A curious feature of evolution by natural selection is that it depends crucially on events that “almost never” happen. For instance, speciation, the process in which a new species is generated by wandering away from its parent species, is an exceedingly rare event, but each of the millions of species that have existed on this planet got its start with an event of speciation. Every birth in every lineage is a potential speciation event, but speciation almost never happens, not once in a million births. Mutation in DNA almost never happens—not once in a trillion copyings—but evolution depends on it. Moreover, the vast majority of mutations are either deleterious or neutral; a fortuitously “good” mutation almost never happens. But evolution depends on those almost rarest of rare events.
Philosopher Daniel Dennett's Book Intuition Pumps