Conventional evolutionary wisdom is that new vertebrate

species normally arise either via a splitting of lineages (cladogenesis) or by gradual transformations through time in ancestral-descendant series of populations (anagenesis). However, all known vertebrate taxa that are constitutively clonal clearly arose via interspecific hybridization events between progenitor species with standard sexuality. The basic suspicion is that normal meiotic and sexual operations became disrupted in hybrid offspring in ways that BAY 57-1293 in vitro precipitated each evolutionary transition to ameiotic asexual reproduction. For several clonal vertebrate taxa, researchers have used molecular markers to help clarify some of the detailed cytogenetic mechanics check details of unisexual origins (Uzzell, 1970; Dawley & Bogart, 1989; Quattro, Avise & Vrijenhoek, 1992a). Molecular markers have also been used to pinpoint the sexual species and the direction(s) of the original cross(es) that produced each unisexual biotype (e.g. Avise et al., 1991). To pick just a few examples, the diploid parthenogenetic rock lizard Darevskia rostombekovi of central Europe apparently arose via a single cross between a sexual D. raddei female and a sexual D. portschinskii male (Moritz, Wright & Brown, 1992; MacCulloch et al., 1997), whereas some other unisexual

taxa such as parthenogenetic lizards Menetia greyii (Adams et al., 2003) and hybridogenetic fishes named Poeciliopsis monacha-lucida (Quattro, Avise & Vrijenhoek, 1991) each encompass multiple evolutionary lineages that originated via separate hybridization events. In the Poeciliopsis case, the hybridizations that give rise to unisexual biotypes triclocarban appear to be ongoing. For these unisexual fish, the interpretation is that each such

event genetically ‘freezes’ a new clonal genotype (Vrijenhoek, 1984), which if lucky might happen to fill an open ecological niche. Thus, overall, many biotypes are generated but probably only a few persist for very long. Another revelation about unisexual origins is that the sexual progenitors that hybridized to produce each clonal lineage usually are not sister species but instead belong to different branches of the phylogenetic tree for that taxonomic genus. Two hypotheses (not mutually excusive) have been advanced for this observation. Under the balance hypothesis, parthenogenesis can arise only when the genomes of parental species are divergent enough to disrupt meiosis in hybrids yet not so divergent as to seriously compromise hybrid viability or fertility. By contrast, the phylogenetic constraint hypothesis posits that genetic peculiarities predispose particular parental species to produce unisexual lineages following hybridization.