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Theory of Sexual Selection

Theory of Sexual Selection - The Human Mind and the Peacock's Tale

by Caspar Hewett

The obvious starting point for any discussion of sexual selection is to note that one of the most common differences between the sexes in sexually reproducing species is that males often have bright colours while their female counterparts do not. The archetype example of this is the peacock’s tail. Generations of biologists and evolutionary theorists have been intrigued by the question; how and why did the gaudy and cumbersome plumage of the peacock evolve? Charles Darwin himself suggested that the answer to this question may lie in female choice in his book The Descent of Man and Selection in Relation to Sex and this is the basic premise of the theory of sexual selection.

For the most part we expect to be able to explain most of the characteristics of an organism as adaptations; that is we assume that they have been ‘designed’ by natural selection – the filter which only allows the genes of successful individuals to reach the next generation. So, when we observe that an organism is well suited to its environment this is the explanation for how its attributes became honed to that particular environment. In the case of animals this argument applies not only to physical attributes, but also to behaviour. This idea was explained first by Darwin in his revolutionary The Origin of Species. However, if this is the case should we not be able to explain the bright colours of peacocks in terms of some survival advantage such as its use as camouflage? Many theorists have attempted to do just this, but the theory of sexual selection suggests an alternative.

First we note that only the genes of those individuals who successfully reproduce can be selected – this depends on both the ability of the individual to survive life’s exigencies for long enough to reach reproductive age and the ability to attract a mate. Thus we would expect that the behaviour and appearance of an animal is adapted not only to helping it survive, but also to helping it acquire the largest number or the highest quality mates. Returning to our example of the peacock and assuming that some time in the past the peacock’s male ancestors were drab colours like their female counterparts. What if, at some point in evolutionary history, there was a tendency for peahens to prefer slightly brighter coloured males? The gaudier males would then be more likely to find mates than their less gaudy rivals which would in turn lead to a tendency for brighter males in the next generation. This would provide a selective advantage for the brightest males which could lead to a runaway process where males would get brighter and brighter with each generation. Even females who bucked the trend would be at a disadvantage for, if they chose to mate with plainer males, they risk producing drab sons who would be less likely to be picked as mates in their turn. Thus selection pushes both sexes into producing and choosing ever-brighter males. Only when the disadvantages in terms of surviving for long enough to reproduce outweigh the advantages in terms of being selected as a mate would this process cease.

This idea of female preference driving the evolution of male gaudiness has been demonstrated to the satisfaction of most. However, it begs the question of why preferences begin in the first place. Echoing Darwin’s earlier argument Ronald Fisher argued in the 1930s that female preference for bright colours was completely arbitrary, and there are those that still hold to this idea, often referred to as Fisherians. There are those who disagree with this theory and argue rather that the peacock’s plumage (or, equivalently, the length of a swallow’s tail, the sweetness of a blackbird’s song) is a signal that the male has high quality genes, and thus that the female’s selection is based on rational criteria (because such behaviour has been selected for – this does not mean that the females are rational or even know that they are choosing). Fisher rather favoured the idea that initially a preference may be based on some indication of health and vigour, but that once the selection process had begun it could run away and result in a preference for something which was unrelated to good genes. Another insight comes from Amotz Zahavi, who suggested that the fact that the tail is a handicap is itself a signal showing how strong and healthy a particular male is; he can survive in spite of his handicap. Regardless of these disagreements, the theory of sexual selection, based on the idea of female choice drives the evolution of particular traits, is accepted my most theorists.

Before we go on to look at the relevance of sexual selection theory to human evolution, we need to address the question; why female as opposed to male choice? In order to explore this, it is worth introducing the term parental investment. Parental investment (PI), an idea first introduced by R. L. Trivers, is defined as any investment by a parent in one of her (his) offspring that increases the chance that the offspring will survive at the expense of that parent’s ability to invest in any other offspring (alive or yet to be born). PI then includes the provision of a wide range of resources such as food, energy and time expended obtaining food and maintaining the home or nest; time spent teaching children and risks taken to protect young. In terms of PI, there is a fundamental asymmetry between the sexes – females have an initial investment in their offspring far greater than that of males because female gametes (eggs) are much more costly to produce than those of males (sperm). This means that a female can have only a limited number of offspring, whereas a male can have a virtually unlimited number, provided that he can find females willing to mate with him. Thus females generally need to be much choosier about who they mate with. The criteria for what constitutes a good choice of male will vary considerably from species to species, but the basic point about female choice remains.

No discussion of peacocks would be complete without some mention of the lek paradox. At mating time peacocks gather in large congregations called leks at which they strut their stuff and show off their wares to peahens. Not all the peacocks get to mate and some get to mate several times. The paradox arises because, if all of the females choose to mate with the same few males – those with the ‘best’ genes, then there will be much less genetic variety in the population in the next generation, and over a number of generations we might expect this to lead to no variety, making it impossible to sustain any choice. Many solutions to this paradox have been suggested, notably that of Hamilton and Zuk, who followed through a particular train of thought regarding sexual reproduction itself. In the 1920s and 30s the ‘modern synthesis’ of the idea of natural selection with the science of genetics (which began with the work of Gregor Mendel) took place. In the 1970s Richard Dawkins popularised one interpretation of the synthesis in his famous book The Selfish Gene. The selfish gene hypothesis is that the gene, as opposed to the individual organism or group of organisms is the level at which natural selection takes place, since only genes are passed into the next generation (replicated). This raised the question; why do so many organisms reproduce sexually rather than asexually when asexual reproduction clearly gets more of an individual’s genes into the next generation than sexual reproduction? This was answered, at least in part by Bill Hamilton, who argues that sex is an important part of the struggle against disease – mixing genes provides greater genetic variety and thus greater chance of resistance to particular diseases. This is where we come back to Hamilton and Zuk’s hypothesis mentioned above. They argued that the tail tells a tale about the health of a male! The state of a peacock’s plumage tells a peahen much about whether or not the male has blood parasites. Thus those males with the best plumage will not necessarily be the descendants of those with the finest tails of the last generation, solving the lek paradox.

Now we come to the question of how sexual selection is relevant to the human mind. Three million years ago our ancestor the upright ape Australopithecus afarensis, also known as Lucy, had a brain size of about 400cc. Modern humans have a brain that is a remarkable 3½ times that size, at 1400cc. This inordinately large brain is very costly to run; the brain consumes 18% of our energy expenditure. From a Darwinian perspective this suggests that there must have been significant and immediate advantages to possessing a larger brain which outweighed the expense.

Thus we come to the question; why did we need to become so intelligent? The answers to this question are many and varied; Was it that accumulated knowledge played a crucial role in enabling humans to develop a rich, varied diet, which in turn required the capacity for language and for a large memory? Or was it that a sexual preference for juvenile features drove us towards prolonged retention of such features (neoteny) which in turn allowed the development of a larger brain as a secondary effect of a longer period of growth?

One idea that has become influential in the last few years is known as the Machiavellian hypothesis, which has its origins in the work of Richard Alexander in the 1970s. He suggested that the main evolutionary pressure for human beings to increase in intelligence was competition with other people, in particular, sexual competition between individuals of the same sex. The argument runs thus; the primary function of most animal communication is to manipulate others, not just to impart information; the ability to deceive and to detect deception in others is highly important for many social animals; thus it is reasonable to assume that this principle underlies the evolution of our highly developed communicative ability. What is more, in order to be really convincing to others in our deception it became necessary to develop the ability for individuals to deceive themselves; Robert Trivers has argued that this may be why we developed a subconscious. In a similar vein Nicholas Humphrey argued that the need to guess the likely actions of other individuals required us to develop the capability to imagine what is in others’ minds; a theory of mind, which was a key factor in the development of self-consciousness.

Matt Ridley does not find any of these explanations satisfactory as they still beg the question why humans, and not other apes, evolved such a large brain. Following Geoffrey Miller he suggests that, in fact, only runaway sexual selection as described by Fisher is sufficient to explain the huge increase in brain size. Being intelligent, witty and entertaining was sexy to our ancestors! This hypothesis requires only that there was an initial preference for more intelligent mates that drove the process from then on. This helps to explain why only humans developed in this way. First, because the initial preference was quite arbitrary and only came about by chance and we can thus assume that an equivalent preference never arose in the other apes. Second, because the human mating system is unique among apes in that it is characterised primarily by monogamous pair bonding (with occasional polygamy) and shared parental effort in child rearing. This leads to a whole set of tendencies untypical of apes such as the preference of males for young or youthful-looking mates and the preference of females for high status, often older, men.

Underlying all of these theories is the hypothesis that, because we have not evolved much in the last hundred thousand years, and because we evolved in a particular environment (sometimes described as the environment of evolutionary adaptedness or EEA which is usually assumed to be the African savannahs) living in a particular way (as social usually monogamous, occasionally polygamous hunter-gatherers) our minds should be adapted to that way of life.

It is worth noting that this is quite different from the approach taken by sociobiology in the 1970s, which attempted to explain modern human behaviour as adaptive. Modern theorists, in particular the proponents of evolutionary psychology (EP), rather attempt to explain the human mind in terms of tendencies which developed in the EEA.

As Matt Ridley puts it in The Red Queen:

There has been no genetic change since we were hunter-gatherers, but deep in the mind of modern man is a simple hunter-gatherer rule: strive to acquire power and use it to lure women who will bear heirs; strive to acquire wealth and use it to buy affairs with other men’s wives who will bear bastards . . . Wealth and power are means to women; women are means to genetic eternity.

Likewise, deep in the mind of modern woman is the same hunter-gatherer calculator, too recently evolved to have changed much: strive to acquire a provider husband who will invest food and care in your children; strive to find a lover who can give those children first-class genes. Only if she is very lucky will they both be the same man . . . Men are to be exploited as providers of parental care, wealth and genes.

However, Ridley tells us that we need not interpret any of these arguments in a deterministic way; none of this denies our free will. To say that a tendency is in our nature is not to say that we cannot overcome that tendency. What is more, the fact that we are so versatile and plastic in terms of behaviour actually depends on us having more instincts, not fewer; the language instinct being a prime example. The vocabulary of language may be infinitely plastic, but our ability to learn languages depends heavily on our hard-wired ability to form generalised rules based on what we hear; to infer the rules of grammar.


C. Badcock., Evolutionary Psychology: A Critical Introduction, Polity Press, Cambridge, 2000
H. Cronin., The Ant and the Peacock, CUP, Cambridge, 1992
C. R. Darwin., On The Origin of Species by Means of Natural Selection, John Murray, London, 1859
C. R. Darwin., The Descent of Man and Selection in Relation to Sex, John Murray, London, 1871
R. Dawkins., The Selfish Gene (new edition), OUP, 1989
R. A. Fisher, The Genetical Theory of Natural Selection, Clarendon Press, Oxford, 1930
G. F. Miller., ‘Sexual Selection for Protean Expressiveness: A New Model of Hominid Encephalization,’ 4th Annual meeting of the Human Behavior and Evolution Society, Albuquerque, New Mexico, July 22-6, 1992
M. Ridley., The Red Queen: Sex and the Evolution of Human Nature, Penguin, 1993 (reissued 2000)
S. Rose and H. Rose (eds.), Alas Poor Darwin, 2000
R. L. Trivers., ‘Parental Investment and Sexual Selection’, Sexual Selection and the Descent of Man, ed. B. Campbell, Aldine-Atherton, Chicago, 1972

Order these books from Amazon
Evolutionary Psychology: A Critical Introduction The Ant and the Peacock The Origin of Species The Descent Of Man The Selfish Gene
The Genetical Theory of Natural Selection Man, Beast and Zombie The Mating Mind The Red Queen: Sex and the Evolution of Human Nature Alas Poor Darwin

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© C J M Hewett, 2003