Wednesday, March 5, 2008
Evolution of Altruism
Today I learned about the evolution of altruism. Darwin admitted that altruistic behavior seemed to go against the idea of evolution. Recently though people came up with the math to show exactly how, even though it has a negative fitness to the individual who practices it, it can actually have an overall positive fitness effect. Here is the formula: rB > C where C is the negative effect to the individual for practicing altruism measured in # of offspring, B is the positive effect of altruism measured in # of extra grandchildren, and r is the proportion of genetic similarity between the altruistic individual and the individual who benefits directly from the altruism. Here is an example: ground squirrels practice altruism by giving warning cries to alert their fellow squirrels about the approach of predators. The squirrel giving the cries puts itself in danger by being an easier target for predation. However more of the offspring will survive if a squirrel does it. So here is the math, say on average a squirrel who has the gene causing it to give warning cries has a shorter life, and on average produces 2 fewer offspring, however the offspring it does produce has higher rates of survival, say 5 of them survive that would have otherwise been eaten. For a given squirrel since half of the DNA comes from mom and half from dad, the r will be equal to 0.5. doing this math we get 2.5> 2 which is true so you would hypothesize that this altruistic behavior would be genetically selected for. This is the same principle that goes into the evolution of extreme altruism as seen in ant colonies. The purest form of altruism is the worker ant who is completely sterile, it has no chance of having offspring. The reason this evolves though is because ants are so inbred that the value for r is extremely high. Any ant in the ant colony shares about 0.8 of its DNA with any other ant so you will hypothesize that any altruistic behavior will be very beneficial to the fitness of the gene that produced that altruistic behavior. Just for a comparison if you compare the entire human population the likelihood that you share genes with any other individual is about 0.0001, which is why we haven't evolved into having classes of drone workers like the ants have.