Sexual Selection is a term applied by Darwin to the process of favouring and eliminating which to some extent occurs in the mating of many animals. It is a special case of natural selection, depending upon a competition between rival males, in which a premium is set upon those qualities which favour their possessors in securing mates. This competition takes two forms: on the one hand, rival males, for instance stags and gamecocks, fight with one another, and the conquerors have naturally the preference in mating; on the other hand, rival males sometimes seem to vie with one another in displaying their attractive qualities before their desired mates, who, according to Darwin, choose those that please them best.
Darwin gives the following summary of his theory: 'It has been shown that the largest number of vigorous offspring will be reared from the pairing of the strongest and best-formed males, victorious in contests over other males, with the most vigorous and best-nourished females, which are the first to breed in the spring. If such females select the more attractive and, at the same time, vigorous males, they will rear a larger number of offspring than the retarded females, which must pair with the less vigorous and less attractive males. So it will be if the more vigorous males select the more attractive and, at the same time, healthy and vigorous females; and this will especially hold good if the male defends the female, and aids in providing food for the young. The advantage thus gained by the more vigorous pairs in rearing a larger number of offspring has apparently sufficed to render sexual selection efficient.'
Where there is direct competition between males, the weakest will tend to be eliminated, either directly by death or injury in the struggle, or indirectly by diminished success in reproduction. In the same way, if a male be lacking in the qualities necessary to find a mate—e.g. in senses acute enough to find out her whereabouts—that male may remain unproductive. But there is not enough of evidence to enable us to compute how many males do remain unmated in consequence of non-success in competition.
In regard to the second aspect of sexual selection, in which the females are believed to exercise some choice, giving the preference to those suitors which have brighter colours, more graceful forms, sweeter voices, or greater charms of some kind, there is no little difference of opinion. Darwin indeed believed strongly in the female's choice, and referred to this process of selection many of the qualities which distinguish male animals. The females 'have by a long selection of the more attractive males added to their beauty or other attractive qualities.' 'If any man can in a short time give elegant carriage and beauty to his bantams, according to his standard of beauty, I can see no reason to doubt that female birds, by selecting during thousands of generations the most melodious or beautiful males, according to their standard of beauty, might produce a marked effect.' On the other hand, Alfred Russel Wallace maintains a very different position. 'There is,' he says, 'a total absence of any evidence that the females admire or even notice the display of the males. Among butterflies there is literally not one particle of evidence that the female is influenced by colour or even that she has any power of choice, while there is much direct evidence to the contrary.' Against this, G. W. and E. G. Peckham, in their careful essay on sexual selection in spiders, state that they have in the Attidae 'conclusive evidence that the females pay close attention to the love dances of the males, and also that they have not only the power, but the will, to exercise a choice among the suitors for their favour.' Some observers of birds are also confident that the females choose the more musical or otherwise attractive males. But again Wallace maintains that the fact that every male bird finds a mate 'would almost or quite neutralise any effect of sexual selection of colour or ornament; since the less highly coloured birds would be at no disadvantage as regards leaving healthy offspring.' In spiders, however, it seems that the more brilliant males may be selected again and again while the mating season lasts.
The theory of sexual selection is of considerable importance in a general theory of evolution. This may be illustrated in reference to the bright plumage of many birds. If we postulate successive crops of variations (which cannot at present be completely rationalised), if we acknowledge that there is really 'preferential mating' among birds (which is not readily proved or disproved), if we believe that the females are sensitive to the slight excellences which distinguish one suitor from another and that their choice of mates is determined by these excellences (which Wallace emphatically denies), then we may say that the greater brightness of male birds may have been evolved by sexual selection. This was Darwin's opinion. The brighter males succeeded better than their rivals in the art of courtship; the variations which gave them success were transmitted to the offspring; gradually the qualities were established and enhanced as secondary sexual characters of the species. But Wallace interpreted the facts quite otherwise. The relatively plain plumage of the female birds was due to natural selection, eliminating those whose conspicuousness during incubation was fatal, fostering those whose colouring was protective. Just as Daines Barrington, a naturalist still remembered as the correspondent of Gilbert White, suggested (1773) that singing-birds were small and hen-birds mute for safety's sake, so Wallace maintained that female birds had forfeited brightness as a ransom for life.
But, leaving the birds, let us take a case which seems to afford better illustration of Darwin's theory of sexual selection—that of spiders. The courtship of these animals has been observed and described by G. W. and E. G. Peckham in a manner so careful that their paper ranks as one of the most important contributions yet made to the theory of sexual selection. 'The fact that in the Attidae the males vie with each other in making an elaborate display, not only of their grace and agility but also of their beauty, before the females, and that the females, after attentively watching the dances and tournaments which have been executed for their gratification, select for their mates the males that they find most pleasing, points strongly to the conclusion that the great differences in colour and in ornament between the males and females of these spiders are the result of sexual selection.' It may be that the American observers have, especially in their psychological language, mingled a little imagination with their induction, but they state a strong case for sexual selection.
The conclusions drawn from the courtship of spiders are not affected by Wallace's criticism so seriously as are those which Darwin drew from the courtship of birds, and this suggests that the wisest position is one of compromise, which recognises that in some cases—e.g. spiders—the external divergence of the sexes may depend upon sexual selection, and that in other cases—e.g. birds—it may depend rather upon natural selection.
But even with this compromise it is difficult to rest satisfied. For before we can believe that attractively bright ornaments could become characteristic of males by sexual selection, or that protectively plain colouring could become characteristic of females by natural selection, we must assume that the qualities of brightness can be entailed in inheritance on the males only, and the qualities of plainness on the females only. But this fundamental assumption has not yet been justified by a sufficiently strong body of facts.
This difficulty arouses scepticism as to the thoroughness of the explanations of secondary sexual characters suggested either by Darwin or by Wallace. We are not surprised, therefore, to find Mivart's explanation of the beauty of males as the direct expression of an internal force, or Mantegazza's hints as to a physiological explanation of the sexual divergence, or Brooks's reference to 'something within the animal which determines that the male should lead and the female follow in the evolution of new breeds.' Geddes advanced further, endeavouring to interpret the secondary sexual characters as outcrops of the relative preponderance of anabolism and katabolism characteristic of females and males respectively. Gay colouring—sometimes at least due to pigmented waste products—is regarded as a characteristic expression of the predominantly katabolic or male sex, and quiet plainness is equally natural to the more anabolic females. But this theory, which seeks to rationalise the variations which Darwin simply postulated, is by no means inconsistent with a recognition of sexual selection as an accelerant directive process in the evolution of male brightness, or of natural selection as a retardative directive process eliminating disadvantageously conspicuous females.
Wallace has also in his work on Darwinism (1889) worked towards a rational interpretation of the variations which he was previously content to postulate as facts. For he says that 'ornament is the natural outcome and direct product of superabundant health and vigour,' and is 'due to the general laws of growth and development.' It seems to some that this mode of interpreting characters is of far-reaching importance, and that it affects not only the theory of sexual selection but that of natural selection as well.
The Peckhams do indeed deny that male spiders possess greater vital activity than the females, and they find no relation in either sex between activity and development of colour. But it must be noted that a predominant katabolic diathesis—which is, according to Geddes, the fundamental characteristic of maleness—may be true of male spiders though not expressed in greater vital activity.
We must of course be careful to distinguish that the suggestions made by Geddes, Wallace, and others as to the physiological meaning of sexual characters have to do with primary factors in evolution—i.e. with those which originate variations—while the explanation of the differences in plumage between male and female birds, either by the theory of sexual selection (according to Darwin) or by natural selection (according to Wallace), have to do with secondary factors in evolution—i.e. with those which foster or eliminate variations. Apart from the problem of the origin of the sexual variations the central question with regard to sexual selection by preferential mating is, as Lloyd Morgan says, what guides the variation along special lines leading to heightened beauty. 'Sexual selection by preferential mating involves a standard of taste; that standard has advanced from what we consider a lower to what we consider a higher æsthetic level, not along one line but along many lines. What has guided it along these lines?'
To sum up, the problems involved in sexual selection are (1) what physiological conditions explain the secondary sexual characters which so often distinguish males and females; (2) to what extent and in what degrees of refinement does preferential mating occur; and (3) to what extent has sexual selection guided the differentiation of the sexes alike in distinctive qualities and in æsthetic sensitiveness? Before these problems can be adequately solved many more facts must be accumulated.
See SEX, DARWINIAN THEORY, EVOLUTION; Darwin, The Descent of Man and Selection in Relation to Sex (1871); A. R. Wallace, Contributions to the Theory of Natural Selection (1871), and Darwinism (1889); St George Mivart, Lessons from Nature (1876); W. K. Brooks, The Law of Heredity (Baltimore, 1883); P. Geddes, article 'Sex,' Encyclo. Brit.; P. Geddes and J. A. Thomson, The Evolution of Sex (1889); G. W. and E. G. Peckham, Observations on Sexual Selection in Spiders of the family Attidæ, Occas. Papers, Nat. Hist. Soc. Wisconsin (Milwaukee, 1889); C. Lloyd Morgan, Animal Life and Intelligence (1890-91).