Evolutionary biologist Ernst Mayr established the biological species concept.
It states that the most similar organisms which are involved in interbreeding natural populations and are reproductively isolated from other such groups, those are belonging to the same species. The species names are given in Latin accordingly.
It is accepted that the same species interbreeds under natural conditions and produces fertile offspring whereas individuals of different species generally do not mate.
If individuals of different species are forced to mate then they reproduce sterile offspring.
If a horse (Equus caballus) is allowed to mate with a donkey (Equus assinus), the result will be a mule and mules are sterile and cannot reproduce.
So, horses and donkeys are classified into different species.
Again, for example, domestic cats, and sand cats are all descended from a common ancestor but belong to different species.
Domestic cat- Felis domestica
Sand cat- Felis margarita
They share a common higher group genus but differ in species.