Evolution in parallel: new insights from a classic system

Neo-darwinists have long argued that parallel evo-lution, the repeated evolution of similar phenotypes in closely related lineages, is caused by the action of simi-lar environments on alleles at many loci of small effect. A more controversial possibility is that the genetic architecture of traits initiates parallelism, sometimes through fixation of alleles of large effect. Recent research (by Cole et al., Colosimo et al., Cresko et al., and Shapiro et al.) offers the surprising insight that reduction in two armor traits of threespine stickleback is governed by independently segregating major loci as well as additional quantitative trait loci (QTL), and that alleles at the same major loci are associated with paral-lel phenotypes in globally distributed populations. This research suggests the emergence of a new and excitin