IB Docs (2) Team
SAQ marking: Evolution
Below you will find three sample SAQs for the question: Explain one study of one evolutionary explanation of one behaviour.
For each of the samples, refer to the rubric to award marks 1 - 9. After each sample, there is a predicted grade as well as feedback on the strengths and limitations of the sample.
SAQ rubricSAQ 1
One of the things that can influence our choice of partner is their MHC - Major Histocompatibility Complex, a group of genes that seem to play an important role in the immune system, and is expressed through body odour. Since the genes for immunity are expressed co-dominantly, it is in the partner's interest to have different versions of the gene to give better immunity to their offsprings.
Wedekind decided to carry out a study and assess if the MHC had an effect on the mating choice of university females. He had 49 females and 44 males take part in this experiment, and at first, assessed each participant for their MHC type and noted if the females were taking oral contraceptives. He then asked the males to sleep in the same shirt for two days and refrain from using scented soap or deodorant, as to preserve their natural body odour on the shirt. He then asked the female students to come to the lab and smell and score 7 different boxes with shirts inside them - three were from males of a different MHC than the female, three were from males of the same MHC as the female, and one was a clean shirt that was used as the control. The females were scoring the intensity and pleasantness of each odour. His results showed that the females preferred the odour of the shirt from a male with a different MHC than they themselves had, but oral contraceptives caused this result to be reversed, implying that the contraceptives change a female's sense of smell and therefore influence her partner choice for the worse.
271 words
The first paragraph is a bit unclear. There is no mention of evolution in setting up the SAQ and no evolutionary argument (natural or sexual selection) is identified. The word “expressed” is incorrectly used. The study is appropriate and described, but not explained in the context of evolution. 4 marks.
SAQ 2
One example of a study of how evolution affects behaviour is Wedekind’s study of MHC and mate selection. Evolution is based on survival of the fittest. Psychologists believe that people choose a mate which will increase the chance of the survival of their offspring. The aim of Wedekind’s study was to examine the role of MHC when females look for a male partner. MHC is a codominant gene is released in sweat. Male participants were given a non-odour shampoo and were told conditions according to which they wore a T-shirt when sleeping. Over the day the T-shirts were kept in an open plastic bag. After multiple nights in the same T-shirt¨ the participants were asked to put the T-shirts into a cardboard box. The female participants were then asked to smell the T-shirts through a smelling hole in the paper box¨ and determine which T-shirt smell they found the most attractive. The results showed that the female participants preferred T-shirt that contained the smell of MHC gene which had the greatest possible difference of sizes of the alleles of the MHC gene in males and females. Since MHC is co-dominant, it is inherited by both parents the difference of sizes in the alleles increases the immunity of the offspring. Since the experiment was double-blind which is a strength of this study¨ the researchers and the female participants did not know which T-shirt belonged to whom. In addition¨ the study was repeated multiple times and always obtained similar results. This study, however, does not fully support the attraction of same-sex attraction as it is only focused on female participants looking for a male partner. The study is reductionist and participants were from a similar culture and had similar ages.
285 words
The SAQ has a good start, with some direct reference to evolution. The study is fairly well described, but there are some errors in the explanation of MHC – e.g. “the size of the alleles.” The response loses focus at the end as it does not link the study back to the question, but instead evaluates it. Evaluation is not part of the SAQ and should be avoided so that the response is focused. 5 marks.
SAQ 3
Evolutionary arguments have been used to explain human mating behaviour. Evolutionary psychologists argue that our behaviours are the result of natural selection – that means that the behaviours that most improve our chances of handing down our genes and producing healthy offspring, these behaviours will continue.
Wedekind (1995) carried out a study to see to what extent MHC alleles play a role in mating behaviour. MHC alleles are responsible for our immune systems. They are inherited from both of our parents – and they are codominant. That means we end up with both immune systems. He argued that our “smell” is based on our MHC and it is best for a woman to choose a mating partner who has a different smell in order to maximize the immune system of her child.
Students were used in the study. The men were asked to wear a t-shirt for two nights. They were also told not to wear any perfume or perfumed soap, to avoid spicy food, smoking and alcohol. They were also told not to have sex. The researchers wanted to make sure that the men would not change their natural smell.
Two days later the women were asked to rank the smell of the t-shirts. They were tested in the second week after the beginning of menstruation when they have a better sense of smell. T-shirts were placed into boxes with a “smelling hole.” 3 boxes contained t-shirts from men with the same MHC as the woman, three were different and one was unworn. Every woman rated the shirts for their “pleasantness.”
The women scored the t-shirts as more pleasant when the MHC was different than their own. This suggests that MHC influences human mate selection. If the woman’s mate has a different MHC allele, then both that allele and her allele will be inherited and expressed by a child, thus increasing its immune system and chances of survival."
316 words
The SAQ addresses the question and there is a clear link of the study back to the question. A good response, but could be even stronger if the link were more developed – e.g. couples with identical MHCs have a higher rate of miscarriage; we see this also in animals, etc. 7 marks.
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