How can reproductive barriers form sympatric populations while their members remain in the same geographic area?

Polyploidy

Explanation:

Polyploidy is defined as the phenomenon by which cells, tissues or organisms with three or more complete sets of chromosomes originate. Such cells, tissues or organisms are called polyploids.

If the genomes of a polyploid species come from the same ancestral species, it is said to be autopolyploid or autoploid. And if they come from two different ancestral species, it is said to be allopolyploid or alloploid. Depending on the number of complete chromosome sets that the species has it is called triploid (3X), tetraploid (4X), pentaploid (5X), hexaploid (6X) and so on, where X is the monoploid number, and not to be confused with the haploid number.

Redundant genetic elements of polyploid organisms can be transformed into single copies by sub-functionalization, neofunctionalization mechanisms or by the appearance of certain mutations.

Two populations are sympatric when they exist in the same area and thus frequently encounter one another. In general, different species cannot interbreed and produce healthy and fertile offspring due to barriers known as reproductive isolation mechanisms.  Polyploidy speciation is common in plants but rare in animals. So, animal species are much less tolerant of changes in ploidy. For example, human embryos that are triploid or tetraploid are not viable, cannot survive.