THE INFLUENCE OF MORPHOLOGICAL VARIATION AND GEOGRAPHIC RANGE SIZE ON SPECIES LONGEVITY

Despite the potential importance of morphological variation in structuring evolutionary patterns, little is known about how relative differences in variation across species contribute to differences in longevity. For example, morphological variation may extend the range of environmental conditions a species can tolerate, improving its likelihood of surviving environmental perturbations, thereby increasing longevity. Conversely, the same morphological variation may facilitate population-level response to local selection pressure, increasing the likelihood of divergence between populations. Because numerous studies show a positive correlation between geographic range size and species duration, the spatial structure of variation among populations may also be important in driving morphological evolution.

The relative influence of both geographic range size and species-level variation on duration is investigated using Pterocephaliid trilobite species from the Steptoean (Furongian, Cambrian) of the Great Basin, USA. Relative durations are estimated using a composite section of continent-wide (Laurentian) occurrences of Steptoean taxa based on correlation using constrained optimization, but are not included in the correlation itself. Variation is assessed using geometric morphometric analysis of fossil material preserved in limestone after correction for allometry. Results thus far confirm that geographic range size and duration are positively correlated for this clade, but suggest that variation is negatively correlated with duration. This pattern may indicate a propensity for lineages with more variation to turnover faster (pseudoextinction). In addition, if species with more variation are characterized by more geographic structure (geographic variation), this pattern may indicate a tendency for lineages with more variation to split more frequently. Conversely, species with less variation may simply have lower evolvability, perhaps (but not necessarily) due to higher phenotypic integration. Lineages with less variation may also have greater longevity if lower variation results from stabilizing selection, thereby promoting stasis.