Taxonomic Problems in Parthenogenetic Vertebrates

The classical definition of a species used by most vertebrate systematists employs interbreeding as a primary criterion. For various reasons this definition is inapplicable and new approaches, defining different kinds of species or eliminating criteria dependent on breeding, are being explored. So far the systematic treatment of uniparental forms has employed the same criteria applied to bisexual species and, by analogy, has recognized them as species too. But in several instances the inability of the uniparental form to breed with morphologically similar and, hence, presumably related species has been used as a species criterion, even though crossbreeding between individuals of the unisexual species is impossible. In other instances supplementary criteria such as continuous range, or distinctive niche, augment the usual criteria. The use of the presumed degree of genetic dissimilarity between two taxa is a hazardous approach to species recognition, for a single gene difference propagated parthenogenetically in a clone might be more distinctive phenotypically than differences involving entire genomes. The best systematic approach to this species problem is still morphological and ecological and infers that the taxon under scrutiny is a lineage evolving separately from other lineages.