Conditional Altruism

Examples of altruism are found in social insects. Young hornets (Vespa crabro) are altruistically raised by workers who are their sisters.

Patrick Kennedy

A widely accepted solution to the evolutionary conundrum of why some organisms help others at a cost to themselves was put forward over a half century ago. In 1964, renowned evolutionary biologist W. D. Hamilton, then at Imperial College, London, published a mathematical model which showed that altruists benefit from their self-sacrifice through “inclusive fitness;” by helping relatives produce more offspring, they perpetuate shared genes. Now, Hamilton’s rule has been revisited in a new study that tests a hypothesis that cooperation is more likely in harsh, unpredictable environments.

Encapsulated in Hamilton’s rule is the idea that altruism evolves, provided the benefit to the recipient—multiplied by relatedness between donor and recipient—is greater than the cost to the donor. Patrick Kennedy, a doctoral student at the University of Bristol, UK, supervised by Seirian Sumner, evolutionary geneticist at University College London, and two other UK-based colleagues, unpacked the costs and benefits in Hamilton’s rule to examine the case of organisms living in highly fluctuating environments. They used life history data from Galápagos mockingbirds and Australian carpenter bees.

Traditionally, scientists have examined altruism’s benefits and costs by looking at the average number of offspring produced. Because of helping, “the altruist has fewer offspring, on average...and the beneficiary has more offspring, on average,” Kennedy says. His team’s model went beyond incorporating average reproductive success; it included variance—the degree of spread around the average—in reproductive success. Counterintuitively, the model found that “costly altruism can evolve even when it produces a net negative effect on the average reproductive success of related recipients.” A benefit can be derived when an act of altruism makes the number of offspring produced per breeding effort more consistent. Altruists provide what Kennedy calls “the gift of certainty”—a buffer in a volatile world.

Now that mathematical models have shown that altruism can evolve when it buffers environmental volatility, Kennedy is hopeful that their model can be validated in the field. He suggests, “One fruitful avenue for further investigation are bacteria, with their high relatedness and ability to live in fluctuating environments.” (Nature

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