|
Females carry on the brunt of reproduction to carry on their genes; the costs are high --
|
Who
needs sex? |
I. Monogamy-
annual or perennial Young need a lot of parental care
Spatial variation in resources -->in those territories which are rich, a single male can support many females , or.. Monogamy is a recent phenomenon for humans; before the Judeo-Christian
tradition dominated, polygyny was most common- 83% of matings at that
time period involved 'harems' of a sort. Resources are very tight, and 2+ related males must share one female to support the offspring or when the female is a better hunter than the male. III. Promiscuity: No need for pairing
Females stray to gather the best possible genes for their offspring, while males are driven to father as many and as often as possible, experts say. New studies using genetic testing techniques show that even the most apparently devoted of partners often fool around, visiting nearby nests or dens or clans to enjoy the sexual company of strangers. Birds do it, apes do it, and, of course, so do some people. "True monogamy actually is rare," said Stephen T. Emlen, an expert on evolutionary behavior at Cornell University. He says there is a great difference between "social monogamy," where mating pairs bond and work together to raise their young, and "genetic monogamy," where parents are faithful sex partners. Social monogamy is relatively common, but genetic monogamy is the exception rather than the rule, the studies report. Emlen said there are only two monkeys -- the marmoset and the tamarin -- that are truly monogamous among the primates, the animal order that includes man. All the others, monkeys, apes and people, often mate outside their partnerships. Most primates, in fact, make no pretense of faithfully bonding for life, and it is difficult to know for sure that males actually know which of the young in the clan are their children, he said. That may even be true for humans. An Oregon study suggested that about 10 percent of children were not sired by the male partner of the parent couple with whom they bonded. Among the birds, faithful sex partnership has been thought for years to be widespread. Some species, such as the eastern bluebird, gained reputations as shining examples of devotion. Male and female partners work together closely to build nests, incubate eggs, then feed and raise their young. The truth is, bluebirds have a sex life that rivals a television soap opera. Patricia Adair Gowarty, a behavioral ecologist at the University of Georgia, has found that 15 percent to 20 percent of chicks cared for by a bonded pair of bluebirds were not fathered by the male. Gowarty reports that of 180 socially monogamous species, only about 10 percent are sexually faithful. Emlen said female birds and mammals that seek sexual partners outside their partnership may be pushed by the biological drive to produce the best possible children. "One of the patterns is that females seek males of high status and high quality," he said. "By doing so, they are able to produce offspring of higher quality that will be able to do better and survive better. There is a lot of research going on to see if these ideas are correct." Several studies have shown that "females socially bonded with very high quality males do not have copulations outside the pair bonds," said Emlen. In effect, such females believe they already have the best, so why look for better? Males, some researchers suggest, are biologically driven to stray by the desire to spread their genes into as many members of the next generation as possible. Among some species, such as lions, mountain gorillas and grizzly bears, this drive to influence the future genetically leads dominant males to kill and even eat the young of competing males. Impulses that drive humans to seek sex outside their partnerships are far more complex. Emlen cautioned against drawing "simplistic" conclusions about human biology from the studies of animals. Researchers generally believe that monogamy originated among species whose young survived best when raised by a bonded pair. This may have led to the rise of monogamy among people, since human children take so long to mature. One of the most sexually faithful of animals is the California mouse, Science reports. These golden brown rodents invariably pair up for life. Genetic testing has shown that both male and female partners ignore sexual temptations outside the nest. David Gubernick of the University of California, Davis, reports in Science that one reason for this fidelity may be that both parents are required to keep newborn pups alive through their birth winter. The parents must take turns cuddling and warming the young for them to survive, Gubernick said. If the male leaves, the mother will kill or abandon the young. If the mother leaves, the nursing young will starve. Copyright 1998, Associated Press
After reading this abstract, what lessons do we gain from nature?
|
|
SEXUAL SELECTION Darwin, C. R., 1871, The Descent of Man and Selection in Relation to Sex. "When the two sexes follow exactly the same habits of life, and the male has sensory or locomotive organs more highly developed than those of the females, it may be that the perfection of these is indispensable to the male for finding the female; but in the vast majority of cases, they serve only to give one male an advantage over another, for with sufficient time, the less well endowed males would succeed in pairing with the females; and judging from the structure of the females, they would be in all other respects equally well adapted for their ordinary habits of life. Since in such cases the males have acquired their present structure, not from being better fitted to survive in the struggle for existence, but from having gained an advantage over other males, and from having transmitted this advantage to their male offspring alone, sexual selection must here have come into action." Sexually selected traits Sexual dimorphism and polymorphism In the most striking examples of sexual selection, the sexes show extreme sexual dimorphism. In the bird families of pheasants, widow birds, birds of paradise and ducks, the males are often brilliantly colored with exaggerated plumes, tail feather, ruffs and crests. The females of different species within a family tend to be much more alike and often cryptic in coloration. For example, the males of the closely related Golden and Lady Amherst's pheasants show brilliant but strikingly different plumage colors; the females are alike and difficult to distinguish. Clearly, as Darwin said, the males' plumage has arisen by selection acting on males alone. It was to explain this sexual dimorphism that Darwin put forward his theory of sexual selection. Sexually selected traits are often said to be secondary sexual characters: although not essential to the immediate process of mating, like the genitals, they play a part in sexual behavior. Polymorphism in males Pheasants, birds of paradise and ducks seldom show polymorphism within the sexes: although the sexes are strikingly different, the males, like the females, are monomorphic. Some species show polymorphism in the males, as well as the sexual dimorphism between the sexes. The Ruff, Philomachus pugnax, is the obvious example. In the breeding season, the males develop a prominent collar (the ruff) and ear tufts. They are extremely polymorphic in plumage coloration: they may be white, dark glossy blue, reddish brown and barred; the ear tufts and ruff may show any combination of these colors. The females are very similar to other species in the sandpiper family. The Ruff, unlike most wading birds, is highly polygynous: this strongly corroborates the view that the male breeding colors evolved by sexual selection in competition for mates. Polymorphism in both sexes Many species are polymorphic in morphology or color pattern, both sexes showing the same polymorphic traits: they are polymorphic, but not sexually dimorphic. Although it was the evolution of sexual dimorphism that Darwin put forward sexual selection to explain, many studies have shown that sexual selection, usually of males, is a factor acting to maintain many polymorphisms, even when sexual dimorphism is absent: in the males, but not in the females, polymorphic phenotypes may differ in their chances of mating. In these cases, the sexual selected phenotypes are never brilliant or striking like the male phenotypes in a sexually dimorphic species. Sexual selection has been demonstrated in the following species, in which the sexes are alike and show the same polymorphic variation. Birds Arctic skua (Stercorarius parasiticus)Eleonora's falcon (Falco eleonorae) Feral pigeon (Columba livia) Snow goose (Anser caerulescens) . Arthropods 2-spot ladybird (Adalia bipunctata) milkweed beetle (Tetraopes tetraophthalmus) flour beetle (Tribolium castaneum) seaweed fly (coelopa frigida) a parasitic wasp (Mormoniella vitripennis) 11 species of Drosophila In Drosophila species, various mutants, especially eye color mutants, have been tested for mating advantage. In Tetraopes, certain allozymes of phosphoglucomutase and leucine naphthylamidase were found to show a mating advantage in populations where the specific allozymes were at low frequencies. The is known as the rare male effect first observed in Drosophila matings. It was later shown that female choice must necessarily give rise to selection that is greater at lower frequencies. Measurement of sexual selection Darwin put forward two mechanisms of sexual selection: male competition (which Darwin called "the Law of Battle") and female choice. Male competition has never been controversial: it has always been accepted that where males fight for females and are much larger in size than the females or have special weapons for fighting, these traits will have evolve by male competition. But female choice, particularly choice for brilliant displays and colours, was thought to imply that females were exercising aesthetic judgments in their choice, which seemed implausible. And Darwin could not explain how the females had acquired their preferences for specific male colours and ornaments. The theory of female choice was rejected by many evolutionists until the publication of R. A. Fisher's The Genetical Theory of Natural Selection (1930) where he stated his theory of the evolution of sexual preference. An important aspect of sexual selection is how the males pass on their advantage. In a polygynous species, the most powerful or most brilliant and attractive males mate with many females, so gaining a great advantage over unsuccessful rivals. Darwin noted that only those few polygynous families of birds - pheasants, widow birds, birds of paradise, ducks - show extreme sexual dimorphism. This strongly corroborates the theory of sexual selection. But Darwin knew that sexual dimorphism is also found in some monogamous birds (about 90% of birds are monogamous) and he formulated a subtle ecological theory to show how sexual selection could operate even in monogamous species. THE RARE MALE EFFECT Sexual selection by female choice is always frequency-dependent. Experiments by Lee Ehrman and others on mate choice in Drosophila showed that when females were given a choice of males at different relative frequencies, the less common males were usually found to have the advantage. This was interpreted as frequency-dependent choice: Ehrman postulated that males would become habituated to the courtship cues of common males and thus respond more readily to a rare male giving a new and unusual courtship cue. However, O'Donald (Genetic Models of Sexual Selection, 1980) showed that all cases of this rare male effect could be explained perfectly well by constant female choice. MATE CHOICE IN THE 2-SPOT LADYBIRD, ADALIA BIPUNCTATA. Ladybirds are ideal for studies of sexual selection: they usually copulate for three or four hours at a time! Many ladybirds in natural population will be collected as copulating pairs. In populations polymorphic for melanic and non-melanic phenotypes, melanic males are often found to be at higher frequency in copulating pairs than their general frequency in the population: they have a mating advantage. Majerus and O'Donald carried out a series of mating tests at different male frequencies. The following is an example of one such series in which matings of the melanic quadrimaculata (Q) and non-melanic typica (T) were observed in population cages (Majerus, M.E.N., O'Donald, P. & Weir, J, 1982, Heredity 49, 37-49). .
Ratio in cage T nonmelanic form males melanic Q Numbers of matings of males advantage 7T : 3Q 87 77 164 1: 2.06 5T : 5Q 32 75 107 1 : 2.34 3T : 7Q 42 182 224 1 : 1.85 . It is obvious that the mating frequencies are not random: they do not fit the expected random mating frequencies 7:3, 5:5 and 3:7 (c2 = 53.22 for 3 degrees of freedom). EVOLUTION OF FEMALE PREFERENCE Fisher's Theory Fisher first put forward his theory in 1915 (The Eugenics Review, October 1915). He gave a more precise account of the theory in The Genetical Theory of Natural Selection (1930). The steps in Fisher's argument may be stated as follows. (i) Some females are assumed to have a genetic preference to mate with males who show a recognizable and advantageous trait. The sons of these preferential matings will tend to inherit both their father's advantageous trait and also the genes for their mother's preference. (ii) Males with the trait increase in frequency because the trait is selectively advantageous. Since these males also carry the preference genes, the preference genes are selected too, hitch-hiking with the genes for the male trait. (iii) As the preference increases by hitch-hiking with the male trait, it adds to the trait's original selective advantage. Thus the preference partly selects itself, further increasing the selection on the trait, hence selection on the preference, and so on. (iv) This may lead to a runaway process as more and more extreme developments of the male trait are selected along with the female preference for them. At some point, the initial advantage of the trait will be lost. (v) The runaway process must eventually be halted when the development of the trait has become so extreme as to be highly deleterious to survival. Natural selection will eventually balance sexual selection at an equilibrium. This is the likely point which the evolution of the widow bird's tail has reached. Andersson showed that females actually preferred males with artificially lengthened tails (Andersson, M., 1982, Nature 299, 818-820); but since longer tails have not evolved, the sexual preference of the females is presumably balanced by natural selection against it. Fisher's theory has three stages: (i) initial selection for preference; (ii) runaway after the preference has evolved; (iii) halting the runaway by adverse natural selection. Fisher thought that in many cases, preference would evolve, but without producing runaway. He predicted that: "Sexual preference originating in this way may be far more widespread than the occurrence of striking secondary sexual characters." This prediction has certainly been verified. Mathematical models of Fisher's theory have shown that selection of preference and male trait can occur as Fisher postulated (See for example: O'Donald, P., 1967, Heredity 22, 499-518; O'Donald, P.,1980, Genetic Models of Sexual Selection, Cambridge University Press; Lande, R., 1981, Proc. Nat. Acad. Sci. USA 78, 3721-3725; Kirkpatrick, M. ,1982, Evolution 36, 1-12). There is still, in all models of the evolution of preference, a problem to be solved: how does the process get started? Many models assume the preference is already in existence. For example, in Lande's model, all females mate preferentially according to a normal distribution of preference. Runaway occurs if there is a sufficiently close genetic association between the male trait and the preference, i.e. if the preference is selected rapidly enough by hitch-hiking with the male trait. Since the preference is assumed to exist, the process starts immediately at full speed. If the preference arises as a new mutation, it will start at a very low frequency, and thus exert a negligible effect. O'Donald found in his models that the process would then take an inordinately long time to get going. In general, models have not shown conclusively whether conditions can exist for Fisherian runaway to occur or not." . |