The structure of species, outcomes of speciation and the ‘species problem’: ideas for paleobiology

Paleobiologists reviewing the recent literature on species concepts are likely to come away with the impression that the fundamental unit of biodiversity is impossible to define in terms of a universal concept that applies to all forms of cellular life and the varied outcomes of species-level evolution. Operational concepts are likely to differ from specialty to specialty in biology, but a general theory of species has been available for nearly 50 yr in the form of G.G. Simpson’s evolutionary species concept. In addition to this, a general picture of species structure has been accepted for at least as long, consisting of demes or habitat clusters that bud off other demes/clusters, split, fuse and often go extinct. More recently, it has become obvious that the outcomes or products of species-level evolution may include any of the following: speciation with morphologic differentiation (producing the so-called ‘good species’ of applied biology and probably the stable entities resolved in patterns of stasis in the fossil record); speciation without pronounced phenotypic differentiation (cryptic species); or acquisition of diagnosable characters without either complete reproductive isolation or pronounced morphologic change. The view that species are lineages containing networks of demes (or habitat clusters of uniparental organisms), each with its own biologic tendencies and role – with a unique place within a clade composed of similar historical entities, each having a certain kind of internal structure and resulting from a different evolutionary outcome – is the ultimate concept many of the operational approaches seem to be aiming to uncover. Considering species structure, speciation outcomes and the evolutionary species concept together brings the nature of species into a much sharper focus.