Species concepts and phylogenetics

Concepts of species proposed within the phylogenetic paradigm are critically reviewed. Most so called `phylogenetic species concepts' rely heavily on factors immaterial to phylogenetic hypotheses. Thus, they have limited empirical content and offer weak bases on which to make decisions about real problems related to species. Any workable notion of species relies on an explicit character analysis, rather than on abstract properties of lineages, narrative predications and speculations on tokogenetic relationships. Species only exist conjecturally, as the smallest meaningful units for phylogenetic analysis, as based on character evidence. Such an idea considers species to be conjectures based on similarity, that are subsequently subject to testing by the results of analysis. Species, thus, are units of phylogenetic analysis in the same way as hypotheses of homology are units of comparable similarities, i.e. conjectures to be tested by congruence. Although monophyly need not be demonstrated for species-level taxa, hypotheses of relationships are the only basis to refute species limits and guide necessary rearrangements. The factor that leads to recognition of species is similarity in observed traits. The concept of life cycle is introduced as an important element in the discussion of species, as an efficient way to convey subsidiary notions of sexual dimorphism, polymorphism, polytypy and clusters of diagnosable semaphoronts. The notion of exemplars is used to expand the concept of species-as-individual-organisms into a more generally usable concept. Species are therefore proposed for a diagnosable sample of (observed or inferred) life cycles represented by exemplars all of which are hypothesized to attach to the same node in a cladogram, and which are not structured into other similarly diagnosable clusters. This definition is character-based, potentially testable by reference to a branching diagram, and dispenses with reference to ancestor-descendant relationships or regression into population concepts. It provides a workable basis on which to proceed with phylogenetic analysis and a basis for that analysis to refute or refine species limits. A protocol is offered for testing hypotheses of species boundaries in cladograms.