how do u know if 2 creatures are the members of the same species?

Answer 1

That depends on which species concept you use. Geneticists use the one that says that if they can interbreed to produce fertile offspring, they are the same species. This is useless where fossil forms are involved and worse than useless for ichthyologists; some minnows produce fertile intergeneric hybrids. My favorite definition came from C. Tate Reagan, an ichthyologist, who said that a species is whatever a competent taxonomist working with the group says is a species. Actually, I find that that definition is quite practical.

Answer 2

The easiest answer if they can breed, but of course there are exceptions going both ways. Horses and Donkeys or Lions and Tigers can breed yet they are not the same species they are just at the limit. In the galapagos there are turtles that can breed with each other from different islands yet they are different species, because naturally they would never breed with each other. There are many examples like the first two, but for most of the time it's if they can breed. Species is just a subjective word. We made it up and it fits the first large step between two ograinisms the best but it is hardly perfect.

Answer 3

You can tell a male and a female are of the same species if they can have sexually viable offspring. Given two males, you'd need a female and assume the transitive law works: if male 1 is same species as a female, and so is male 2 with the same female or another of that species, then both males are of the same species. Reverse male and female in the previous argument for 2 females.

Answer 4

If two creatures can successfully breed and produce viable offspring, they are of the same species. =)
Viable meaning that the offspring are fertile and are able to reproduce successfully themselves. Ligers (cross between a Lion and a Tiger) along with Mules (offspring from a horse and a donkey) are infertile, therefore they are not viable and the parents are not of the same species.