Altruism is “self-sacrificing” behavior

-Lowers the helper’s reproductive success while increasing the reproductive success of the individual being helped

Territories of an Australian songbird

  • Only territory owners breed, most of the other birds do not breed but help the others find food

So how does altruism evolve?

Coefficient of relatedness=r

  • The probability that any two individuals will share a copy of a particular genes
  • in a diploid species any given allele has a 50% change if segregation …..

Direct Selection: acts on traits that promote success in personal reproduction

Direct Fitness: A measure of personal reproduction your own offspring that survive and reproduce

Indirect selection (kin selection): acts on traits that promote success in the reproduction of non descendant relatives

indirect fitness: a measure of the number of relatives that the altruist helps to survive and reproduce

Inclusive Fitness: Direct and indirect fitness a total measure of the genetic success of an individual

Hamilton’s Rule

-Hamilton proposed that an altruistic gene will be favored by natural selection when the indirect fitness gained by altruist is greater then the direct fitness it loses as a result of its self-sacrificing behavior


B- the extra number of relatives that exist thanks to the altruists actions

rb- the coefficient of relatedness between the altruist and the extra individuals

C- the number of offspring not produced by the altruist

rc- the coefficient of relatedness between parent and offspring

Ex: Alarm calling Belding’s ground squirrel sentry


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