Natural Hybridization as an Evolutionary Process

Abstract

Studies of natural hybridization have generally addressed evolutionary questions using one of the three following frameworks: (i) taxonomy or systematics (57, 59, 73, 79, 111, 172); (ii) mechanisms of reproductive isolation and speciation (21, 23, 24, 26, 32, 34, 47, 56, 60, 82, 91, 110, 132, 134, 136, 141, 143, 157, 158, 167, 169, 173); or (iii) natural hybridization as a fundamental evolutionary process that produces consequences that are significant in their own right (1-4, 6, 31, 62, 77, 90, 100, 116, 122, 128, 130, 162, 166, 174). This review emphasizes the conceptual and empirical basis for the last of these contexts. The evolutionary significance of natural hybridization is thus addressed through two general questions: To what extent has this process been involved in the evolutionary history of plant and animal species? and How does the fitness of individuals compare with that of the parental taxa? Natural hybridization and introgression (the transfer of genetic material between the hybridizing taxa through backcrossing; 2, 3) have been ascribed varying levels of importance with regard to the genetic makeup of species and the evolutionary history of species complexes (2, 4, 6, 24, 61, 62, 72, 86, 89, 90, 97, 100, 116, 122, 124, 136, 156, 170, 172). In the extreme, introgression may lead to either the merging of the hybridizing forms (61, 178) or the reinforcement of reproductive barriers through selection for assortative (conspecific) mating (45, 76; but see 37, 136). Another potential consequence is the production of more or less fit introgressed genotypes (107; 87), allowing the expansion of the introgressed form into a novel habitat (89; but see 33, 55). Hybrid individuals may also act as a hybrid sink to which pest species are preferentially attracted, thus limiting the ability of pest species