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Anthropology 1500
Department of Anthropology, University of Missouri-Columbia

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Sexual selection

Readings: Boyd and Silk Chapter 7.

Darwin considered differences between the sexes in morphology and behavior to be a most interesting and important outcome of reproductive competition, and devoted a large volume (The descent of man and selection in relation to sex, 1871) to the subject. Darwin recognized that different selective forces act upon the sexes, especially in regard to competition for mates. He viewed antlers and horns, for example, as having evolved in response to selection for fighting abilities among males; the most successful fighters gained access to females, and therefore passed on the characteristics that led to success in fighting to the next generation.

This kind of selection is usually termed "sexual selection." Darwin described the process of sexual selection as follows: "sexual selection depends on the success of certain individuals over others of the same sex, in relation to the propagation of the species; while natural selection depends on the success of both sexes, at all ages, in relation to the general conditions of life. It is a struggle between individuals of one sex, generally the males, for the possession of the other sex. The result is not death to the unsuccessful competitor, but few or no offspring." (Darwin, 1859)

More controversial was Darwin's concept of sexual selection resulting from mate 'choice.' bright coloring such as the spectacular ornamentation displayed by the male peacock, while presenting no obvious function in male-male combat (except perhaps as a bluff), was hypothesized by Darwin to have evolved in response to selective pressures resulting from the choice by females of mates that possessed ever more extravagant displays.

The mechanism by which this "runaway process" could occur, however, was uncertain until Fisher (1930) proposed that inherited female preference for males exhibiting the relative extreme would result in directional selection for the attributes favored by female choice. Fisher noted that if some females prefer mates of a certain type, and other females mate at random, the "choosy" females and the "chosen" males will be favored by selection, because 1) the "chosen" males will outreproduce "unchosen" males, and 2) "choosy" females will have "chosen" sons and hence more grandchildren (consider, however, the problems with inbreeding avoidance!).

Considerable confusion surrounds the distinction between "sexual" and "natural" selection. Darwin perceived that the two processes could be in opposition, as for example with coloration: "obscure tints have often been developed through natural selection for the sake of protection, and the acquirement through sexual selection of conspicuous colors [to attract mates] appear to have been sometimes checked from the danger thus incurred." (Darwin, l859:230) For example, although a brightly colored bird might attract more mates and be favored by "sexual" selection, it might also be more visible to predators and hence disfavored by "natural" selection. Note that here Darwin is equating "natural" selection with "survival" selection. More modern treatments usually consider sexual selection to be a sub-category of natural selection.

Darwin's main points concerning sexual selection (Otte, 1979):

1) Sexual selection accounts for differences between the sexes that appear unrelated to survival.2) Although the two primary mechanisms of sexual selection are: a) combat for access to members of the opposite sex, and b) mate choice, other mechanisms, such as selection for abilities to locate mates, also are important (especially in the development of sensory organs and locomotory structures) .3) Sexual selection operates on females as well as males.4) Sexual selection operates in monogamous species as well as polygamous ones.5) Sexual selection and "natural selection" may be in opposing directions.

6) It is difficult to distinguish whether a characteristic is the result of natural or sexual selection.

Sexual selection and mating systems

The critical observation linking sexual selection to mating systems was made by Huxley (1938), who noted that among polygynous species, the variance in male reproductive success is likely to be greater than the variance in female reproductive success. This observation was further developed by Bateman (1948:365), who noted that:

"in most animals the fertility of the female is limited by egg production which causes a severe strain on their nutrition. In mammals the corresponding limiting factors are uterine nutrition and milk production, which together may be termed the capacity for rearing young. In the male, however, fertility is seldom likely to be limited by sperm production but rather by the number of inseminations or the number of females available to him... In general, then, the fertility of an individual female will be much more limited than the fertility of a male... This would explain why in unisexual organisms there is nearly always a combination of an undiscriminating eagerness in the males and a discriminating passivity in the females."

Bateman conducted a simple experiment demonstrating the different reproductive strategies of the sexes. Equal numbers of each sex of drosophila were placed together, and the number of matings by each individual recorded. Whereas most females mated at least once but fewer than four times, some males were able to mate many times, and some others were unable to mate at all. Male mating success was more variable than that of females (see Daly and Wilson: 80-81). A male's reproductive success increases with the number of females he mates with, whereas a female's reproductive success does not increase with the number of males she mates with (note that this is not always true for all species). All of a female's eggs can be inseminated by one male, so additional matings have little effect -- in contrast to males, who can inseminate the eggs of many females. There is much stronger selection on males to maximize matings, generating a difference in selective pressures on the sexes, and consequently sexual dimorphisms. Bateman's principle posits that the sex that invests more in the production of offspring becomes a "resource" for which members of the less parental sex compete. Bateman's principle has relevance to what has been termed the "Coolidge effect," a somewhat sexist anecdote from a visit by President and Mrs. Coolidge to a chicken farm. While touring the farm, Mrs. Coolidge asked about the sexual capabilities of a rooster and was told that he would mate several times a day. "tell that to Mr. President," she replied. Upon hearing Mrs. Coolidge's remark, President Coolidge inquired whether or not it was with the same hen. The farmer replied, "oh no, always with a different hen" "tell that to Mrs. Coolidge," responded the President.

An exception that proves the rule for Bateman's principle is the "sex reversal" that occurs in polyandrous mating systems. Among the polyandrous jacana, Wilson's phalarope, and some species of seahorses, the females are slightly larger and more brightly colored than the males. Males tend the eggs, whereas females jealously compete for additional mates.

The advantages of discriminating mate choice by females were examined by Maynard Smith's (1956) experiments with drosophila mating behavior. He was able to demonstrate that (1) females choose certain genotypes over others, and that (2) the choice tended to be adaptive for those females.

The importance of selection resulting from competition for mates remained somewhat obscure until Williams (1966) noted that "all adaptation must relate to reproduction," i.e. survival is irrelevant unless it involves reproduction. Williams' focus upon the individual as the primary level of adaptation by natural selection is most directly responsible for the current explosion in theory and research concerning the evolution of sexual differences, indeed, the recent theoretical attention to the evolution of sex itself (Williams, 1975)! Williams advanced Bateman's arguments concerning the differences in the amounts of parental nurturance provided by the sexes:

"It is a common observation that males show a greater readiness for reproduction than females. This is understandable as a consequence of the greater physiological sacrifice made by the females for the production of each surviving offspring... The traditional coyness of the female is thus easily attributed to adaptive mechanisms by which she can discriminate the ideal moment and circumstances for assuming the burdens of motherhood. One of the most important circumstances is the inseminating male. It is to the female's advantage to be able to pick the most fit male available for fathering her brood. Unusually fit fathers tend to have unusually fit offspring. One of the functions of courtship would be the advertisement, by a male, of how fit he is... Inevitably there is a kind of evolutionary battle of the sexes. If a male attempts to reproduce at all in a certain breeding season, it is to his advantage to pretend to be highly fit whether he is or not. If a weak and unresourceful male successfully coaxes a female to mate with him he has lost nothing, and may have successfully reproduced. It will be to the female's advantage, however, to be able to tell the males that are really fit from those that merely pretend to be. In such a population genic selection will foster a skilled salesmanship among the males and an equally well-developed sales resistance and discrimination among the females." (Williams 1966: 163, 183-4)

Williams identifies the two critical components of mate choice: (1) choice based on the genetic composition of a potential mate (estimated from phenotype), and (2) choice based on resources provided and/or controlled by a potential mate.

Verner and Wilsson (1966) and Orians (1969) elaborate on the effects of this second component of adaptive mate choice by females. They identify resource distribution as a critical variable influencing mating systems. Their models are germane to the analysis of human mating systems because they provide clear predictions concerning correlations between resource structure and reproductive variance. Verner and Wilsson (1966) define the "polygyny threshold" as the point at which it is adaptive for a female to share a male (and his territory, or other resources controlled by him) with another female, rather than choosing an unmated male who controls a much smaller territory.

 

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