SPECIES INDIVIDUALITY, PUNCTUATED EQULIBRIUM, AND GOULD'S PARADOX

A major goal of Gould’s The Structure of Evolutionary Theory is to show that macroevolution is a viable concept that is directly comparable to, but not predictable from, microevolution. His claim is based on two major assertions: 1) species are individuals, and 2) punctuated equilibrium (P.E.) represents the predominant way in which species arise. Assertion 1 is important to Gould because it means that species can participate directly in evolution, whereas assertion 2 allows him to recast major evolutionary patterns in terms of differential reproductive success of species. Gould lists two sets of criteria, called vernacular and evolutionary, that species must meet to be considered individuals. We only consider his vernacular criteria, which are 1) a discrete beginning or birth, 2) a discrete end or death, and 3) “sufficient coherence of substance and constancy of form during the individual’s lifetime to merit continuous recognition as the ‘same thing’”. Using criteria 1 and 2, Gould defines a species as a lineage segment bounded by two branching events, implying that an ancestral species can never survive a branching event. Although theoretically sound, this (Hennigian) species definition is divorced from practical criteria (e.g., morphology) used to recognize “same things”. Criterion 3 introduces a practical component for recognizing “same things,” but this may contradict criteria 1 and 2, e.g., when the “same —morphological— thing” persists through a branching event. Because the persistence of an ancestor after a branching event is critical to recognizing phylogenetic patterns consistent with P.E., Gould’s views on species individuality and P.E. are paradoxical.

We can escape from Gould’s Paradox if we carefully distinguish the defining properties of species and the data we use in practice to infer the existence of “same things.” No species individual can survive a branching event, and we must apply different names to lineage segments before and after such events regardless of their morphologic similarity. However, the persistence of an ancestral morphology in one of the new species permits us to infer that branching has occurred, allowing the recognition of patterns consistent with P.E. Other means to escape from Gould’s Paradox are possible if different theoretical definitions of species are used.