THE SHAPE OF SPECIES: A THEORETICAL EXCURSION INSPIRED BY PROFESSOR KEN MCKINNEY

To most neo- and paleobiologists the "species problem" (controversies associated with defining species and identifying their primary properties) seems irresolvable and interminable. Preferred concepts vary with major taxonomic groups (bacteria vs. mammals) and disciplines (population genetics, phylogenetic systematics, paleontology). An alternative stance is that the species problem has been solved--about 50 years ago by George Gaylord Simpson, when he proposed the Evolutionary Species Concept (ESC): "...a lineage (an ancestral-descendant sequence of populations) evolving separately from others and with its own unitary evolutionary role and tendencies." Despite criticism, many modern operational concepts converge on the ESC. Considering ESC as an ultimate picture of species involves an excursion into the land of scientific realism and core concepts, a journey I began three decades ago as an undergraduate student in Prof. Ken McKinney's invertebrate paleontology class.

In this view, species have as primary features a unique position in the genealogic hierarchy (they consist of demes and at the same time are parts of clades), unique evolutionary and ecologic roles and potentials, and unique historical patterns involving originations, component population structures, and extinctions. They are the smallest more or less permanently branching "twigs" within clades. Secondary features include reproductive isolation and morphologic differentiation, considered primary properties in operational approaches that do not take into account varied outcomes of speciation or historical patterns--but not occurring in all species. With the ESC in mind, the contentious issues of prevalence of punctuated equilibria, varied speciation "products" including cryptics, rare-common differences, and the possibility of speciation pulses come into clearer focus.