Evolution by natural selection
Evolution can be defined as the change in the frequency of a phenotype in a population over many generations.
Variation
Although members of a speciesA type of organism that is the basic unit of classification. Individuals of different species are not able to interbreed successfully. have many features in common they are not identical to each other. Variation occurs as a result of mutations that create new alleles of genes.
Mutations can be inherited if they occur in DNA that is passed on to the next generation. In populations that produce offspring by sexual reproduction, variation also occurs as a result of individuals receiving different combinations of alleles from their parents.
New alleles produced by mutations can result in plants and animals becoming better adapted to their environment. Variation within a population makes it possible for a population to evolve over time in response to changing environmental conditions.
Natural selection
Individuals produce more offspring than their environment can support, and some die because of factors such as predation, food shortage or disease. These factors are known as environmental selection pressureA factor such as predation or disease that affect a population resulting in the death of some individuals and the survival of others. and they determine which individuals will do best at surviving and reproducing.
Some individuals are better adapted to their environment than others as a result of characteristics they have inherited from their parents. The genes that produce these characteristics give them a selective advantageA favourable characteristic which gives an organism an increased chance of survival.. They may have a better ability to avoid predators, compete for food, or resist disease.
These individuals are therefore better able to survive and reproduce than less well-adapted individuals. They pass on the versions of genes that produce their adaptationA feature of an organism's body which helps it to survive. and the frequency of these alleles will increase within the population. Predation, competition and disease are examples of selection pressures.
