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’
- 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)
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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.
Again, in humans, this system if physical signals of the ability to provide resources, be healthy and have good genes are not all that reliable
- We are capable of hiding the truth and giving off dishonest signals about our wealth, health and physical appearance – we have our hair coloured, use make up, may take medication and wear good clothes to give a certain impression
- So in humans we have a relatively long period of courtship that enables us to ‘see through’ any fake signals so that we can make a more informed choice
- This is also necessary because of the extended period of care required by our children. The female needs a male who really can provide resources and will stick around. The male needs a caring female who will provide healthy, well cared for children. Both genders need to ensure that they are sharing their genes with someone who can also provide good genetic stock for the children
Other Evidence for the Evolutionary Place of Monogamy or Mild Polygamy in Humans
Examining the relative size of testis:body in mammalian males gives an idea of how promiscuous the females are likely to be
- An animal where females are likely to be mated by more than one male will need to develop large testis and poduce large quantities of sperm per ejaculate in order for the male to ‘drown out’ any other males’ sperm and to be the one to father the child
- Animlas who live in groups where one male controls all the females have a large amount of sexual dimorphism (as the male has to defend the females), but the male needs only small testis and to make a small amount of sperm per ejaculate – as he has no other competition. Gorillas are an example of this
- Animals where males and females have access to each other freely require large testis and produce large amounts of sperm – chimpanzees are examples
- In orang-utans, one male has to fight continually to secure his females and there is always the chance that another male may gain access. Testis size is smaller than in chimps but larger than in gorillas
- In humans, the male testis is similar in proportionate size to that of the orang-utan, but they produce more sperm
- This suggests that humans have evolved for one male to compete to possess a small group of females
- However, other males may still possibly access the females, either willingly or by force – so it is still necessary to ensure that enough sperm are produced to increase the ‘owner’s’ chance of fathering any children – especially seeing as it is he who is going to be providing resources and care for it
