Lecture 15: Sexual Selection PDF

Lecture 15: Sexual Selection BIO 4083: Evolution Today’s Lecture Evolution of sexual reproduction Sexual selection Male-male competition Female choice Cost of ornaments and weapons Reproductive mechanisms are diverse Asexual reproduction Sexual reproduction Sexual/Asexual Hermaphrodite Parthenogensi...

Lecture 15: Sexual Selection BIO 4083: Evolution Today’s Lecture Evolution of sexual reproduction Sexual selection Male-male competition Female choice Cost of ornaments and weapons Reproductive mechanisms are diverse Asexual reproduction Sexual reproduction Sexual/Asexual Hermaphrodite Parthenogensis Twofold cost of sex: sexual reproduction is costly Limited number of reproducing individuals John Maynard Smith (1920-2004) Asexual lineages multiply faster than sexual lineages Consequences of Sexual Reproduction Disadvantages Twofold cost of sex Search cost Reduced relatedness Risk of sexually transmitted infections Ploidy: beneficial mutations go to fixation faster in haploid organisms Advantages Combining beneficial mutations Generation of novel genotypes Faster evolution Clearance of deleterious mutations Ploidy: diploidy and polyploidy masks deleterious mutations We have to think about evolutionary trade-offs Sexual reproduction enables organisms in finite populations to better adapt to changing environments Otto 2008 Sexual reproduction would allow for ++++ individuals Limitations to asexual reproduction Muller’s ratchet: genomes of an asexual population accumulate irreversible, deleterious mutations over generations Genetic load: the burden of accumulated deleterious mutations increases over time Why? Limitations to asexual reproduction Muller’s ratchet: genomes of an asexual population accumulate irreversible, deleterious mutations over generations Genetic load: the burden of accumulated deleterious mutations increases over time Why? No recombination Deleterious alleles in sexual and asexual populations Note how deleterious alleles become fixed in asexual populations but are purged from sexual populations Anisogamy results in differential investment in reproduction Kiwi Limitations on reproductive success differ for the sexes Females are limited by fecundity Males are limited by the number of mates they can obtain Fecundity: reproductive capacity of an individual; for example, number and quality of eggs or sperm. As a measure of relative fitness, refers to the number of offspring produced by an individual Investment differences can extend past fertilization Differences in parental care: uncertain paternity may explain why male parental care is less common Females have certain maternity Males have uncertain paternity Parental care/investment of resources could be directed toward offspring that is not their own Asymmetrical parental care Asymmetrical parental care alters the operational sex ratio Operational sex ratio (OSR): ratio of males to females capable of reproducing at a given time Slower rate of reproduction by females leads to male-biased OSR Sexual selection Differential reproductive success resulting from competition for mates Intrasexual selection (Male-male competition) Intersexual selection (Female choice) Sexual selection Differential reproductive success resulting from competition for mates Intrasexual selection (Male-male competition) Intersexual selection (Female choice) Still need variation, heritability, and differential fitness Heritable differences between sexes and among individuals of the same sex Limited number of mating opportunities Selection for traits that increase reproductive success Male-male competition (intrasexual selection) Sexual selection is commonly a stronger effect for males Male-biased OSR leads to sexual selection favoring males that can outcompete other males for access to females More males competing for mates: higher sexual selection High variance in male reproductive success leads to intense sexual selection Some males compete for territory Red-winged blackbird Agelaius phoeniceus Sexual dimorphisms result from intrasexual selection Ornaments: attractive traits that increase mating success Armaments: weaponry used to outcompete other individuals Cherry barb Golden metallic stag beetle Red-necked phalarope Sexual dimorphisms result from intrasexual selection Ornaments: attractive traits that increase mating success Armaments: weaponry used to outcompete other individuals Cherry barb Golden metallic stag beetle Red-necked phalarope Variance in reproductive success = prediction of which sex will experience stronger sexual selection High variance in male reproductive success leads to intense sexual selection Variance in reproductive success = prediction of which sex will experience stronger sexual selection Female intrasexual selection: higher OSR and reproductive variance in females Brennan 2010 Female choice (intersexual selection) Benefits Direct benefits: benefit the female directly For example, food, nest sites, protection Indirect benefits: benefits that affect the genetic quality of the female’s offspring Direct benefits Number of spermatophores eaten by females (given as gifts by males) positively correlates with egg weight and number of eggs Some females benefit from cannibalism The rules of attraction Females may gain indirect benefits by being selective about mate choice What traits can females rely upon as indicators of genetic quality in males? Some males display elaborate ornaments Related to female choice Female preferences are often consistent We can predict and measure female choice Female stalk-eyed flies prefer males with longer eye stalks Dances and displays Red-capped manakin https://www.youtube.com/watch?v=Sxl_bvCRgyE&t=9s Vogelkop superb bird of paradies https://www.youtube.com/watch?v=nPhVOZiPokA Ornaments can be honest signals of better health Red-collard widowbirds: healthiest males had the longest tails and more active nests “Sexy Sons Hypothesis”: females choose mates to maximize fitness of sons Explanation for crazy ornaments in males “Fisherian runaway” selection R.A. Fisher Photo: Trina Anderson Hamilton-Zuk Hypothesis: female choice related to parasites A “Good Genes” model W.D. Hamilton Marlene Zuk The Red Queen effect makes sex beneficial Where does variation come from in asexually reproducing organisms? Sexually reproducing organisms? Example of the Red Queen effect More males associated with more parasite infections: evidence that selection from parasites can drive the evolution of sexual reproduction Parthenogenic (asexual) and sexually reproducing populations But do we really understand female choice? Good genes Arbitrary choice? What about sexual selection in plants? Tonnabel et al. 2021 Pollen-pistil interactions What about sexual selection in fungi? Nieuwenhuis and Aanen 2012 Pheromone signaling The costs of sexual selection Competing for mates is costly Costly ornaments can be costly: fitness tradeoffs Red-collard widowbirds: longer tails also associated with a rapid deterioration in body condition over a breeding season https://www.youtube.com/watch?v=MXroJpZS1JQ Development of weapons can involve trade-offs Onthophagus dung beetles: larger horns associated with smaller eyes Weapons are difficult to fake: an honest signal of fitness Assignments Exam 2 on Friday, March 8