entiation between species cannot be investigated with the classical methods of genetics. With Mendelian methods, the presence of a gene is established by observing segregation in the progenies of crosses; but crosses between individuals from different species are characteristically not possible. Fortunately, the advent of molecular biology has opened up the investigation of the genetic changes associated with the speciation process. Thus, the proportion of genes that change during speciation has been ascertained in a variety of organisms (Ayala et al., 1974; Ayala, 1975; Bullini, 1982; Avise, 1994). Moreover, DNA and protein sequence information has facilitated the reconstruction of phylogenetic relationships. Yet the DNA in the gene pool of species holds a largely untapped wealth of genetic and evolutionary information. In this paper we explore DNA sequence polymorphism in current human populations in order to shed light on some aspects of human evolution, particularly on the size of human ancestral populations as they evolved from one to another species.

Speciation by Founder Effect

The theories of ''founder effect" speciation propose that speciation often occurs after a founder event or bottleneck; that is, when a new population is established by a pair or very few individuals, as may happen in the colonization of an island, or when an established population declines severely so that extremely few individuals survive, from which a population expands again (Mayr, 1954, 1963, 1982; Carson, 1968, 1975; Templeton, 1980; Carson and Templeton, 1984). These theories claim that founder events and drastic bottlenecks are associated with random genetic drift, inbreeding, and selective changes that refashion the gene pool ("genetic revolution," Mayr, 1963) and thus increase the likelihood that a new species will arise.

The prevalence of founder events (or severe bottlenecks) in speciation is a matter of great interest and acrimonious debate. Some authors argue that founder effects prevail, whereas others reject on theoretical grounds the purported genetic consequences associated with drastically reduced population numbers (Barton and Charlesworth, 1984; Giddings et al., 1989; Otte and Endler, 1989; Galiana et al., 1993).

The evolution from hominoid ancestors that lived a few million years ago to modern humans involved several transitional species, characterized by important biological changes, such as the evolution of bipedalism and a large brain. The issue arises whether these changes may have been associated with, and perhaps a consequence of, extreme population constrictions. It has been claimed, in particular, that a severe population bottleneck of only one pair or very few individuals preceded

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