SEXUAL CONFLICT AND SEXUAL SELECTION

Pre-Darwinian writings on Sex. John Hunter -distinguished primary sexual characteristics (those present at birth) from secondary sexual characteristics (those that appear with sexual maturity) John Hunter (1728-1793)

Erasmus Darwin Would it be too bold to imagine, that in the great length of time, since the earth began to exist, perhaps millions of ages before the commencement of the history of mankind, would it be too bold to imagine, that all warm-blooded animals have arisen from one living filament, which THE GREAT FIRST CAUSE endued with animality, with the power of acquiring new parts, attended with new propensities, directed by irritations, sensations, volitions, and associations; and thus possessing the faculty of continuing to improve by its own inherent activity, and of delivering down those improvements by generation to its posterity, world without end! Erasmus Darwin (1731-1802)

Natural and Sexual Selection Charles Darwin 1809 - 1882 Alfred Russel Wallace 1823 – 1913

The Ternate Letter Ternate Island    We believe we have now shown that there is a tendency in nature to the continued progression of certain classes of varieties further and further from the original type--a progression to which there appears no reason to assign any definite limits--and that the same principle which produces this result in a state of nature will also explain why domestic varieties have a tendency to revert to the original type. This progression, by minute steps, in various directions, but always checked and balanced by the necessary conditions, subject to which alone existence can be preserved, may, it is believed, be followed out so as to agree with all the phenomena presented by organized beings, their extinction and succession in past ages, and all the extraordinary modifications of form, instinct, and habits which they exhibit.

Besides this natural means of selection, by which those individuals are preserved, whether in their egg, or larval, or mature state, which are best adapted to the place they fill in nature, there is a second agency at work in most unisexual animals, tending to produce the same effect, namely, the struggle of the males for the females. These struggles are generally decided by the law of battle, but in the case of birds, apparently, by the charms of their song, by their beauty or their power of courtship, as in the dancing rock-thrush of Guiana. The most vigorous and healthy males, implying perfect adaptation, must generally gain the victory in their contests. This kind of selection, however, is less rigorous than the other; it does not require the death of the less successful, but gives to them fewer descendants. The struggle falls, moreover, at a time of year when food is generally abundant, and perhaps the effect chiefly produced would be the modification of the secondary sexual characters, which are not related to the power of obtaining food, or to defence from enemies, but to fighting with or rivalling other males. The result of this struggle amongst the males may be compared in some respects to that produced by those agriculturists who pay less attention to the careful selection of all their young animals, and more to the occasional use of a choice mate. Darwin, C. & A. Wallace. 1858. On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection. J. Proc. Linn. Soc. Lond. 3:45-50

Problems with Sexual Selection - Traits involved are - energetically wasteful - overly elaborate - contrary to ideas of natural selection

Costs of sex 1. Cost of meiosis must combine genes with another organism lose 50% every generation relative to asexuals -often referred to as cost of males - females can always do better asexually Sex but no males (isogamy) Sex with males (anisogamy) No sex 5 offspring each 100% relatedness 5 offspring each 50% relatedness 5 offspring 50% relatedness

Reproductive Behaviour Costs of sex 2. Cost of recombination take a functioning genome, split it and combine it with another one -chance of deleterious combinations

Reproductive Behaviour Benefits of sex 1. Produce new combinations Unique offspring can cope with different niches x

Reproductive Behaviour Benefits of sex 2. Muller’s Ratchet - in asexual species, deleterious mutations accumulate A Frequency of mutant gene A+B Mutation A A+B+C Mutation B B+C Mutation C C Time

Reproductive Behaviour Benefits of sex 2. Muller’s ratchet - in sexual species, such mutations tend to be eliminated Frequency of mutant gene Mutation A Mutation B Mutation C Time

Darwin’s Dilemma ? ? ?

And this leads me to say a few words on what I call Sexual Selection And this leads me to say a few words on 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. Generally, the most vigorous males, those which are best fitted for their places in nature, will leave most progeny. Charles Darwin, 1859. Origin of species

Darwin - Theory of sexual selection Two parts 1. Intrasexual selection (Male-male competition) But in many cases, victory will depend not on general vigour, but on having special weapons, confined to the male sex. The war is, perhaps, severest between the males of polygamous animals, and these seem oftenest provided with special weapons. 2. Intersexual selection (Female choice) …but if man can in a short time give elegant carriage and beauty to his bantams, according to his standard of beauty, I can see no good reason to doubt that female birds, by selecting, during thousands of generations, the most melodious or beautiful males, according to their standard of beauty, might produce a marked effect. Charles Darwin, 1859. Origin of species

Darwin - Theory of sexual selection Two parts 1. Intrasexual selection (Male-male competition) All those structures and behaviour patterns employed by males to fight other males for the chance to mate with females - claws, antlers etc.

Darwin - Theory of sexual selection Two parts 2. Intersexual selection (Female choice) All those structures and behaviour patterns employed by males to attract females - plumage, song.

Reproductive Behaviour Males and females have different reproductive interests What does each sex have to provide to produce offspring? Females Males Manufacture eggs (expensive) Manufacture sperm (cheap) 2. Incubate eggs or fetus (expensive) 3. Post partum care (expensive) 4. Loss of mating opportunity when pregnant (expensive) Males invest less in any offspring

Reproductive Behaviour In more general terms Females Difference in reproductive investment Males higher investment lower investment lower reproductive rate higher potential reproductive rate lower levels of mating activity higher levels of mating activity Biased operational sex ratio Selection among potential mates Competition for mates Best mate = best fitness benefit Achieve greatest number of matings

Reproductive Behaviour Operational vs Numerical Sex ratio Numerical Sex Ratio The ratio of the number of males to the number of females in the population Operational Sex Ratio The ratio of the number of males to the number of females in the population who are available for reproduction

Reproductive Behaviour Operational vs Numerical Sex ratio Numerical Sex Ratio (close to 1:1) Males Females Operational Sex Ratio (very different from 1:1) Competition in more abundant sex Selectivity in less abundant sex

Reproductive Behaviour Evidence?? Bateman (1948) -Drosophila melanogaster - nearly all females mated but not all males -variance in male reproductive success - higher Males Bateman gradient # of offspring Females # of mates

Reproductive Behaviour Maximum # offspring produced (lifetime) Species Male Female Ratio Elephant seal 100 8 12.5 Red deer 24 14 1.7 Human 888 69 12.9 Kittiwake gull 26 28 0.96

(usually male-biased) OSR and strength of sexual selection Is = measure of strength of directional sexual selection = (coefficient of variation in mating success)2 Assumption: Is determined by number of individuals available for mating (usually male-biased) Also need to know the ability of males to monopolize mates

OSR and strength of sexual selection Mean = .75 Variance = 1.69 Is = 3.0 Increase monopolization +1 Male OSR = 1.33 Mean = .75 Variance = .19 Is = 0.33 OSR = 1 Mean = 1 Variance = .67 Is = .67 Decrease monopolization

Intersexual Selection Why are females choosy? ✓ How is this choosiness exhibited? Genetic models for mate choice 1. Direct Benefits 2. Good Genes 3. Runaway Selection

Intersexual Selection Genetic Models for Mate Choice 1. Direct Benefits -females choose males that give them a concrete resource e.g. Nuptial gifts

Intersexual Selection Genetic Models for Mate Choice 1. Direct Benefits

Intersexual Selection Genetic Models for Mate Choice 1. Direct Benefits -females choose males that give them a concrete resource Crotolaria Utethesia ornatrix

Intersexual Selection Genetic Models for Mate Choice 1. Direct Benefits -females choose males that give them a concrete resource

Intersexual Selection Alkaloid retained on molt to adult male Larva ingests alkaloid Some alkaloid converted to male pheromone Attracts females Mating Females mate with males will more pheromone Greater amount of alkaloid transferred Greater protection for eggs

Intersexual Selection 2. Good genes models (indirect benefits) Female should look for best genetic complement in male Traits should reflect genetic quality

Intersexual Selection 2. Good genes models (indirect benefits) Female should look for best genetic complement in male Traits should reflect genetic quality Cost to males Year 1 -mated faster -preferred by females seeking EPC’s -poorer foragers -caught small prey Year 2 -shorter tails -poor quality plumage -lower mating success

Intersexual Selection 2. Good genes models (indirect benefits) Female should look for best genetic complement in male Red bishop Traits should reflect genetic quality Cost to males Collar functions in territory defense Red-collared widowbird Tail is to attract females

Intersexual Selection 2. Good genes models (indirect benefits) Female should look for best genetic complement in male Traits should reflect genetic quality

Intersexual Selection Equilibrium 3. Runaway Selection Female Preference Female Preference Male Character Male Character Viability selection is strong Viability selection is weak After Lande (1981)

Intersexual Selection 3. Runaway Selection Assume 2 genes Gene A - Trait ‘X’ in males - more colourful feathers Gene B - In females - preference for trait ‘X’ A+ B Expressed in males as the trait A+ B Expressed in females as a preference for the trait

Intersexual Selection 3. Runaway Selection Assume 2 genes Gene A - Trait ‘X’ in males - more colourful feathers Gene B - In females - preference for trait ‘X’ Frequency in males Frequency in females

Intersexual Selection 3. Runaway Selection Example - stalk-eyed flies - Diopsidae Variation in males Female

Intersexual Selection 3. Runaway Selection Example - stalk-eyed flies - Diopsidae Two experiments 1. Standard disruptive selection every generation - allow shortest and longest stalked males to mate with randomly selected females Short stalked Long stalked 13 generations Stalk length Stalk length

Intersexual Selection 3. Runaway Selection Example - stalk-eyed flies - Diopsidae Two experiments 2. Female choice Female offspring prefer short stalked males Female offspring prefer long stalked males Stalk length

Male Strategies