[fishtank]

If they can interbreed, they are still the same species.

[KarlInOhio]

If they can interbreed, they are still the same species.

Their offspring have to be fertile to count as the same species. You can get close genetic relatives like horses and donkeys mating to produce sterile mules.

[allmendream]

A Lion and a Tiger can (and do) produce a Liger or a Tigon depending upon which species was the father. A Tigon can (and did) interbreed with a Tiger to produce a Ti-Tigon. This Ti-Tigon could conceivably breed back into the Tiger population, mixing in Lion genetics.

Tigers and Lions are not the same species.

All things that can interbreed are not the same species.

A species is defined as an interbreeding population, not a population that is capable of interbreeding.

In this particular case the black squirrels are a more aggressive and dark colored “phenotype” of the original grey population. Some day they might become a “sub-species” if the black and greys start to differentiate their mating habits or mating season or start mating assortively. This may one day be classified as a different “species” if this reproductive isolation continues. Some day perhaps the differences will accumulate to the point where they not only do not interbreed in the wild, but are incapable of producing fertile offspring.