Sexual selection and the evolution of sex differences

Transcription

1 Sexual selection and the evolution of sex differences Males and females have the same genes. Why do the sexes often look and act so differently? Why is the male often insanely ornamented? (Or simply insane?) Darwin recognized that differential mating success gives rise to a powerful form of selection ( sexual selection ) that may seem to oppose natural selection. Charles Darwin Long-tailed widowbird Biol 3410, 2 March 09 Species differ both in the degree to which the sexes differ, and in the ways they differ. In some species, males are larger than females, more combative, and usually duller. In others, males are smaller than females, more solicitous, and usually brighter. These syndromes are thought to arise from two distinct forms of sexual selection: male-male competition, and female choice. Both derive from a fundamental asymmetry in the limits on reproductive success (RS) first identified by A.J. Bateman in 1948, and developed into the theory of parental investment by Robert Trivers in Female RS is usually limited by ability to invest in offspring (resources), but male RS is limited by reproductive access to females (matings). Bateman s experiment with fruit flies Robert Trivers 1

2 The same is true in plants, and for the same reason: Male gametes (pollen) are inexpensive. Female gametes (eggs) and seed production are costly to the plant as a female. Thus male RS is limited by access to female gametes (parental investment). But female RS is not limited (at least not so strongly) by access to male gametes. Flowers of the wild radish Raphanus raphanistrum are white (WW, Ww) or yellow (ww). Pollinators prefered yellow flowers in a population (50:50 yellow and white) studied by Maureen Stanton (1986). This preference strongly affected male RS (c), but not female RS (b). The variance of RS is usually much greater for males than for females In rough-skinned newts most males have no mates and no offspring, but all females mate and reproduce. There is a strong positive relationship between number of mates and number of offspring for males, but not for females. Thus selection on males, to succeed in competition with other males, is very strong. 2

3 But these relationships are reversed in species where males make most of the parental investment For example, in pipefish the females are more likely not to reproduce at all, and they have greater variance of RS than do males, who brood the young. Predicted consequences of asymmetric limits on reproductive success: In most species (where females make the sole or major investment in offspring): males should be competitive (male-male competition: intrasexual selection), and/or females should be choosy (female choice: intersexual selection). But in species where sex roles are reversed (males invest, females court), these predictions are also reversed. Sex differences should be minimal in highly monogamous species where both sexes invest about equally (e.g., many songbirds, some primates). 3

4 Where females are highly concentrated during the mating season, males may achieve enormous reproductive success by excluding other males from access to those females. For example, female elephant seals need to haul out on safe beaches to give birth and nurse their young pups. Male elephant seals are many times larger than females. They engage in effectively lethal fights for control of prime rookery beaches. The winners mate with many females, but most males mate with none. Females have little say in the matter. Males senesce totally after breeding, and in any case much more rapidly than females. 4

5 Milder forms of female-defense polygyny are common If females have small home ranges, then males can potentially defend several female territories. As female home ranges become larger, they should become harder (less profitable) to defend. Judy Stamps collected data on female home-range sizes and male/female body-size ratios for many species of lizards. As expected, there is a strong negative correlation. Thus, to the degree that males can monopolize mating opportunities by excluding other males, they tend to evolve relatively larger sizes. In other words, the optimal size for a male becomes larger than it otherwise would be, given the species ecology. Sexual dimorphism (male/female body length) Female home range size (m 2 ) Anolis allisoni, Honduras Judy Stamps Male-male competition by other means Male infanticide in lions and other species with harem polygyny. Note: This is not good in any sense for the infants, the mothers or the species! But it does tend to increase the frequencies of alleles carried by the infanticidal male! Sperm competition in flying foxes (bats) and other species with multi-male groups. 5

6 But in some species females can choose among several potential mates. Males are often ornamented in such species, and females seem to prefer males with brighter, more elaborate, or more symmetrical ornaments. Why do females care? One hypothesis is that brighter males have better genes. For example, both male and female barn swallows have elongated outer tail feathers, but males have longer tails than females do. Female and male barn swallows, Hirundo rustica Males with longer tails are preferred by females, and they have more offspring Anders Møller surgically shortened and lengthened the outer tail feathers of male barn swallows, and then asked how long they took to pair, how often the resulting couple produced a second clutch, and how many offspring they fledged from their nest(s). Male barn swallows help their mates with feeding and defense of nestlings. Do females choose for good genes, or good dads? Control I birds were mockaltered. Control II birds were not manipulated. 6

7 Male displays really are costly! Sarah Pryke and Steffan Andersson (2005) shortened the tails of randomly selected male red-collared widowbirds. They maintained better body condition than control males and they attracted many fewer nesting females. So why do females (in these species) prefer expensive displays? One hypothesis: they get better genes for their offspring. A test: male gray tree frogs (Hyla versicolor) sing long, loud, difficult songs. 7

8 Female gray tree frogs prefer long, fast male calls And the offspring of long-calling males tend to have higher fitness Better larval growth and survival Female hangingflies choose mates on the basis of gifts they offer Males catch insect prey, then sit on a twig and release a pheromone. Females approach, and if they accept the prey, copulate for as long as it takes them to eat the prey. Sperm transfer is proportional to time. 8

9 Male pipefish prefer females that are large, showy, and apparently parasite-free. Females can make eggs faster than males can brood and hatch them, so female reproductive success is limited by access to males. Are males choosing females for traits that signal better than average genetic quality? Humans are modestly dimorphic for size and many other traits The pattern of sex differences and studies of human behavior both suggest that male-male competition and female choice have both been significant processes (though not extreme) in our evolutionary history. Lower left: Polish men with children are taller than those without children. Right: Yanomamo men who have killed other men (Unokais) have more wives and more children than those who have not killed. Don t despair! Culture makes a huge difference, and we can change our cultures! 9

10 Summary In most species, female and male reproductive success are limited by different factors. Female RS is typically limited by access to resources for parental investment in offspring. Male RS is typically limited by mating access to females (because males do not invest in offspring). This asymmetry gives rise to male-male competition and/or female choice, with intensities that depend on many details of the species ecology. In species where females may select among a number of potential mates, they appear to be choosing on the basis of the males apparent genetic quality and/or abilities to provide useful resources, again depending on many details of the species biology. In sex-reversed species where males make the larger investments in offspring, females tend to be larger, showier, and more aggressive than males, because their RS is limited in a male-like way, by access to mates more than by access to resources. Magnificent frigatebirds, Fregata magnificens 10