fisherian geometry
Barton NH 2001 The role of hybridization in evolution. Mol Ecol 10:551-568.
- Orr's analysis assumes that substitutions occur rapidly, so that the population is usually fixed
- this requires that 2sPNU << s
- s is the typical selective advantage of a favourable mutation
- P is the proportion of mutations which are favourable
- the restriction may not be as severe as this argument suggests
- if several favourable mutations occur at each locus in every generation (2PNµ > 1, where µ is the per-locus mutation rate), then the results will be qualitatively different
- these alternative alleles compete with each other
- the first favourable mutation to establish may have substantial deleterious side-effects, and so may later be displaced by more favourable alleles
- the new allele may itself mutate to a more favourable variant before it fixes
- the direction of selection may change as the environment fluctuates and as alleles substitute at other loci
- a low rate of intragenic recombination or gene conversion may bring different favourable alleles together
- in a spatially extended population, different alleles will fix in different regions
- for Nµ sufficiently large, evolution is described by a deterministic continuum-of-alleles model
- populations are typically highly polymorphic
- the distinction between sexual and asexual reproduction is crucial
- an unreasonably large number of alleles are required to support the continuum-of-alleles approximation when many traits are under selection
- the behaviour of large but finite populations (2PNµ ≈ 1) remains an interesting open question
- stabilizing selection on diploids can generate overdominance at individual loci, and so can maintain polymorphism
- with n traits, up to n loci can be polymorphic at linkage equilibrium
