A discussion on hybrid vigour - On the biochemistry of heterosis, and the stabilization of polymorphism

The participants in this discussion have mainly considered two theories of heterosis. One approximates to the old view that heterosis increases the probability of the presence of at least one allele of the optimal type at each locus. This is probably true for loci where mutation is constantly producing suboptimal but not lethal alleles. These will only slowly be eliminated by natural selection, particularly when they are closely linked with optimal alleles. The second theory is that a heterozygote may be more versatile, that is to say, adapted to a wider range of environments, either internal, that is to say, in different tissues of the same organism, or external, than either of the corresponding homozygotes. I have held this view for some time (Haldane 1949a) and Lewis’s contribution strongly supports it.