Unification of Speciation Theory in Plants and Animals

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Abstract
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. Founder 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 phenotypically 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 fwdarw 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 reinforcing reproductive isolation. Rather, it will maximize fitness by making reproduction more efficient through mate recognition and sexual selection. If the order is B fwdarw 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 speciation encompassing plants and animals is possible.

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