Natural selection is an evolutionary process in which individuals with survival-promoting traits reproduce at higher rates. These favorable traits become more common within a population or species. Naturally selected traits initially arise via random genetic mutations. In order for selection to occur, there must be variation within a population, the trait controlling the variation must be heritable, and there must be an evolutionary advantage for variation in the trait.
Natural selection is a phenomenon that favors individuals that are better adapted to their environment. Charles Darwin described the process in his 1859 publication, On the Origin of Species:
“Owing to this struggle for life, any variation, however slight and from whatever cause proceeding, if it be in any degree profitable to an individual of any species, in its infinitely complex relations to other organic beings and to external nature, will tend to the preservation of that individual, and will generally be inherited by its offspring. The offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection, in order to mark its relation to man's power of selection”.
It is also worth noting that the phrase “survival of the fittest,” commonly used when referring to natural selection and often misattributed to Darwin, was coined by his contemporary, Herbert Spencer. Darwin later adopted this phrasing in his works.
At the time of the inception of his theory of evolution by natural selection, Darwin did not know that the heritable traits acted upon by selection were genes. This knowledge would come soon after, when Gregor Mendel published his “Experiments in Plant Hybridization” in 1865, introducing the world to genetics.
For natural selection to occur, there needs to be trait variation. Snowshoe hares, for example, are brown in the spring and summer months, but during the winter, their coat changes to white. There also needs to be an evolutionary advantage to the trait; any hares that do not change color and remain brown are more easily visible to predators in the snow and are therefore less likely to survive and reproduce. The third condition is that the trait needs to be heritable. For the snowshoe hares, it is variation at a single location on the Agouti gene that natural selection acts upon.
Interestingly, there is some evidence that the effects of climate change on snow seasons are shifting selection pressures; individuals in some hare populations are reverting back to keeping their brown coat year-round. This process, typically occurring over many generations within a population, is referred to as adaptive evolution.
