Genes may interact additively, multiplicatively, or epistatically Epistatic selection favors individuals with specific combinations of alleles at different loci Epistasis is suggested by violation of two-locus hwe Linkage disequilibrium is the non-random association of alleles at different loci; D measures the degree of non-random association, scaled by allele frequencies in the population Transient LDE can be produced by drift or admixture; Permanent LDE is caused by non-random mating or selection Lde may be relatively uncommon; But direct estimation from pairs Of loci likely to interact is difficult

Sexual reproduction 1) Physical recombination of chromosomes - DNA repair - allelic shuffling 2) Outcrossing - meiosis - syngamy (genes in zygotes are from two unrelated individuals Consensus: recombination initially a DNA repair mechanism; allelic recombination has been favored subsequently

Evolution of outcrossing 1) group selection explanations - short term advantage to asexual taxa (cost of meiosis), but adapted to a specific environment

Cost of Meiosis -- Maynard Smith (1978) in a few generations, an asexual species will numerically supplant a sexual one!

Evolution of outcrossing 1) group selection explanations - short term advantage to asexual taxa (cost of meiosis), but adapted to a specific environment -long term certainty of environmental change -predicts lifespan of asexual << sexual species

Whiptail Lizards Cneimodophorus e.g. “uniparens”

Bdelloid rotifers asexual 40 million years diversify into 360 species gene diversity in 4 species vs sexual rotifers (Welch and (Meselson 2003)

Evolution of outcrossing 1) group selection explanations - short term advantage to asexual taxa (cost of meiosis), but adapted to a specific environment -long term certainty of environmental change -predicts lifespan of asexual << sexual species BUT -relies on selection above the individual level (species competition) -selection acts in the present, not the future (short term advantage ??) -why not an asexual world with high species turnover??

selection is operating on the individual but many species are capable of both sexual and asexual reproduction sexual reproduction occurs when the environment becomes unstable asexual reproduction under stable environmental conditions selection is operating on the individual

Evolution of outcrossing 1) group selection explanations - short term advantage to asexual taxa (cost of meiosis), but adapted to a specific environment -long term certainty of environmental change -predicts lifespan of asexual << sexual species BUT -relies on selection above the individual level (species competition) -selection acts in the present, not the future (short term advantage ??) -why not an asexual world with high species turnover?? short-term, individual advantage to sexual reproduction

Linkage disequilibrium arises from: genetic drift selection avoid deleterious combine favorable Muller’s ratchet mutational load* mutation rate environmental heterogeneity

Individual selection models 1) Muller’s ratchet asexual species - clones vary in the total number of deleterious mutations they carry - each generation, the clone with the highest fitness (lowest number of mutations) may be lost by drift or additional mutation - the average number of deleterious mutations in the population will continually increase - the average relative fitness will continually decline; eventually the population becomes extinct sexual species - recombination generates individuals with a decreased number of deleterious alleles - recombination reconstitutes the highest fitness categories (zero mutations)

How the ratchet works: in any generation the class with the fewest mutations may be lost by chance (drift) or by acquisition of another mutation forward mutation is more common than reversion; the distribution will continue to shift to the right Number of Individuals Number of deleterious mutations

444 lines of Salmonella typhimurium Andersson and Hughes 1996 444 lines of Salmonella typhimurium ~1700 generations (60 growth cycles) 5 lines (1%) had significantly slower growth than ancestor Wt 23.2 + 0.7 mut9 25.0 + 1.1 mut4 25.1 + 1.0 mut20 27.0 + 1.3 mut5 46.5 + 1.1 mut3 47.5 +3.5 28 generations

Linkage disequilibrium arises from: genetic drift selection avoid deleterious combine favorable Muller’s ratchet mutational load* mutation rate environmental heterogeneity

Individual selection models 2) environmental heterogeneity assumes: trade-off in performance in different environments asexual species – offspring competitively superior in parent’s environment, but only in that environment sexual species – genetically diverse offspring can compete in a large number of environments requires rapid change in the environment ---> acquisition of new combinations of alleles faster by recombination than mutation

A1B1 - + 0 frequency of A1 1 + - 0 frequency of B1 1 A2B2

fitness of host genotype with two types of parasites H1 H2 parasite P1 0.9 1 genotype P2 1 0.9 fitness of parasite genotype in two types of host parasite genotype P1 P2 host H1 1 0.9 genotype H2 0.9 1

H1 Frequency of H2 H2 Frequency of P2 P2 P1 Host Parasite

Red Queen Hypothesis Parasite Host “Now here, you see, it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that.”

Lively 1992 freshwater snail, Potamopyrgus antipodarum two types of females: obligate sexuals ---> %%, && obligate parthenogens ---> && trematode parasite, Microphallus infected snails are sterile lakes (snail popns) vary in: relative frequency of sexuals vs. asexuals relative frequeny of trematode infections

frequency of sexual reproduction increased with incidence of parasite

Dybdhal and Lively 1998 Snail and Trematode host Red Queen model predicts negative frequency dependent selection - oscillations in host genotype frequency are a consequence of selection by parasites Lake Poerua asexuals only - measure abundance of 112 snail clones over 5 years - only four are common (> 15% of total; 50-60% of sample) - frequency of the 4 common clones in random and parasite infested samples varied significantly among years

* *** *

Dybdhal and Lively 1998 snail and trematode host Red Queen model predicts negative frequency dependent selection - oscillations in host genotype frequency are a consequence of selection by parasites Lake Poerua asexuals only - measure abundance of 112 snail clones over 5 years - only four are common (> 15% of total; 50-60% of sample) - frequency of the 4 common clones in random and parasite infested samples varied significantly among years - significant overinfection was time-lagged

Dybdhal and Lively 1998 snail and trematode host Red Queen model predicts negative frequency dependent selection - oscillations in host genotype frequency are a consequence of selection by parasites Lake Poerua asexuals only - measure abundance of 112 snail clones over 5 years - only four are common (> 15% of total; 50-60% of sample) - frequency of the 4 common clones in random and parasite infested samples varied significantly among years - significant overinfection was time-lagged - the prevalence of infection in rare (n=40) clones was significantly lower than in common clones

cost of meiosis suggests that asexually reproducing taxa should be more common than sexually reproducing ones group selection models explain the advantages of sex by long-term pay-offs, but cannot account for a short-term advantage most asexual taxa are relatively young (vs. related sexual species) Muller’s ratchet works, but is too slow for other than microbes hypotheses that depend on envionmental heterogeneity currently have the best empirical support parasites and pathogens may be an important selective force favoring sexual reproduction; host-parasite systems are inherently frequency-dependent

Evolution of Quantitative Traits Phenotype Frequency Phenotypes are often (usually) continuous (rather than discrete) and often have a normal distribution Height, weight, bill depth, resistance to a parasite, etc. Why?? Several to Many genes may affect phenotype Effect of environment as well as the effect of genes

P = G + E The Phenotype is produced by the Genotype, and the Environment P = G + E Genotype 0 1 2 Phenotype

P = G + E The Phenotype is produced by the Genotype, and the Environment P = G + E Genotype 0 1 2 Phenotype

P = G + E The Phenotype is produced by the Genotype, and the Environment P = G + E Genotype 0 1 2 Phenotype

P = G + E The Phenotype is produced by the Genotype, and the Environment P = G + E We will always look at the Phenotype, the Genetic component and the Environmental component with respect to the population mean Genotype 0 1 2 Phenotype Pi = pi - p _ Gi = gi - g Ei = ei - e