Stabilizing selection favors average or intermediate traits in a population, reducing variation. This causes the population to stay relatively the same over time. Directional selection favors one extreme trait, causing the population to evolve in one direction. Disruptive selection favors two extreme traits, leading to the emergence of two new species separated by intermediate forms. Geographic isolation occurs when a physical barrier prevents contact between populations, allowing them to evolve independently and potentially speciate. Adaptive radiation is a type of divergent evolution where a species evolves to occupy different niches in an environment.