The recurrent origin of plant races and species

Whereas there is increasing evidence that some polyploid taxa have multiple independent origins, little attention has been given to the possibility that taxa arising through selective processes without ploidal shifts also may be the products of recurrent evolution. I argue that there are no necessary reasons why ecologically distinct races and species could not arise from separate lineages at different times and in different places. Similarity of the genetic structure of progenitor populations combined with parallel selective pressures would make this possible, as would evolution in the direction of greatest genetic variance. Putative examples of multiple independent origins of single and multiple adaptations are reviewed. The possibility that some taxa are polyphyletic is suggested by the presence of isolated populations in different clades. I also consider the possibility of the recurrent origins of reproductive isolating mechanisms and of species.