Genetic differentiation, local life history adaptations, and geographic distribution in subspecies of the grasshopper Melanoplus sanguinipes in California

Strong selective pressures can result in local adaptation of populations, which allows for their success and persistence, and can, in turn, lead to reproductive incompatibility between populations. However, gene flow between populations acts to maintain reproductive compatibility, even in the face of strong local selection, by insuring similar evolutionary trajectories. This study examines life history adaptations and genetic differentiation in populations of two nominal species of acridid grasshopper, Melanoplus sanguinipes and M. devastator, throughout California in an effort to understand how the forces of local selection and gene flow are shaping these populations. Analysis of the COII and ND5 regions of the mitochondrial DNA revealed no significant differentiation between populations, regardless of species type, suggesting that high levels of gene flow occur among these populations. Laboratory experiments reveal that, although no genetic differentiation was apparent between populations, significant local adaptation occurs with regard to the number of immature instars, the time to adulthood, and the adult body size. These adaptations differ between populations in a manner consistent with local adaptation to changes in selection pressure from the hot desert-like long season environment of the Central Valley to the cooler short season environment at higher elevations. Differences in male genitalia, used in the past to classify these grasshoppers, showed high levels of fidelity within populations even though genetic studies suggest high levels of gene flow. Some correlation of genitalic differences with climatological factors exists, but this pattern does not fully explain the distribution of morphotypes in California. The evidence suggests that, although genitalic differences are genetically based, they are not indicative of separate species status. Instead these populations should be regarded as member of subspecies within a single diverse genetic pool.