Charles Darwin (1809-1882) – within each animal species, including humans, there is variation, making people not identical. 
Part of the variation comes from differences in individuals genes, 50% inherited from each parent, but genes (strands of DNA) can also undergo mutation, a random change affecting an individual’s physiology and behaviour, which sometimes gives individual an advantage when competing for resources such as food, territories and mates. Such individuals stand more chance of surviving into adulthood and reproducing offspring who will also have the mutated gene, with the characteristic determined by the mutation becoming more widespread over time. This evolutionary process of gradual changes to our genetic makeup is known as natural selection. One important way in which evolution works to shape human reproductive behaviour is that of sexual selection.
Evolution – the process of adaptation through natural selection
Human reproductive behaviour – the different mating strategies used by males and females
Sexual selection – the selection of characteristics increasing reproductive success
Socio-biological explanation – a theory of relationships based on biological determinants
Anisogamy
Anisogamy means two sex cells (or gametes) that are different coming together to reproduce. Men have sperm cells, which are able to reproduce quickly with little energy expenditure and once they start being produced they do not usually stop until the man dies. Female gametes (eggs or ova) are, in contrast, much less plentiful; they are released in a limited time frame (between puberty and menopause) and require much more energy to produce. This difference (anisogamy) means that men and women use different strategies when choosing their partners.
Females lose more resources than men if they choose a sub-standard partner, so are pickier about who they select. They are more likely to pick a partner who is genetically fit and willing to offer the maximum resources to raise their offspring (a man who will remain by her side as the child grows to protect them both and potentially provide more children).If they have made a good choice, then their offspring will inherit the positive features of their father and are therefore also more likely to be chosen by women or men in the next generation.
Intra-sexual Selection
Whilst females prefer quality over quantity, anisogamy suggests that men’s best evolutionary strategy is to
have as many partners as possible. To succeed, men must compete with other males to present themselves as the most attractive mate, encouraging features such as muscles which indicate to the opposite sex an ability to protect both them and their offspring.
The relationship between sexual selection and human reproductive behaviour
Males produce lots of small, highly mobile sperm, about 110 million sperm per ejaculation- around enough to populate Britain twice over – and males can fertilise many females at little cost to reproductive potential. Before the advent of DNA testing, they could not be sure of paternity (that the child is theirs), so natural selection favours male behaviours maximising the number of potential pregnancies, resulting in Intrasexual competition between males (where males compete for mating opportunities with females) and polygamy, where one male mates with more than one female. Therefore, a male’s best strategy to heighten his chance of reproducing genes into the next generation is to have as much sex as possible with as many females as possible. Various male strategies have arisen, for instance seeking females displaying signs of fertility, such as health, youth and childbearing hips, as mating with fertile females enhances the chances of successful reproduction.
Females produce a few, relatively large eggs, each one representing a sizeable reproductive investment, though she is always sure of maternity. Females are fertile for about 25 years- ovulating one egg a month they therefore have only 300 opportunities to reproduce. Males can, in theory, reproduce as many as three times a day and remain fertile for longer – the oldest documented father was Nanu Ram Jogi, an Indian father who fathered a child at the age of 90. The oldest documented mother conceiving without hormone treatment was Dawn Brooke of Guernsey, who
gave birth in 1997 ages 59- her pregnancy was so unexpected it was believed to be cancer. Therefore females must be more selective about who they mate with, as each mating involves a sizeable part of reproduction potential compared with that of males.
Attractiveness
Physical attractiveness in females is valued by males as an indicator of health and fertility, two of the qualities needed to produce and raise children. Young women are seen as more attractive, as they tend to be more fertile. Females are more attracted to men, often older, who have access to resources, as this indicates an ability to provide for a female and her children. Although physical attractiveness is less important, females are choosier in selecting mates, as their investment is greater. Females are also attracted to kindness in males, as it indicates a willingness to share resources.
Body symmetry and waist to hip ratio
Body symmetry and waist to hip ratio are forms of physical attractiveness that indicate genetic fitness, with males and females possessing near- perfect body symmetry having 2-3 times as many sexual partners as those with asymmetrical bodies. Facial symmetry is especially seen as attractive, as it is regarded as the best predictor of body symmetry. Symmetry is particularly attractive in males, as symmetry requires genetic precision and only males with good genetic quality can produce it. Generally symmetry itself is not directly attractive, but other characteristics related to body symmetry like being more dominant or having high self-esteem, are.
Waist to hip ratio is an important aspect of female attractiveness, as females with a larger waist to hip ratio are associated with greater reproductive ability – they have ‘child-bearing hips’. A small waist also suggests a woman is not carrying another man’s child.