Lecture 13 - The Evolution of Sex, or Why have we not died 100 times over?
1. Sexual reproduction, i.e., meiosis, is an ancestral trait in Eukaryotes (i.e., the Most
Recent Common Ancestor of all extant Euks appears to have engaged in meiosis).
whole point of meiosis is recombination.
(Prokaryotes and viruses also recombine, and
in one sense a lot more than do eukaryotes, but the mechanism is not canonical).
- Asexual (non-recombining taxa) have evolved many, many times in the eukaryotes,
but they don't last long over evolutionary time (a couple of putative exceptions: bdelloid
2. Sexual reproduction (i.e., meiosis with anisogamy, i.e., differently sized gametes in
males and females) has a cost.
Specifically there is a two-fold reduction in fitness
relative to parthenogenesis (females beget only females).
Ancestral state was
isogamy, why anisogamy evolves is a different question.
3. What does sex (= meiosis, ~ recombination) do?
Heuristic argument first due to
Weissman (ca. 1890s): recombination increases genetic variation, thereby facilitating
But, paradoxically, recombination breaks up combinations of favored
alleles, which reduces adaptation in the short term.
Implies "evolution can see into the
future", sounds bogus.
i) Within a single locus in diploids, it returns the population to H-W equilibrium.
ii) Between loci, in haploids and diploids, it returns the population to linkage equilibrium,
breaks up linkage disequilibrium
D = -rD).
This is the key fact!
e.g., three three-locus genotypes, in LD: Abc, aBc, abC; capital letters are (equally)
Without recombination, fitness is equal, which allele fixes at any locus is
determined solely by drift.
With recombination, LD breaks down, A, B, C are favored by selection and capital
letters are expected to (eventually) fix at all loci.
So, recombination (~sex) is favored when linkage equilibrium is favored.
question becomes: under what circumstances is linkage equilibrium favored?
4. What about environmental variability, i.e., fluctuating selection?
combinations of alleles favored in different environments.
E.g., two environmental
variables, temperature and humidity: hot/cold, wet/dry.
Two loci, one affects
temperature response, other affects humidity response.
i. Temporally fluctuating selection won't generally maintain variation**, but frequency-