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Sexual Selection. Selection for traits which are solely concerned with increasing mating success is usually referred to as sexual selection. Can Work in Two Ways:.

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Sexual Selection

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Sexual Selection

Selection for traits which are solely concerned with increasing mating success is usually referred to as sexual selection

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Can Work in Two Ways:

1. By favoring the ability of one sex (usually males) to compete directly with one another for fertilizations, for example by fighting – intra-sexual selection

2. By favoring traits in one sex which attracts the other sex – inter-sexual selection

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The intensity of sexual selection depends on the degree of competition for mates & this in turn depends on two factors

1. The difference in parental effort between the sexes

2. The ratio of males to females available for mating at any one time (known as the operational sex ratio)

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Strength of Sexual Selection

When parental effort is more or less equal, as in monogamous birds, for example, where both male and female feed the young, sexual selection is less intense than in species with very different levels of parental effort

If equal numbers of the two sexes come into breeding condition at the same time, the degree of sexual selection is reduced because there is less chance for a few males to control access to very large numbers of females

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Strength of Sexual Selection

In contrast, when females come into breeding condition asynchronously there’s a chance for a small number of males to control many females one after the other

With such high potential payoffs, sexual competition should be intense

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Sexual Selection



Very Strong Less Strong

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If sexual selection is to explain the differences between the sexes, it will have to act on the sexes differently

  • A.J. Bateman (1948) predicted that sexual selection – variation in mating success – will usually be a more potent force in the evolution of males than in the evolution of females

  • In other words, access to mates will be a limiting resource for males but not females

  • Of course this prediction is central to the theory of sexual selection

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Direct Tests

Jones, A.G., J.R. Arguello, and S.J. Arnold. 2002. Validation of Bateman’s principles: a genetic study of sexual selection and mating patterns in the rough-skinned newt. Proceedings of the Royal Society of London 269:2533-2539.

Jones, A.G., G. Rosenqvist, A. Berglund, et al. 2000. The Bateman gradient and the cause of sexual selection in a sex-role-reversed pipefish. Proceedings of the Royal Society of London 267:677-680.

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Rough-Skinned Newt

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Broad-nosed Pipefish

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Sexual Selection Theory

  • Jones et al.’s study on pipefish shows that greater sexual selection on males than on females is not inherent in the identity of the sexes themselves

  • When access is limiting for females instead of males we predict that females will compete with each other over access to males and males will be choosy

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Competition for Mates

The most dramatic and obvious way in which males compete for mates is by fighting and ritualized contests, and males often have evolved weapons for fighting

Males may dispute over direct access to females or over places where females are likely to go

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Male-Male Competition: Intrasexual Selection

Wikelski, M. and F. Trillmich. 1997. Body size and sexual size dimorphism in marine iguanas fluctuate as result of opposing natural and sexual selection: an island comparison. Evolution 51:922-936

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Although intense fighting over females can occur, males often compete in less conspicuous ways

Abele, L.G. and S. Gilchrist. 1977. Homosexual rape and sexual selection in acanthocephalan worms. Science

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Abele and Gilchrist (1977)

The males of Moniliformes dubius, a parasitic acanthocephalan worm found in the intestine of rats cements up the females genital opening after copulation to prevent other males from fertilizing her

In addition to sealing up the female after copulation, the male sometimes “copulates” with rival males and applies cement to their genital region to prevent them from mating again

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Carayon (1974)

Shows that the male hemipteran Xylocoris maculipennis pierces the body wall of the female and injects sperm during copulation fertilizing her eggs

Like with acanthocephalan worms, males sometimes engage in homosexual “copulation”. A male injects his sperm into a rival male where they wait to be passed on to a female next time the victim mates

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Female Choice

Since females in the majority of species invest more than males in each offspring, they might be expected to be choosier in selecting their mates

Females often select males on the basis of material resources that they can offer or females may select males on the basis of genetic benefits for their offspring

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Female Choice

Non-genetic benefits or material resources might include male defended breeding territories containing resources that play a crucial role in the survival of a female’s eggs or young

Females may also choose whether or not to mate with a male on the basis of his ability to provide food. In some birds and insects, for example, males may provide food for the female during courtship making a significant contribution to her eggs

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For example, female hanging flies, Hylobittacus apicalis, will mate with a male only if he provides a large insect for her to eat during copulation

The larger the insect the longer the male is allowed to copulate and the more eggs he fertilizes. The female gains from a large insect by having more food to put into her eggs

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Genetic Benefits

If some males have better genes than others, could a female improve the success of her progeny by choosing males with good genes?

Good genes are the ones which increase the ability of her offspring to survive, compete and reproduce

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Good Genes

Welch, A., R.D. Semlitsch and H. C. Gerhardt. 1998. Call duration as an indicator of genetic quality in male gray tree frogs. Science 280:1928-1930.

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Fitness of long-calling frogs vs. short-calling frogs

1995 1996

Fitness Measure HF LF HF LF

Larval Growth NS LCB LCB LCB

Time to Metamorphosis LCB NS LCB NS

Mass at Metamorphosis NS LCB NS NS

Larval Survival LCB NS NS NS

Postmetamorphic growth - - NS LCB

HF – High food

LF – Low food

NS – non-significant

LCB – Long call better

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Elaborate Displays

The theory of sexual selection is most famous as an attempt to explain the evolution of exclusively elaborate adornments and displays

Although some elaborate displays may have evolved for use in contests between males, some have certainly evolved as a result of selection by females for genetic benefits

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Two Hypotheses to Explain how Selection for Genetic Benefits might Produce Elaborate Traits

1. Fisher’s hypothesis (also called the runaway process)

2. The Handicap hypothesis

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Many studies have shown a relationship between male mating success or female preference and male sexual displays

Andersson, M. 1982. Female choice selects for extreme tail length in a widowbird. Nature

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Andersson (1982)

Showed that females of the long tailed widow bird in Kenya prefer males with long tails

This is a highly polygynous species making it an ideal candidate for sexual selection; the male is a sparrow sized bird with a tail up to 50cm long

The female’s tail is about 7cm long, presumably close to the optimum for flight purposes

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Andersson (1982)

Studied 36 males which he divided into 4 groups

One group he docked the tails to about 14cm

Another group he increased tail lengths by 25cm, gluing on cut feathers

The remaining two groups served as controls - one left untouched & the other has their tails cut and glued without altering length

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Male ornaments are favored by female mate choice and probably evolved through it

Not due to intra-sexual selection among males competing for territories or hierarchy ranks - all males held territories equally

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Fishers Hypothesis (1930) - postulates runaway feedback between female preference and male displays

Imagine at the start there was a range of tail lengths and female preferences in the population

Females with a preference for slightly longer than average tails would be mated to males with longer tails

The crucial fact to note is that offspring of these mating would have both the tail and preference genes

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Fishers Hypothesis

  • The preference is expressed only in females and the tail in males, but everyone carries both kinds of genes

  • In short, there will arise an association or covariance between tail and preference genes

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The Handicap Hypothesis - Zahavi (1975) suggested an alternative view of elaborate male sexual displays

  • He suggested that females prefer long tails (or other equivalent traits) because they are handicaps and therefore act as reliable signals of a males genetic quality

  • The tail demonstrates a males ability to survive in spite of the handicap, which means that he must be extra good in other respects

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Zahavi’s Hypothesis

  • If any of this ability is heritable, then the tendency to be good at surviving will be passed on to the offspring

  • Therefore females select for good genes by selecting to mate only with males whose displays honestly indicate their genetic quality

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It is important to note that in this hypothesis the good genes are genes for survival and reproduction, rather than genes purely for attracting females, as assumed in Fisher’s hypothesis

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Evidence for the Fisher and Handicap Hypotheses

To demonstrate that a trait had evolved by Fisher’s process, it would be necessary to show that there is genetic variation for both female preference and the male trait and that genes tend to covary

Because Fisher’s hypothesis assumes that the only benefit of the selected trait is increased mating success, it would be necessary to show that expression of the male trait did not correlate with any inherited aspect of fitness

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Support for Fisher’s Hypothesis

Houde, A.E. and J.A. Endler. 1990. Correlated evolution of female mating preferences and male color patterns in the guppy Poecilia reticulata. Science

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Houde and Endler (1990)

Males from different populations differ greatly in the extent to which they develop bright orange and blue spots, which are a stimulus for females during courtship

Females from streams with large spotted males have stronger preferences for males with large orange spots than streams with small-spotted males

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Houde and Endler (1990)

The difference between populations in both male sexual color pattern and female preference are genetic: they persist in the lab for many generations

The fact that the differences persist under lab conditions suggests that the expression does not depend on the ability to gather food or on disease resistance - i.e., results are consistent with Fisher’s hypothesis

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Handicap Hypothesis

Mainly focused on

Hamilton, W.D. and M. Zuk. 1982. Heritable true fitness and bright birds: a role for parasites? Science

- namely that sexual displays are reliable indictors of genetic resistance to disease

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One of the most detailed studies to date

Moller, A.P. 1988. Female choice selects for male sexual tail ornaments in the monogamous swallow. Nature

Moller, A.P. 1990. Effects of a haematophagous mite on the barn swallow: a test of the Hamilton and Zuk hypothesis. Evolution

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Moller (1988)

Moller demostrates that females prefer males with longer tails

Males with experimentally elongated tails paired up more quickly and were also preferred by females seeking extra-pair matings

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Why do females prefer males with longer tails?

Moller (1990) - Could the ornament signal a males genetic quality in terms of his ability to resist parasites?

Swallows carry a blood-sucking mite, Ornithonyssus bursa, which infects both adults and nestlings

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Moller (1990)

The life cycle of the mite, from egg to adult, lasts just 5-7 days so one reproductive cycle of the swallow provides time for 8-10 generations of mites (up to 14,000 mites)

Moller showed that nestlings reared in nests with lots of mites were lighter and smaller and suffered increased mortality. Removal experiments confirmed that mites were the cause of the reduced growth

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Moller (1990)

There was large variation in the population in the degree of parasite infection

To test whether parasite resistance was heritable, Moller exchanged half the nestlings between pairs of nests

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Nestling parasite burden was correlated with that of its parents, even when it was reared in another’s nest - suggests that resistance is at least partly genetic

# of mites

on own


in other


# of mites on

male parent

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Link with the swallow’s tail

# mites

on own


in other


Moller found that parents with longer tails had offspring with smaller mite loads, even when their offspring were raised in another nest

males tail length (mm)

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Moller (1990)

This suggests that the length of a male’s tail signals his degree of parasite resistance

This fits the Hamilton-Zuk/Handicap hypothesis that females are choosing males able to pass on good genes to their offspring

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Female Choice & Sexual Selection

Seehausen, O., J.M. van Alphen Jacques, and F. Witte. 1997. Cichlid fish diversity threatened by eutrophication that curbs sexual selection. Science 277:1808-1811.

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Female Choice

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Female Mate Choice in Plants

Waser, N.M., M.V. Price, A.M. Montalvo, and R.N. Gray. 1987. Female mate choice in a perennial herbaceous wildflower, Delphinium nelsonii. Evolutionary Trends in Plants 1:29-33.

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Waser et al. (1987)

  • The success of a cross between two plants should depend in part on their genetic similarity - i.e., an optimal separation or optimal outcrossing distance; an index of genetic similarity

    • the optimal outcrossing distance for Delphinium nelsonii is 3-10m

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Offspring of crosses over the optimal outcrossing distance survive best under field conditions

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Waser et al. (1987)

  • These effects of outcrossing distance on fitness components are mirrored in the performance of male gametophytes

  • In a series of controlled crosses, pollen donors 10m from the recipient had a significantly higher probability of delivering a pollen tube to the ovary than did pollen from donors 1m or 100m away

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Waser et al. (1987)

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Waser et al. (1987)

  • This suggests a physiological interaction between pollen and pistil that increases the probability of producing offspring with high viability

  • Such interaction can be interpreted as adaptive discrimination or choice of mates on the part of the female

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