

Context

Charles Darwin published On the Origin of Species in 1859, arguing that evolution in nature must be driven by natural selection, just as breeds of domestic animals and cultivars of crop plants were driven by artificial selection. Darwin's theory radically altered popular and scientific opinion about the development of life. However, he lacked evidence and explanations for some critical components of the evolutionary process. He could not explain the source of variation in traits within a species, and did not have a mechanism of heredity that could pass traits faithfully from one generation to the next. This made his theory vulnerable; alternative theories were being explored during the eclipse of Darwinism; and so Darwinian field naturalists like Wallace, Bates and Müller looked for clear evidence that natural selection actually occurred. Animal coloration, readily observable, soon provided strong and independent lines of evidence, from camouflage, mimicry and aposematism, that natural selection was indeed at work. The historian of science Peter J. Bowler wrote that Darwin's theory "was also extended to the broader topics of protective resemblances and mimicry, and this was its greatest triumph in explaining adaptations".

Camouflage

Snow camouflage

Convergent evolution of snow camouflage in Arctic hare, ermine, and ptarmigan provided early evidence for natural selection.

In his 1889 book Darwinism, the naturalist Alfred Russel Wallace considered the white coloration of Arctic animals. He recorded that the Arctic fox, Arctic hare, ermine and ptarmigan change their colour seasonally, and gave "the obvious explanation", that it was for concealment. The modern ornithologist W. L. N. Tickell, reviewing proposed explanations of white plumage in birds, writes that in the ptarmigan "it is difficult to escape the conclusion that cryptic brown summer plumage becomes a liability in snow, and white plumage is therefore another cryptic adaptation." All the same, he notes, "in spite of winter plumage, many Ptarmigan in NE Iceland are killed by Gyrfalcons throughout the winter."

More recently, decreasing snow cover in Poland, caused by global warming, is reflected in a reduced percentage of white-coated weasels that become white in winter. Days with snow cover halved between 1997 and 2007, and as few as 20 percent of the weasels had white winter coats. This was shown to be a result of natural selection by predators making use of camouflage mismatch.

Coincident disruptive coloration

Hugh Cott's drawings of 'coincident disruptive coloration' formed "persuasive arguments" for natural selection. Left: active; right: at rest, marks coinciding.

In the words of camouflage researchers Innes Cuthill and A. Székely, the English zoologist and camouflage expert Hugh Cott's 1940 book Adaptive Coloration in Animals provided "persuasive arguments for the survival value of coloration, and for adaptation in general, at a time when natural selection was far from universally accepted within evolutionary biology."

In particular, they argue, "Coincident Disruptive Coloration" (one of Cott's categories) "made Cott's drawings the most compelling evidence for natural selection enhancing survival through disruptive camouflage."

Cott explained, while discussing "a little frog known as Megalixalus fornasinii" in his chapter on coincident disruptive coloration, that "it is only when the pattern is considered in relation to the frog's normal attitude of rest that its remarkable nature becomes apparent... The attitude and very striking colour-scheme thus combine to produce an extraordinary effect, whose deceptive appearance depends upon the breaking up of the entire form into two strongly contrasted areas of brown and white. Considered separately, neither part resembles part of a frog. Together in nature the white configuration alone is conspicuous. This stands out and distracts the observer's attention from the true form and contour of the body and appendages on which it is superimposed".

Cott concluded that the effect was concealment "so long as the false configuration is recognized in preference to the real one". Such patterns embody, as Cott stressed, considerable precision as the markings must line up accurately for the disguise to work. Cott's description and in particular his drawings convinced biologists that the markings, and hence the camouflage, must have survival value (rather than occurring by chance); and further, as Cuthill and Székely indicate, that the bodies of animals that have such patterns must indeed have been shaped by natural selection.

Bernard Kettlewell claimed that changes in the frequencies of light and dark morphs of the peppered moth, Biston betularia were direct evidence of natural selection.

Industrial melanism

Between 1953 and 1956, the geneticist Bernard Kettlewell experimented on peppered moth evolution. He presented results showing that in a polluted urban wood with dark tree trunks, dark moths survived better than pale ones, causing industrial melanism, whereas in a clean rural wood with paler trunks, pale moths survived better than dark ones. The implication was that survival was caused by camouflage against suitable backgrounds, where predators hunting by sight (insect-eating birds, such as the great tits used in the experiment) selectively caught and killed the less well-camouflaged moths. The results were intensely controversial, and from 2001 Michael Majerus carefully repeated the experiment. The results were published posthumously in 2012, vindicating Kettlewell's work as "the most direct evidence", and "one of the clearest and most easily understood examples of Darwinian evolution in action".

Mimicry

Common Mormon (Papilio polytes)
Common rose (Pachliopta aristolochiae)
The butterfly Papilio polytes (left) mimics the unpalatable Pachliopta aristolochiae (right).

Batesian

Batesian mimicry, named for the 19th century naturalist Henry Walter Bates who first noted the effect in 1861, "provides numerous excellent examples of natural selection" at work. The evolutionary entomologist James Mallet noted that mimicry was "arguably the oldest Darwinian theory not attributable to Darwin." Inspired by On the Origin of Species, Bates realized that unrelated Amazonian butterflies resembled each other when they lived in the same areas, but had different coloration in different locations in the Amazon, something that could only have been caused by adaptation.

Müllerian

Müllerian mimicry, too, in which two or more distasteful species that share one or more predators have come to mimic each other's warning signals, was clearly adaptive; Fritz Müller described the effect in 1879, in an account notable for being the first use of a mathematical argument in evolutionary ecology to show how powerful the effect of natural selection would be.

Warning coloration protects poison dart frog Dendrobates leucomelas.

Aposematism

In 1867, in a letter to Darwin, Wallace described warning coloration. The evolutionary zoologist James Mallet notes that this discovery "rather illogically" followed rather than preceded the accounts of Batesian and Müllerian mimicry, which both rely on the existence and effectiveness of warning coloration. The conspicuous colours and patterns of animals with strong defences such as toxins are advertised to predators, signalling honestly that the animal is not worth attacking. This directly increases the reproductive fitness of the potential prey, providing a strong selective advantage. The existence of unequivocal warning coloration is therefore clear evidence of natural selection at work.

Defence of Darwinism

Warning coloration of the "Brazilian Skunk" in Edward Bagnall Poulton's The Colours of Animals, 1890

Edward Bagnall Poulton's 1890 book, The Colours of Animals, renamed Wallace's concept of warning colours "aposematic" coloration, as well as supporting Darwin's then unpopular theories of natural selection and sexual selection. Poulton's explanations of coloration are emphatically Darwinian. For example, on aposematic coloration he wrote that

At first sight the existence of this group seems to be a difficulty in the way of the general applicability of the theory of natural selection. Warning Colours appear to benefit the would-be enemies rather than the conspicuous forms themselves, and the origin and growth of a character intended solely for the advantage of some other species cannot be explained by the theory of natural selection. But the conspicuous animal is greatly benefited by its Warning Colours. If it resembled its surroundings like the members of the other class, it would be liable to a great deal of accidental or experimental tasting, and there would be nothing about it to impress the memory of an enemy, and thus to prevent the continual destruction of individuals. The object of Warning Colours is to assist the education of enemies, enabling them to easily learn and remember the animals which are to be avoided. The great advantage conferred upon the conspicuous species is obvious when it is remembered that such an easy and successful education means an education involving only a small sacrifice of life."

Poulton summed up his allegiance to Darwinism as an explanation of Batesian mimicry in one sentence: "Every step in the gradually increasing change of the mimicking in the direction of specially protected form, would have been an advantage in the struggle for existence".

The historian of biology Peter J. Bowler commented that Poulton used his book to complain about experimentalists' lack of attention to what field naturalists (like Wallace, Bates, and Poulton) could readily see were adaptive features. Bowler added that "The fact that the adaptive significance of coloration was (sic) widely challenged indicates just how far anti-Darwinian feeling had developed. Only field naturalists such as Poulton refused to give in, convinced that their observations showed the validity of selection, whatever the theoretical problems."

Saltation (biology)

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Saltation_(biology)

In biology, saltation (from Latin saltus 'leap, jump') is a sudden and large mutational change from one generation to the next, potentially causing single-step speciation. This was historically offered as an alternative to Darwinism. Some forms of mutationism were effectively saltationist, implying large discontinuous jumps.

Speciation, such as by polyploidy in plants, can sometimes be achieved in a single and in evolutionary terms sudden step. Evidence exists for various forms of saltation in a variety of organisms.

History

Prior to Charles Darwin most evolutionary scientists had been saltationists. Jean-Baptiste Lamarck was a gradualist but similar to other scientists of the period had written that saltational evolution was possible. Étienne Geoffroy Saint-Hilaire endorsed a theory of saltational evolution that "monstrosities could become the founding fathers (or mothers) of new species by instantaneous transition from one form to the next." Geoffroy wrote that environmental pressures could produce sudden transformations to establish new species instantaneously. In 1864 Albert von Kölliker revived Geoffroy's theory that evolution proceeds by large steps, under the name of heterogenesis.

With the publication of On the Origin of Species in 1859 Charles Darwin wrote that most evolutionary changes proceeded gradually but he did not deny the existence of jumps.

From 1860 to 1880 saltation had a minority interest but by 1890 had become a major interest to scientists. In their paper on evolutionary theories in the 20th century Levit et al wrote:

The advocates of saltationism deny the Darwinian idea of slowly and gradually growing divergence of character as the only source of evolutionary progress. They would not necessarily completely deny gradual variation, but claim that cardinally new ‘body plans’ come into being as a result of saltations (sudden, discontinuous and crucial changes, for example, the series of macromutations). The latter are responsible for the sudden appearance of new higher taxa including classes and orders, while small variation is supposed to be responsible for the fine adaptations below the species level.

In the early 20th century a mechanism of saltation was proposed as large mutations. It was seen as a much faster alternative to the Darwinian concept of a gradual process of small random variations being acted on by natural selection. It was popular with early geneticists such as Hugo de Vries, who along with Carl Correns helped rediscover Gregor Mendel's laws of inheritance in 1900, William Bateson, a British zoologist who switched to genetics, and early in his career Thomas Hunt Morgan. Some of these geneticists developed it into the mutation theory of evolution. There was also a debate over accounts of the evolution of mimicry and if they could be explained by gradualism or saltation. The geneticist Reginald Punnett supported a saltational theory in his book Mimicry in Butterflies (1915).

The mutation theory of evolution held that species went through periods of rapid mutation, possibly as a result of environmental stress, that could produce multiple mutations, and in some cases completely new species, in a single generation. This mutationist view of evolution was later replaced by the reconciliation of Mendelian genetics with natural selection into a gradualistic framework for the neo-Darwinian synthesis. It was the emergence of population thinking in evolution which forced many scientists to adopt gradualism in the early 20th century. According to Ernst Mayr, it wasn't until the development of population genetics in the neo-Darwinian synthesis in the 1940s that demonstrated the explanatory power of natural selection that saltational views of evolution were largely abandoned.

Saltation was originally denied by the "modern synthesis" school of neo-Darwinism which favoured gradual evolution but has since been accepted due to recent evidence in evolutionary biology (see the current status section). In recent years there are some prominent proponents of saltation, including Carl Woese. Woese, and colleagues, suggested that the absence of RNA signature continuum between domains of bacteria, archaea, and eukarya constitutes a primary indication that the three primary organismal lineages materialized via one or more major evolutionary saltations from some universal ancestral state involving dramatic change in cellular organization that was significant early in the evolution of life, but in complex organisms gave way to the generally accepted Darwinian mechanisms. The geneticist Barbara McClintock introduced the idea of "jumping genes", chromosome transpositions that can produce rapid changes in the genome.

Saltational speciation, also known as abrupt speciation, is the discontinuity in a lineage that occurs through genetic mutations, chromosomal aberrations or other evolutionary mechanisms that cause reproductively isolated individuals to establish a new species population. Polyploidy, karyotypic fission, symbiogenesis and lateral gene transfer are possible mechanisms for saltational speciation.

Macromutation theory

The botanist John Christopher Willis proposed an early saltationist theory of evolution. He held that species were formed by large mutations, not gradual evolution by natural selection.

The German geneticist Richard Goldschmidt was the first scientist to use the term "hopeful monster". Goldschmidt thought that small gradual changes could not bridge the hypothetical divide between microevolution and macroevolution. In his book The Material Basis of Evolution (1940) he wrote "the change from species to species is not a change involving more and more additional atomistic changes, but a complete change of the primary pattern or reaction system into a new one, which afterwards may again produce intraspecific variation by micromutation." Goldschmidt believed the large changes in evolution were caused by macromutations (large mutations). His ideas about macromutations became known as the hopeful monster hypothesis which is considered a type of saltational evolution.

Goldschmidt's thesis however was universally rejected and widely ridiculed within the biological community, which favored the neo-Darwinian explanations of R.A. Fisher, J. B. S. Haldane and Sewall Wright. However, there has been a recent interest in the ideas of Goldschmidt in the field of evolutionary developmental biology as some scientists are convinced he was not entirely wrong.

Otto Schindewolf, a German paleontologist, also supported macromutations as part of his evolutionary theory. He was known for presenting an alternative interpretation of the fossil record based on his ideas of orthogenesis, saltational evolution and extraterrestrial impacts opposed to gradualism but abandoned the view of macromutations in later publications.

Søren Løvtrup, a biochemist and embryologist from Denmark, advocated a similar hypothesis of macromutation to Goldschmidt's in 1974. Lovtrup believed that macromutations interfered with various epigenetic processes, that is, those which affect the causal processes in biological development. This is in contrast to the gradualistic theory of micromutations of Neo-Darwinism, which claims that evolutionary innovations are generally the result of accumulation of numerous very slight modifications. Lovtrup also rejected the punctuated equilibria of Stephen Gould and Niles Eldredge, claiming it was a form of gradualism and not a macromutation theory. Lovtrup defended many of Darwin's critics including Schindewolf, Mivart, Goldschmidt, and Himmelfarb. Mae Wan Ho described Lovtrup's theory as similar to the hopeful monster theory of Richard Goldschmidt.

Goldschmidt presented two mechanisms for how hopeful monsters might work. One mechanism, involved “systemic mutations”, rejected the classical gene concept and is no longer considered by modern science; however, his second mechanism involved “developmental macromutations” in “rate genes” or “controlling genes” that change early development and thus cause large effects in the adult phenotype. These kind of mutations are similar to the ones considered in contemporary evolutionary developmental biology.

On the subject of Goldschmidt Donald Prothero in his book Evolution: What the Fossils Say and Why It Matters (2007) wrote:

The past twenty years have vindicated Goldschmidt to some degree. With the discovery of the importance of regulatory genes, we realize that he was ahead of his time in focusing on the importance of a few genes controlling big changes in the organisms, not small-scales changes in the entire genome as neo-Darwinians thought. In addition, the hopeful monster problem is not so insurmountable after all. Embryology has shown that if you affect an entire population of developing embryos with a stress (such as a heat shock) it can cause many embryos to go through the same new pathway of embryonic development, and then they all become hopeful monsters when they reach reproductive age.

In 2008 evolutionary biologist Olivia Judson in her article The Monster Is Back, and It’s Hopeful listed some examples which may support the hopeful monster hypothesis and an article published in the journal Nature in 2010 titled Evolution: Revenge of the Hopeful Monster reported that studies in stickleback populations in a British Columbia lake and bacteria populations in a Michigan lab have shown that large individual genetic changes can have vast effects on organisms "without dooming it to the evolutionary rubbish heap". According to the article "Single-gene changes that confer a large adaptive value do happen: they are not rare, they are not doomed and, when competing with small-effect mutations, they tend to win. But small-effect mutations still matter — a lot. They provide essential fine-tuning and sometimes pave the way for explosive evolution to follow."

A paper by (Page et al. 2010) have written that the Mexican axolotl (Ambystoma mexicanum) could be classified as a hopeful monster as it exhibits an adaptive and derived mode of development that has evolved rapidly and independently among tiger salamanders. According to the paper there has been an interest in aspects of the hopeful monster hypothesis in recent years:

Goldschmidt proposed that mutations occasionally yield individuals within populations that deviate radically from the norm and referred to such individuals as "hopeful monsters". If the novel phenotypes of hopeful monsters arise under the right environmental circumstances, they may become fixed, and the population will found a new species. While this idea was discounted during the Modern synthesis, aspects of the hopeful monster hypothesis have been substantiated in recent years. For example, it is clear that dramatic changes in phenotype can occur from few mutations of key developmental genes and phenotypic differences among species often map to relatively few genetic factors. These findings are motivating renewed interest in the study of hopeful monsters and the perspectives they can provide about the evolution of development. In contrast to mutants that are created in the lab, hopeful monsters have been shaped by natural selection and are therefore more likely to reveal mechanisms of adaptive evolution.

Günter Theissen, a German professor of genetics, has classified homeotic mutants as "hopeful monsters" and has documented many examples of animal and plant lineages that may have originated in that way. American biologist Michael Freeling has proposed "balanced gene drive" as a saltational mechanism in the mutationist tradition, which could explain trends involving morphological complexity in plant and animal eukaryotic lineages.

Current status

Known mechanisms

Examples of saltational evolution include cases of stabilized hybrids that can reproduce without crossing (such as allotetraploids) and cases of symbiogenesis. Both gene duplication and lateral gene transfer have the capacity to bring about relatively large changes that are saltational. Polyploidy (most common in plants but not unknown in animals) is saltational: a significant change (in gene numbers) can result in speciation in a single generation.

Claimed instances

Evidence of phenotypic saltation has been found in the centipede and some scientists have suggested there is evidence for independent instances of saltational evolution in sphinx moths. Saltational changes have occurred in the buccal cavity of the roundworm Caenorhabditis elegans. Some processes of epigenetic inheritance can also produce changes that are saltational. There has been a controversy over whether mimicry in butterflies and other insects can be explained by gradual or saltational evolution. According to Norrström (2006) there is evidence for saltation in some cases of mimicry. The endosymbiotic theory is considered to be a type of saltational evolution. Symonds and Elgar, 2004 have suggested that pheromone evolution in bark beetles is characterized by large saltational shifts. The mode of evolution of sex pheromones in Bactrocera has occurred by rapid saltational changes associated with speciation followed by gradual divergence thereafter. Saltational speciation has been recognized in the genus Clarkia (Lewis, 1966). It has been suggested (Carr, 1980, 2000) that the Calycadenia pauciflora could have originated directly from an ancestral race through a single saltational event involving multiple chromosome breaks. Specific cases of homeosis in flowers can be caused by saltational evolution. In a study of divergent orchid flowers (Bateman and DiMichele, 2002) wrote how simple homeotic morphs in a population can lead to newly established forms that become fixed and ultimately lead to new species. They described the transformation as a saltational evolutionary process, where a mutation of key developmental genes leads to a profound phenotypic change, producing a new evolutionary lineage within a species.

Explanations

Reviewing the history of macroevolutionary theories, the American evolutionary biologist Douglas J. Futuyma notes that since 1970, two very different alternatives to Darwinian gradualism have been proposed, both by Stephen Jay Gould: mutationism, and punctuated equilibria. Gould's macromutation theory gave a nod to his predecessor with an envisaged "Goldschmidt break" between evolution within a species and speciation. His advocacy of Goldschmidt was attacked with "highly unflattering comments" by B. Charlesworth and Templeton. Futuyma concludes, following other biologists reviewing the field such as K.Sterelny and A. Minelli, that essentially all the claims of evolution driven by large mutations could be explained within the Darwinian evolutionary synthesis.

Alternatives to Darwinian evolution

From Wikipedia, the free encyclopedia
The mediaeval great chain of being as a staircase, implying the possibility of progress: Ramon Lull's Ladder of Ascent and Descent of the Mind, 1305

Alternatives to Darwinian evolution have been proposed by scholars investigating biology to explain signs of evolution and the relatedness of different groups of living things. The alternatives in question do not deny that evolutionary changes over time are the origin of the diversity of life, nor that the organisms alive today share a common ancestor from the distant past (or ancestors, in some proposals); rather, they propose alternative mechanisms of evolutionary change over time, arguing against mutations acted on by natural selection as the most important driver of evolutionary change.

This distinguishes them from certain other kinds of arguments that deny that large-scale evolution of any sort has taken place, as in some forms of creationism, which do not propose alternative mechanisms of evolutionary change but instead deny that evolutionary change has taken place at all. Not all forms of creationism deny that evolutionary change takes place; notably, proponents of theistic evolution, such as the biologist Asa Gray, assert that evolutionary change does occur and is responsible for the history of life on Earth, with the proviso that this process has been influenced by a god or gods in some meaningful sense.

Where the fact of evolutionary change was accepted but the mechanism proposed by Charles Darwin, natural selection, was denied, explanations of evolution such as Lamarckism, catastrophism, orthogenesis, vitalism, structuralism and mutationism (called saltationism before 1900) were entertained. Different factors motivated people to propose non-Darwinian mechanisms of evolution. Natural selection, with its emphasis on death and competition, did not appeal to some naturalists because they felt it immoral, leaving little room for teleology or the concept of progress (orthogenesis) in the development of life. Some who came to accept evolution, but disliked natural selection, raised religious objections. Others felt that evolution was an inherently progressive process that natural selection alone was insufficient to explain. Still others felt that nature, including the development of life, followed orderly patterns that natural selection could not explain.

By the start of the 20th century, evolution was generally accepted by biologists but natural selection was in eclipse. Many alternative theories were proposed, but biologists were quick to discount theories such as orthogenesis, vitalism and Lamarckism which offered no mechanism for evolution. Mutationism did propose a mechanism, but it was not generally accepted. The modern synthesis a generation later claimed to sweep away all the alternatives to Darwinian evolution, though some have been revived as molecular mechanisms for them have been discovered.

Unchanging forms

Aristotle did not embrace either divine creation or evolution, instead arguing in his biology that each species (eidos) was immutable, breeding true to its ideal eternal form (not the same as Plato's theory of forms). Aristotle's suggestion in De Generatione Animalium of a fixed hierarchy in nature - a scala naturae ("ladder of nature") provided an early explanation of the continuity of living things. Aristotle saw that animals were teleological (functionally end-directed), and had parts that were homologous with those of other animals, but he did not connect these ideas into a concept of evolutionary progress.

In the Middle Ages, Scholasticism developed Aristotle's view into the idea of a great chain of being. The image of a ladder inherently suggests the possibility of climbing, but both the ancient Greeks and mediaeval scholastics such as Ramon Lull maintained that each species remained fixed from the moment of its creation.

By 1818, however, Étienne Geoffroy Saint-Hilaire argued in his Philosophie anatomique that the chain was "a progressive series", where animals like molluscs low on the chain could "rise, by addition of parts, from the simplicity of the first formations to the complication of the creatures at the head of the scale", given sufficient time. Accordingly, Geoffroy and later biologists looked for explanations of such evolutionary change.

Georges Cuvier's 1812 Recherches sur les Ossements Fossiles set out his doctrine of the correlation of parts, namely that since an organism was a whole system, all its parts mutually corresponded, contributing to the function of the whole. So, from a single bone the zoologist could often tell what class or even genus the animal belonged to. And if an animal had teeth adapted for cutting meat, the zoologist could be sure without even looking that its sense organs would be those of a predator and its intestines those of a carnivore. A species had an irreducible functional complexity, and "none of its parts can change without the others changing too". Evolutionists expected one part to change at a time, one change to follow another. In Cuvier's view, evolution was impossible, as any one change would unbalance the whole delicate system.

Louis Agassiz's 1856 "Essay on Classification" exemplified German philosophical idealism. This held that each species was complex within itself, had complex relationships to other organisms, and fitted precisely into its environment, as a pine tree in a forest, and could not survive outside those circles. The argument from such ideal forms opposed evolution without offering an actual alternative mechanism. Richard Owen held a similar view in Britain.

The Lamarckian social philosopher and evolutionist Herbert Spencer, ironically the author of the phrase "survival of the fittest" adopted by Darwin, used an argument like Cuvier's to oppose natural selection. In 1893, he stated that a change in any one structure of the body would require all the other parts to adapt to fit in with the new arrangement. From this, he argued that it was unlikely that all the changes could appear at the right moment if each one depended on random variation; whereas in a Lamarckian world, all the parts would naturally adapt at once, through a changed pattern of use and disuse.

Alternative explanations of change

Where the fact of evolutionary change was accepted by biologists but natural selection was denied, including but not limited to the late 19th century eclipse of Darwinism, alternative scientific explanations such as Lamarckism, orthogenesis, structuralism, catastrophism, vitalism and theistic evolution were entertained, not necessarily separately. (Purely religious points of view such as young or old earth creationism or intelligent design are not considered here.) Different factors motivated people to propose non-Darwinian evolutionary mechanisms. Natural selection, with its emphasis on death and competition, did not appeal to some naturalists because they felt it immoral, leaving little room for teleology or the concept of progress in the development of life. Some of these scientists and philosophers, like St. George Jackson Mivart and Charles Lyell, who came to accept evolution but disliked natural selection, raised religious objections. Others, such as the biologist and philosopher Herbert Spencer, the botanist George Henslow (son of Darwin's mentor John Stevens Henslow, also a botanist), and the author Samuel Butler, felt that evolution was an inherently progressive process that natural selection alone was insufficient to explain. Still others, including the American paleontologists Edward Drinker Cope and Alpheus Hyatt, had an idealist perspective and felt that nature, including the development of life, followed orderly patterns that natural selection could not explain.

Some felt that natural selection would be too slow, given the estimates of the age of the earth and sun (10–100 million years) being made at the time by physicists such as Lord Kelvin, and some felt that natural selection could not work because at the time the models for inheritance involved blending of inherited characteristics, an objection raised by the engineer Fleeming Jenkin in a review of Origin written shortly after its publication. Another factor at the end of the 19th century was the rise of a new faction of biologists, typified by geneticists like Hugo de Vries and Thomas Hunt Morgan, who wanted to recast biology as an experimental laboratory science. They distrusted the work of naturalists like Darwin and Alfred Russel Wallace, dependent on field observations of variation, adaptation, and biogeography, as being overly anecdotal. Instead they focused on topics like physiology and genetics that could be investigated with controlled experiments in the laboratory, and discounted less accessible phenomena like natural selection and adaptation to the environment.

Theory Date Notable
proponent
Species
can change?
Mechanism
of change
Mechanism
is physical?
Extinction
possible?
Notes
Scala naturae c. 350 BC Aristotle No None N/A No Characteristics of groups do not fit on linear scale, as Aristotle observed. Teleology and homology recognised but not connected as evolution with adaptation; not spiritual
Great chain of being 1305 Llull, Ramon;
scholastics
No None N/A No Aristotelian, fitted into Christian theology
Vitalism 1759 Wolff, Caspar Friedrich Yes A life force in embryo No No? Varieties of theory from Ancient Egypt onwards, often spiritual. Dropped from biology with chemical synthesis of organic molecules e.g. of urea in 1828
Theistic evolution 1871–6 Gray, Asa
Mivart, St George J.
Yes Deity supplies beneficial mutations (Gray 1876), or sets (orthogenetic) direction (Mivart 1871) No Yes "Failed the test of methodological naturalism that had come to define science". Discounted by biologists by 1900
Orthogenesis 1859 Baer, Karl von Yes "Purposeful creation" No Yes? Many variants in 19th and 20th centuries
Orthogenesis
inc. emergent evolution
1959 Teilhard de Chardin, Pierre Yes "Inherent progressive tendency" (teleological, vitalist) No Yes Spiritual theory, emergence of mind, Omega Point
Lamarckism 1809 Lamarck, Jean-Baptiste Yes Use and disuse; inheritance of acquired characteristics So it was thought, but none was found No Part of his view of orthogenesis. Dropped from biology as Weismann barrier prevents changes in somatic cells from affecting germ line in gonads
Catastrophism 1812 Cuvier, Georges No Extinctions caused by natural events such as volcanism, floods Yes, for reducing number of species Yes To explain extinctions and faunal succession of tetrapods in fossil record; repopulation by new species after such events noted but left unexplained
Structuralism 1917 Thompson, D'Arcy Yes Self-organization, physical forces Yes Yes? Many variants, some influenced by vitalism
Saltationism
or Mutationism
1831 Geoffroy Saint-Hilaire, Étienne Yes Large mutations Yes Yes? Sudden production of new species under environmental pressure
Neutral theory of molecular evolution[31] 1968 Kimura, Motoo Yes Genetic drift Yes Yes Only at molecular level; fits in with natural selection at higher levels. Observed 'molecular clock' supports neutral drift; not a rival to natural selection, as does not cause evolution of phenotype
Darwinian evolution[32]
1859 Darwin, Charles Yes Natural selection Yes Yes Lacked mechanisms of mutation and heredity until birth of genetics, 1900; Darwin instead proposed pangenesis and some degree of inheritance of acquired characteristics

Vitalism

Louis Pasteur believed that only living things could carry out fermentation. Painting by Albert Edelfelt, 1885

Vitalism holds that living organisms differ from other things in containing something non-physical, such as a fluid or vital spirit, that makes them live. The theory dates to ancient Egypt. Since Early Modern times, vitalism stood in contrast to the mechanistic explanation of biological systems started by Descartes. Nineteenth century chemists set out to disprove the claim that forming organic compounds required vitalist influence. In 1828, Friedrich Wöhler showed that urea could be made entirely from inorganic chemicals. Louis Pasteur believed that fermentation required whole organisms, which he supposed carried out chemical reactions found only in living things. The embryologist Hans Driesch, experimenting on sea urchin eggs, showed that separating the first two cells led to two complete but small blastulas, seemingly showing that cell division did not divide the egg into sub-mechanisms, but created more cells each with the vital capability to form a new organism. Vitalism faded out with the demonstration of more satisfactory mechanistic explanations of each of the functions of a living cell or organism. By 1931, biologists had "almost unanimously abandoned vitalism as an acknowledged belief."

Theistic evolution

The American botanist Asa Gray used the name "theistic evolution" for his point of view, presented in his 1876 book Essays and Reviews Pertaining to Darwinism. He argued that the deity supplies beneficial mutations to guide evolution. St George Jackson Mivart argued instead in his 1871 On the Genesis of Species that the deity, equipped with foreknowledge, sets the direction of evolution by specifying the (orthogenetic) laws that govern it, and leaves species to evolve according to the conditions they experience as time goes by. The Duke of Argyll set out similar views in his 1867 book The Reign of Law. According to the historian Edward Larson, the theory failed as an explanation in the minds of late 19th century biologists as it broke the rules of methodological naturalism which they had grown to expect. Accordingly, by around 1900, biologists no longer saw theistic evolution as a valid theory. In Larson's view, by then it "did not even merit a nod among scientists." In the 20th century, theistic evolution could take other forms, such as the orthogenesis of Teilhard de Chardin.

Orthogenesis

Henry Fairfield Osborn claimed in 1918 that Titanothere horns showed a non-adaptive orthogenetic trend.

Orthogenesis or Progressionism is the hypothesis that life has an innate tendency to change, developing in a unilinear fashion in a particular direction, or simply making some kind of definite progress. Many different versions have been proposed, some such as that of Teilhard de Chardin openly spiritual, others such as Theodor Eimer's apparently simply biological. These theories often combined orthogenesis with other supposed mechanisms. For example, Eimer believed in Lamarckian evolution, but felt that internal laws of growth determined which characteristics would be acquired and would guide the long-term direction of evolution.

Orthogenesis was popular among paleontologists such as Henry Fairfield Osborn. They believed that the fossil record showed unidirectional change, but did not necessarily accept that the mechanism driving orthogenesis was teleological (goal-directed). Osborn argued in his 1918 book Origin and Evolution of Life that trends in Titanothere horns were both orthogenetic and non-adaptive, and could be detrimental to the organism. For instance, they supposed that the large antlers of the Irish elk had caused its extinction.

Support for orthogenesis fell during the modern synthesis in the 1940s when it became apparent that it could not explain the complex branching patterns of evolution revealed by statistical analysis of the fossil record. Work in the 21st century has supported the mechanism and existence of mutation-biased adaptation (a form of mutationism), meaning that constrained orthogenesis is now seen as possible. Moreover, the self-organizing processes involved in certain aspects of embryonic development often exhibit stereotypical morphological outcomes, suggesting that evolution will proceed in preferred directions once key molecular components are in place.

Lamarckism

Jean-Baptiste Lamarck, drawn by Jules Pizzetta, 1893

Jean-Baptiste Lamarck's 1809 evolutionary theory, transmutation of species, was based on a progressive (orthogenetic) drive toward greater complexity. Lamarck also shared the belief, common at the time, that characteristics acquired during an organism's life could be inherited by the next generation, producing adaptation to the environment. Such characteristics were caused by the use or disuse of the affected part of the body. This minor component of Lamarck's theory became known, much later, as Lamarckism. Darwin included Effects of the increased Use and Disuse of Parts, as controlled by Natural Selection in On the Origin of Species, giving examples such as large ground feeding birds getting stronger legs through exercise, and weaker wings from not flying until, like the ostrich, they could not fly at all. In the late 19th century, neo-Lamarckism was supported by the German biologist Ernst Haeckel, the American paleontologists Edward Drinker Cope and Alpheus Hyatt, and the American entomologist Alpheus Packard. Butler and Cope believed that this allowed organisms to effectively drive their own evolution. Packard argued that the loss of vision in the blind cave insects he studied was best explained through a Lamarckian process of atrophy through disuse combined with inheritance of acquired characteristics. Meanwhile, the English botanist George Henslow studied how environmental stress affected the development of plants, and he wrote that the variations induced by such environmental factors could largely explain evolution; he did not see the need to demonstrate that such variations could actually be inherited. Critics pointed out that there was no solid evidence for the inheritance of acquired characteristics. Instead, the experimental work of the German biologist August Weismann resulted in the germ plasm theory of inheritance, which Weismann said made the inheritance of acquired characteristics impossible, since the Weismann barrier would prevent any changes that occurred to the body after birth from being inherited by the next generation.

In modern epigenetics, biologists observe that phenotypes depend on heritable changes to gene expression that do not involve changes to the DNA sequence. These changes can cross generations in plants, animals, and prokaryotes. This is not identical to traditional Lamarckism, as the changes do not last indefinitely and do not affect the germ line and hence the evolution of genes.

Georges Cuvier, shown here with a fossil fish, proposed catastrophism to explain the fossil record.

Catastrophism

Catastrophism is the hypothesis, argued by the French anatomist and paleontologist Georges Cuvier in his 1812 Recherches sur les ossements fossiles de quadrupèdes, that the various extinctions and the patterns of faunal succession seen in the fossil record were caused by large-scale natural catastrophes such as volcanic eruptions and, for the most recent extinctions in Eurasia, the inundation of low-lying areas by the sea. This was explained purely by natural events: he did not mention Noah's flood, nor did he ever refer to divine creation as the mechanism for repopulation after an extinction event, though he did not support evolutionary theories such as those of his contemporaries Lamarck and Geoffroy Saint-Hilaire either. Cuvier believed that the stratigraphic record indicated that there had been several such catastrophes, recurring natural events, separated by long periods of stability during the history of life on earth. This led him to believe the Earth was several million years old.

Catastrophism has found a place in modern biology with the Cretaceous–Paleogene extinction event at the end of the Cretaceous period, as proposed in a paper by Walter and Luis Alvarez in 1980. It argued that a 10 kilometres (6.2 mi) asteroid struck Earth 66 million years ago at the end of the Cretaceous period. The event, whatever it was, made about 70% of all species extinct, including the dinosaurs, leaving behind the Cretaceous–Paleogene boundary. In 1990, a 180 kilometres (110 mi) candidate crater marking the impact was identified at Chicxulub in the Yucatán Peninsula of Mexico.

Structuralism

In his 1917 book On Growth and Form, D'Arcy Thompson illustrated the geometric transformation of one fish's body form into another with a 20° shear mapping. He did not discuss the evolutionary causes of such a change, raising suspicions of vitalism.

Biological structuralism objects to an exclusively Darwinian explanation of natural selection, arguing that other mechanisms also guide evolution, and sometimes implying that these supersede selection altogether. Structuralists have proposed different mechanisms that might have guided the formation of body plans. Before Darwin, Étienne Geoffroy Saint-Hilaire argued that animals shared homologous parts, and that if one was enlarged, the others would be reduced in compensation. After Darwin, D'Arcy Thompson hinted at vitalism and offered geometric explanations in his classic 1917 book On Growth and Form. Adolf Seilacher suggested mechanical inflation for "pneu" structures in Ediacaran biota fossils such as Dickinsonia. Günter P. Wagner argued for developmental bias, structural constraints on embryonic development. Stuart Kauffman favoured self-organisation, the idea that complex structure emerges holistically and spontaneously from the dynamic interaction of all parts of an organism. Michael Denton argued for laws of form by which Platonic universals or "Types" are self-organised. In 1979 Stephen J. Gould and Richard Lewontin proposed biological "spandrels", features created as a byproduct of the adaptation of nearby structures. Gerd Müller and Stuart Newman argued that the appearance in the fossil record of most of the current phyla in the Cambrian explosion was "pre-Mendelian" evolution caused by plastic responses of morphogenetic systems that were partly organized by physical mechanisms. Brian Goodwin, described by Wagner as part of "a fringe movement in evolutionary biology", denied that biological complexity can be reduced to natural selection, and argued that pattern formation is driven by morphogenetic fields. Darwinian biologists have criticised structuralism, emphasising that there is plentiful evidence from deep homology that genes have been involved in shaping organisms throughout evolutionary history. They accept that some structures such as the cell membrane self-assemble, but question the ability of self-organisation to drive large-scale evolution.

Saltationism, mutationism

Hugo de Vries, with a painting of an evening primrose, the plant which had apparently produced new species by saltation, by Thérèse Schwartze, 1918

Saltationism held that new species arise as a result of large mutations. It was seen as a much faster alternative to the Darwinian concept of a gradual process of small random variations being acted on by natural selection. It was popular with early geneticists such as Hugo de Vries, who along with Carl Correns helped rediscover Gregor Mendel's laws of inheritance in 1900, William Bateson, a British zoologist who switched to genetics, and early in his career, Thomas Hunt Morgan. These ideas developed into mutationism, the mutation theory of evolution. This held that species went through periods of rapid mutation, possibly as a result of environmental stress, that could produce multiple mutations, and in some cases completely new species, in a single generation, based on de Vries's experiments with the evening primrose, Oenothera, from 1886. The primroses seemed to be constantly producing new varieties with striking variations in form and color, some of which appeared to be new species because plants of the new generation could only be crossed with one another, not with their parents. However, Hermann Joseph Muller showed in 1918 that the new varieties de Vries had observed were the result of polyploid hybrids rather than rapid genetic mutation.

Initially, de Vries and Morgan believed that mutations were so large as to create new forms such as subspecies or even species instantly. Morgan's 1910 fruit fly experiments, in which he isolated mutations for characteristics such as white eyes, changed his mind. He saw that mutations represented small Mendelian characteristics that would only spread through a population when they were beneficial, helped by natural selection. This represented the germ of the modern synthesis, and the beginning of the end for mutationism as an evolutionary force.

Contemporary biologists accept that mutation and selection both play roles in evolution; the mainstream view is that while mutation supplies material for selection in the form of variation, all non-random outcomes are caused by natural selection. Masatoshi Nei argues instead that the production of more efficient genotypes by mutation is fundamental for evolution, and that evolution is often mutation-limited. The endosymbiotic theory implies rare but major events of saltational evolution by symbiogenesis. Carl Woese and colleagues suggested that the absence of RNA signature continuum between domains of bacteria, archaea, and eukarya shows that these major lineages materialized via large saltations in cellular organization. Saltation at a variety of scales is agreed to be possible by mechanisms including polyploidy, which certainly can create new species of plant, gene duplication, lateral gene transfer, and transposable elements (jumping genes).

Genetic drift

Many mutations are neutral or silent, having no effect on the amino acid sequence that is produced when the gene involved is translated to protein, and accumulate over time, forming a molecular clock. However this does not cause phenotypic evolution.

The neutral theory of molecular evolution, proposed by Motoo Kimura in 1968, holds that at the molecular level most evolutionary changes and most of the variation within and between species is not caused by natural selection but by genetic drift of mutant alleles that are neutral. A neutral mutation is one that does not affect an organism's ability to survive and reproduce. The neutral theory allows for the possibility that most mutations are deleterious, but holds that because these are rapidly purged by natural selection, they do not make significant contributions to variation within and between species at the molecular level. Mutations that are not deleterious are assumed to be mostly neutral rather than beneficial.

The theory was controversial as it sounded like a challenge to Darwinian evolution; controversy was intensified by a 1969 paper by Jack Lester King and Thomas H. Jukes, provocatively but misleadingly titled "Non-Darwinian Evolution". It provided a wide variety of evidence including protein sequence comparisons, studies of the Treffers mutator gene in E. coli, analysis of the genetic code, and comparative immunology, to argue that most protein evolution is due to neutral mutations and genetic drift.

According to Kimura, the theory applies only for evolution at the molecular level, while phenotypic evolution is controlled by natural selection, so the neutral theory does not constitute a true alternative.

Combined theories

Multiple explanations have been offered since the 19th century for how evolution took place, given that many scientists initially had objections to natural selection (dashed orange arrows). Many of these theories led (blue arrows) to some form of directed evolution (orthogenesis), with or without invoking divine control (dotted blue arrows) directly or indirectly. For example, evolutionists like Edward Drinker Cope believed in a combination of theistic evolution, Lamarckism, vitalism, and orthogenesis, represented by the sequence of arrows on the extreme left of the diagram.

The various alternatives to Darwinian evolution by natural selection were not necessarily mutually exclusive. The evolutionary philosophy of the American palaeontologist Edward Drinker Cope is a case in point. Cope, a religious man, began his career denying the possibility of evolution. In the 1860s, he accepted that evolution could occur, but, influenced by Agassiz, rejected natural selection. Cope accepted instead the theory of recapitulation of evolutionary history during the growth of the embryo - that ontogeny recapitulates phylogeny, which Agassiz believed showed a divine plan leading straight up to man, in a pattern revealed both in embryology and palaeontology. Cope did not go so far, seeing that evolution created a branching tree of forms, as Darwin had suggested. Each evolutionary step was however non-random: the direction was determined in advance and had a regular pattern (orthogenesis), and steps were not adaptive but part of a divine plan (theistic evolution). This left unanswered the question of why each step should occur, and Cope switched his theory to accommodate functional adaptation for each change. Still rejecting natural selection as the cause of adaptation, Cope turned to Lamarckism to provide the force guiding evolution. Finally, Cope supposed that Lamarckian use and disuse operated by causing a vitalist growth-force substance, "bathmism", to be concentrated in the areas of the body being most intensively used; in turn, it made these areas develop at the expense of the rest. Cope's complex set of beliefs thus assembled five evolutionary philosophies: recapitulationism, orthogenesis, theistic evolution, Lamarckism, and vitalism. Other palaeontologists and field naturalists continued to hold beliefs combining orthogenesis and Lamarckism until the modern synthesis in the 1930s.

Rebirth of natural selection, with continuing alternatives

By the start of the 20th century, during the eclipse of Darwinism, biologists were doubtful of natural selection, but equally were quick to discount theories such as orthogenesis, vitalism and Lamarckism which offered no mechanism for evolution. Mutationism did propose a mechanism, but it was not generally accepted. The modern synthesis a generation later, roughly between 1918 and 1932, broadly swept away all the alternatives to Darwinism, though some including forms of orthogenesis, epigenetic mechanisms that resemble Lamarckian inheritance of acquired characteristics, catastrophism, structuralism, and mutationism have been revived, such as through the discovery of molecular mechanisms.

Biology has become Darwinian, but belief in some form of progress (orthogenesis) remains both in the public mind and among biologists. Ruse argues that evolutionary biologists will probably continue to believe in progress for three reasons. Firstly, the anthropic principle demands people able to ask about the process that led to their own existence, as if they were the pinnacle of such progress. Secondly, scientists in general and evolutionists in particular believe that their work is leading them progressively closer to a true grasp of reality, as knowledge increases, and hence (runs the argument) there is progress in nature also. Ruse notes in this regard that Richard Dawkins explicitly compares cultural progress with memes to biological progress with genes. Thirdly, evolutionists are self-selected; they are people, such as the entomologist and sociobiologist E. O. Wilson, who are interested in progress to supply a meaning for life.

Coloration evidence for natural selection

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