Evolution means "A change in the gene frequencies in a population over
time."

Evolution happens through many different processes, including mutation, gene
flow, genetic drift, and natural selection.  The rate varies depending on
the difference forces, the generation length of the population, the size of
the population, etc.

It can happen instantaneously, as when you have a new mutation occur in a
baby who is born to the population, or when a group of new immigrants joins
the population.

It can happen over just a few decades, such as the evolution of domesticated
strawberries from their wild forebears, or the development of a new breed of
dogs, or cows that produce 10X as much milk as their own offspring need.

It can happen in just a few days, such as the administration of antibiotics
that kill 99.999% of the bacteria causing an infection, but leave the other
.001% of the population that is naturally resistant to repopulate the host
with a resistant infection.

Other evolutionary changes take millions of years, such as the evolution of
modern human brain size (1350 cc average) from the 450 cc average of the
Australopithecines 4 million years ago.

Kathy Dettwyler, anthropologist



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