Abstract
Gradual evolution is a common phenomenon in the fossil record of marine microplankton, yet no theoretical model has so far been presented to explain the observed pattern of unidirectionality in trait evolution lasting over tens of millions of generations. Recent molecular genetic data show that the majority of microfossil-producing plankton groups harbors substantial cryptic diversity. Here, we examine the effect of cryptic diversity on apparent rates of lineage evolution. By using a theoretical approach, we show that under resource competition, an increasing number of sibling species within a hypothetical lineage leads to an exponential slowdown of the apparent rate of evolution. This mechanism explains both the remarkable variation in apparent rates of evolution observed in marine plankton, as well as the presence of long gradual evolutionary trends.