Post-Mating Reproductive Isolation and Hybrid Pollen Inviability Between Two Subspecies of Clarkia xantiana (Onagraceae)

Abstract

The origin of new species of organisms (speciation) is a source of continual debate in the realm of evolutionary biology. When two diverging populations of the same species become reproductively isolated from each other speciation can occur because of phenotypic or genetic changes. Phenotypic changes prevent mating from happening (pre-mating changes) and genetic changes render hybrids, or the product of the two populations, sterile (post-mating changes). For my study, I examined the post-mating isolation between two subspecies of Clarkia xantiana, an endemic, native herb of California. The species is currently divided into two subspecies Clarkia xantiana ssp. xantiana and Clarkia xantiana ssp. parviflora. These two subspecies overlap in distribution in a narrow contact zone (sympatric zone). In sympatry, these subspecies rarely form hybrids in nature but can be forced to produce hybrids in the lab. My study focused on the fertility of pollen grains (male fertility) in the parents and hybrids of these two subspecies to assess post-mating isolation. Answering these questions provides insight into the mechanisms behind speciation due to post-mating changes, an understudied but fundamental process of evolutionary biology.