Long-time readers may recall that one thing I wish I did active research on is hybridization: the crossing of divergent species or lineages, the developmental abnormalities arising from hybridization, and the potential role of hybridization in human evolution. One such developmental abnormality is “heterosis,” a.k.a. ‘hybrid vigor.’ In general, heterosis refers to any trait in hybrids that is larger than the average of the two parents’ (or the parents’ species) values for that trait. The phenomenon was recognized in plant domestication as far back as the 19th century – crosses between different plant (namely corn) strains produced hybrid strains with much greater yield than their parent species.
The advisability of applying the term “heterosis” to cases in which heterozygotes are larger in body size, or show “increases” in any “traits,” but no evidence of higher adaptive value compared to the corresponding homozygotes, is open to question. Perhaps the word “luxuriance” would be a better designation for such cases, the word “heterosis” or “euheterosis” to be used for adaptive superiority of heterozygotes to homozygotes. . . . it is clear that the mechanisms underlying euheterosis and luxuriance are quite different.