Limited divergent adaptation despite a substantial environmental cline in wild pea

Isolation by environment (IBE) is a wide spread phenomenon in nature. It is commonly expected that the degree of differences among environments is proportional to the level of divergence between populations in these environments. Consequentially, it is assumed that species' genetic diversity displays pattern of IBE in the presence of a strong environmental cline if geneflow does not mitigate isolation. We tested this common assumption by analyzing the genetic diversity and demographic history of Pisum fulvum. P. fulvum inhabits very contrasting habitats in the southern Levant and is expected to display only minor migration rates between populations what makes it an ideal test case. Ecogeographic and subpopulation structure was analyzed and compared. Correlation of genetic with environmental distances was calculated to test the effect of IBD and IBE and detect the main drivers of these effects. Historic effective population size was estimated using stairwayplots. Limited overlap of ecogeographic and genetic clustering was observed, and correlation of genetic with environmental distances was statistically significant yet small. We detected a sharp decline of effective population size during the last glacial period. The low degree of IBE may be the result of genetic drift due to the past bottleneck. Our findings contradict the expectation that strong environmental clines cause IBE in the absence of extensive geneflow.