What is the term for speciation that occurs without geographic isolation?

The term for speciation that occurs without geographic isolation is sympatric speciation. In this process, populations remain in the same broad region while ecological preferences, behavior, mating choices, chromosome changes, or other genetic differences reduce gene flow between them.

One important route occurs in plants through polyploidy, when chromosome duplication creates individuals that can no longer produce fertile offspring with the original population. Because the new forms arise in the same environment, this is a clear example of sympatric speciation. Host-specific insects and fish with different feeding or mating preferences provide other possible cases.

Sympatric speciation is contrasted with allopatric speciation, which begins after a physical barrier separates populations. Parapatric speciation involves neighboring populations with limited contact, while peripatric speciation starts in a small isolated population at the edge of a species’ range. Biologists once debated whether sympatric speciation was possible, because ongoing gene flow tends to blend populations; current research supports it under strong ecological or reproductive selection, though its frequency remains uncertain.