Meeting Abstract
Long-term stasis seems paradoxical, but several ecological hypotheses could explain persistence of a morphology for millions of years despite dramatic environmental change. Testing those hypotheses, however, can be complicated owing to processes that either mimic or mask long-term stasis. What might seem like long-term stasis could instead be due to a rapid, recent adaptive radiation; conversely, a few highly divergent forms might mask persistence of an ancestral morphology in otherwise static lineages. We examine diversification and divergence of jaw shape in three tree squirrel lineages, one considered a living fossil (Sciurini), which underwent a recent Neotropical radiation. Another (Callosciurinae), was tentatively interpreted as a living fossil but it contains several distinctive morphologies including some extreme specialized forms. The third (Pteromyini) has never been viewed as static. We find that Sciurini is half as disparate as Callosciurinae and Pteromyini. Apart from two ecologically specialized morphs, Sciurini occupies a single, stable adaptive peak. Despite its greater disparity, Callosciurinae is nearly identical to Sciurini in its stationary variance; its disparity is elevated by a larger number of adaptive peaks, many unique to a single species, and more extreme forms. In both lineages, the static morphology is specialized but is functionally versatile; these squirrels can eat the hardest nuts of the tropical rainforest although most have broad diets, also eating soft fruits, insects and small seeds. In contrast, Pteromyini is not static; in this lineage, disparity accumulates over time, at a far lower rate in a clade of specialized folivores. Our analysis supports a variant of classic hypothesis: static lineages comprise functionally versatile specialists within a broad adaptive zone because those specialists behave as ecological generalists much of the time.
