Question: where does all the natural variation necessary for natural selection come from

title: Natural selection
text: The concept of fitness is central to natural selection . In broad terms , individuals that are more `` fit '' have better potential for survival , as in the well - known phrase `` survival of the fittest '' , but the precise meaning of the term is much more subtle . Modern evolutionary theory defines fitness not by how long an organism lives , but by how successful it is at reproducing . If an organism lives half as long as others of its species , but has twice as many offspring surviving to adulthood , its genes become more common in the adult population of the next generation . Though natural selection acts on individuals , the effects of chance mean that fitness can only really be defined `` on average '' for the individuals within a population . The fitness of a particular genotype corresponds to the average effect on all individuals with that genotype .