Genetic Variation: Evolution begins with genetic variation within a population. Mutations, genetic recombination, and other factors introduce new traits, and natural selection acts on these traits. Over time, some individuals are better suited to their environment than others, and their traits are more likely to be passed down.

Isolation: For speciation to occur, populations of the same species must become isolated from each other. This isolation can be geographic $($e$.$g$.,$ a river forms between populations$),$ ecological $($e$.$g$.,$ different niches or habitats$),$ or behavioral $($e$.$g$.,$ different mating rituals$).$ This separation prevents gene flow between the populations.

Differentiation: Once isolated, the populations begin to evolve independently. The genetic differences between them accumulate over generations due to mutations, genetic drift, and natural selection. These differences may be in traits like morphology, behavior, or physiology, making each population adapt to its specific environment.

Reproductive Isolation: Over time, the genetic changes may lead to reproductive isolation, where individuals from the two populations can no longer interbreed successfully. This can happen through mechanisms such as different mating seasons, incompatible mating behaviors, or even genetic incompatibilities that prevent viable offspring.

Speciation: Once reproductive isolation is established, the two populations are considered distinct species. They are no longer able to produce fertile offspring together, even if they come into contact again, thus completing the process of speciation.