sympatic speciation, because it doesn't require a geographic barrier, can occur in a number of ways, some of which include:
polyploidy-a mutation during meiosis in which gamete cells receive either an extra chromsome (2n + 1) or fewer chromosomes (2n - 1); if either of these mutated gametes fertilizes an egg, you will have a zygote/embryo that has either extra chromosomes or fewer chromosomes as well. This zygote can actually end up being fertile and only mating with fellow mutated zygotes. eventually these mutated species will only mate with each other and eventually separate the gene pool, giving rise to a new species
habitat isolation- some organisms might choose a particular habitat within the same geographic area (i.e. they prefer trees over low grasses) and thus they separate themselves and eventually give rise to a new species over time
hope that helps
