Unification of Speciation Theory in Plants and Animals

Most diploid species have unique polygenic systems; since they are complex and coadapted throughout the genome, their integrity is not easily perturbed. A recent theory calls for the formation of new species to proceed in two phases: 1) disorganization of the polygenic system by genetic recombination under relaxed selection in a subpopulation of the parent species and 2) a major selective reorganization in the same subpopulation to form a new coadapted system. Found- er events, hybridization, in situ reduction of population size and shifting balance can contribute to the disorganization phase. The selection in the second phase makes the population phenotypi- cally recognizable as a new species. Two quite different character sets may be the object of the second or selective phase: either A) adaptation altering the system of sexual reproduction or B) adaptation to the ambient (non-sexual) environment. In animals there is a general tendency for A to occur first; in plants, the reverse tendency is seen. If the order is A B, reproductive isolation may result as a by-product. This has led the zoologist to overemphasize the role of reproductive isolation as a criterion for recognizing a new species. Selection is rarely directed towards reinforc- ing reproductive isolation. Rather, it will maximize fitness by making reproduction more efficient through mate recognition and sexual selection. If the order is B - A, as in many plants, we see that species are rarely swamped out of existence by hybridization; normalizing selection prevents this. If the zoologist would abandon insistence on reproductive isolation as a prime species criterion and substitute the notion of a positive fitness system under stabilizing selection, a theory of spe- ciation encompassing plants and animals is possible.