Darwin noticed that there were certain features that he could not explain by Natural Selection Such as differences in size, body form, colouration and behaviour between males and female of the same species – why are females often smaller and less colourful than males? Infact, in some species, the male seems at a real disadvantage in that he would show up to predators by being brightly coloured, he might have huge antlers that mean he can hardly lift his head, or he may have a huge tail that would get caught in the undergrowth
Why would it make evolutionary sense to have such features that are only going to get you eaten??? Differences in the appearance etc. between males and females of the same species are referred to as SEXUAL DIMORPHISM Humans are sexually dimorphic to a certain extent with females being generally smaller, less muscular and without features such as facial hair Sexual Selection (as opposed to Natural Selection) was Darwin’s solution to this problem: He suggested that the differences must be due to an advantage they give in passing genes into the next generation
Females are ‘in demand’, so sexual selection will mostly operate on the males who have to work to get access to the females The features will either help the males to compete with other males and ‘fight them off’ for access to the females, or they will make them sexually desirable to the females, so they get access to more mates INTRAsexual selection is when features have been selected by how well they helped to fight of the other males (body size, muscularity, fighting ability, aggression etc.) INTERsexual selection is when features have been selected by how well they made bearers attractive to the opposite sex (peacock’s tail, fidelity? Kindness/nurturing? Body shape?)
Intra – within own sex Inter – outter apperance The human situation is slightly different to that of the other animals due to the need for extended care of children. Males need to stay around to ensure a child is to survive, and so he needs to be more discriminating in his choice of females – so both male and female humans undergo intrasexual selection (competing with members of their own gender for access to mates)
Intersexual Selection – the problem Darwin struggled to explain why females found certain features so attractive that thye always ‘went for’ males carrying them – even though the feature could be seen as a ‘handicap’ to the male (peacock’s tail) This has been explained by two ‘schools of thought’ 1. Good Sense School This idea suggests that the preference starts of as an indicator of something of evolutionary significance to the female
The male with the best feathers has the best food (can provide resources), has not been beaten up in fights (can protect) and does not have parasites (is healthy). The fact that he is capable of all this also suggests that he has good genes, so will be a good choice to share her genes with Good Taste School (Fisher, 1930’s) Fisher suggested that the preference may have started with ‘good taste’ – where the female was choosing the best mate and best genes However, this then underwent a ‘runaway process’, where the original reason for admiring the feature was ‘forgotten’ and the features became a ‘fashion accessory’
Females blindly chose males with more and more exaggerated versions of the feature, until it ceased to indicate it’s original meaning For the male, the biological expense of producing and maintaining the feature becomes such a disadvantage this it outweighs the advantages of attracting more mates
However, the feature is still necessary to attract females (who will want to pass it onto THEIR sons, in order to get genes into the NEXT generation) and so will continue It has further been suggested that females look for slight handicaps in males as a sign of ‘strength’ – if a male can survive even with this huge tail, then he may have really excellent capabilities in all other aspects.