Meeting Abstract
Diversity declines are well documented and widespread in the fossil record, but the mechanisms that drive the observed dynamics remain unclear. Macroevolutionary ratchets have been proposed as a potential driver of decline, where selection favors the loss of early generalized forms, which results in more vulnerable specialized forms late in the history of the clade. While empirical evidence for the presence of a macroevolutionary ratchet has been demonstrated in the North America canid record, the general clade dynamics accompanying this pattern have not yet been explored. We used simulations to explore the relationship between trait evolution, extinction, and clade decline under macroevolutionary ratchet-type scenarios. To simulate the macroevolutionary ratchet, we modeled the extinction rate of each lineage as a function of its trait value. We used a Brownian motion model with trend to reflect the ratchet-like effect of specialization, where clade is forced to become more specialized over time. Our simulations confirm that a macroevolutionary ratchet-type scenario will lead to a diversity decline trajectory. However, the diversity trajectory after peak diversity is not always unidirectional, suggesting that clades may be able to recover from short periods of decline. Additionally, clades where the relationship between trait value and extinction was high were unable to generate enough diversity in the rise phase in order to experience decline. Inferences from these simulations can be used to detect the signature of the macroevolutionary ratchet in empirical clades. Our results clarify how trait evolution can drive decline under a range of parameter values and provides additional insights on a mechanistic hypothesis of clade decline.
