Why does it take many many generations for evolution by natural selection to occur?

Answer 1

Basically because populations evolve and individuals are selected. Let us assume that a beneficial mutation occurs in a individual organism. Even if beneficial in the immediate environment it must be in the germ line and passed on to progeny who mate with other organisms whose genes must work in concert with the new mutation that then needs to spread far enough into the populations gene pool so that there is a change in allele frequency that leads to, perhaps, some morphological or behavioral change. This can take uncounted generations, or can be a bit quicker. But, never in one jump.

Answer 2

Evolution by natural selection involves gradual changes in the frequency of traits in a population over generations. This process takes time because it relies on the accumulation of beneficial genetic variations through random mutations and the differential survival and reproduction of individuals with those variations. It often requires many generations for these changes to become widespread in a population.