4Advantages of asexuality: which sex limits population growth? Sexual lineageAsexual lineageNote: no figure, but attempted to convey same idea with letters.What would happen if an asexual mutant occurred in a population? Would it increase or decrease?
5Advantages of asexuality: fitness of individualsFrequency
699.9% of species can’t be wrong So, is sex good?99.9% of species can’t be wrongCnemidophorusSexualAsexualDandelionsDaphnia
7(but a few asexual lineages persist) Bdelloid rotifersfreshwater filter feedersThis implies that asexuality should have some major problems, or sex some major advantages.What exactly?
8Hypotheses to explain the maintenance of sex Harmful mutations: Mller’s ratchetSpeed of adaptation and the Red Queen
9Disadvantages of asexuality: Müller’s ratchet Mutations happenand accumulatemutation-free chromosome
10# of deleterious mutations # of deleterious mutations Müller’s ratchetLLC: “least-loaded class”“CLICK”frequencyfrequency# of deleterious mutations# of deleterious mutations
11Müller’s ratchet in sexuals? No. “CLICK”frequencyfrequency# of deleterious mutations# of deleterious mutationsfrequency# of deleterious mutations
12Müller’s ratchet : an experiment Set up 444 cultures of SalmonellaTransfer one individual every 24 hours1700 generationsPrediction: lower fitnessTest: generation time:parental:Experimental populations:
13Evidence for Müller’s ratchet : the human Y chromosome In XX females, recombinationY chromosome does not pair with X
15Reason’s for sex: adaptation B1A2B2Low fitnessHigh fitnessA1B1parentalA1B2A1B1recombinantmeiosisA2B1A2B2recombinantA2B2parental
16Does recombination increase genetic variance for fitness?
17The results of sex: cost of recombination If parents have higher than average fitness, what effect of recombination?The results of sex: cost of recombinationA1B1meiosisA2B2parentalrecombinant
18Experimental test of recombination and adaptation Adaptation may require new combinations of allelesAsexuality does not allow thisExperiment: flour beetles (Tribolium)Have stock populationAllow one population to evolveThe other is restocked from original population, as if asexual.Asexual has 3x reproductive advantageAsexuals start 0.5 of populationSelection: pesticide MalathionWhat proportion are sexual?
19Advantage of sex: adaptation Malathion concentrationProportion sexual30Generationsfigure 7.18
20Do organisms need to adapt? Red queen hypothesis Red Queen to Alice:“Now, here, you see, it takes all the running you can do, to keep in the same place.”--Lewis Carrol,Through the Looking Glass (1872)Concept: constant adaptation needed. Why?
21Parasites and hostsImagine four parasite genotypes, four host defense genotypesParasiteHost
22Parasites and host: Red queen startafter selection on host
23Parasites and host: Red queen after selection on hostafter selection on parasite
29Why two sexes? Most species have two sexes Some have multiple sexes (mating types): mating type 1 can mate with anyone except mating type 1, etc.Advantage: higher proportion of population available for matingWhy only two mating types??
30Implication of two sexes Sexual selectionSexual conflict
31Intrasexual competition: sperm competition When females mate with multiple males, sperm can competeExample: yellow dung fly Scathophaga stercoraria
32Yellow dung flies Females mate with multiple males Last male to mate fertilizes most ovulesSelection experiment: compare wild to 10 generations monogamous, 10 generations polyandrousExpected effect on sperm competition?Monogamous♀ + ♂Polygamous♀ + 3♂10 gen.10 gen.
33Yellow dung flies: polyandrous males vs. monogamous males Mate each female with two males: one from polyandrous line, one from monogamous lineHosken & Ward % offspring sired by second male to mate
34Sexual selection and intersexual conflict evolution Traits that favour male success may harm female successExample: sperm competitionfaster sperm win: higher fitness for maletoo many sperm: polyspermy – egg is losthuman spontaneous abortions: 3% of conceptions end due to triploid embryos (1 egg, 2 sperm)
35Female evolutionary response: slow the sperm Yellow dung fly experiment
36Evolution of sex ratios Which sex should be more common?Why aren’t there many females per male?imagine monogamous speciesfemales become more commonwhich would be better to have has offspring: male or female?
37Sex bias? If females could choose: What if polygamous species?some males have many matesmost males do not mateIf in excellent condition, should a female produce male or female offspring?If in poor condition, should a female produce male or female offspring?
38Sex bias: data Case 1: Kakapo (NZ parrot) Captive breeding, plenty of food.When well fed: 70% sonsAdequately fed: 50% sons
39Sex bias in humans: data Mormon second (or higher) wivesSons Daughters RatioOne co-wife 4,217 3,More than one
40ReferencesHoskins, Garner, and Ward Sexual conflict selects for male and female reproductive characters. Current Biology 11:Hoskins and Ward Experimental evidence for testes size evolution via sperm competition. Ecology Letters 4:10-13.Lively, Craddock, & Vrijonhoek Red queen hypothesis supported by parasitism in sexual and clonal fish. Nature 344:Ridley, M The red queen: sex and the evolution of human nature. Harper. Nice job reviewing Muller’s ratchet and other hypotheses for the maintenance of sex. Goes boldly into explaining human nature, far beyond the evidence.Robertson et al Sex allocation theory aids species conservation. Biology Letters 2:
41Readings and questions 1. In the beetle evolution experiment (figure 8.18) Dunbrack et al did not actually asexual beetles, as there aren't any. Instead they used two different lines of beetles that differed in color, treating one line as if it were asexual by replacing individuals with individuals from a stock population. The researcher's simulated asexual population was not allowed to evolve at all in response to competition and the presence of the insecticide. Is this realistic?2. In general, would you expect asexual lineages to persist longer with small population sizes or large population sizes? Why?3. If the offspring of sexual and asexual individuals have equivalent fitness, why would asexuals take over a population?4. Explain how Muller's ratchet affects sexual populations differently from asexual populations.Explain how the Red Queen hypothesis relates to the maintenance of sex. Why might sex be advantageous in the face of parasites or disease?In most species, the sex ratio is 50 / 50 male: female. Why wouldn’t evolution favor a higher proportion of females, since one male could mate with many females? Discuss using the idea of frequency dependent selection.