NATURAL SELECTION 24
Plato: hierarchy of being, species are unchanging
Darwin and Wallace: a population consists of individuals of the same species that are living in the same
area at the same time. Evolution occurs because traits vary among individuals in a population, and
because individuals with certain traits leave more offspring than others do.
Species change through time and they are related by a common ancestor. Extinction and transitional forms
provide evidence that species change. Geographic relationships and homology provide evidence that all
species share a common ancestor.
Evolution by natural selection occurs when heritable variation leads to differential success in survival and
An adaptation is a heritable trait that increases the fitness of an individual in a particular environment
relative to individuals lacking the trait. Adaptations increase fitness the ability to produce offspring.
Individuals themselves do not evolve. Natural selection acts on individuals, because individuals experience
differential success. But only populations evolve. Allele frequencies change in populations, not individuals.
EVOLUTIONARY PROCESSES 25
1) Sexual selection is a form of natural selection which arises from differences in mating success.
2) Intrasexual selection takes place between members of the same sex.
3) Intersexual selection takes place between different sexes.
4) There are numerous possible causes of and balances to sexual selection.
1. Mutation: modifies allele frequencies by continually introducing new alleles. The alleles created by
mutation may be beneficial or detrimental or have no effect on fi