pleiotropy
Chevin L-M, Martin G & Lenormand T 2010 Fisher's model and the genomics of adaptation: restricted pleiotropy, heterogenous mutation, and parallel evolution. Evolution 64:3213-3231.
- if all loci exhibit a similar and large degree of pleiotropy, many loci may produce beneficial mutations and respond to selection, so alleles of small effects will tend to accumulate at different loci, resulting in the steady regime
- (predictable trajectory with unpredictable genetic architecture)
- if mutations at each locus only affect a very limited number of traits, only a few candidate loci may be able to respond to selection, resulting in the erratic regime
- (unpredictable trajectory but with a more predictable genetic architecture)
- the idea that genes vary in their adaptive potential is strongly supported by the large body of evidence for parallel genetic evolution
- that is, the repeated and independent fixation of mutations at the same gene (or of the same mutation) in different populations in response to similar environmental challenges
- this phenomenon has been documented in the wild
- some of these cases are caused by the recurrent use of the same allele from standing variation (Barrett and Schluter 2007)
- those that imply very distant taxa rather reflect genuine parallel evolution, where beneficial mutations arose several times independently at the same gene
- repeated use of the same gene more than predicted by chance, as demonstrated statistically by Woods et al. (2006) for instance, indicates that at any given time, all genes are not equivalent for adaptation
- how much evolution is determined and repeatable, or merely historical and contingent, impinges on the nature of evolutionary biology as a historical or predictive science
- the selection coefficient of a mutation in isolation may not be relevant to predict its actual dynamics and fate in very polymorphic populations of sexual organisms, where mutations may segregate simultaneously at many loci
