Sympatric speciation occurs when populations of the same species become reproductively isolated from each other due to breeding preferences. This most often occurs in plants that exhibit polyploidy, a disorder in which chromosomes are doubled in offspring. These mutated offspring are only able to reproduce with others in the species that have the same chromosome count, making them reproductively independent.
Apple maggot flies are examples of sympatric speciation. These flies originally only reproduced on hawthorns, a species of tree native to North America, before Europeans brought their varieties of apple trees to the New World. Certain flies bred only with flies who preferred the same tree, which isolated one population over another. Eventually, the apple maggot flies either preferred hawthorn trees or domestic apple trees for their breeding ground. Two hundred years after Europeans brought apple trees to North America, apple maggot flies differed genetically based upon the type of tree they prefer.
Scientists believe sympatric speciation is rare. This type of evolution occurs due to behavioral choices in animals, not physical barriers. Preferences for mating include mating-call patterns, nocturnal versus diurnal behavior, feeding choices and habitat preferences. This type of evolutionary process differs from allopatric speciation, wherein a species develops divergent characteristics based upon a physical separation that splits populations of the same species.