1 10 SEXUAL SELECTION AND BEHAVIORAL EVOLUTION In this course, you have already discussed how natural selection favors alleles that increase survival and reproduction. However, one can observe traits in some organisms that seem clearly to reduce some aspects of fitness. For example, in social insects, many individuals are sterile workers, and they feed and rear the offspring of other individuals. Another example occurs in many animals in which individuals of one sex have conspicuous traits or behaviors that make them vulnerable to predators, are energetically expensive to produce, and don’t contribute to the well-being of offspring. In this lecture, we will concentrate on traits in this second category, the evolution of conspicuous, sexually dimorphic traits. Darwin was at first puzzled by conspicuous traits such as bright colors, horns, and displays of males of many species—seemingly lower survival of the bearers (predation, energy). His solution—Sexual Selection (he devoted most of a book—The Descent of Man, and Selection in Relation to Sex—to this topic). De finition: Selection that arises from differences in mating success (# of mates that bear progeny over a defined time interval) (Arnold and Wade 1984). It is a kind of natural selection (vs. ecological selection) Sexual Selection exists because one gender is a limiting resource for the other. In many species, females are a limiting resource for males b/c females produce few, large, gametes that are energetically expensive, while males produce many, small gametes that are not expensive. In some species, females provide more parental care than males, which also makes females a limiting resource. However, males can be a limiting resource for females in species in which males provide parental care (many insects, fish, birds, and mammals, including primates). Two examples: 1) Female red phalarope s lay more eggs than a male can tend, they are more brightly colored than males and compete with other females for mates (Oring 1986). 2) Pipefish: eggs deposited in the male's brood pouch, or attached to his ventral surface. Males fertilize eggs, developing embryos are protected, aerated, osmoregulated and nourished by the male brooding structure—males are choosy sex (they prefer females with blue coloration. Berglund et al., (1986), Berglund & Rosenqvist (1993, 1994) Berglund (1991), Rosenqvist(1990). 3) Midwife Toad 4) Giant Water bug Also, in many species, it takes two parents to successfully raise offspring, so males and females are equally limiting (many primates, many birds). 10.1 KINDS OF SEXUAL SELECTION Darwin proposed two kinds of Sexual Selection:2 10.1.1 Int rasexua l Selection— contests between male (females) for access to females (males). a) Male blackbirds defend territories using song, and display (epaulets). Anderson showed that obscuring epaulets led to more trespasses by other males (Andersson 1994). 10.1.2 Int erse xual Sel ecti on: Sexual selection via mate choice. Females of many species mate preferentially with males with particular phenotypes (size, color pattern, behavior). Jumping spider courtship: http://tolweb.org/tree/eukaryotes/animals/arthropoda/arachnida/araneae/salticidae/++salticidae/movies/tarsalis.mov Courtship behavior in Japanese crane: see http://www3.famille.ne.jp/~ochi/eng/kushiro2.html Striking how numerous and seemingly arbitrary these traits are. In some species, females prefer males with bizarre artificial traits. WHY would females prefer these strange traits? Several Hypotheses: 1) Direct Benefits 2) Indirect Benefits 3) Sensory Bias 10.1.2.1 Direct Benefits Male provides a resource that directly benefits the female (food, territory, parental care). Male house finches vary from orange to bright red. Redder males bring food to the nest at a higher rate (Hill 1991), and are preferred by females. Not controversial, but does not explain evolution of ornaments in species in which males provide nothing to females or offspring (except gametes). 10.1.2.2 Indirect Benefits Most difficult to explain are those species in which male provides no resources to the female, and does not participate in parental care. Male provides only genes. What could be the cause of female preference in such species? a) “Good Genes” Models. Females choose mates of high genetic quality so that their offspring inherit this “quality”. Females choose based on some trait that indicates high fitness (both trait and fitness must be heritable, VA > 0). Mutation, changing environments, or parasite and pathogen co-evolution could contribute to long-term maintenance of heritable variation for fitness. a) Runaway Sexual Selection.3 First model described by R.A. Fisher (1930), developed mathematically by O’Donald (1962), (in O'Donald 1980), (Lande 1980; Kirkpatrick 1982). Male trait and female preference, once initiated, become self reinforcing. Can lead to continual co-evolution and elaboration of both trait and preference. Kirkpatrick’s genetic model of Fisher’s Runaway process: Genotypes Females P1 (no preference) P2 (preference) Males T2 (no trait) T2 (trait) P2 females tend to mate with T2 males. P1 females mate randomly. P1 females mate randomly. Offspring of P2 females inherit both the T2 trait and the P2 preference—generates a genetic correlation between preference and trait. Since T2 males have higher mating success, on average, trait increases in frequency in population. Preference increases in population b/c of genetic correlation. Other genes affecting trait and preference behave similarly. Can lead to elaboration of trait and preference until the mating advantage of trait is outweighed by survival cost of the trait. c) Sensory B ias M odels. Female preference for a trait may be a by-product of the female sensory system that has adapted to be sensitive to certain kinds of signals for reasons that are unrelated to mating. Also, some species display preferences for novel stimuli for reasons that are not understood (e.g., grass finches prefer artificial crests). Platyfish and swordtails are related species of Poecillid fish, both in genus Xiphophorus. Swordtails (obviously) have sword-like extensions on their caudal fins. Basolo (1990; 1995) attached artificial swords to the tails of platyfish, and showed that female platyfish preferred males with the artificial swords over those that