E E Sotka, College of Charleston, Charleston, SC, USA © 2008 Elsevier B.V. All rights reserved.
Cline Examples Further Reading
A cline is a gradient of a phenotypic or genetic character within a single species. The geographic distances across which characters shift can range from meters to thousands of kilometers. Clines are especially frequent within geographically widespread species. There is strong evidence that natural selection plays a central role in maintaining clines, in part because much of the spatial variation in a given trait reflects shifts in the biotic and abiotic environments. Clines are known as 'taxonomist's nightmares and evolutionist's delights' because their evolution informs several contentious issues in ecology and evolution, including the degree and nature of natural selection, the process of dispersal and gene flow, historical demography, and speciation. Below, we briefly describe a few examples of clinal variation, outline the theoretical frameworks that underlie modern analyses of genetic clines, and describe the role of clines in understanding parapatric speciation.
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