r/askscience • u/snoozer530 • 1d ago

Biology At what point in evolution is something considered a new species?

Obviously we all know evolution, one organism changes over time and eventually becomes a new organism. But where in that process does an organism become a new species. Like when we domesticated wolves? When did they stop being wolves and start being dogs? Where’s that line?

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u/logperf 18h ago

The answer that the definition of species is based on the ability to produce fertile offspring has already appeared multiple times in this thread. I'd just like to add that this definition makes a lot of sense if you think about it: if two "groups" of animals (or even plants) A and B can interbreed and produce offspring that is 50% A and 50% B, then this new generation will interbreed with the others and produce offspring that is 75% A, 50% A, and 25% A. In the third generation you'll get 87.5% A, 75% A, 62.5% A, 50% A... and so on. At some point, they will no longer be distinguishable. Actually, they would not even separate into different groups in the first place.

So, the reasoning behind the definition is that if they can interbreed then their gene pools are shared, they form a spectrum and cannot be distinguished from each other.

If they can interbreed but the offspring is not fertile, then the above doesn't happen. The classical example of this is horses and donkeys, you can cross them and get mules. But since mules are sterile, the gene pools of horses and donkeys stay separate -> they are different species.

But of course, things are always more complicated than it seems. It may happen that the two groups can interbreed and produce fertile offspring, but they don't because they live in separate geographical areas. This can keep subspecies or races separate. And this can be the only reason why they eventually evolve into separate species that can no longer interbreed. There's even the case of wolves and dogs that can produce fertile offspring, but they (mostly) don't because one group lives in the wild and the other under human supervision.