Download Natural selection/sexual selection

Sexual selection and natural selection
Jean Gayon
Institute of History and Philosophy of science & Techniques
Un. Paris 1-Panthéon Sorbonne
1. Introduction: the two sides of Darwin’s theory of
selection in nature
Natural selection/sexual selection: “two natural means of
selection”
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Different fates of the two theories
Two questions:
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On the Origin of Species by Means of Natural Selection (1859):
491 pages
The Descent of Man, and Selection in Relation to Sex (1871):
571 pages (over 830) devoted to sexual selection
Why was sexual selection so important to Darwin?
Why was it so much deemphasized by Darwin followers?
Outline
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Sexual selection in Darwin’s thought
Wallace’s criticism of sexual selection
Sexual selection as an indicator of different conceptions of
natural selection
2. The rise of sexual selection in Darwin’s thought
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“Old and Useless Notes” (1838-1840)
Darwin sensitive to two problems:

How does a female recognize a beautiful male?
“How does Hen determine most beautiful cock, which best singer?”
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Similarity between:
 Choice of a sexual partner by an animal
 Artificial selection
2. The rise of sexual selection in Darwin’s thought
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Essays of 1842 and 1844
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The term “sexual selection” does not yet appear
Distinction between two kinds of “struggle of the males for the females”:
“battle” vs “courtship”
In 1844, this process is presented as another “natural means of selection”
In this text, this other process is presented as another way for producing
adaptations
2. The rise of sexual selection in Darwin’s thought
« Besides this natural means of selection, by which those individuals are
preserved, whether in their egg or seed or in their mature state, which are
best adapted to the place they fill in nature, there is a second agency at
work in most bisexual animals tending to produce the same effect, namely
the
struggle
of
the
males
for
the
females
»
(Darwin, 1844 Essay)
2. The rise of sexual selection in Darwin’s thought

On the Origin of Species (1859)

Sexual selection disconnected from struggle for existence:
« ...what I call Sexual Selection. This depends, not on a struggle for
existence, but on a struggle between the males for possession of the
females; the result is not death to the unsuccessful competitor, but few or
no offspring. Sexual selection is, therefore, less rigorous than natural
selection.

1st
mode:
direct
rivalry
between
males:
« ...Generally, the most vigorous males, those which are best fitted for their
places in nature, will leave most progeny. But in many cases, victory will
depend not on general vigour, but on having special weapons, confined to
the male sex. » (OS, p. 88)
2. The rise of sexual selection in Darwin’s thought

On the Origin of Species (1859)
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2nd
mode:
female
choice
« I can see no reason to doubt that female choice, by selecting,
during thousands of generations, the most melodious or beautiful
males, according to their standard of beauty, might produce a
marked
effect.
»
(OS,
p.
88)

This sentence was the main reason of the controversy between
Wallace and Darwin over sexual selection, because of its
anthropomorphic connotations:
 cognitive abilities and aesthetic sense in animals
2. The rise of sexual selection in Darwin’s thought

Descent of Man (1871)
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Huge documentation on secondary sexual characters
Darwin’s method: consilience of inductions (explaining a vas range
of previously uncorrelated phenomena). Predictions:
 Struggle more severe if sex ration ≠ 1
 Struggle more severe in polygamous species
 Struggle more severe in animals with more cognitive abilities.
 ...
2. The rise of sexual selection in Darwin’s thought

Descent of Man (1871)
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Question: why did Darwin put in the same book his reflection over the
origin of Man and that on sexual selection?
Darwin’s answer to the question of the origin of man is subtle:
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Origin of generic physical traits: individual natural selection
Origin of mental characters: individual natural selection (but also benefits to
the group)
Origin of moral characters: “tribal” (or group) natural selection
Origin of human races (physical traits): sexual selection
Origin of mental and moral traits among races: natural selection (tribes)
2. The rise of sexual selection in Darwin’s thought

Descent of Man (1871)
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Content of the sexual selection hypothesis :
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Definition:
“[it] depends on the advantage which certain individuals have over either
individuals of the same sex and species, in exclusive relation to
reproduction”
Two modes:
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rivalry between males founded on vigor and strength;
mate choice (mainly female choice, but not exclusively)
These two modes are not sharply divided
Neither is the distinction natural/sexual selection absolute
3. The Darwin-Wallace controversy
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Overview of the disagreements between Wallace and Darwin
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Theory of selection
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Environmental vs competitive concept of natural selection
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Group and species selection
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Sexual selection
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Role of natural selection in the explanation of particular
phenomena
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Origin of hybrid sterility
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Origin of sexual dimorphism
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Origin of Man and of human races
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Place of the principle of natural selection in evolutionary
theory
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Wallace more radical than Darin on natural selection as an
all-sufficient principle for evolutionary theory
3. The Darwin-Wallace controversy

Overview of the disagreements between Wallace and Darwin

Theory of selection

Environmental vs competitive concept of natural selection

Group and species selection

Sexual selection

Role of natural selection in the explanation of particular
phenomena

Origin of hybrid sterility

Origin of sexual dimorphism

Origin of Man and of human races

Place of the principle of natural selection in evolutionary
theory

Wallace more radical than Darin on natural selection as an
all-sufficient principle for evolutionary theory
3. The Darwin-Wallace controversy

Overview of the disagreements between Wallace and Darwin

Theory of selection

Environmental vs competitive concept of natural selection

Group and species selection

Sexual selection

Role of natural selection in the explanation of particular
phenomena

Origin of hybrid sterility

Origin of sexual dimorphism

Origin of Man and of human races

Place of the principle of natural selection in evolutionary
theory

Wallace more radical than Darin on natural selection as an
all-sufficient principle for evolutionary theory
3. The Darwin-Wallace controversy

Overview of the disagreements between Wallace and Darwin

Theory of selection

Environmental vs competitive concept of natural selection

Group and species selection

Sexual selection

Role of natural selection in the explanation of particular
phenomena

Origin of hybrid sterility

Origin of sexual dimorphism

Origin of Man and of human races

Place of the principle of natural selection in evolutionary
theory

Wallace more radical than Darin on natural selection as an
all-sufficient principle for evolutionary theory
3. The Darwin-Wallace controversy

Overview of the disagreements between Wallace and Darwin

Theory of selection

Environmental vs competitive concept of natural selection

Group and species selection

Sexual selection

Role of natural selection in the explanation of particular
phenomena

Origin of hybrid sterility

Origin of sexual dimorphism

Origin of Man and of human races

Place of the principle of natural selection in evolutionary
theory

Wallace more radical than Darin on natural selection as an
all-sufficient principle for evolutionary theory
3. The Darwin-Wallace controversy

Wallace’s attacked sexual selection on two fronts
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Direct rivalry between males
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Does exist, but superfluous in most cases. Natural selection
can account for the observed phenomena
e.g. male weapons are useful for defending the group (or the
species) against predators)
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Female (or mate) choice
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Does not exist. Two lines of arguments:
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Superfluous
e.g. The dull appearance of many female birds is best
explained by the necessity of protecting the nest against
predators, while the conspicuous appearance of males
allows them to attract predators and divert them from the
nest
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“Choice”, “beauty”, “aesthetic taste” are anthropomorphic
notions. Such notions have no room in biological theory.
3. The Darwin-Wallace controversy
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Wallace’s final judgement in his Darwinism (1889):
“My whole work tends forcibly to illustrate the overwhelming
importance of natural selection over all agencies... even in rejecting
that phase of sexual selection depending on female choice. I insist
on the greater efficacy of natural selection. This is pre-emintently the
Darwinian doctrine, and I therefore claim for my book the position of
being
the
advocate
of
pure
Darwinism”
4. Conclusion:striking the balance
Retrospectively, Wallace and Darwin were both right and wrong
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Wallace was right to emphasize the adaptive importance of a
number of aspects of sexual dimorphism
Zahavi’s theory of “honest signaling” (1975) illustrates this point
But Wallace was unable to account for the most spectacular
aspects of sexual dimorphism: extravagant colors, ornaments,
and courtships.
Darwin was right to emphasize the proper evolutionary dynamics
based on mate choice.
But his explanation based on purely psychological notions
(choice and aesthetic sense) lacked evolutionary generality
R.A. Fisher (1930) was the first author to provide a convincing
argument
4. Conclusion:striking the balance
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Fisher’s argument (GTNS, 1930)
In a population where there is a “majority preference” for anything
whatsoever, the best reproductive strategy for a female is to adopt
this preference, because “the next generation of daughters will
inherit the mother’s preference whilst her sons will inherit their
father’s attractive feature”
4. Conclusion:striking the balance
Sexual selection as an indicator of Wallace’s and Darwin’s concept
of natural selection
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Wallace: environmentalist conception of natural selection
Darwin: competitive conception of natural selection
Sexual selection as a key test for Darwin’s conception of natural
selection, based upon intraspecific competition