Comparative physiology of ecological races in Minulus

[First paragraph...]
Perhaps most important among the causes of the distribution patterns studied by ecologists are the underlying physiological mechanisms which lead the organisms to prefer the conditions they do. These ecological aspects of physiology are not easily investigated, because so many factors are usually interacting that straightforward separation into specific physiological questions is not usually possible. However in occasional rather rare situations only a small part of this complex needs to be considered and the ecological questions can be posed in more clearcut physiological terms. This happens when, for example, a pair of distinct but otherwise closely related kinds of organism have a mutually exclusive distribution coupled with a simple but definite environmental change. A case of this sort is found in the two species of Mimulus, M. cardinalis and M. lewisii of the western states of North America. Although very distinct in morphology and distribution these two are genetically close enough to be regarded from an evolutionary viewpoint as sister ecological races.