The origin of marine invertebrate species: a critical review of microevolutionary transformations

The ‘classic” view of the origin of species is that new forms originate by slow and gradual transformation (phyletic gradualism). The alternative view is that new species arise abruptly and once established, persist with little change for long periods of time (punctuated equilibria). Whereas the latter model makes sense in terms of population dynamics there are several constraints which limit the testing of either model against the rock record. These include (a) theory influencing observation; (b) confusion arising from taxonomic practice; (c) uncertainty about true ancestor/descendant relationships; (d) cryptic breaks in the sedimentary record; (e) the difficulty of achieving fine stratigraphie resolution. A critical analysis of several now classic case histories reveals that some, at least, are ambiguous. Moreover whereas stasis can readily be demonstrated, relative rapidity of origin of species is harder to pinpoint since a sufficiently fine focus can seldom be obtained. The most reliable data are based on micro-stratigraphic sampling and on finely resolved biostratigraphy. Such information indicates that both gradual and rapid evolutionary changes have taken place; a whole spectrum of microevolutionary transformations is evident. New views on microevolutionary change can be accommodated within the neo-Darwinian framework, and do not, by themselves, call for a new paradigm for evolution theory.