Love is in the air

Evolution is all about being selfish. By natural selection, genes that increase an individual’s chance of survival or fitness accumulate in the population, resulting in individuals that appear to behave for their own selfish interests. Yet humans are inherently social, so making sense of random acts of kindness is something that has puzzled evolutionary biologists for many years.

W_D_HamiltonBiologist W. D. Hamilton realised that while the best way to pass on your genes is through personal reproduction, social reproduction is also important. We all share our genes with our family, so by helping them increase their fitness we are indirectly increasing our own. Following Hamilton’s work, biologists recognised that natural selection depends on your level of relatedness to others. The numerous cooperative acts seen throughout the animal world can be understood if individuals are related. However, this still fails to explain the social interactions encountered on a daily basis between unrelated strangers. Why do we give up our seat on the tube? Why do we donate clothing and food to benefit people we may never meet? To understand these peculiarly altruistic human habits we have to consider that it is not only family that we may share our genes with. To a certain extent we share our genetic makeup with all members of the population. All that is required is a high enough relatedness between two individuals.

Today with the latest gene sequencing technology we are able to measure the level of relatedness accurately. But are we as human capable of scrutinising relatedness and adjusting our behaviour accordingly? It seems impossible that we are able to unravel another’s genetic makeup.

kibbutz_2_shutterstock_242816407However, recent studies have shown that humans are capable of using both genetic and environmental signals to discriminate in favour of those who we are more related to. Using environmental signals is a very simple way to distinguish relatives. Those that share environmental features like dialect are more likely to originate from the same area and share a similar gene pool. This is especially important for avoiding inbreeding. In a number of cultures in which children spend a long period of childhood living together it has been observed that humans unconsciously avoid reproducing with genetically similar individuals. For example, in Kibbutz in Israel, families often share a limited gene pool due to little migration. Children growing up together in the kibbutz were found to be less sexually attracted to one another. It suggests we instinctively try to maximise our reproductive success, preventing the deleterious consequences of inbreeding. This active seeking of genetically diverse partners has also been observed in China. In some regions it is traditional for child brides to live with their husband from a very young age. These couples are generally more genetically similar than couples that meet later life and were born in different villages, and as a result the birth rate was lower. Sexual satisfaction decreased the longer the couple had grown up together.

body odour Elias SchewelThe most interesting of cues studied in great detail is smell. In a study of students at Edinburgh, odour was found to be an important and reliable indicator of an individual’s genes. Male students were given t-shirts and asked to wear them for 48 hours continually. The t-shirts were then collected and women asked to smell them judging the best smell and which t-shirt they would go on a date with. Incredibly researchers found that the shirts judged best by each women corresponded to a male with the greatest difference in the Major Histocompatibility Complex (MHC) gene to themselves. MHC has an important role in determining your immunity. By selecting males with the most dissimilar form of the MHC gene, women are ensuring that their offspring have strong immunity. Smells really can determine sexiness.

Evolutionary biologists are increasingly finding further evidence of genetic and environmental cues of relatedness. We appear to be able to unconsciously measure the extent to which we share our genes with other individuals, which influences how we behave and even partner choice. In some cases it seems genes are more important than personality in determining who we love.

This is an online-only feature from Issue 30, Spring 2015, themed on ‘Who are we’

Images: WD Hamilton (Wikimedia Commons); Children from Kibbutz gathering almonds (1936) (Shutterstock); Body odor is cool by Elias Schewel (Flickr Creative Commons)

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