Natural selection is the process by which the forms of organisms in a population that are best adapted to the environment increase in frequency relative to less well adapted forms over a number of generations. Charles Darwin (pictured opposite) was the first to see that this process can explain both evolution and adaptation.

Natural selection produces evolution when the environment changes; it will also produce evolutionary change in a constant environment if a new form arises that survives better than the existing forms of the species. If this process was to continue for the thousands of millions of years since life originated, large evolutionary changes could be accomplished, possibly responsible for the whole diversification of life from a simple common ancestor.

Natural selection can operate in two ways:

• by differences in survival among genotypes;

• by differences in fertility.

There are two theoretical extremes:

• the surviving individuals of all genotypes produce the same number of offspring, and selection operates only on survival;

• individuals of all genotypes have the same survival, but differ in the number of offspring they produce (that is, their fertility).

Natural selection is a mechanism which operates provided certain conditions are met and can take three distinct forms: directional, stabilizing or disruptive.

Although natural selection is central to the theory of evolution, biologists such as Richard Lewontin have reminded us that it cannot explain every feature of an organism.