Human population growth fits an exponential pattern

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#Unit 11. Evolution and Behavior

Selfish (cheater) genotypes are expected to increase within populations, a trait that benefits the group would have to evolve by selection among groups, rather than by selection among individual organisms within the groups.

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#Unit 11. Evolution and Behavior

Some cells carry a cheater mutation that makes spores but that avoids contributing to the stalk.

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#Unit 11. Evolution and Behavior

Reciprocity

When one individual provides a fitness benefit to another, as long as the second individual is likely to return the favor later.

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#Unit 11. Evolution and Behavior

Altruism

Altruistic acts serve to benefit the individual’s close relatives, or kin. They enhance the fitness of other individuals—their offspring—at a cost to themselves.

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#Unit 11. Evolution and Behavior

An individual carrying the allele can pass copies of it to his or her own children. This is the allele’s direct fitness. The allele can also pass extra copies of itself to the next generation as the result of the increased fitness of relatives that benefit from the altruistic individual’s actions. This is the allele’s indirect fitness. 

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#Unit 11. Evolution and Behavior

Allele’s inclusive fitness = Direct fitness + Indirect fitness