How can natural selection lead to the formation of new species?

Natural selection leads to the formation of new species—a process known as speciation—primarily when populations of a single species become isolated and evolve independently.12

When populations are separated, such as by geographic barriers (allopatry), they may face unique environmental conditions. As natural selection favors individuals with traits best suited to their specific surroundings, these groups accumulate distinct genetic and physical differences over many generations.123

Eventually, these differences can become so significant that the populations are no longer able to interbreed, even if they come into contact again. This reproductive isolation may arise as an accidental by-product of genetic divergence or be directly promoted by natural selection, which favors the development of mechanisms that prevent the production of less-fit hybrid offspring.124

In some cases, such as adaptive radiation, a single ancestral lineage may rapidly diversify into multiple species as populations adapt to exploit various unoccupied ecological niches, a phenomenon famously observed by Charles Darwin in the Galapagos finches.52