BARRIERS TO GENE EXCHANGE BETWEEN MEMBERS OF THE MIMULUS GUTTATUS COMPLEX (SCROPHULARIACEAE)

A host of stages of active evolution is exhibited by the Mimulus guttatus complex of related species. This group is a living example of Wright's (1932) hypothetical model for rapid evolution and speciation. Its tens of thousands of populations are frequently small and partially or completely geographically isolated from one another just as Wright postulated. The marked morphological and physiological differences between the populations (Grant, 1924; Pennell, 1951) indicate that much evolutionary diversification has al-. ready occurred. However, diversification alone does not lead to the evolution of new species. Speciation requires the development of isolating mechanisms that prevent gene exchange between populations (Dobzhansky, 1951). The evolution of isolating mechanisms may be concurrent with morphological and physiological differentiation, or may be independent of such diversification. The purpose of this study is to identify and investigate those isolating mechanisms which may be present within the Mimulus guttatus complex. The populations selected for investigation well represent the obvious morphological and physiological diversity of the group. They include the main species of the complex (M. guttatus Fischer), its two most common relatives (M. nasutus Greene and M. laciniatus Gray), and two relatively rare members of the group (M. platycalyx Pennell and M. glaucescens Greene). The individual plants as well as the different populations of the species M. guttatus may be from a centimeter to over a meter in height. Their leaves vary in shape and size from one population to the next. The leaves of individual plants range from relatively large and petiolate near the base to comparatively small and sessile at the stem apex (fig. 1). The flowers are conspicuous, yellow usually dotted with red, and strongly bilabiate. The most noticeable difference between M. nasutus and the other members of the complex is its nose-like upper calyx lobe. One M. nasutus population has n = 13 chromosomes instead of n = 14, the characteristic number of the complex (Mukherjee and Vickery, 1960). Mimulus laciniatus differs from the rest of the complex by its deeply laciniate leaves. Mimulus platycalyx is characterized by its open-throated corollas and its cup-shaped to funnel-form calyces. Furthermore, it has n = 15 chromosomes. Mimulus glaucescens can be easily recognized by its connate floral leaves. These are the most conspicuous differences which separate the species chosen for study. The species also differ in other flower and vegetative characteristics as well as in their physiological requirements and adaptations (Grant, 1924; Pennell, 1951). Furthermore, each selected population differs from the others of the same species in smaller, but still obvious morphological and physiological traits (table 1 and fig. 1). For this investigation, the Mimulus 1 Approximately one-fourth of the work reported here was carried out at the Carnegie Institution of Washington's Department of Plant Biology Laboratory, Stanford, California, and was included in the author's dissertation submitted to the faculty of Stanford University in partial fulfillment of the Ph.D. requirements. The rest of the work has been carried out subsequently in the Division of Biology, University of Utah, Salt Lake City, Utah where it has been supported by grants from the University of Utah Research Fund and from the National Science Foundation. The author wishes to thank his colleague, W. W. Newby, for his careful reading of the manuscript and many helpful suggestions.