Definition
Individual selection is natural selection operating on individual organisms: traits that raise an individual's own reproductive success become more common because their bearers leave more surviving offspring. It is the baseline mechanism of evolution and, for most of biology, the level at which selection is assumed to act.
Sapolsky uses individual selection as the foundation for explaining social behavior. Whenever an animal helps, competes, or mates, the first evolutionary question is how the behavior served the actor's own reproductive interest.
Why it matters
How it works
In a population with heritable variation, individuals whose traits yield more surviving, reproducing offspring pass those traits on at higher rates. Over generations the population shifts toward those traits. Because the unit that survives or fails to reproduce is the individual, selection most directly tracks individual reproductive success.
This is why claims that an animal acts "for the species" are treated with suspicion: a self-sacrificing trait is usually outcompeted by a selfish variant within the same population. Apparent altruism therefore demands an explanation grounded in individual or genetic advantage.