***30th Anniversary Edition*** Cover note: Each copy of the anniversary edition of The Blind Watchmaker features a unique biomorph. No two covers are exactly alike. Acclaimed as the most influential work on evolution written in the last hundred years, The Blind Watchmaker offers an inspiring and accessible introduction to one of the most important scientific discoveries of all time. A brilliant and controversial book which demonstrates that evolution by natural selection - the unconscious, automatic, blind yet essentially non-random process discovered by Darwin - is the only answer to the biggest question of all: why do we exist?

The Blind Watchmaker

Beginning with Emlen (1966) and MacArthur and Pianka (1966) and extending through the last ten years, several authors have sought to predict the foraging behavior of animals by means of mathematical models. These models are very similar,in that they all assume that the fitness of a foraging animal is a function of the efficiency of foraging measured in terms of some "currency" (Schoener, 1971) -usually energy- and that natural selection has resulted in animals that forage so as to maximize this fitness. As a result of these similarities, the models have become known as "optimal foraging models"; and the theory that embodies them, "optimal foraging theory." The situations to which optimal foraging theory has been applied, with the exception of a few recent studies, can be divided into the following four categories: (1) choice by an animal of which food types to eat (i.e., optimal diet); (2) choice of which patch type to feed in (i.e., optimal patch choice); (3) optimal allocation of time to different patches; and (4) optimal patterns and speed of movements. In this review we discuss each of these categories separately, dealing with both the theoretical developments and the data that permit tests of the predictions. The review is selective in the sense that we emphasize studies that either develop testable predictions or that attempt to test predictions in a precise quantitative manner. We also discuss what we see to be some of the future developments in the area of optimal foraging theory and how this theory can be related to other areas of biology. Our general conclusion is that the simple models so far formulated are supported are supported reasonably well by available data and that we are optimistic about the value both now and in the future of optimal foraging theory. We argue, however, that these simple models will requre much modification, espicially to deal with situations that either cannot easily be put into one or another of the above four categories or entail currencies more complicated that just energy.

Citation classic - optimal foraging - a selective review of theory and tests

It has been argued that females should be able to choose parasite-resistant mates on the basis of the quality of male secondary sexual characters and that such signals must be costly handicaps in order to evolve. To a large extent, handicap hypotheses have relied on energetic explanations for these costs. Here, we have presented a phenomenological model, operating on an intraspecific level, which views the cost of secondary sexual development from an endocrinological perspective. The primary androgenic hormone, testosterone, has a dualistic effect; it stimulates development of characteristics used in sexual selection while reducing immu- nocompetence. This "double-edged sword" creates a physiological trade-off that influences and is influenced by parasite burden. We propose a negative-feedback loop between signal intensity and parasite burden by suggesting that testosterone-dependent signal intensity is a plastic re- sponse. This response is modified in accordance with the competing demands of the potential costs of parasite infection versus that of increased reproductive success afforded by exaggerated signals. We clarify how this trade-off is intimately involved in the evolution of secondary sexual characteristics and how it may explain some of the equivocal empirical results that have surfaced in attempts to quantify parasite's effect on sexual selection.

Parasites, bright males, and the immunocompetence handicap

A DNA test to sex most birds.

Biological signals as handicaps.

The past two decades have seen extensive growth of sexual selection research. Theoretical and empirical work has clarified many components of pre- and postcopulatory sexual selection, such as aggressive competition, mate choice, sperm utilization and sexual conflict. Genetic mechanisms of mate choice evolution have been less amenable to empirical testing, but molecular genetic analyses can now be used for incisive experimentation. Here, we highlight some of the currently debated areas in pre- and postcopulatory sexual selection. We identify where new techniques can help estimate the relative roles of the various selection mechanisms that might work together in the evolution of mating preferences and attractive traits, and in sperm‐egg interactions.

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Sexual selection and mate choice.

Darwin's1 hypothesis that male secondary sexual ornaments evolve through female preferences is theoretically plausible2–7, but there is little experimental field evidence that such preferences exist8–10. I have studied female choice in relation to male tail length in the long-tailed widowbird, Euplectes progne, and report here that males in which the tail was experimentally elongated showed higher mating success than males having normal or reduced tails. The possibility that intrasexual competition among males maintains the long tail was not supported: males with shortened tails held their territories as long as did other males. These results suggest that the extreme tail length in male long-tailed widowbirds is maintained by female mating preferences.

Female choice selects for extreme tail length in a widowbird

Songs are complex acoustic signals produced mainly during the mating season. Such signals are found in many taxa but are most typical of birds, frogs, and insects. Darwin (38) noted that in these groups males are the predominant singers, and for this reason he attributed the evolution of song to sexual selection, that is, selection due to competition for mates. Darwin could muster no direct evidence that song or other display influences mating suc­ cess; only recently has such evidence begun to accumulate. Our fIrst goal here is to review and evaluate evidence that song influences mating success, via either female choice or male contests. Although convinced that sexual selection shapes display behavior, Darwin could not explain the evolution of a relationship between display and mating success. Fisher (50) first gave such an explanation in his discussion of female choice and male threat display. Our second goal is to review ideas and evidence on the evolution of relationships between song and female choice, and between song and success in male contests. We focus on the question of why listeners have evolved to respond to song in the contexts of female choice and male contest.

Sexual selection and the evolution of song

The possibility that the evolution of mating preferences and secondary sex traits can be based on heritable differences in viability is examined with a three-locus model. Earlier genetic models suggested that viability-based processes alone cannot explain the evolution of mate choice and sex ornaments that reduce survival; a Fisherian mating advantage seemed necessary. The present model is based on a monogamous mating system that precludes such a mating advantage. A key assumption is that ornament development depends on the phenotypic condition and overall genotype of the possessor; there is evidence that secondary sex traits often mirror nutritional status and health, sometimes through hormonal mediation. Ornament and preference can then hitchhike slowly to high frequency with alleles that confer a slight survival advantage, provided that such alleles become available often enough. The evolution of mating preferences and secondary sex traits that reflect overall genotypic constitution therefore can be based solely on viability differences, no Fisherian mating advantage being required. In practice, these and several other mechanisms of sexual selection may occur together.

Evolution of condition-dependent sex ornaments and mating preferences: sexual selection based on viability differences.

In Darwin's and Fisher's theory of sexual selection, females prefer ornamented males; the evolution of larger ornaments is limited by increased mortality, lor example through predation. An adornment of given size should often raise mortality more in low than in high quality phenotypes. Possible consequences for the evolution and optimal size of ornaments are here examined with mathematical models.

Fisher suggested that an ormamem may evolve if it initially improves male survival. Female preference then spreads for adorned males. Their consequent mating advantage furthers propagation of the preferred trail. An alternative, the ‘handicap mechanism’, suggests that only those males best able to survive can do so with a large, handicapping ornament. Choosing adorned males, females might therefore bear offspring with high general survivorship, but sons also inherit the handicapping ornament. When it reduces survival more in low than in high quality phenotypes, the handicap mechanism (in conjunction with the Fisherian mating advantage) becomes powerful with lower heritability of fitness than previously supposed. It still requires that fitness has some heritability, for which there is indirect evidence, but field measurements are lacking.

In Darwin's and Fisher's theory, both mating advantage and mortality disadvantage increase with ornament size, and balance at its optimal development. If a given adornment reduces survival most in low quality phenotypes, the optimum increases with phenotypic quality. Ornament size then may provide a measure of fitness, and can be used in mate choice. A male can usually not gain fitness by developing larger adornments than other males of the same quality.

Ornaments can probably also evoke through direct competition between males, without female preferences for adorned males. Field experiments are needed to clarify this and other aspects of sexual selection.

Sexual ornaments form part of the reproductive effort. Certain predictions from life-history theory should therefore apply. One is that ornaments will increase in size over the first few reproductive seasons; this is the case in many species.

Malte Andersson

Papers

Sexual selection and mate choice.

Female choice selects for extreme tail length in a widowbird

Sexual selection and the evolution of song

Evolution of condition-dependent sex ornaments and mating preferences: sexual selection based on viability differences.

Sexual selection, natural selection and quality advertisement