Evolution

Evolution (also known as biological, genetic or organic evolution) is the change in the inherited traits of a population of organisms through successive generations.^[1] This change results from interactions between processes that introduce variation
into a population, and other processes that remove it. As a result,
variants with particular traits become more, or less, common. A trait is
a particular characteristic—anatomical, biochemical or behavioural—that is the result of gene–environment interaction.

The main source of variation is mutation, which introduces genetic changes. These changes are heritable (can be passed on through reproduction), and may give rise to alternative traits in organisms. Another source of variation is genetic recombination,
which shuffles the genes into new combinations which can result in
organisms exhibiting different traits. Under certain circumstances,
variation can also be increased by the transfer of genes between species,^[2][3] and by the extremely rare, but significant, wholesale incorporation of genomes through endosymbiosis.^[4][5]

Two main processes cause variants to become more common or rarer in a population. One is natural selection,
through which traits that aid survival and reproduction become more
common, while traits that hinder survival and reproduction become rarer.
Natural selection occurs because only a small proportion of individuals
in each generation will survive and reproduce, since resources are
limited and organisms produce many more offspring than their environment
can support. Over many generations, heritable variation in traits is
filtered by natural selection and the beneficial changes are
successively retained through differential survival and reproduction.
This iterative process adjusts traits so they become better suited to an organism's environment: these adjustments are called adaptations.^[6]

However, not all change is adaptive. Another cause of evolution is genetic drift,
which leads to random changes in how common traits are in a population.
Genetic drift is most important when traits do not strongly influence
survival—particularly so in small populations, in which chance plays a
disproportionate role in the frequency of traits passed on to the next
generation.^[7][8] Genetic drift is important in the neutral theory of molecular evolution, and plays a role in the molecular clocks that are used in phylogenetic studies.

A key process in evolution is speciation, in which a single ancestral species splits
and diversifies into multiple new species. There are several modes
through which this occurs. Ultimately, all living (and extinct) species
are descended from a common ancestor via a long series of speciation
events. These events stretch back in a diverse "tree of life" which has grown over the 3.5 billion years during which life has existed on Earth.^{[9][10][11][12]} This is visible in anatomical, genetic and other similarities between groups of organisms, geographical distribution of related species, the fossil record and the recorded genetic changes in living organisms over many generations.