The Genetics of Speciation at the Diploid Level

A hypothesis is advanced that a diploid species has two differing systems of genetic variability. The "open" system consists of polymorphic gene loci which recombine freely without drastic effects on viability. Examples are electrophoretic, polygenic, clinal, and subspecific variability. The "closed" system consists of blocks of genes forming coadapted, internally balanced gene complexes with or without the presence of inversions as a stabilizing mechanism. Perturbation of these blocks (supergenes) by crossing over results in greatly reduced viability under normal natural selection. These blocks vary between but not within species. When natural selection is relaxed, as during a populational flush-crash-founder cycle, the coadaptive balances of the closed system may become disorganized and one or more discordant individuals may survive. Speciation may occur as selection operates on the perturbed genetic system to organize new coadapted closed systems which come to characterize the new species.