Charles Darwin's theory of evolution by natural selection suggest that the physical environment exerts selective pressure upon adaptive characteristics, which are traits that increase the survival potential of an individual because they enable them to better adapt to their ecological niche. The adaptive traits are genetically transmitted (inherited) because the individual survives to reproduce and this is known as 'survival of the fittest', where fitness is measured by the number of genes present in the next generation.
However, natural selection cannot account for characteristics that seem disadvantageous, for example, the peacock's tail and the stag's large antlers, both of which would inhibit escape from predators.
Darwin solved these problems in 1871 when he proposed the theory of sexual selection e.g. large antlers increase the chances of survival and reproductive success, as they are better at fighting for the harem.
Intra means within and refers to the competition within a species to attract mates. The sex that invests the least (which in humans is males), will compete over the sex that invests the most (i.e. females they invest more because they only ovulate once a month and once pregnant, can't reproduce successfully again for 9months). Thus, males compete to achieve the dominant position of the alpha male and so have exclusive access to all the females. This is also known as intrasexual selection. But, if females choose the best males, it is known as intersexual selection.
Dimorphism (the difference between the sexes) arises because of intrasexual selection. This is because of intrasexual competition. If males have to compete, they need to be able to