A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can survive bouts of intense selection. Meanwhile, low genetic diversity (see inbreeding and population bottlenecks) can cause reduced fitness (biology)|biological fitness and an increased chance of extinction.
When all individuals in a population are identical with regard to a particular phenotypic trait, the population is said to be ''monomorphic''. When the individuals show several variants of a particular trait they are said to be polymorphism (biology)|polymorphic.
