Natural hybridization and species concepts

Abstract

Abstract Is not the species concept that the species includes all individuals that together are capable of producing completely fertile offspring, and must we not then consider groups whose individuals can produce new species by hybridization as partial groups of one species? (Hennig, 1966) Although the definition of hybridization used in this book is independent of species concepts, most studies of natural hybridization have used frameworks that reflect certain concepts. Even a cursory examination of the relevant scientific literature reveals a close association between these studies and debates concerning the nature of species and the process of speciation. Indeed, natural hybridization has alternatively been described as unimportant or of profound importance in plant and animal evolution based largely on the researcher’s underlying definition of species (Mayr, 1963; Wagner, 1970; Anderson and Stebbins, 1954; Arnold, 1992; Dowling and DeMarais, 1993). For example, it has been argued that the formation of new evolutionary lineages through hetero-specific hybridization is insignificant (or nonexistent) because species are completely reproductively isolated (“Biological Species Concept”; Mayr, 1963) or because new lineages cannot be polyphyletic in origin (“Phylogenetic Species Concept”; Hennig, 1966; Mishler and Donoghue, 1982; Cracraft, 1989; see Nixon and Wheeler, 1990, for another variant of the PSC). In contrast, it has been estimated that from 30% to 70% of all angiosperm species are polyploid relative to one of their ancestral lineages (Grant, 1981; Ehrlich and Wilson, 1991; Whitham et al., 1991; Masterson, 1994). Furthermore, it has been suggested that most of the occurrences of polyploidy in angiosperms reflect allo-polyploidy (Fig. 2.1) and that the allopolyploid derivatives have originated largely from crosses between different species or genera (Whitham et al., 1991).