Evolution in changing environments: the "synthetic" work of Clausen, Keck, and Hiesey

The studies of Clausen, Keck, and Hiesey (CKH) have been widely cited as exemplars of ecotypic differentiation in textbooks and in the primary literature. However, the scope of their findings and achievements is significantly greater than this. In this paper we analyze the research program of CKH, highlighting their major findings during the years when the modern synthesis of evolution was taking shape. That synthesis, curiously, drew little from their examples, although their studies at the Carnegie Institution represent conceptual and methodological work that is still relevant. The works of CKH not only embodied the principles of the nascent synthesis, but often provided needed supporting data. Their classic work, especially on Achillea and Potentilla, produced abundant evidence on population differentiation of many quantitative traits and plant phenotypes, as well as demonstrating the now commonly reported distinction between environmental and genetic determination of traits. Their ecological genetic investigations of quantitative traits in plants were in sharp contrast to contemporaneous animal studies on adaptation that focused on discrete polymorphisms--with correspondingly little influence of the environment on phenotypic expression. Of utmost importance was the demonstration by CKH of adaptive differentiation by natural selection and their approaches to understanding the genetic structure of populations.