Natural selection is the process by which organisms better adapted to their environment tend to survive and produce more offspring. Over generations, heritable traits that improve survival or reproduction become more common in a population.
The process requires variation among individuals, inheritance of at least some of that variation, and differences in reproductive success. The environment does not consciously choose organisms; conditions simply make some traits more advantageous than others. “Fitness” therefore means reproductive success, not necessarily strength, size, or longevity.
Charles Darwin and Alfred Russel Wallace jointly presented the idea in 1858. Darwin developed it extensively in On the Origin of Species, published in 1859. Modern evolutionary biology combines natural selection with genetics, explaining how mutations and recombination supply variation while selection changes the frequency of inherited traits.
Natural selection differs from artificial selection, in which humans deliberately breed organisms for preferred traits. It also differs from genetic drift, in which chance—not adaptation—changes trait frequencies. Antibiotic resistance is a modern example: resistant bacteria are more likely to survive treatment and reproduce.