SEXUAL SELECTION



EEB 210/396

Spring 2007

Class #4: Sexual selection and sex differences

Sexual selection: This phenomenon was first suggested by Darwin, especially in his book, The Descent of Man. Sexual selection occurs when (phenotypic) traits are selected because they are attractive to members of the opposite sex. Often such traits may actually reduce an individual’s potential for survival, while at the same time increasing its potential to obtain opportunities for reproduction. For example, the showy tail of the male peacock is attractive to the female, but also makes the male more visible to predators---to say nothing of the energy required for producing and carrying all those extra long feathers. Sexual selection can be viewed as a special case of natural selection, where individuals of the opposite sex are seen as a very important part of an organism’s environment, exerting selection pressure that contributes to the evolution of various traits. So it may be that the disadvantages of the male peacock’s big tail are outweighed by the advantage it provides him as he competes with other males to obtain mates. Female peacocks must surely gain in at least one way by mating with males with the showiest tails: To the extent that showiness is heritable, females that mate with showy males will produce sons with showy tails who will be more likely to succeed in competition for mates as compared to males that are less showy.

Sexual selection can be responsible for patterns of evolution of:

a. morphological traits, such as bright colors in many male birds and fishes, large antlers in deer and elk (The extinct Irish elk appears to be an especially spectacular example.)

b. behavioral traits, such as elaborate mating displays

There continues to be debate over the question of how sexual selection begins; that is, why are particular traits ‘attractive’ to members of the opposite sex? One idea is that a trait like, say, the male peacock’s showy tail may be linked to another trait that has significant survival value, such as a robust immune system. This link could be via one or more genes with pleiotropic effects. (Pleiotropism refers to the ability of a single allele to produce more than one overt effect. For example, the gene that codes for the production of the peptide, oxytocin, has multiple effects because oxytocin is a hormone that stimulates ejection of milk from the mammaries during nursing and also stimulates uterine contractions during labor. In addition, oxytocin is also produced in certain brain regions where it acts as a neurotransmitter; its functions as a neurotransmitter are not as well understood as are its endocrine functions, but there is evidence that it may be involved in certain social behaviors.) Thus, it might be that a particular gene that contributes to immune function is also responsible for the production of bright eyespots in the tail. In this event, females that choose to mate with the males that have showy tails would be (unwittingly) getting mates with genes for good immune systems and would pass these good genes on to both their male and female offspring. In addition, females that mate with ‘showy’ males would also tend to produce male offspring that would develop showy tails. These male offspring would have an advantage in the competition for mates as compared to males with less elaborate plumage. For these reasons, a selective advantage would be conferred on the females that make a choice of mates with showy tails. Once this pattern becomes established with a genetic basis, then males with even more showy tails would likely have an advantage in the competition for mates, and this would now be the case even if the additional ‘showiness’ of the tail was not linked to a further improvement in immune function. That is, the showy tail itself has now become a force for selection; females choose males with the showiest tails, and by mating with such males they will produce sons with showy tails which will tend to win competitions for mating opportunities---and on it goes. This escalating process would be expected to go on until the disadvantages (to the male) of having too big a tail balance the advantages that accrue in the competition for female mates.

Zahavi proposed an alternative hypothesis to explain sexual selection---the so-called handicap hypothesis. He suggested that some sexually selected traits evolved because they serve as an ‘advertisement’ or ‘proof’ that their bearer has been able to survive despite carrying a handicap; thus, in this view the male peacock is demonstrating that he must have superior traits in that he has survived despite his unwieldy and showy tail. Zahavi proposes that females that mate with such males are ‘choosing’ males with superior characteristics as demonstrated by their ability to survive despite a handicap.

Think about what features are generally considered attractive in humans who are looking for mates. Might some of these features be subject to the process of sexual selection as originally postulated by Darwin? Keep in mind that though sexual selection is most often discussed with respect to masculine traits that are important in male-male competition for mates, the phenomenon logically could apply to female traits that become attractive to males. The reason for the emphasis on male traits is that, for most species, male-male competition for mates is more intense than female-female competition. This is because (1) males produce more gametes and thus a male can inseminate multiple females in a short period of time and (2) females generally invest more energy in the offspring, whether simply by making large eggs (as compared to much smaller sperm) or by providing care for newborns as well. Since the sex ratio is usually close to 1:1, if some males mate with multiple females, this must mean that some males fail to father any offspring; hence the competition between males can be very intense.

A strongly monogamous system may tend to increase competition among females for choosing the best mates, as compared to the situation in a polygamous species---this would tend to balance the forces of sexual selection with selection acting more equally on both males and females to choose best partners and therefore acting to produce sexually selected traits in both sexes.

Sex differences: In any given sexually reproducing species, there are a number of morphological and behavioral differences between the sexes. Some of these differences may be at least partly the result of sexual selection, but many are not. Most sex differences presumably evolve in relation to the different reproductive strategies of the two sexes. In many organisms, there are sex differences in body size, aggressive behaviors, and sex behaviors.

In mammals, there appears to be an interesting correlation between the degree of sex dimorphism in body size and the reproductive pattern. Specifically, species that are polygamous tend to show marked sex dimorphisms in size, with males typically larger than females. This type of dimorphism is especially pronounced in species where the males establish territories and attract harems; examples would be sea lions and elk. The large size of the males in these species is likely related to the intense male-male competition for territories and for females, with larger males presumably having an advantage in this competition. In contrast, monogamous species tend to show less sex difference in body size, and this may be related to a reduced degree of male-male competition. Humans are sexually dimorphic with respect to body size, but much less so than gorillas and other primate species that are polygamous. It has been suggested that this may indicate that early humans were either monogamous or at least largely monogamous.

Is there a relation between sexual selection and sexual differentiation? There is no direct relation between these two processes, one of which refers to an evolutionary process and the other a developmental process. Not all sexually differentiated traits are sexually selected. (For example, many internal structures are sexually differentiated but are not subject to sexual selection. Male mammals develop glands like the prostate that secrete substances into the semen and females do not have such glands.) However, a great many sexually selected traits are sexually differentiated. Sexually selected traits generally either appear in only one sex (i.e., specific patches of bright color in males of many bird species that are attractive to females of the same species) or are markedly more pronounced in one sex (i.e., larger body size in males of some species where females prefer to mate with the largest males). Thus, a general understanding of mechanisms of sexual differentiation is bound to be useful in understanding some of the developmental mechanisms that underlie cases of sexual selection.

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