Sexual selection. 1) Sexual dimorphism. 2) Variation in mating success. 3) Sexual selection. 4) Female choice based on male ornaments

Transcription

1 Sexual selection 1) Sexual dimorphism 2) Variation in mating success 3) Sexual selection 4) Female choice based on male ornaments 5) The evolution of female preference

2 1) Sexual dimorphism

3 1) Sexual dimorphism Lesser bird of paradise Superb bird of paradise upload/ wilsons_bop.jpg

4 1) Sexual dimorphism Catasetum saccatum orchid male female

5 2) Variation in mating success Ÿ Matings are not apportioned randomly within species --- some individuals obtain more mates than others. Ÿ Consider the differences between males and females of most species: males produce many tiny mobile gametes (sperm), whereas females produce comparatively large, nutrient-rich gametes (eggs). Ÿ Consequently, reproductive success of females is limited more by her ability to produce and nourish gametes than by the number of mates obtained. Ÿ Males are able to produce many more sperm than there are eggs. The reproductive success of males is likely to be limited by the number of matings obtained.

6 2) Variation in mating success No. of progeny Drosophila pipefish females males females In Drosophila, the relationship between number of offspring and number of mates is steeper in males than in females. However, in the pipefish Sygnathus typhle, the relationship is reversed: it is steeper in females than in males. Males provide all of the parental care, supplying offspring with nutrients and oxygen through a placenta-like connection. males Mating success (no. mates) Jones et al (2000)

7 2) Variation in mating success The result in the typical case is that abundant sperm compete for much less numerous eggs, creating competition between males for fertilizations. This leads to sexual selection on traits that enhance fertilization success in males. Mating can be costly in males too, so the above arguments don t apply in all species. Males of many species are choosy.

8 2) Variation in mating success Males are choosy in pipefish, which have reverse sexual dimorphism choice by males choice by females Berglund & Rosenqvist 2003 Adv. Study Behav Berglund et al 1986

9 3) Sexual selection Nonrandom association between a trait and mating success Sexual selection in plants

10 3) Sexual selection Male-male competition: combat Male-male competition is thought to explain why males are larger than females in many species. Sexual selection in forked fungus beetle, Bolitotherus cornutus Traits Selection differential s Selection gradient b elytra length horn length weight Connor (1988)

11 3) Sexual selection Male-male competition: sperm competition Adaptations for sperm removal and sperm displacement in male damelflies Robinson & Novak (1997) Waage (1979)

12 3) Sexual selection Male-male competition: scramble competition for pollinators

13 3) Sexual selection Mate choice: resources Female preference of males based on resource provisioning

14 3) Sexual selection Mate choice: male ornaments [placeholder page for video]

15 The terms choice and preference here refer to a mating bias (not necessarily active choice)

16 Before Darwin proposed that such traits were elaborated in males because females preferred them Mean number of nests per male After

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18 Yet, the preferred male traits are costly (to the male)

19 The evolution of female preference for male ornaments Hypotheses fall into two categories A) Preference is nonadaptive, arbitrary Ÿ Fisher runaway process Ÿ Latent preferences / sensory exploitation B) Preference is adaptive Ÿ Direct natural selection on the preference Ÿ Indirect natural selection ( good genes )

20 The Fisher process 1. Imagine a population with an initial bias in the female population: a slight, genetically-based tendency to prefer males having a slightly elaborated trait, such as a long tail. Imagine also some genetically-based variation in males in tail length. 2. Assume no natural selection on this preference. Females preferring long tails produce no more nor fewer offspring than females who do not prefer long tails. 3. Males with longer tails will then experience slightly higher mating success. 4. The sons of such matings will inherit long tails and also the genes causing a preference for longer tails. This establishes a nonrandom association in the population (a genetic correlation) between genes for tails and preferences. 5. Because of the bias in favor of longer tails, these sons will have higher than average mating success, which indirectly favors the genes for the preference. 6. This self-reinforcing process favors ever-longer tails and preferences for longer tails until the mating advantage to males is counteracted by the costs of the trait.

21 Illustration of the Fisher process at equilibrium Males survive best that have a moderate trait value The equilibrium trait value in the male is greater than this Survival selection favors the moderate male each generation Males with a large trait value are most attractive to females The mating advantage of the large trait value offsets the survival disadvantage, which maintains the male mean above the survival optimum

22 Multiple equilibria possible under the Fisher process (2) Tail length preferred by females (3) Where the two forces are balanced (1) Tail length maximizing survival

23 Male trait and female preference often evolve in tandem among populations This is a prediction of Fisher s hypothesis (although other hypotheses make the same prediction) Slope refers to strength of female preference for orange in a population of males

24 Sensory exploitation hypothesis A) Preference is nonadaptive, arbitrary Ÿ Fisher runaway process Ÿ Latent preferences / sensory exploitation B) Preference is adaptive Ÿ Direct natural selection on the preference Ÿ Indirect natural selection ( good genes )

25 Sensory exploitation?

26 Female preference for swords in swordless species Priapella body_bestandsliste.html Xiphophorus

27 Latent preferences in females for male ornaments Burley found that red and black leg bands were more attractive than blue or green bands Zebra finches Files/Color%20Banding%20in%20Zebra%20Finches.ppt

28 Chase-away hypothesis An extension of the idea of sensory bias

29 Evidence for a direct advantage of female preference A) Preference is nonadaptive, arbitrary Ÿ Fisher runaway process Ÿ Latent preferences / sensory exploitation B) Preference is adaptive Ÿ Direct natural selection on the preference Ÿ Indirect natural selection ( good genes )

30 The good genes model of sexual selection A) Preference is nonadaptive, arbitrary Ÿ Fisher runaway process Ÿ Latent preferences / sensory exploitation B) Preference is adaptive Ÿ Direct natural selection on the preference Ÿ Indirect natural selection ( good genes )

31 Assumptions of hypotheses of direct vs indirect advantage Direct: Ÿ Males vary in their quality or condition. Ÿ Males vary in a secondary sexual trait preferred by females. Ÿ Degree of male ornamentation and male quality/condition are positively correlated ( honest indicator, expected to evolve only if male trait is costly). Ÿ Females who choose males having higher ornamentation obtain direct benefits (higher survival, more, plumper offspring) via his higher than average quality (fewer STDs, better paternal care). Indirect: Ÿ Males vary in their quality or condition. Ÿ Variation in male quality is heritable ( good genes ). Ÿ Males vary in a secondary sexual trait preferred by females Ÿ Degree of male ornamentation and male quality/condition are positively correlated ( honest indicator, expected to evolve only if male trait is costly). Ÿ Females who choose males having higher ornamentation obtain indirect benefits (offspring inherit good genes, and so have higher survival and reproductive success)

32 Evidence for an indirect advantage of female preference Males of the species court by tapping females at her back using head and forelegs. Mating mating success increases with courtship rate, and so the trait is preferred by females. Kotiaho et al (2001) Nature

33 Degree of male ornamentation and male condition are positively correlated ( honest indicator ). Demonstrated by manipulating food and showing that this affected courtship rate positively. This was a non-heritable form of condition natural variation in condition was not evaluated here. Left, mean s.e. of courtship rate per minute (log + 1 transformed); right, the same after five days of manipulation of food availability. Solid symbols, constant food treatment; open symbols, no food treatment. Kotiaho et al (2001) Nature

34 Natural variation in male condition is heritable. Demonstrated by showing that male scrawniness is heritable. They did not test whether this influenced offspring survival and reproduction (it is a lab population). 3) Condition is heritable Conclusion: females mating with high-courting males transmit high condition to their offspring Kotiaho et al (2001) Nature Condition is measured as the standardized residual mass from a linear regression of log body mass on log pronotum width. The different symbols represent the 12 sires. The three replications within each sire cluster represent the three dams for each sire. Sires are ranked according to the mean condition of their offspring.

35 The evolution of female preference for male ornaments Interim conclusion: There are several hypotheses for the evolution of female preferences for male traits. None has been conclusively ruled out by data. Some make similar predictions, making it difficult to tease apart the most important cause. A) Preference is nonadaptive, arbitrary, possibly even hazardous Ÿ Fisher runaway process Ÿ Latent preferences / sensory exploitation B) Preference is adaptive Ÿ Direct natural selection on the preference Ÿ Indirect natural selection ( good genes )