Patterns of temporal change in single morphological characters of a Miocene stickleback fish

Abstract

Patterns of temporal variation of six characters in a Miocene stickleback (Gasterosteus doryssus) are presented. Most pairs of characters tend to be correlated, but these correlations account for only about 26% of the observed variation, and thus the characters are studied separately. All character state frequencies exhibit temporal heterogeneity, and their means have temporal trends. Regardless of these overall trends, reversals of the trends cause end members of four time series not to differ significantly from each other. We argue that most observed temporal variation represents intrapopulation evolutionary change. Although most time series have somewhat stepped patterns, complete stasis is absent. Gradual variation is predominant, and in one case is nearly linear, but very rapid evolutionary bursts are also seen. The most rapid stratigraphic change appears to represent a local extinction, followed by recolonization by a differentiated population, but another episode of rapid change probably represents evolution in situ by means of standard neo-Darwinian mechanisms, without involvement of “macromutations” or true saltation. The irregular patterns and great magnitude of phenotypic change that are observed indicate that conventional paleontological samples may miss important evolutionary phenomena and are not comparable to shorter-term evolution in extant populations.