Which scenario best illustrates sympatric speciation?

• A. A polyploid plant arises within a population in the same geographic area.
• B. A population splits due to a river forming a barrier.
• C. Two populations diverge due to habitat isolation across mountains.
• D. Different species occupy different habitats but still interbreed.

Answer: (A)

Sympatric speciation happens when a new species arises in the same geographic area, usually through mechanisms that create reproductive isolation without any physical barrier. The strongest example is a polyploid plant that develops in the population in the same area. Doubling the chromosome number produces individuals with a different chromosome set that cannot produce viable offspring with the original diploid plants, so a distinct lineage forms right where the population lives, without moving to a new location.

The other scenarios rely on physical separation: a river forming a barrier splits the population, and mountains separating groups create geographic isolation, both typical of allopatric speciation. The last scenario describes different species occupying different habitats but still interbreeding, which means gene flow can continue and does not demonstrate the complete reproductive isolation needed for a new species to emerge in the same place.