Natural Hybridization and Hybrid Zones

Woodruff, David S. (The Biological Laboratories, Harvard University, Cambridge, Massachusetts 02138). 1973. Natural Hybridization and Hybrid Zones. Syst. Zooal. 22: 213-218.-New criteria are proposed for describing cases of natural hybridization that overcome the difficulties inherent in schemes based on the history of an interaction (primary or secondary contact) or the taxonomic status of the interacting forms. On the basis of distributional data alone the following types of natural hybrid zones may be recognized: allopatric (corresponding to zones of intergradation), parapatric, peripheral sympatric, localized sympatric, and widespread sympatric. Artificial (extralimital) sympatric hybridization also occurs in nature. [Hybridization; allopatric; sympatric.] Several generations of workers have been confused by the semantic and conceptual problems presented by natural hybridization. In this paper I attempt to extract the concept of hybridization (an essentially genetic phenomenon) from the matrix of taxonomy in which it has unfortunately been long embedded. Two developments make it necessary to review the meanings of terms used to describe situations involving intergradation and hybridization. First, it has become increasingly apparent that the nature of the interaction between taxa may vary geographically (Short, 1969a:97-100; Highton and Henry, 1970). This suggests that our terminology should not depend on the taxonomic status of the populations involved or on the probable outcome of their interactions. Second, recent work on parapatric divergence (see reviews by Ehrlich and Raven, 1969; Antonovics, 1971; Endler, 1973; and White, 1973) has made the distinction between primary and secondary intergradation (Mayr, 1942:99; 1963:381382) increasingly hard to draw. Accordingly, our teminology should not rest on generally untestable hypotheses about the histories of interactions. In this paper I review the terms in current use and show how they may be redefined on the basis of the geographical relationship of the ranges of the interacting forms. This simple revision overcomes the above mentioned objections to the present nomenclature and improves the operational utility of the terms involved. NATURAL HYBRIDIZATION The term hybridization is now applied to several different phenomena. Here we are concerned only with its applicability to natural situations. Several authors have restricted the term to cases of secondary intergradation (Mayr, 1963:110; Short, 1969a:84), and others have limited its use to intergradations between certain specified taxa, usually species (see Mayr, 1969:405). The latter restriction is often accomplished by using the expression "interspecific hybridization." Such usage implies that hypotheses about the history and outcome of an interaction have been shown to be valid, and that appropriate taxonomic conclusions have been drawn. In most cases, however, all that is really known is that two populations, presently assigned to different species, are hybridizing. Recently, several attempts have been made to redefine the term hybridization without reference to the taxa involved or the history of the contact. Bigelow (1965: 458), for example, defined hybridization as the "crossing between natural populations that are sufficiently divergent to render the effects of genetic incompatibility recognisable as such." This definition is unfortunately difficult to apply in practice. While I agree with the desirability of distinguishing hybridization between full species in secondary contact from hybridization between conspecific populations in primary or secondary contact, the term