EXPLORING MACROEVOLUTIONARY RATCHETS AS A POTENTIAL DRIVER OF CLADE DECLINE

Diversity declines are well documented and widespread in the fossil record, but the mechanisms that drive the observed dynamics are still unclear. Macroevolutionary ratchets have been proposed as a potential driver of decline, where selection favors the loss of early generalized forms resulting in more vulnerable, specialized forms late in the history of the clade. While empirical evidence for the presence of a macroevolutionary ratchet has been found in the North America canid record, the general clade dynamics that accompany the pattern have not yet been explored. Here, 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. Furthermore, we used a Brownian motion model with drift to reflect the ratchet-like effect of specialization, in which the clade is forced to become more specialized over its evolutionary history. After simulating clade histories, we tested for differences in rise and decline durations as well as compared the clade durations with null clades generated under a non-trait dependent scenario. 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. Clades generated under the macroevolutionary ratchet-type scenarios also exhibit asymmetry in the rise and decline durations with directionality dependent on extinction strength. 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.