Natural selection occurs when individuals differ in heritable traits and those differences are associated with differences in reproductive success.

Three ingredients are sufficient for the basic logic:

1. individuals vary;
2. some of that variation is heritable;
3. variants differ in fitness in the current environment.

If those conditions persist, variants associated with greater reproductive contribution tend to become more common across generations.

Example

Suppose a beetle population contains two heritable color variants. On dark soil, birds detect pale beetles more easily. If dark beetles leave on average 6 surviving offspring while pale beetles leave 3, the dark phenotype has greater relative fitness in that environment.

If color differences are heritable, alleles contributing to dark coloration can increase in frequency over generations.

The birds are not selecting alleles directly. Selection acts through the phenotypes of whole organisms and their consequences for survival and reproduction; allele frequencies change because those phenotypes have a genetic basis.

Natural selection is not random with respect to fitness, although the mutations that provide variation do not arise because they would be useful. Nor does selection guarantee progress toward perfection: environments change, trade-offs exist, and other evolutionary forces can oppose or overwhelm selection.

The compact causal chain is

$$\boxed{\text{heritable variation} \rightarrow \text{fitness differences} \rightarrow \text{frequency change}.}$$