Transcript 3000_3F
3000-3f Hosler: “Optical Allusions” another graphic novel exploring the evolution of eyes... QuickTime™ and a decompressor are needed to see this picture. evolution: sex (a mode of inheritance) and competition for resources (ability to have sex) why is sex bad? why is sex bad? • in addition to providing a common means of pathogen transfer... • in addition to the energy spent finding a mate... • offspring have only HALF of a parent’s alleles • typically only one mating type CAN reproduce, slows growth of sexual population and its diversity simple model: two-fold cost of sex • asexual and sexual females produce the same number of offspring • offspring of asexual and sexual females have same fitness • if both of these are true, sex cannot be maintained: one must be violated • but we know sex is very common despite costs how does selection work? 1.individuals are variable 2.variation is heritable 3.some don’t survive to reproduce 4.those that survive and reproduce have inherited variation that was advantageous variation • mutation causes variation (sexual or asexual) • recombination reshuffles that variation: largely random crossover events that allow different parts of genome evolve independently (sexual or asexual) • genotypic variation through independent assortment of parental gametes (sexual) simple model: two-fold cost of sex • asexual and sexual females produce the same number of offspring • offspring of asexual and sexual females have same fitness • which of these is more likely to be violated, allowing sex to persist? conceptual model: sex produces variation faster note asex needs 2 sequenti al mutation s • mutation and physical processes alone create variation in asexual populations • sex brings different alleles together faster than mutation alone • this is presumed to have huge fitness implications beetle experiment: one color can have sex and keep new variation, other can not... which wins? selection favors sex violates 2nd assumption: sexual offspring survive better • sex reduces linkage disequilibrium • generates variation that otherwise wouldn’t exist • remember • so... variation is Darwin’s first postulate! selection acts on variation and (in this case) has increased variation! • Muller’s ratchet: start a population where all individuals identical • eventually mutations occur. most we assume to be deleterious. • without sex, populations keep accumulating more and more deleterious mutations (genetic load) • average • sex fitness declines breaks the ratchet! recreates genotypes if recombination happens between the mutations some gametes are cleared of mutations! • experiment: Mullers ratchet with bacterial populations • take them through bottlenecks so drift is strong • 1700 generations of clonal reproduction (asexual) • fitness • also was reduced in many cultures done naturally: endosymbiont bacteria living in insect guts can no longer compete • those experiments suggest drift and mutation alone can explain sex • but the “cost of sex” suggests asexuals could take over very quickly, and the flour beetle experiment suggests that sexual populations take over quickly instead: so is selection the preferred explanation? • Red Queen hypothesis: rapidly changing environment (or parasite) requires rapid evolutionary response Potamopyrgus trematode parasite • thought problem: New Zealand mud snails have been introduced accidentally into coastal Oregon • in many species introductions it has been shown that - because of the same sampling/bottleneck process associated with drift, and lack of coadapted parasites in new habitat - the parasite load in non-native populations is lower • what would you predict about the frequency when sex happens • now the players have different things at stake: energetics of reproduction vary between males and females • gamete size and number, investment in offspring... • leads us to major variant on natural selection: sexual selection responsible for many traits that otherwise seem unassociated with “fitness” sexual selection: not by gender, by resources given to reproduction asymmetry and fitness • sexual selection represents the fitness consequences of reproduction • • • agent of sexual selection is generally the gender that provides most of the resource for offspring (thus, the limiting resource for reproduction) members of the sex subject to strong sexual selection will be competitive members of the sex subject to weak sexual selection will be choosy where is the sexual selection? • where are the limiting resources, who puts in the most resource? • males gather in ponds (large numbers) and wait for females (smaller numbers at any one time) • females lay ~300 eggs, no parental care (cost of eggs and sperm is all parental investment) • female investment >> male 25 IN THIS CASE, sexual selection is more potent in evolution of males heritable traits associated with failure to mate will disappear (successful newts tend to have larger tail crests during breeding season, for example) where is the sexual selection? • resource use very different! father provides parental care via brood pouch (O2 and nutrients) • so for same paternity tests with genotype data, what difference do you expect from the newts? –eggs more expensive than sperm –parental care more expensive than none 29 sexual selection is more potent in evolution of females heritable traits associated with failure to mate will disappear leads to asymmetric limits on fitness asymmetry and fitness • sexual selection represents the fitness consequences of reproduction • • • agent of sexual selection is generally the gender that provides most of the resource for offspring (thus, the limiting resource for reproduction) members of the sex subject to strong sexual selection will be competitive members of the sex subject to weak sexual selection will be choosy