John A. Endler

Speciation and its consequences.

Speciation is a subject of great interest because it represents both the formation of units of evolution and the connection between microevolution and macroevolution. The purpose of this book is to illustrate how different patterns of speciation and differentiation have occurred among diverse taxa.

A Predator’s View of Animal Color Patterns

It has long been known that the general colors and tones of animals tend to match their backgrounds (E. Darwin, 1794; Poulton, 1890). The adaptive significance of this has been borne out in numerous experimental studies (DiCesnola, 1904; Sumner, 1934, 1935; Isley, 1938; Popham, 1942; Dice, 1947; Turner, 1961; Kettlewell, 1956, 1973; Kaufman, 1974; Wiklund, 1975; Curio, 1976). There is also a good understanding of warning coloration (Cott, 1940; Wickler, 1968; Edmunds, 1974; Rothschild, 1975). However, the determinants of color pattern are poorly known, although it is known in a general way that the patterns and forms of animals are similar to their backgrounds (Poulton, 1890; Thayer, 1909; Cott, 1940; Wickler, 1968; Robinson, 1969; Edmunds, 1974; Fogden and Fogden, 1974). It is the purpose of this paper to explore the factors that determine color patterns under various specific conditions. The basic assumption is that a color pattern must resemble a random sample of the background seen by predators in order to be cryptic, and must deviate from the background in one or more ways in order to be conspicuous. As a result, the actual pattern evolved in a particular place represents a compromise between factors which favor crypsis and those which favor conspicuous color patterns.

Natural and sexual selection on color patterns in poeciliid fishes

SynopsisIn poeciliid fishes, sexual dichromism is associated with larger size and larger broods, but there is no relationship between sexual size dimorphism and sexual dichromism, or between degree of dichromism and color pattern polymorphism. Factors are discussed which influence the evolution of color pattern polymorphisms, sexual dimorphism and dichromism. Detailed studies of South American species have shown that the color patterns of poeciliid fishes have predictable effects in (1) avoiding diurnal visually hunting predators; (2) mating success; and (3) species recognition. Data from some Central American species indicate that some color pattern elements may be closely linked to physiologically variable loci, which further affect the variation in color patterns. Different elements of any given color pattern can be influenced by different modes of natural selection; in guppies the relationship between predation intensity and color pattern is different for melanin, carotenoid, and structural colors. Different color patterns have different degrees of conspicuousness on different backgrounds, and may appear differently to predators and mates with differing visual abilities.

Sensory ecology, receiver biases and sexual selection

During courtship, signals are sent between the sexes, and received signals contain information that forms the basis of decision making. Much is known about signal content, but less is known about signal design-what makes signals work efficiently? A consideration of design not only gives new insights into the evolution of signals (including novelty), but also allows the development of specific and testable predictions about the direction of evolution. Recently there has been increased interest in signal design, but this has resulted in some apparently divergent views in the literature.

Geographic variation in female preferences for male traits in Poecilia reticulata

We examined the preferences of female guppies (Poecilia reticulata) from 11 localities in Trinidad with respect to male color‐pattern elements, body shape and size, and overall color and brightness contrast. Females are on average more attracted to males from their own population than from alien populations, and populations appear to vary in the criteria used in female choice. Multiple‐regression analysis suggests that mate‐preference criteria vary among localities in intensity, sign, and the number of traits used. Although preference estimators and color‐pattern parameters are unique to each population, only orange, black, and color contrast showed a correlation between degree of male trait and degree of preference for that trait. There is a clear effect of water color and a possible effect of predation intensity. The results are discussed in light of various models of sexual selection and the early stages of speciation.

Interacting Effects of Lek Placement, Display Behavior, Ambient Light, and Color Patterns in Three Neotropical Forest-Dwelling Birds

Forests exhibit a mosaic of different spectral environments that arise from forest geometry and weather. If visual signals are used in mate choice, then forest geometry and weather will affect reproductive behavior because the appearance of a visual signal depends on the joint effects of ambient light and the animals reflectance spectra. We investigated three lekking birds at Nourages field station, French Guiana: Rupicola rupicola, Corapipo gutturalis, and Lepidothrix serena. Conspicuousness is a function of ambient light spectra during displays and the reflectance spectra of color pattern elements of the birds and their visual backgrounds. Each species places its lek and performs its lek displays in only one or two of the available light environments, and some may specialize in the more extreme spectra even within each light environment. The color patterns and behavior of each species maximize its visual contrast during its display and reduce it off the lek or on the lek but not displaying. Each species does this with a different combination of colors and light environments. If this phenomenon is general, then it has important implications for the evolution of color patterns and display behavior.

Variation in the appearance of guppy color patterns to guppies and their predators under different visual conditions

Color patterns of natural populations of guppies (Poecilia reticulata) are a compromise between sexual selection and predation avoidance. Field data on ambient light spectra, water transmission spectra, courtship and attack distances, and cone pigments of guppies and their predators were used to calculate measures of conspicuousness of guppies under various combinations of visual conditions and vision. The results suggest that color patterns are relatively more conspicuous to guppies at the times and places of courtship and relatively less conspicuous at the times and places of maximum predator risk. Some implications to the evolution of vision, visual communication and behavior are discussed.

Correlated Evolution of Female Mating Preferences and Male Color Patterns in the Guppy Poecilia reticulata.

Sexual selection may explain why secondary sexual traits of males are so strongly developed in some species that they seem maladaptive. Female mate choice appears to favor the evolution of conspicuous color patterns in male guppies (Poecilia reticulata) from Trinidad, but color patterns vary strikingly among populations. According to most theory, correlated evolution of female mating preferences and preferred male traits within populations could promote this kind of divergence between populations. But mating preferences could also constrain the evolution of male traits. In some guppy populations, females discriminate among males based on variation in the extent of orange pigment in male color patterns, and populations differ significantly in the degree offemale preferences for orange area. In a comparison ofseven populations, the degree offemale preference based on orange is correlated with the population average orange area. Thus male traits and female preferences appear to be evolving in parallel.

Predation, light intensity and courtship behaviour in Poecilia reticulata (Pisces: Poeciliidae)

The colour patterns of guppies, Poecilia reticulata, represent a balance between selection for crypsis (by predators) and selection for conspicuousness (by females). To a predator or mate, the conspicuousness of a colour pattern depends on environmental light conditions at the time of viewing. In this study light intensity and predation had significant effects on male courtship behaviour in guppies. Males courted less and used visually conspicuous behavioural elements less often in the presence of predators or under high light levels. Maximum risk due to predation naturally occurs at the highest light intensities, so either light intensity or the presence of a predator can be used as a cue for predator avoidance. Guppies minimized predator risk during courtship by shifting visually conspicuous displays to times of day when visual predation is minimal, and using the visually less conspicuous sneak copulation strategy when predation risk is greatest. Because ambient light conditions vary with time of day, the same colour patterns are seen under different visual conditions by predators and mates. Such context-dependent conspicuousness reduces the need for an evolutionary compromise between colour patterns that reduce the risk of visual predation and those that increase visibility to females.

Gene flow and population differentiation.

There are many possible spatial patterns of selection and gene flow that can produce a given cline structure; the actual geography of natural selection and gene flow must be worked out before an attempt is made to explain a given natural cline in terms of a model. The results of experimental and theoretical models show that it is possible for local differentiation to evolve parapatrically in spite of considerable gene flow if the selection gradients are relatively uniform. Irregularities in environmental gradients increase the sensitivity of clines to the effects of gene flow in proportion to the increase in the differences in gene frequencies between the emigrants and the demes receiving the immigrants. It is not necessary for a sharp spatial environmental change to be present for distinct differentiation to occur. In some cases even a gentle environmental gradient can give rise to marked spatial differentiation along a genetically continuous series of demes; such environmental differences may be below the practical limits of resolution in field studies. Any asymmetry in gene flow does not lead to dedifferentiation if the environmental gradient is smooth; it merely shifts the position of the transition zone between the differentiated areas from that which would be expected if there were no asymmetry. Abrupt geographic differences in gene, genotype, or morph frequencies should not, therefore, be interpreted as evidence for environmental changes in the immediate vicinity of the steepest part of the cline; neither should they be interpreted as evidence for geographic barriers, sharp environmental differences, or sexual isolation among the differentiated groups of populations when there are no other sources of evidence for these phenomena. Gene flow may be unimportant in the differentiation of populations along environmental gradients.