what factor indicates that a population has evolved into 2 seperate species?

Answer 1

Reproductive isolation.

The members of both populations have separated to a degree that members of one group simply *do not* even attempt to breed with members of another. And yes, that separation needs to be genetic too, to the extent that interbreeding no longer (usually) works, but the main thing is that it has to be complete enough that the populations no longer *recognize* each other as being co-members of the same kind of animal.

For example....dogs will generally attempt to copulate with anything that strikes them as "dog", regardless of how physically mis-matched the breeds are. They'll even attempt to hump humans if they are sufficiently confused about that boundary (pack instincts, bad training, all that).

But you don't see dogs and cats humping each other. Their instincts simply don't register that way. Well, with usual, typically healthy members of the species. They simply don't see each other (or scent each other?) as the same kind of animal.

Hope this helps. :)

Answer 2

When the two populations occupy different niches in the environment so that they do not normally breed with each other. Possibilities include one set becoming active only in the morning the other in the evening; or one set becoming adept at picking seeds out of cones while the other becomes good at finding seeds on the ground; or picking bugs out of tree bark vs finding them in ground clutter. Or, as Darwin observed, Galapagos finches with beaks matched to available foods.

Answer 3

when certain members of the population can either not produce offspring with the others or produce offspring that is infertile.

Answer 4

when its reproductive habits isolate it from other populations, or prevent it from reproducing with other populations.

Answer 5

The two species do not interbreed. (At least, without human coercion.)