Animal coloration provided important early
evidence for evolution by
natural selection
Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Charle ...
, 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
Evidence for a proposition is what supports this proposition. It is usually understood as an indication that the supported proposition is true. What role evidence plays and how it is conceived varies from field to field.
In epistemology, evidenc ...
of selection:
camouflage
Camouflage is the use of any combination of materials, coloration, or illumination for concealment, either by making animals or objects hard to see, or by disguising them as something else. Examples include the leopard's spotted coat, the ...
(protective coloration);
mimicry
In evolutionary biology, mimicry is an evolved resemblance between an organism and another object, often an organism of another species. Mimicry may evolve between different species, or between individuals of the same species. Often, mimicry f ...
, both
Batesian and
Müllerian; and
aposematism
Aposematism is the advertising by an animal to potential predators that it is not worth attacking or eating. This unprofitability may consist of any defences which make the prey difficult to kill and eat, such as toxicity, venom, foul taste or ...
.
Charles Darwin
Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended fr ...
's ''
On the Origin of Species
''On the Origin of Species'' (or, more completely, ''On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life''),The book's full original title was ''On the Origin of Species by Me ...
'' 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
Genetic variation is the difference in DNA among individuals or the differences between populations. The multiple sources of genetic variation include mutation and genetic recombination. Mutations are the ultimate sources of genetic variation, ...
or for
heredity
Heredity, also called inheritance or biological inheritance, is the passing on of traits from parents to their offspring; either through asexual reproduction or sexual reproduction, the offspring cells or organisms acquire the genetic inform ...
, 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
Henry Walter Bates (8 February 1825, in Leicester – 16 February 1892, in London) was an English naturalist and explorer who gave the first scientific account of mimicry in animals. He was most famous for his expedition to the rainforests of ...
and
Fritz Müller
Johann Friedrich Theodor Müller (31 March 1822 – 21 May 1897), better known as Fritz Müller, and also as Müller-Desterro, was a German biologist who emigrated to southern Brazil, where he lived in and near the German community of Blumenau, ...
. 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
Alfred Russel Wallace (8 January 1823 – 7 November 1913) was a British naturalist, explorer, geographer, anthropologist, biologist and illustrator. He is best known for independently conceiving the theory of evolution through natural se ...
and
Edward Bagnall Poulton
Sir Edward Bagnall Poulton, FRS HFRSE FLS (27 January 1856 – 20 November 1943) was a British evolutionary biologist, a lifelong advocate of natural selection through a period in which many scientists such as Reginald Punnett doubted its ...
, and in the 20th century
Hugh Cott
Hugh Bamford Cott (6 July 1900 – 18 April 1987) was a British zoologist, an authority on both natural and military camouflage, and a scientific illustrator and photographer. Many of his field studies took place in Africa, where he was espec ...
and
Bernard Kettlewell, sought evidence that natural selection was taking place. Wallace noted that
snow camouflage
Snow camouflage is the use of a coloration or pattern for effective camouflage in winter, often combined with a different summer camouflage. Summer patterns are typically disruptively patterned combinations of shades of browns and greys, up to ...
, 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
''The Colours of Animals'' is a zoology book written in 1890 by Sir Edward Bagnall Poulton (1856–1943). It was the first substantial textbook to argue the case for Darwinian selection applying to all aspects of animal coloration. The book a ...
'', 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
Disruptive coloration (also known as disruptive camouflage or disruptive patterning) is a form of camouflage that works by breaking up the outlines of an animal, soldier or military vehicle with a strongly contrasting pattern. It is often comb ...
in frogs convinced other biologists that these deceptive markings were products of natural selection. Kettlewell experimented on
peppered moth evolution
The evolution of the peppered moth is an evolutionary instance of directional colour change in the moth population as a consequence of air pollution during the Industrial Revolution. The frequency of dark-coloured moths increased at that time ...
, showing that the species had
adapted as pollution changed the environment; this provided compelling evidence of Darwinian evolution.
Context
Charles Darwin
Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended fr ...
published ''
On the Origin of Species
''On the Origin of Species'' (or, more completely, ''On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life''),The book's full original title was ''On the Origin of Species by Me ...
'' in 1859, arguing that
evolution
Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
in nature must be driven by
natural selection
Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Charle ...
, just as breeds of
domestic animal
This page gives a list of domesticated animals, also including a list of animals which are or may be currently undergoing the process of domestication and animals that have an extensive relationship with humans beyond simple predation. This includ ...
s and cultivars of
crop plant
A crop is a plant that can be grown and harvested extensively for profit or subsistence. When the plants of the same kind are cultivated at one place on a large scale, it is called a crop. Most crops are cultivated in agriculture or hydroponics ...
s were driven by
artificial selection
Selective breeding (also called artificial selection) is the process by which humans use animal breeding and plant breeding to selectively develop particular phenotypic traits (characteristics) by choosing which typically animal or plant m ...
.
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
Heredity, also called inheritance or biological inheritance, is the passing on of traits from parents to their offspring; either through asexual reproduction or sexual reproduction, the offspring cells or organisms acquire the genetic inform ...
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
Julian Huxley used the phrase "the eclipse of Darwinism" to describe the state of affairs prior to what he called the "modern synthesis". During the "eclipse", evolution was widely accepted in scientific circles but relatively few biologists b ...
; 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
Peter J. Bowler (born 8 October 1944) is a historian of biology who has written extensively on the history of evolutionary thought, the history of the environmental sciences, and on the history of genetics. His 1984 book, ''Evolution: The His ...
wrote that Darwin's theory
Camouflage
Snow camouflage
In his 1889 book ''Darwinism
Darwinism is a scientific theory, theory of Biology, biological evolution developed by the English naturalist Charles Darwin (1809–1882) and others, stating that all species of organisms arise and develop through the natural selection of smal ...
'', the naturalist Alfred Russel Wallace
Alfred Russel Wallace (8 January 1823 – 7 November 1913) was a British naturalist, explorer, geographer, anthropologist, biologist and illustrator. He is best known for independently conceiving the theory of evolution through natural se ...
considered the white coloration of Arctic
The Arctic ( or ) is a polar regions of Earth, polar region located at the northernmost part of Earth. The Arctic consists of the Arctic Ocean, adjacent seas, and parts of Canada (Yukon, Northwest Territories, Nunavut), Danish Realm (Greenla ...
animals. He recorded that the Arctic fox
The Arctic fox (''Vulpes lagopus''), also known as the white fox, polar fox, or snow fox, is a small fox native to the Arctic regions of the Northern Hemisphere and common throughout the Arctic tundra biome. It is well adapted to living in co ...
, Arctic hare
The Arctic hare (''Lepus arcticus'') is a species of hare highly adapted to living in the Arctic tundra and other icy biomes. The Arctic hare survives with shortened ears and limbs, a small nose, fat that makes up close to 20% of its body, and ...
, ermine and ptarmigan
''Lagopus'' is a small genus of birds in the grouse subfamily commonly known as ptarmigans (). The genus contains three living species with numerous described subspecies, all living in tundra or cold upland areas.
Taxonomy and etymology
The ge ...
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 Gyrfalcon
The gyrfalcon ( or ) (), the largest of the falcon species, is a bird of prey. The abbreviation gyr is also used. It breeds on Arctic coasts and tundra, and the islands of northern North America and the Eurosiberian region. It is mainly a reside ...
s throughout the winter."
More recently, decreasing snow cover in Poland, caused by global warming
In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
, 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
In the words of camouflage researchers Innes Cuthill Innes C. Cuthill (born 1960) is a professor of behavioural ecology at the University of Bristol. His main research interest is in camouflage, in particular how it evolves in response to the colour vision of other animals such as predators.
Life
In ...
and A. Székely, the English zoologist and camouflage expert Hugh Cott
Hugh Bamford Cott (6 July 1900 – 18 April 1987) was a British zoologist, an authority on both natural and military camouflage, and a scientific illustrator and photographer. Many of his field studies took place in Africa, where he was espec ...
's 1940 book ''Adaptive Coloration in Animals
''Adaptive Coloration in Animals'' is a 500-page textbook about camouflage, warning coloration and mimicry by the Cambridge zoologist Hugh Cott, first published during the Second World War in 1940; the book sold widely and made him famous.
The ...
'' provided "persuasive arguments for the survival value of coloration, and for adaptation
In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the po ...
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.][
]
Industrial melanism
Between 1953 and 1956, the geneticist Bernard Kettlewell experimented on peppered moth evolution
The evolution of the peppered moth is an evolutionary instance of directional colour change in the moth population as a consequence of air pollution during the Industrial Revolution. The frequency of dark-coloured moths increased at that time ...
. He presented results showing that in a polluted urban wood with dark tree trunks, dark moths survived better than pale ones, causing industrial melanism
Industrial melanism is an evolutionary effect prominent in several arthropods, where dark pigmentation (melanism) has evolved in an environment affected by industrial pollution, including sulphur dioxide gas and dark soot deposits. Sulphur diox ...
, 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 tit
The great tit (''Parus major'') is a passerine bird in the tit family Paridae. It is a widespread and common species throughout Europe, the Middle East, Central Asia and east across the Palearctic to the Amur River, south to parts of North Af ...
s used in the experiment) selectively caught and killed the less well-camouflaged moths. The results were intensely controversial, and from 2001 Michael Majerus
Michael Eugene Nicolas Majerus (13 February 1954 – 27 January 2009) was a British geneticist and professor of evolution at the University of Cambridge. He was also a teaching fellow at Clare College, Cambridge. He was an enthusiast in Darwin' ...
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
Batesian
Batesian mimicry
Batesian mimicry is a form of mimicry where a harmless species has evolved to imitate the warning signals of a harmful species directed at a predator of them both. It is named after the English naturalist Henry Walter Bates, after his work on bu ...
, named for the 19th century naturalist Henry Walter Bates
Henry Walter Bates (8 February 1825, in Leicester – 16 February 1892, in London) was an English naturalist and explorer who gave the first scientific account of mimicry in animals. He was most famous for his expedition to the rainforests of ...
who first noted the effect in 1861, "provides numerous excellent examples of natural selection" at work. The evolutionary entomologist James Mallet James Mallet (born 15 March 1955 in London) is an evolutionary zoologist specialising in entomology.
He was educated at Winchester College.
He became professor of biological diversity at the Department of Biology, University College London. He wa ...
noted that mimicry
In evolutionary biology, mimicry is an evolved resemblance between an organism and another object, often an organism of another species. Mimicry may evolve between different species, or between individuals of the same species. Often, mimicry f ...
was "arguably the oldest Darwinian
Darwinism is a theory of biological evolution developed by the English naturalist Charles Darwin (1809–1882) and others, stating that all species of organisms arise and develop through the natural selection of small, inherited variations that ...
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
Müllerian mimicry is a natural phenomenon in which two or more well-defended species, often foul-tasting and sharing common predators, have come to mimic each other's honest warning signals, to their mutual benefit. The benefit to Müllerian ...
, 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
Johann Friedrich Theodor Müller (31 March 1822 – 21 May 1897), better known as Fritz Müller, and also as Müller-Desterro, was a German biologist who emigrated to southern Brazil, where he lived in and near the German community of Blumenau, ...
described the effect in 1879, in an account notable for being the first use of a mathematical argument in evolutionary ecology
Evolutionary ecology lies at the intersection of ecology and evolutionary biology. It approaches the study of ecology in a way that explicitly considers the evolutionary histories of species and the interactions between them. Conversely, it can ...
to show how powerful the effect of natural selection would be.[
]
Aposematism
In 1867, in a letter to Darwin, Wallace described warning coloration. The evolutionary zoologist James Mallet James Mallet (born 15 March 1955 in London) is an evolutionary zoologist specialising in entomology.
He was educated at Winchester College.
He became professor of biological diversity at the Department of Biology, University College London. He wa ...
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
Advertising is the practice and techniques employed to bring attention to a product or service. Advertising aims to put a product or service in the spotlight in hopes of drawing it attention from consumers. It is typically used to promote a ...
to predator
Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...
s, signalling honestly that the animal is not worth attacking. This directly increases the reproductive fitness
Fitness (often denoted w or ω in population genetics models) is the quantitative representation of individual reproductive success. It is also equal to the average contribution to the gene pool of the next generation, made by the same individua ...
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
Edward Bagnall Poulton
Sir Edward Bagnall Poulton, FRS HFRSE FLS (27 January 1856 – 20 November 1943) was a British evolutionary biologist, a lifelong advocate of natural selection through a period in which many scientists such as Reginald Punnett doubted its ...
's 1890 book, ''The Colours of Animals
''The Colours of Animals'' is a zoology book written in 1890 by Sir Edward Bagnall Poulton (1856–1943). It was the first substantial textbook to argue the case for Darwinian selection applying to all aspects of animal coloration. The book a ...
'', renamed Wallace's concept of warning colours "aposematic" coloration, as well as supporting Darwin's then unpopular theories of natural selection and sexual selection
Sexual selection is a mode of natural selection in which members of one biological sex mate choice, choose mates of the other sex to mating, mate with (intersexual selection), and compete with members of the same sex for access to members of t ...
. Poulton's explanations of coloration are emphatically Darwinian. For example, on aposematic coloration he wrote that
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 science 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."
Notes
References
{{evo ecol
Evolution
History of evolutionary biology
Camouflage
Mimicry