14: Conspecific sperm precedence in two species of tropical sea urchins, Geyer, et al. (2005) Evolution 59:1

Free spawning urchin species which can interbreed don't because of egg based sperm selection. This explains how speciation might occur in species with external fertilization. While the species are fully interfertile in the lab, when given a mixture of conspecific and heterospecific sperm, eggs are always fertilized by the same species. The question is driven by hypotheses of common descent and broaden our understanding of macroevolutionary processes.