At What point is one species different from another philosophically speaking?

It is important to understand that all biological classifications, including species, are artificial. They are groupings that WE humans made up and imposed on the natural world. When animal A encounters animal B in the wild, it doesn't stop to consider anatomic similarity and genetic divergence and probability of cross-fertility. It just tries to figure out whether to eat it, run from it, fight it, ignore it, or have sex with it.

That said, I'm not implying that species divisions, in particular, are not real. They are. The most commonly used definition of a species is a population of organisms that can and regularly do interbreed in the wild. A species represents a pool of genetic information that flows freely within itself, but is isolated from the rest of the world's genetic information. More or less.

What you choose to do with that information, ethically, is not a discussion of science. It is philosophy.

I think the best way to understand a species is whether individuals can reproduce together or not. Now of course the nitpicking skeptic will come along and pull that definition apart with more questions like "What if someone is infertile? What about the same sex? What if we splice genes?" etc. But if we want to use this word "species" to describe ourselves as "human" we have to have some sort of meaning for it, so lets just say it means we can reproduce together.

Why don't animals have rights? Well in order to have rights, you need to know you have them and you need to be able to utilize them, stop other people from violating them. Otherwise they are meaningless.

So a mosquito can't have rights because if I violate its rights, nobody is realistically going to do anything about it.

But what if we are in a situation when nobody is doing anything about people's rights being violated? Well, I'd say that as people have speech and can potentially learn to read, then they have the possibility of exercising their own rights which a mosquito does not. On that basis, they SHOULD have rights, even if they are being denied now.

Again, there will be nitpicking skeptics who find fault with this, but we've got to work with something.

Good question. I think speciation begins gradually with one group of a species (race, possibly) no longer wanting to have sex with another group. If that happens, then over time, the tiny ongoing changes in DNA would eventually cause the two groups to not be able to have viable offspring anymore. I'm not that kind of scientist, but as a lay person, I would think that (inability to have offspring together) would be a good definition of them being two different species. It may be underway now if you look at the increasingly strident social divides in this world between some groups.

Philosophy is not biology. Philosophically speaking the difference between one species and aborted is not philosophy.