In two crater lakes of East Africa there are small fish called cichlids. Species in each lake are most likely descended from a single ancestor. There are no barriers within either lake. The species in each lake are alike in their mitochondrial DNA and unlike the species in neighboring lakes and streams. The lakes are small so the fish must live in sympatry.
Polyploidy, is a change in chromosome number (3n, 4n, etc.). Offspring with altered chromosome number cannot breed with the parent population. This is a common mechanism of speciation in flowering plants. The inheritance of three or more of each type of chromosome is due to improper separation of chromosomes during meiosis or mitosis, and results in rather instantaneous speciation for plants. The possible evolution of wheat is an example of polyploidy.
This animation (No Audio) describes the speciation of wheat.