Evolution as a self-limiting process

Evidence is presented that diploid evolution is largely finished and that s exual reproduction is incapable of supporting progressive evolution. Since the first meiotic division is a form of diploid reproduction, it is propose d that this presexual (gynogenetic) event provides the basis for macroevolu tionary change. The first meiotic division can serve both to preserve the o riginal genome and also produce in a single step new paired chromosome rear rangements. Chromosome restructuring characterizes the differences which di stinguish species such as our closest living primate relatives. Since chrom osome restructuring is not likely to be precisely reversed, this hypothesis offers an explanation for the irreversible nature of organic evolution. Th e semi-meiotic hypothesis finds support from the nonhomologous devices whic h have evolved to determine the sexes, as well as the nonhomologous origins of the sex cells in the various vertebrate taxa. The parthenogenetic turke y and the diploid commensal protozoon Spirotrichosoma both offer living exa mples of the semi-meiotic mechanism. A brief critique of the Darwinian hypo thesis is presented. Since the karyotypic differences which characterize sp ecies were apparently not produced sexually, I propose they were produced s emi-meiotically as I have suggested.