Sexual selection is a special type of natural selection that is concerned with an organisms ability to successfully reproduce. Survival is no guarantee of passing gene variants to the next generation, that can only happen if the animal reproduces.
Males usually compete to mate with females
According to Trivers (1972), "Where one sex invests considerably more than the other, members of the latter will compete among themselves to mate with members of the former."
In most species, females invest considerably more than males; however, in humans, there is a large variation in the amount that males invest. Some males invest a great deal, while others invest nothing.
Parental Investment and Sexual Selection
Females usually contribute more to the physical development their offspring. In humans a new born baby is hundreds of billions of times heavier than the fertilised ovum from which it develops. All of the additional weight is provided by the mother (Coen, 1999). Male humans provide only the sperm, which is the smallest cell in the human body.
However, to ensure that their progeny survive to reproductive age, humans also provide food, shelter, protection, education etc.
Nevertheless, the more time, effort and resources that parents invest in their children, the less they can invest in further reproduction. Therefore, there is a trade off between parental investment and reproduction; either many children and very little investment or fewer children with greater investment. Females do not really have a choice, however, because of their limited reproductive potential (see box 1). This means that females need to be more choosy in their mate selection than males. Nevertheless, when males adopt a long term mating strategy of forming a relationship and investing in few offspring, they also need to make wise mating decisions if their genes are to survive.
Sexual selection, therefore, often takes the form of female mate choice, whereby characteristics that females find attractive are those that are selected. Nevertheless, sexual selection in humans can also operate through male mate choice - males adopting a long term mating strategy may be choosy because they are limited in the number of offspring that the relationship can produce. Recent research by Finkel and Eastwick (2009) has supported the notion that males are choosier than females in certain social situations.
Evolutionary Psychology and Mate Choice
Genes can affect behaviour
According to evolutionary psychologists, the human brain has evolved "mental modules" to solve adaptive problems, such as avoiding predators, eating the right food and finding mates. These modules have evolved because the genes that produce them increased our ancestors chances of surviving and reproducing.
Mate Selection Modules
Sexual selection may be influenced by mental modules that influence the characteristics that are prefered in a mate; for example, females prefer symmetrical males with waist-hip ratios of 0.9 and immune systems that are different to their own, they also prefer males who have accumulated resources and are kind and generous.
Good Genes v Bad Genes
Good genes are those that increase the probability of survival and reproduction, while bad genes are those that reduce the probability of survival and reproduction. If a person mates with someone with good genes, then their children will have a good chance of surviving and passing on their genes again. If a person mates with someone with harmful genes, their children will be less likely to survive and pass on their genes. Evolutionary Psychologists believe that humans have evolved mental modules that enable us to identify potential mates with good genes and avoid those with harmful genes
How Mental Modules Identify Good Genes
Gangstead and Thornhill (1993) claim that both males and...