This document discusses mechanisms of species diversification through genetic drift versus divergent selection. It outlines how genetic drift can lead to speciation through random processes that increase trait variance over time without a correlation between traits and the environment. Divergent selection, which is more complex, can also lead to speciation by favoring different trait values in different environments, changing both the mean trait value and variance between populations. The author develops a mathematical model combining genetic drift and selection based on the Ornstein-Uhlenbeck process to test these mechanisms on artificial and real biological datasets.