Species have three general requirements...
Now, all species concepts, it seems to me, must have three general requirements. It must say something about what I call reproductive cohesion. That really means that we are not going to put males and females in different species. In other words, that we use reproductive cohesion within groups, males and females, such that they're not going to be put them in different species except by mistake. Second, there are lots of these kinds of examples, until scientists found out that there was reproductive cohesion. And there also must be some criterion of diagnosibility. In other words, you ought to be able, you have to be able to say "that species is different from that species."
Third is the criterion for ranking. By this I mean you have to be able to say, "that diagnosable entity is a species and not a genus, not a family, not an order, not a class." In other words, what its taxanomic ranking is. It's where I placed those entities - those real world entities - into the Linnaen hierarchy at the level of species. And we'll go through this
The most common species concept that everybody learns about that's in virtually all the textbooks now, is the biological species concept. It says that a biological species is a group of interbreeding, natural populations that are reproductively isolated from other such groups. There are only two key elements - two of those three key elements in this. One is reproductive cohesion, that is, interbreeding natural populations. This definition says something about reproductive cohesion - that we would put males or females together in the same species. The second is that they're reproductively isolated. That is how biological species concepts diagnose species. It says that if this group of organism will not interbreed with this other group of organisms, they're separate species. If they interbreed significantly, they're the same species.