A FURTHER STEP IN EVOLVING IDEAS ON POPULATION GENETICS

Abstract

Endler makes a sterling effort to incorporate information from biogeography and field studies to support his models. For example, in Chapter 1 he examines biogeographic information on species distribution and morphology to find support for parapatric speciation. However, in Chapter 6 he rightly points out that it is generally impossible to distinguish between primary intergradation (the prerequisite for parapatric speciation) and secondary intergradation (the result of recontact of differentiated populations). Perhaps more successful is the effort to understand natural clines with gene flow and selection models. There is a useful summary of the information on natural clines and a good attempt to predict the width of specific clines with two basic models. Yet when Endler suggests that gene flow is underestimated here and overestimated there, it should remind the reader that there are probably many unknown factors involved and that biological reality is much more complicated than these models imply (see also the recent discussion of geographical variation in Cepaea by Jones et al., 1977). One of the major strengths of evolutionary biology is that evolutionary change is the result of genetic phenomena and that these genetic phenomena can then be incorporated into explanatory or predictive genetic models. However, demonstrating exactly which genetic phenomena are important in particular evolutionary events is a very difficult task indeed. It entails finding populations undergoing particular changes which can be monitored and tested: a circumstance which is often not possible because of the time involved or financial limitations. Otherwise, specific models can be tested in laboratory situations which are often suspect by many because of the artificial nature of the system. Furthermore, laboratory testing can be quite frustrating since evolutionary events such as speciation do not have to occur frequently to be important. Endler has done a fine service by documenting and clarifying some important aspects of evolutionary biology, although he could have given more suggestions for testing these models. Perhaps now others can further quantify the importance of parapatric speciation and how the interaction of selection and gene flow results in different patterns of genetic variation. This latest Princeton monograph is a well-written, thoroughly organized, and stimulating presentation of several difficult and controversial areas of evolutionary biology. Now if only someone had temporary ominiscience, they could give us the facts.