The Problem of Species in Oenothera

Abstract

The members of the subgenus Euoenothera which grow from the Rocky Mts. eastward do not show a clear-cut segregation into readily recognizable spp. Instead, the population consists of a multitude of geographical races which breed true because of balanced lethals and which maintain a considerable degree of isolation from each other because of their self-pollinating habit. Each race is a complex-heterozygote, its complexes differing both in segmental arrangement and in genic composition. These races are of hybrid origin, owing their origin to the infrequent transgressions of the reproductive barriers, or they have been derived from such hybrids through subsequent alteration in segmental arrangement or in genic composition. One complex of a race is often masked in its pheno-typic effect by the other, so that its presence, and the relationships which it bears to complexes in other races, can be shown only by outerosses. The multitudinous geogr. races fall into several large groups on the basis of cytogenetic behavior. These groups cannot always be distinguished from each other with certainty on the basis of phenotypic characters, but their cytogenetic behavior is distinctive. The cytogenetic situation in the S. American oenotheras, so far as this is known, is briefly summarized and compared with the situation in N. America.