Species
A species is a group of organisms capable of interbreeding and producing fertile offspring, and which does not normally breed with other groups.
Dogs show significant variation but can all interbreed and produce fertile offspring. Dogs are therefore one species.
Lions and tigers can breed but the offspring, like this ligress, cannot reproduce
Lions and tigers are able to interbreed producing a hybrid called a liger or tigon. However, ligers and tigons are infertile and cannot produce their own offspring. Lions and tigers are therefore different species.
As long as a population has the opportunity to interbreed and exchange genes, they remain one species.