Mimicry.notes1

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Mimicry:
Natural Selection or Genetic Mutation?
Mimicry.
Noun.
A
superficial
resemblance of one organism to another
or to natural objects among which it lives
that secures it a selective advantage (as
protection from predation)
For more information on mimicry:
http://fig.cox.miami.edu/Faculty/Tom/bil101sp99/23_mimicry.html
Examples of Mimicry
Which one is tasty? Which one is nasty?
Which one is poisonous? Which one is harmless?
The Debate: Mendelians versus Selectionists
1) Theoretical problem: How did mimic species (e.g., Viceroy) come to resemble the model
species (e.g., Monarch)?
Darwin’s solution- Potential mimic and model originally had similar dull patterns, then the model gradually
developed brighter colors pulling the mimic with it.
Poulton argues that a mimicry is primarily developed by the forces of natural selection. Hence, he focuses on
small, gradual changes creating the mimic species.
Punnett argues that a very small amount of color on a butterfly (mimic) should not be expected to fool the
bird, therefore there is an “unbridgeable gap” between model and mimic. He concludes that mimicry arose in
one large step (genetic mutation), not gradually in many, many small steps (natural selection). How could a single
mutation could create the same pattern in distantly related species? Punnett argues that model and mimic shared
enough common genetic factors for a mutation to produce resemblance.
2) Experimental evidence
According to Punnett, some species of butterflies confirm the existence of one mutational step. Furthermore, if
a pattern had been built up gradually, then intermediates should appear in crossing mimetic with non-mimetic clearly
as single alleles.
Selectionists- small variations were key to natural selection and creating new
species/varieties
Mendelians- small variations were not produced by mutation, nothing came of them
The Sources: Punnett versus Poulton
1) Punnett, Mendelism (1911)
Contrasts Darwinian with Mendelian view of the evolution of mimicry. Unknown conditions brought about similar
mutations in widely disparate species of butterflies that already contained a similar set of pattern factors. Sudden
rather than gradual change is stressed. Thus, Punnett rejects Natural Selection, favoring genetic mutations.
2) Poulton, “Darwin and Bergson on the Interpretation of Evolution,” Bedrock 1: 48-65
(April 1912)
Poulton tests four theories explaining mimicry [i.e., Batesian (1862), Müllerian (1879), External Forces (local
climate, diet, physico-chemical), Internal Forces (Bergson- internal creative drives)]. He argues that small variations
arose to sufficient degree to give adaptive advantage. Natural selection gradually produces a detailed likeness of the
model. Poulton concludes that the “essential cause” of mimicry is natural selection. Mimicry was built up of
“small differences by the accumulative power of selection.” Even the first essential cause was “a very small
affair- a minute transformation.”
3) Poulton, “Mimicry, Mutation and Mendelism,” Bedrock 2: 42-56 (April 1913)
Poulton accepts Mendelian relationships illustrated in laboratory breeding experiments. However, he rejects
Punnett’s view of mimicry arising suddenly and complete as a mutation calling it an “astounding hypothesis.”
Poulton claims that Punnett is ignoring evidence of transitional forms: “Transitional forms are common near the zone
where the one model passes into the other.” Small changes in wing patterns are observed in biogeographical regions.
Poulton concludes that “mimetic resemblance has not been produced from the original non-mimetic form by a
sudden mutation,” it is the result of a series of transitional steps, some of which are preserved.
4) Punnett, “Mendelism, Mutation and Mimicry,” Bedrock 2: 146-164 (July 1913)
Current mimicry theory of Poulton and Weismann is “too facile.” Many difficulties, new experimental facts do not
support natural selection. Punnett attacks Poulton's view of natural selection with numerous questions. Where is the
evidence of the adaptive value of minute variations? Why make the "sweeping assumption" of the inheritance of
minute variations? By what “utterly unknown and mysterious process” are these minute variations built up to bridge
the unbridgeable gap between model and mimic? Why is mimicry frequently absent in males of certain species of
mimics? How can polymorphic mimicry be explained by natural selection?
He concludes saying that mimicry does not occur through gradual, continuous change, instead there is
“strong evidence for supposing evolution to have proceeded by definite mutational stages.”
5) Poulton, “Mimicry and the Inheritance of Small Variations,” Bedrock 2: 295-312
(October 1913)
William Bateson and Punnett imply Darwin was a “hasty generalizer.” Both are “dazzled and confused” by
Mendelism. They have “lost all perspective and all sense of proportion” and their “distorted vision is
unfortunate for English biological science.” Poulton asks: Does Punnett really doubt the inheritance of small
variations? Does he doubt family resemblance passed from parents to children (e.g., shapes of nose, face, etc.)?
Poulton calls Punnett’s language “an irresponsible utterance of one who has not thought out the consequences
of his own words.” He ignores the evidence of American geneticist, C. B. Davenport (1909) who showed that
small variations reappear in offspring according to Mendelian laws.
Poulton admits that the initial variation must be something appreciable, but natural selection is still the primary
reason for the development of mimicry.
6) Punnett, “More Mendelism and Mimicry,” Bedrock 2: 496-514 (January 1914)
Punnett claims that Poulton is stirring up prejudice with his last article. Darwin should not be above criticism.
Punnett argues that mutations arise in germ-plasm and are subject to the rules of genetic inheritance. Punnett
concludes: “No new type is created by selection. It merely selects a pre-existing variation, and eventually
disentangles it from others with which it was mixed up.”
Mutations may be “large” or “small” but this merely records “ocular sense impressions” (i.e., changes in physical
appearance) it says nothing about size of change in the germ-plasm (i.e., changes in genes).
Poulton has not answered Punnett's seven difficulties. Maybe because he cannot?
7) Poulton, “The Evolution of Mimetic Resemblance,” Bedrock 3: 34-45 (April 1914)
Poulton attempts to answer Punnett's seven objections from an earlier article:
1) Most naturalists believe that birds can see as well or better than humans. And even humans can
easily appreciate minute variations. Therefore, minute variations do have selective value.
2) He gives examples which Punnett ignored (e.g., family likeness, mimetic butterflies). Inheritance of
minute variations does occur and there is evidence.
3) The “unknown, mysterious process” referred to by Punnett is Natural Selection. Through selection
small variations are built up to create new species.
4) Segregation occurs according to Mendelian theory, but it operates at many successive stages in
developing mimetic resemblance. There are transitional forms which exist.
5) Females’ colors and patterns are more genetically variable then males’, therefore females more
frequently supply the material for selection. Hence, more mimicry appears in females than in males.
6) Hereditary variation is one chief explanation of Natural Selection and mimicry. Females are more
prone to polymorphism than males, although the cause is unknown.
7) There is much indirect evidence that enemies are fooled by mimics and that birds learn from unpleasant
experiences.
Therefore, Punnett has made “rash and unsupported” critiques of Darwin. Hereditary transmission of small
variations is not a “sweeping assumption.”
Poulton closes his argument with William Castle’s experimental research on hooded rats. Castle concludes that
Mendelian characters can be modified by selection. Unit characters are changeable, they can be modified in several
ways. Castle writes, and Poulton agrees with him:
“Occasionally a unit-character is lost altogether or profoundly modified at a single step. This is mutation.
But more frequent and more important, probably, are slight, scarcely noticeable modifications of unitcharacters that afford a basis for a slow alteration of the race by selection. . .”
(quoted from Castle's Hereditary and Eugenics, 1912)
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