Final answer:
Plant species can evolve in sympatry through a chromosomal mechanism known as polyploidy, where gametes with extra chromosomes create a reproductively isolated new species within the same geographical area.
Step-by-step explanation:
Sympatric Speciation in Plants
The chromosomal mechanism by which plant species evolve in sympatry, without the need for a geographical barrier, is primarily through polyploidy. This process occurs when a serious chromosomal error during cell division, such as during meiosis, leads to gametes with extra chromosomes. This can result in the formation of a new species capable of self-reproduction but unable to interbreed with the parent population, thereby achieving reproductive isolation within the same geographical area.
A classic example of sympatric speciation can be found among East African cichlid fish, where differences in coloration, influenced by light penetration in the water, lead to selective mating and reproductive isolation. Although more common in plants, this mechanism has been observed in animal species such as cichlids as well.
Sympatric speciation showcases the remarkable ability of species to diverge and form new species without physical barriers, purely through reproductive isolation mechanisms and genetic changes.