When the term variation is discussed in evolutionary biology, it can take two meanings.

• Firstly it can refer to the characters of an organism which can vary among individuals at both the phenotypic and the genetic levels. The image, opposite for instance, shows the enormous variation in coloring of the Hawaiian Happy-face spider Theridion grallator .

• Secondly, biologists talk about the variation in the fitness of individuals. For natural selection to take place, there must not only be variation in characteristics: the variation must be for fitness.

It is another condition of natural selection that the characteristics and hence the fitness of organisms is heritable. Not all characteristics of organisms are inherited and natural selection will not adjust the frequency of non-inherited characters. But many are inherited, on these natural selection can potentially work. Since inheritance is produced through genes via the Mendelian process, there must be sufficient genetic variation for natural selection to operate.

How much do natural populations show variation?