Sexual Selection
Contents Updated: Monday, September 13, 1999
The Mating Game
An organism has to reproduce as individuals before it can survive as a species. If an individual is highly successful but fails to reproduce, its successful characteristics cannot be passed on. Selection is not selection merely to survive but selection to reproduce.
It is survival until reproduction that is necessary for the continuation of the species. Darwin’s dictum would be better expressed as reproduction of the fittest rather than survival of the fittest.
Reproduction has its own necessities. Creatures must first find suitable mates. Then they must be preferred by their intended mates above other competing members of the species. This element of choice of mate is where sexual selection comes in.
Usually it is the female that does the selecting. Why? Dawkins invites us to think of the whole cycle of courtship and mating as a game played each mating season. Impregnation puts the females out of the game because she has to concentrate on giving birth, but not so for the male. He can stay in for another round.
The mating game consists of a first round where the most desirable females select the most desirable males as a mates. The less desirable females are not immediately successful because they also are on the look out for very desirable partners. They delay their choice hoping for a desirable mate but tend not to be chosen while there are some of the best females still around. Having secured a desirable mate, the desirable females become pregnant and drop out of the game for the rest of this season.
The desirable males are then free to rejoin the less desirable males not yet selected by the less desirable females who are still delaying their choice. Of course, while the most desirable males were courting some of the less desirable males and females will have tired of looking for perfection and will have also paired off. The females remaining are not so desirable but the most desirable males returning to the mating game will again have their pick of them. The desirable males play the field.
The net effect of this is that some of the other males do not get mates because the most desirable ones have had several goes and each time a female is eliminated from the game. The least desirable males do not breed. Males are selected for their desirability, whatever it is. The obvious advantage to a male of being desirable is that he can impregnate several of the females thus passing on his genes and his particular brand of desirability. The males that fail to attract a mate do not pass on their genes.
On the other hand females have little need to evolve obviously desirable traits. They only have one turn and even the undesirable ones will be in demand by desirable males when all the more attractive females are pregnant. The females then are normally the sex that does the choosing, needing not to be unusually desirable as long as they are not repugnant.
The males benefit by being desirable in some way. Whatever is desirable about them will tend to be enhanced by sexual selection because those that are insufficiently endowed with it have no offspring.
Polygenes
Many characteristics or qualities of individuals within a species, such as height in humans, do not depend upon the interaction of only one gene with the environment but upon many acting together. These are called polygenes. Female preference is a characteristic controlled by a polygene. So also is the set of male features that the female finds rather alluring. The main point of such features are that they are continuous rather than having discrete values.
Consider a male hairy chest. Woman tend to find hairy chests, to a greater or lesser degree, attractive or repugnant. Chests themselves can, of course, be, to a greater or lesser extent, hairy. Some women may have genes giving them a preference for chests which are to a degree hairy while others may be endowed with a preference for the more smooth chested males. If, for some reason, the preference for smooth chests became dominant then the most hairy chested men would find it difficult to find a mate.
In the story of the aquatic ape, hydrodynamics determined that smoothness would be advantageous to a swimming primate. The smoother apes would tend to survive and once sexual selection started favoring smoothness then smoothness would prevail even without the influence of the water.
You may protest, "what is to stop women being perverse enough to prefer hairy chested men even though smooth men are in the majority?
The answer is smooth men have the genes for smoothness, plainly enough, but they also have the genes for preferring smoothness. These will be passed on to their daughters who will tend to select smooth chested men as mates!
But why? you may persist. "Surely a smooth chested man could have the genes for the preference of hairy chests which he would pass on to his daughters.
He could. But is he likely to? The answer is no. A smooth man is more likely to attract a woman who prefers smoothness and a hairy chested man is more likely to attract a woman who prefers hairiness.
Linkage Disequilibrium
I am hairy chested like my father. It is more likely—in so far as hairiness was a factor at all in my mother’s choice of partner (assume for the sake of this argument that it was)—that my mother preferred hairy chests. It follows that the genes I inherit from my mother are likely to be those for female preference for hairy chests. My daughters will be likely to inherit my mother’s preference for hairiness through me or my wife’s preference for hairiness (again assuming that my hairiness was influential in my wife’s choice of mate).
You can see that hairiness and the preference for hairiness tend to link together through sexual selection. In a species which is predominantly smooth we can be sure that the majority of women prefer smoothness. This linkage of genes is called linkage disequilibrium.
When genes are in equilibrium in a population, no particular pairings of genes are favored and the distribution of features is even.
The interesting thing about linkage disequilibrium is that it can form a positive feedback cycle leading to explosive evolution. In the case of smoothness, it could lead to such a selection against hairiness that in a very short period in evolutionary terms, men became smooth—other things being equal. That all men are not smooth shows that we are dealing with a gene complex and there are other factors. Perhaps hairy men are more dominant and some females forgive the less-than-ideal hairiness to associate with a dominant male.
Or many other things! Sexual selection and the linkage disequilibrium it causes have led to bizarre effects like the elaborate displays of the birds of paradise, or the helmets and crests of the hadrosaurs.
Evolution of the Human Penis
Human males have the largest penis of all the great apes though some of the apes have much larger bodies. Sexual selection with positive feedback associated with the human change to an upright posture could have been the reason.
A positive feedback loop leading to explosive evolution simply requires sexual selection for a particular feature more exaggerated than the average, and the development of a linkage disequilibrium. The degree of exaggeration controls the rate of evolution with slow evolution accompanying a slight exaggeration and rapid evolution accompanying a large exaggeration.
Imagine, for the sake of illustration, how the large penis of the naked ape might have developed according to this hypothesis. Ignore cultural influences because culture, including religion and indeed clothing, had not arisen when this selection was initially taking place.
The early hominids, whether by the aquatic route or the savannah route, had begun to walk upright. They had accordingly begun to adapt to copulating face-to-face. The change was not easy. Females did not find it very satisfactory in most instances because their sexual organs were still oriented rearwards. Males with longer penises could effect face-to-face intercourse more successfully.
Suppose that the average early hominid had a penis similar to a chimpanzee’s but our female ancestors had more sexual satisfaction from those few males having larger penises. Males with larger penises would tend to be sexually selected, would be more successful in reproducing and would have more offspring. In time, the whole population of males would tend to have larger penises and, through linkage disequilibrium, would carry genes for the preference for larger penises among females. At the same time the females would tend to prefer males with larger penises and would carry the unexpressed genes for large penises.
What is happening is that the two sets of genes (for large penises and for the preference for large penises) are selecting each other using the size of the penis as a signal. A female chooses a male with a large penis because his large penis advertises the fact that he carries genes for the preference for a large penis. The female is choosing a male who has the same or related polygene to her own.
So, after some evolutionary time penises will be larger. The preference for large penises will also be stronger. This may even be the case though the average penis length has now become sufficient to satisfy the female when copulating in the newly adopted face-to-face position.
The cycle apparently could continue locked in its positive feedback loop with ultimately bizarre consequences. In reality, of course, the feedback loop is broken and an equilibrium established when the environmental disadvantages of the feedback begin to outweigh the advantages of further response to sexual selection. For the birds of paradise and the hadrosaurs the equilibrium was sufficiently tilted to the sexually selected side that exotic features were the result, despite their apparent disadvantages for survival.
It might be interesting to know whether penises are still tending to get bigger—perhaps 30 per cent of human females still do not get adequate satisfaction from orthodox sex, suggesting that we have still not fully adapted to face-to-face copulation—but it might not be so easy to test.
It is more acceptable to test tail feathers on birds.
Malte Anderson, a Swedish ethologist cut off the end of the tail feathers of some male Kenyan long-tailed widow birds. The trimmed-off ends he stuck with superglue to the ends of the tails of other birds. Thus some of the male birds had artificially long tails and some had artificially short tails. There were also controls that had had their tails cut off then stuck on again (to test whether the operation itself had any effect), and some untouched birds.
The males with the artificially long tails attracted four times as many females as the artificially short tailed birds. This proves that sexual selection is still propelling the birds towards longer tail lengths, although in practice a maximum has been reached. Longer tails render the male more liable to predation and failure to breed. But Anderson did not check whether the longer tailed birds survived less well than their short tailed rivals as the theory would predict.
So, sexual selection is one way of developing a grossly enlarged feature.
Does the overdeveloped tail of a widow bird remind you of the human brain? In the sense that the human brain is another grossly overdeveloped organ and might have been produced by the effects of sexual selection, perhaps it should.
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