Polyploidy, Hybridization, and the Invasion of New Habitats

Abstract

Experiments are described showing that most artificial autopolyploids derived from native or introduced perennial grass species from California are far inferior in behavior under field conditions than their diploid ancestors. In a single species, Ehrharta erecta in two out of 22 localities, the autotetraploid maintained itself for, respectively, 19 and 39 years, but remained either in the exact locality of planting or in nearby localities having very similar ecological conditions. The control diploids, direct descendants of the progenitors of the autopolyploids, spread more widely and evolved more variation with respect to growth habit among each progeny. These results, along with other evidence derived from several literature sources, strengthen the hypothesis that successful polyploids among natural populations are usually or almost always the result of increased heterozygosity accompanying either interracial or interspecific hybridization. The hypothesis that polyploids succeed because of their greater tolerance of severe ecological or climatic conditions is again rejected, and that which postulates secondary contacts between previously isolated populations as the principal cause for their high frequencies in some groups of angiosperms is favored. The unusually high frequency of polyploids in the Gramineae is attributed to the probable fact that habitats to which they are best adapted have changed in extent and position repeatedly during the geological periods since the initial evolution of the family. The family Gramineae contains higher percentages of species and races or cytotypes of polyploid origin than any other large family of angiosperms. More than 80% of its species have undergone polyploidy some time during their evolutionary history. Polyploidy is expressed by four different kinds of numerical series, as fol-