John A. Endler

Natural selection is an immense and important subject, yet there have been few attempts to summarize its effects on natural populations, and fewer still which discuss the problems of working with natural selection in the wild. These are the purposes of John Endler's book. In it, he discusses the methods and problems involved in the demonstration and measurement of natural selection, presents the critical evidence for its existence, and places it in an evolutionary perspective. Professor Endler finds that there are a remarkable number of direct demonstrations of selection in a wide variety of animals and plants. The distribution of observed magnitudes of selection in natural populations is surprisingly broad, and it overlaps extensively the range of values found in artificial selection. He argues that the common assumption that selection is usually weak in natural populations is no longer tenable, but that natural selection is only one component of the process of evolution; natural selection can explain the change of frequencies of variants, but not their origins.

Geographic Variation, Speciation and Clines explores the origins and development of geographic variation, divergence, and speciation. In particular it is concerned with genetic divergence as it is usually found on continents, among groups of populations isolated only by distance. Although earlier writers on this topic considered the effects of geography and dispersal, intense geographic differentiation and speciation were thought to require complete isolation. Professor Endler shows how geographic differentiation and speciation may develop in spite of continuous gene flow. Following a review of the diverse and scattered literature on gene flow and population differentiation, the author discusses the relationships among gene flow, dispersal, and migration. He then summarizes the factors which limit the geographic extent of gene flow, and those which allow steep clines to develop in the absence of barriers to gene flow. His analysis draws on examples from the field, experiments, and single- and multiple-locus models. The mechanism and conditions for parapatric speciation are presented: steepening clines, development into hybrid zones, and the evolution of sexual isolation. In the final chapter the author considers the interpretation of natural clines and the associated geographic patterns of subspecies and species.

There is a bewildering diversity of signals, sensory systems, and signaling behavior. A consideration of how these traits affect each other's evolution explains some of this diversity. Natural selection favors signals, receptors, and signaling behavior that maximize the received signals relative to background noise and minimize signal degradation. Properties of sensory systems bias the direction of evolution of the signals that they receive. For example, females may prefer males whose signals they can perceive more easily, and this will lead to the spread of more easily perceived male traits. Environmental conditions during signal transmission and detection also affect signal perception. Specific environmental conditions will bias the evolutionary direction of behavior, which affects the time and place of signaling as well as microhabitat preferences. Increased specialization of microhabitats and signaling behavior may lead to biased evolution of the sensory systems to work more efficiently. Thus, sensory systems, signals, signaling behavior, and habitat choice are evolutionarily coupled. These suites of traits should coevolve in predictable directions, determined by environmental biophysics, neurobiology, and the genetics of the suites of traits--hence the term "sensory drive." Because conditions vary in space and time, diversity will be generated

Forests exhibit much variation in light environments, and this can affect communication among animals, communication between animals and plants, photosynthesis, and plant morphogenesis. Light environments are caused by, and can be predicted from, the geometry of the light paths, the weather conditions, and the time of day. The structure of forests leads to four major light habitats when the sun is not blocked by clouds: forest shade, woodland shade, small gaps, and large gaps. These are characterized by yellow—green, blue—gray, reddish, and "white" ambient light spectra, respectively. When the sun is blocked by clouds, the spectra of these four habitats converge on that of large gaps and open areas, so the single light environment during cloudy weather will be called open/cloudy. An additional light environment (early/late) is associated with low sun angles (near dawn or dusk); it is purplish. Each light environment is well defined and was found in forests of Trinidad, Panama, Costa Rica, Australia, California, and Florida. Scattered literature references suggest similar patterns elsewhere in North America, Europe, and Java. Perceived colors of animals, flowers, and fruits depend upon the interaction between ambient light color and the reflectance color of the animal or plant parts. As a result, an animal or plant may have a different appearance in each environment, i.e., a color pattern may be relatively cryptic in some light environments while relatively conspicuous in others. This has strong implications for the joint evolution of visual signals and vision, as well as microhabitat choice. Plant growth and form may also be affected by variation in the color of forest light.

NATURAL SELECTION ON COLOR PATTERNS IN<i>POECILIA RETICULATA</i>