Chromosome Variation Versus Chromosome Fixation

Abstract

As we have discussed in previous chapters, the phylogeny of human and great ape chromosomes can be explained by chromosome rearrangement; nevertheless we do not yet know whether such a change led to speciation or whether it took place after speciation. In this respect, a great controversy exists among evolutionary biologists on the roles of chromosome change and geographical isolation in speciation. On one side, Mayr (1963) has defined a species as a population separated from others by discontinuity, the three main isolating mechanisms producing such discontinuity being (1) geographical, (2) ecological, and (3) reproductive. Obviously the ultimate mechanism that keeps species as separate taxa is reproductive isolation (Chap. 4). Now, according to Mayr (1963) geographical isolation is the essential mechanism by which speciation is produced; the other isolating mechanisms being only secondarily developed. These secondary mechanisms by which species are kept as separate populations may operate at a later stage in the event that these species might be again brought into contact. They could act at the pre, intra, or post mating levels, but their main effect is to avoid cross-hybridisation between different species, so that each of them may conserve its own genetic pool. Thus, the role of chromosome change in speciation has been categorically denied by Mayr (1963), since chromosome change has occurred, in his opinion, after speciation was produced by geographical isolation.