THE ESTABLISHMENT OF CHROMOSOMAL VARIANTS.

Abstract

Closely related species frequently differ by some chromosomal change such as a difference in chromosomal number, an inversion, or a translocation, although it is not clear whether this change is a cause of speciation or coincidental with other genetic change. For example, Wright (1940) and Bush et al. (1977) suggested that both speciation and chromosomal evolution are enhanced by small effective population size (see also Bengtsson, 1980). In any case, it seems the genetic changes which occur during speciation may take many forms with the primary prerequisite being that they cause reproductive isolation between the new species and its ancestor. White (1978) and others have advocated that chromosomal changes may provide a barrier between a new population and its ancestral one because chromosomal hybrids generally have reduced fertility due to segregation abnormalities in meiosis. A problem arises, however, as discussed by Wright (1941) and by Bengtsson and Bodmer (1976) in that the probability of fixation of a new chromosomal type, given lowered heterokaryotic fertility, is generally very small. When there is a heterokaryotic disadvantage and the frequency of the new chromosomal type is below the unstable equilibrium generated by such a fitness array, selection will reduce its frequency further and lead to the elimination of the new variant. As a result, there appears to be a basic paradox: there are generally chromosomal changes associated with speciation but there is no genetic model which permits the establishment of these new chromosomal types in a popu-