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.

Evolution and common ancestry

Populations can change over many generations as inherited characteristics become more or less common. This change is called evolution. It occurs in populations over generations; an individual organism does not evolve during its lifetime.

Natural selection can contribute to evolution. Individuals in a population have inherited differences. If a characteristic helps individuals survive and reproduce in their environment, they may pass it on to more offspring. Over generations, that characteristic can become more common in the population.

Evolution and shared ancestry

Populations descended from the same ancestral population can gradually become different from one another. This divergence can eventually give rise to different species. The formation of different species contributes to biodiversity, the variety of living organisms.

Living organisms are related through common ancestry: their ancestral lineages connect at some point in the past. Organisms that share a more recent common ancestor are more closely related than organisms whose most recent common ancestor lived further in the past.

Speciation

Speciation is the formation of new species as populations become increasingly different over generations. If populations of the same species experience different conditions, natural selection can favour different inherited characteristics in each population. Over generations, their phenotypes—their observable characteristics—can become more different.

Reproductive isolation and new species

Populations exchange genes when individuals from them interbreed and produce offspring. This exchange is called gene flow. When populations become reproductively isolated, gene flow between them stops, so inherited differences can accumulate independently.

If the populations diverge until they can no longer interbreed to produce fertile offspring, they are regarded as separate species. Fertile offspring are able to reproduce themselves.

Speciation and the diversity of life

Divergence and speciation have occurred repeatedly over the history of life, producing the diversity of species seen today. All present-day species are descended from simple life forms that first developed more than three billion years ago.