Evidence exists that during the millions of years in which the Grand Canyon formed, the canyon divided a single population of tufted-eared squirrels into two populations. One of these squirrel populations, the Kaibab squirrel, now lives in isolation from the other tufted-ear squirrels on the North Rim of the canyon. The other population, the Abert’s squirrel lives on the South Rim. Some biologists think that the two populations of squirrels have evolved into two separate species. Other biologists think that the Kaibab squirrel is a subspecies of the Abert’s squirrel. Which of the following factors would make the evolution of the squirrel population into separate species less likely?