Natural selection

Individuals in a population can have different inherited characteristics. If a characteristic helps an individual survive or reproduce in its environment, that individual may leave more offspring than others. Over generations, the inherited characteristic can become more common in the population. This process is called natural selection.

A phenotype is an individual's observable characteristics. Some differences in phenotype are heritable: they are influenced by alleles that can be passed from parents to offspring. Natural selection acts when individuals with different heritable phenotypes differ in their survival or reproductive success.

Selection pressures

A selection pressure is a feature of the environment that affects survival or reproduction. Predation, disease and competition can all be selection pressures. For example, if predators more easily spot some individuals than others, individuals with inherited colouring that makes them harder to see may be more likely to survive and reproduce.

An advantageous phenotype is one that improves survival or reproductive success under the prevailing conditions. If conditions change, the same phenotype may no longer provide an advantage.

Natural selection and evolution

Individuals with an advantageous phenotype are more likely to reproduce and pass the alleles contributing to that phenotype to their offspring. Not every individual with the advantage will reproduce, but if they leave more offspring on average, those alleles can become more common over generations.

The gene pool is the collection of alleles in a population. Evolution is a change in the frequencies of alleles in a population over time. Natural selection is one process that can produce this change: individuals are selected through differences in survival and reproduction, while allele frequencies change in the population across generations.

Common descent

The earliest simple life forms developed more than three billion years ago. All living species have evolved from these early forms through descent with inherited change: organisms pass characteristics to their offspring, while inherited characteristics can change across many generations.

A branching history of life

An ancestral population can give rise to different evolutionary lineages. As these lineages change over many generations, they can become distinct species. Each lineage can also branch again, producing a branching history of life rather than a single straight line.

Species that share an ancestor have inherited characteristics from that ancestral lineage. These shared characteristics provide a connection between the species, even though each lineage may have changed since it separated from the others.

Present-day species are descendants of earlier organisms, not necessarily unchanged copies of them. Each living species belongs at the end of a lineage that has continued to change over many generations.

Speciation

Speciation is the formation of new species from an existing species. It can happen when populations of the original species become separated and gradually develop inherited differences.

When separated populations exchange few or no alleles—different versions of genes—changes in one population are unlikely to be shared with the other. Over many generations, inherited differences can accumulate. If the populations live in different environments, natural selection may favour different inherited characteristics in each population. Their phenotypes, or observable characteristics, may therefore become increasingly different.

If the populations change so much that they can no longer interbreed to produce fertile offspring, they are regarded as distinct species. Two populations of an original species can therefore form two new species. A difference in appearance alone does not establish that speciation has occurred: the populations must no longer be able to produce fertile offspring together.