Müller’s ratchet frequency # of deleterious mutations LLC: “least-loaded class” frequency # of deleterious mutations “CLICK”
Müller’s ratchet in sexuals? No. frequency # of deleterious mutations frequency # of deleterious mutations frequency # of deleterious mutations “CLICK”
Müller’s ratchet : an experiment Set up 444 cultures of Salmonella Transfer one individual every 24 hours 1700 generations Prediction: lower fitness Test: generation time: –parental: –Experimental populations:
Evidence for Müller’s ratchet : the human Y chromosome In XX females, recombination Y chromosome does not pair with X
Does recombination increase genetic variance for fitness? D = 0.25D = -0.25D = 0
If parents have higher than average fitness, what effect of recombination? The results of sex: cost of recombination A1A1 B1B1 meiosis A2A2 B2B2 A1A1 B1B1 A2A2 B2B2 A1A1 B2B2 A2A2 B1B1 parental recombinant parental
Experimental test of recombination and adaptation Adaptation may require new combinations of alleles Asexuality does not allow this Experiment: flour beetles (Tribolium) Have stock population Allow one population to evolve The other is restocked from original population, as if asexual. Asexual has 3x reproductive advantage Asexuals start 0.5 of population Selection: pesticide Malathion What proportion are sexual?
Advantage of sex: adaptation Proportion sexual Malathion concentration Generations 30 figure 7.18
Do 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?
Parasites and hosts Imagine four parasite genotypes, four host defense genotypes ParasiteHost
Parasites and host: Red queen ParasiteHost start after selection on host
Parasites and host: Red queen ParasiteHost after selection on host after selection on parasite
Evolution over time: cycling genotypes
Red queen, evidence: topminnows of Mexico Interspecific hybrids from asexual triploids or: sexual diploid fish Infected by parasites that cause spots
Data I: sexuals vs. asexuals: which has more parasites? Parasites Fish length (mm) Asexuals have more parasites
What if there is no variation in sexual population? Heart pool: dried up in 1976 Recolonized by just a few sexual minnows and a few asexual minnows. Which will have the higher fitness?
Why sex, summary
Why 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 mating Why only two mating types??
Implication of two sexes Sexual selection Sexual conflict
Intrasexual competition: sperm competition When females mate with multiple males, sperm can compete Example: yellow dung fly Scathophaga stercoraria
Yellow dung flies Females mate with multiple males Last male to mate fertilizes most ovules Selection experiment: compare wild to 10 generations monogamous, 10 generations polyandrous Expected effect on sperm competition? Monogamous ♀ + ♂ Polygamous ♀ + 3♂ 10 gen.
Yellow dung flies: polyandrous males vs. monogamous males Mate each female with two males: one from polyandrous line, one from monogamous line Hosken & Ward % offspring sired by second male to mate
Sexual selection and intersexual conflict evolution Traits that favour male success may harm female success Example: sperm competition faster sperm win: higher fitness for male too many sperm: polyspermy – egg is lost human spontaneous abortions: 3% of conceptions end due to triploid embryos (1 egg, 2 sperm)
Evolution of sex ratios Which sex should be more common? Why aren’t there many females per male? –imagine monogamous species –females become more common –which would be better to have has offspring: male or female?
Sex bias? If females could choose: What if polygamous species? some males have many mates most males do not mate If in excellent condition, should a female produce male or female offspring? If in poor condition, should a female produce male or female offspring?
Sex bias: data Case 1: Kakapo (NZ parrot) Captive breeding, plenty of food. When well fed: 70% sons Adequately fed: 50% sons
Sex bias in humans: data Mormon second (or higher) wives SonsDaughtersRatio One co-wife4,2173, More than one
References Hoskins, 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: 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:
Readings 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. 5.Explain how the Red Queen hypothesis relates to the maintenance of sex. Why might sex be advantageous in the face of parasites or disease? 6.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.