An analysis of the fitness of interspecific, intraspecific and nonhybrid Raphanus populations in Michigan
Muir, Kathryn B.
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Speciation is a continuous process; yet the study of speciation can be problematic as it generally occurs in evolutionary time. It is possible however, to postulate when a group of organisms is in the process of speciation based on factors such as partial reproductive barriers, gene flow, and hybridization. Hybridization can be used as a tool for elucidating the genetic and evolutionary relationships between the species and sub-species of a given genus. Within the genus Raphanus, hybridization is known to occur both inter- and intraspecifically. Using a Michigan field study, we have quantified the fitness of Raphanus hybrids made from crossing six parental populations. We found that some interspecific hybrids had higher fitness than several intraspecific hybrids, as well as several pure parental populations. Using previously collected molecular data, we compared fitness levels found in this study to gene flow between the parental populations of radish. Several of the most distantly related radish populations formed hybrid offspring that were more fit than some of the pure parental populations. This brings into question the existence of two distinct species within the genus; perhaps R. sativus (cultivated radish) and R. raphanistrum (wild and weedy radish) should actually be classified as the same species. In addition, molecular data shows significantly more gene flow within weedy radish populations, probably due to worldwide grain shipping, than between weedy radish and natural populations. The high genetic differentiation between native populations suggests incipient speciation. These data taken together indicate that perhaps R. raphanistrum is currently undergoing speciation. This hypothesis is supported by data collected during this field study.