The question of whether two separated squirrel populations would interbreed touches on the fundamental process of speciation, the formation of new species. When a single population is physically divided, the two resulting groups begin to evolve independently, slowly accumulating genetic differences. Reproductive incompatibility determines if they are no longer the same species. Biologists use the concept of reproductive isolation to determine if interbreeding would be successful if the geographic separation were suddenly removed.
How Geographic Barriers Drive Genetic Change
A physical barrier, such as a new river or a mountain range, halts the free flow of genes between the two populations. This initial separation initiates a process called allopatric speciation, where divergence is driven by two main evolutionary forces. Genetic drift involves random changes in allele frequencies that are more pronounced in smaller, isolated groups. The small population on one side of the barrier may, purely by chance, lose certain gene variants or have others become fixed, establishing a unique genetic signature distinct from the larger parent population.
The two isolated habitats often present different environmental challenges, which drives local adaptation through natural selection. One side of a valley may have a colder climate or different local predators, favoring squirrels with thicker fur or better camouflage. Over many generations, these differing selective pressures cause populations to diverge in measurable traits, including body size, coat color, or even foraging behaviors.
Reproductive Isolation
Whether interbreeding is possible rests on the establishment of reproductive isolation, the mechanisms that prevent successful gene exchange. These mechanisms are broadly categorized based on whether they act before or after the formation of a fertilized egg, or zygote. Pre-zygotic barriers are the most efficient from an evolutionary standpoint because they prevent the costly waste of reproductive resources on unsuccessful mating attempts. For example, the separated squirrel populations may have developed different mating rituals or communication calls, leading to behavioral isolation where they fail to recognize each other as potential mates.
Another pre-zygotic mechanism is temporal isolation, where the two groups might begin to breed at different times of the year. If divergence includes significant changes in physical structure, mechanical isolation could prevent successful copulation. If these initial barriers are incomplete and mating occurs, post-zygotic barriers come into play after fertilization. These barriers include hybrid inviability, where the resulting hybrid offspring are weak and do not survive to maturity.
Post-zygotic failure can manifest as hybrid sterility, where the hybrid offspring are robust and healthy but are incapable of producing viable gametes. This sterility typically results from incompatible chromosome numbers or genetic architectures that fail to align correctly during the creation of sperm and eggs. When a combination of pre-zygotic and post-zygotic barriers is fully established, the two populations are considered distinct species that cannot interbreed successfully.
Potential Outcomes When the Barrier is Removed
When the geographic barrier separating the squirrel populations is removed, the result is one of three evolutionary scenarios. If the populations separated only recently and reproductive isolation is minimal, the two groups will interbreed freely, leading to fusion. The gene pools will merge, and the genetic differences that accumulated during isolation will be quickly erased, returning them to a single, mixed species.
If the separation was long enough for complete reproductive isolation, the populations will generally ignore each other, an outcome known as reinforcement or full speciation. Mating attempts may be rare or nonexistent due to established pre-zygotic barriers like different mating calls. The third possibility occurs when reproductive isolation is partial, often involving a failure of post-zygotic barriers, meaning hybrids are produced but are less fit than the purebred individuals.
This partial isolation typically results in the formation of a stable hybrid zone, a narrow geographic area where the two populations meet and interbreed. Within this zone, hybrids are continuously produced but rarely survive or reproduce effectively, preventing the full merger of the two species.