Speciation

Speciation is the evolutionary process by which one ancestral species splits into two or more new species. It occurs when a population diverges enough (often via reproductive isolation) that its members can no longer interbreed with the original group.

• Reproductive isolation is key: something (geography, behavior, genetics) stops gene flow between groups, allowing them to evolve separately into distinct species.
• Speciation can be allopatric (different locations, e.g., separated by a physical barrier) or sympatric (in the same location, via other mechanisms like genetic changes or niche differences).
• Hybrid offspring between related species (if they occur at all) are often inviable or sterile - this is evidence that speciation has occurred (e.g., a mule from a horse and donkey is sterile).

• If a question describes populations separated by a barrier (mountain, river, island) evolving differences such that they can't interbreed, that is allopatric speciation in action.
• Be prepared to identify mechanisms of reproductive isolation - e.g., a question might ask how things like different mating seasons or distinct mating calls can lead to speciation (they prevent interbreeding).
• A classic example often cited is Darwin's finches on the Galápagos: one ancestral species gave rise to many species on different islands (speciation by geographic isolation and adaptive radiation).