- Ring species. A can mate with B, and B with C, but A cannot mate with C
- How do you test it? What about the case where A and B can in molecular terms reproduce (ie the sperm and egg make a viable fetus by IVF) but they can't come together naturally because there's some behavioural issue (different mating rituals)?
- What about things that don't have sexual reproduction?
Eg each human can breed with a human in the previous generation. Each chimp can breed with a chimp in the previous generation. Right back to their common ancestor.
As long as there's a common ancestor, there should be a viable chain to be followed in most cases.
Presumably, this is one way in which speciation happens. You have a group. Some members of the group start adapting one way, some another way. All the while, interbreeding is possible. Eventually, the behavioral differences become great enough that even though interbreeding is possible, it tends not to happen. The two groups, which generally do not interbreed, will continue adapting and experiencing genetic drift. Over a long enough time, the groups may diverge enough that they no longer can interbreed.
The separation can happen because of one part of the group going after a different ecological niche (such as with dogs). Or the groups can end up being separated geographically.
If we defined "species" to only be if they can interbreed, we would miss this distinction between dogs and wolves (and other species). And it's a useful distinction: dogs and wolves have different behaviors, and if we did not call them different species, we would have to invent a new word and classification to capture the difference. This does mean that it is quite difficult, and perhaps even impossible, to pin-point the exact moment when a new "species" arises. That may seem arbitrary, which it is. We impose our orderly classification system on the disorder of evolution. If we did not, we could not reason about it.
No we don't? Domestic dogs and most wolves fall under Canis lupus, they're differentiated as a subspecies level but they're the same species.
That's probably the best way at this point to test for it, if it's there or not.
So all your ancestors count as the same species. And all their descendents count as the same species. So there is a single species.
https://news.ycombinator.com/item?id=10478762
In particular, take this example:
> Consider the following: imagine that you assembled a gathering of all your ancestors in the male line for say, the past two million years. Let's say, for illustrative purposes, that the gap between each generation is twenty years. So you'd have 5 people to represent a century - you, your father, your grandfather, your great-grandfather, and your great-great grandfather. That makes 50 people per millennium, 500 people per ten thousand years, 5,000 people per hundred thousand years, and 50,000 people per million years. So two million years of ancestry gets you 100,000 people, the size of a small city, or about enough people to fit in the Rose Bowl stadium.
> So really, not that many people in the grand scheme of things. Now line all those people up in order going all the way back to the first guy, who we'll just call Adam. The difference between Adam N and Adam N+1 is very small. There's no point at which you can say "Adam N is this species and Adam N+1 is that other species." But there's a hell of difference between you and Adam.
We can have a useful handwavy definition of "species", but there's never going to be a way to define the word in such a way that you can always filter every organism that ever lived into one clear bucket or another.
Lots of species can interbreed. Are lions and tigers the same species? OK, maybe we can modify that and say "with fertile offspring." But sometimes the offspring are fertile among couplings we would say are definitely interspecies. For example, some mules are actually fertile!
On the other hand, there's species that are very genetically similar that can't interbreed. And there's species that are very genetically similar that simply don't interbreed or maybe can't without human intervention. (Silly example: teacup breed of dog and huge breed of dog.)
For basically every definition of species you can think of, nature will throw an example at you that makes it look kind of silly. I suppose you could just deal with the silliness, put your foot down, and say "lions and tigers are the same species, deal with it." But what if something like the humanzee is proven possible? (https://en.wikipedia.org/wiki/Humanzee#Feasibility)
I honestly didn't think it needed to be such a big debate and thought this was long settled. "Can they interbreed?" and if the answer is no, it's a different species.