Fine-scale local adaptation and genetic differentiation : case study of an intertidal copepod

Divergence of conspecific populations can occur by mutation, natural selection, and genetic drift. Such divergence may result in local adaptation to the environment as well as reproductive incompatibility. These processes can ultimately lead to speciation; to what extent they contribute to speciation is an area of interest. Past studies of the marine copepod, Tigriopus californicus, show F1 hybrids of populations 8 km apart (Bird Rock and San Diego) undergo hybrid breakdown and are genetically divergent (10.4% on CYTB). Although the BRxSD F1 hybrids have reduced fitness, they also exhibit transgressive segregation in thermal tolerance, implying different mechanisms of adaptation. Thermal adaptation has been measured for other T. californicus populations but only at larger scales. At what scale such reproductive incompatibilities and differences in thermal tolerance occur has yet to be investigated. This study looks into micro-scale differentiation and determines its consequences. Every inhabited outcrop was sampled from La Jolla (LJ) to Bird Rock (BR) (San Diego County, CA) to determine genetic differentiation and population differences in thermal tolerance. To determine consequences of such divergence, several hybrid crosses were produced to find the spatial scale at which genetic incompatibilities and transgressive segregation may occur. This study found fine-scale genetic differentiation and significant differences in thermal tolerance at a new spatial scale. Despite these differences, hybrid breakdown was not observed at a smaller scale; however, evidence for hybrid breakdown was observed in the LJ and SD (̃13 km distant). Our preliminary results suggest that differences in thermal tolerance occur at the fine scale before reproductive incompatibilities arise. The finding of such differences in thermal tolerance at this scale has significance for fine-scale studies and implications for future work in understanding if there are effects of the microclimate