Day 054 - Dying to save your family

Submitted by Sam on 13 July, 2011 - 21:34

The most extraordinary thing about genes is that they can programme their survival machines to behave in seemingly altruistic ways, even to the point of that machine's self-destruction. A particularly extreme example would be an individual sacrificing itself in order to save the lives of others. Dawkins' selfish gene theory gives an insight into how such an apparently altruistic trait could develop to the gene's advantage by showing that the closer the genetic relationship between two individuals the more sense it makes for them to behave altruistically towards each other, as each will be statistically likely to be carrying a good proportion of the other's genes.

As close relatives have a greater average chance of sharing genes, it could conceivably benefit the spread of a particular individual's genes if it died in order to rescue ten close relatives from drowning, for instance, as it would be likely that whilst one copy of the 'kin-altruism' gene would be lost in the process, many more may be preserved through the apparently selfless act, thereby increasing the 'save close relatives from drowning, even if it kills me' gene's prevalence in the gene pool. It should be cautioned that there is of course no such gene, merely a gene which, when present and expressed in the complex matrix of other genes in a body, induces a greater than usual tendency to save people from drowning than if it were not present or expressed.

Crucial to an understanding of this example is the fact that whilst such a gene may be rare in the wider population, it is likely to be common within a family. There is a 50% chance that your siblings contain the same particular rare gene that you do, as they would have received the gene either from your father or your mother. This means that the 'relatedness' between two siblings is ½, since on average, half of the genes each has will also be possessed by the other. The relatedness between a parent and a child will always be exactly half (barring any mutation), because the process of meiotic division ensures that 50% of a child's genes are received from its father's sperm and 50% are received from its mother's egg. An individual's relatedness to itself is 1, because it can be sure it has 100% of its own genes.

Through this index of relatedness, it can be shown that a gene for suicidally saving five of your cousins (who each have a 1/8th relatedness to you, producing a total relatedness of 5/8ths) will not spread throughout the gene pool, whereas a gene for saving five brothers (each with a ½ relatedness, totalling a 2 ½ relatedness), or ten cousins (1 2/8ths) would spread throughout the population. This is because a suicidal altruistic gene only has to be successful if it can save a minimum total relatedness of more than 1, because this is the total (arbitrary) 'value' of the genes which will be lost through the altruistic suicide. If a gene can, on average, save more than a minimum of two brothers or sisters or parents, more than four uncles, aunts, grandparents (and so on), then it will persist in the bodies of the saved individuals with a frequency great enough to compensate the loss of the altruistic individual.

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