How Kin Selection Works

Competition is key to Darwin's theory of natural selection. In nature, members of the same species ruthlessly compete over limited resources. Without competition, the genetically weak would have the same chance of survival and reproduction as the strong, and evolution would stall. For evolution to work — for organisms to becoming increasingly more fit over time — there must be winners and losers.

But there's a problem. In nature, there are some species that refuse to play the competition game. Instead of fighting tooth and nail in order to survive and reproduce, these animals dedicate their lives to helping others survive and reproduce. In evolutionary biology, such behavior is called biological altruism.

What do we make of the female worker bee that spends every moment of her short life collecting nectar to feed the hive without ever mating herself? Or the bachelor bird that volunteers to help build nests and protect other birds' hatchlings, but never has his own family?

Altruism in nature seems to go against the primal tenets of natural selection — how do the genes for altruistic behavior get passed from one generation to the next if those altruistic individuals never produce offspring of their own? The so-called "problem of altruism" puzzled scientists for a century after Darwin.

The leading solution, hatched in the 1960s by a little-known graduate student named William Hamilton, is called kin selection [source: Bourke]. Hamilton proposed that altruistic behavior in the natural world wasn't random. A helper-bird doesn't randomly pick a couple of strangers and offer protection to their young. Instead, altruistic behavior in animals is more likely to be expressed toward kin, organisms related to the do-gooder by blood.

By helping a close blood relative, the altruistic organism ensures that at least some of its shared genetic material will be passed on the next generation. According to Hamilton's Rule, the altruistic cost of not reproducing is more than compensated by the increased reproductive success of the extended family [source: Rausher]. The genes for altruism are passed on because they improve the inclusive fitness of the group.

Here's what Hamilton's Rule looks like mathematically [source: Okasha]:

b > c/r

c is the cost incurred by the altruist

b is the benefit enjoyed by others

r is the "co-efficient of relationship," with higher values signaling closer blood ties

Confused? Maybe it will help if we look at some specific examples of altruistic behavior in animals and how they are driven by kin selection.