Speciation — the formation of new species — occurs through several mechanisms depending on the geographic context of populations.
Allopatric: A geographic barrier (mountain, river, ocean) physically separates a population. Independent drift and selection diverge the two gene pools. Genetic incompatibilities accumulate; upon secondary contact, hybrids may be unfit (reinforcement).
Sympatric: Speciation without geographic isolation. Ecological niche partitioning creates disruptive selection — intermediate phenotypes are less fit. Assortative mating (like-mates-with-like) evolves, reducing gene flow. Polyploidy in plants is a rapid sympatric mechanism.
Parapatric: Adjacent populations with limited dispersal across a gradient (cline). Partial barriers exist.
Fst → 1 means complete differentiation (full speciation). Watch how different modes drive trait divergence over time.