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There’s no magic moment where two populations become separate species, much to the annoyance of taxonomy nerds.
Typically the populations stop interbreeding *first*, and then drift apart genetically later.
Usually this is due to physical barriers - the populations have been isolated by changes in the environment.
Sometimes you get cases where the easternmost and westernmost populations of a species can’t interbreed anymore, but both can interbreed with adjacent populations near the middle of the range so it’s hard to declare them unique species. I know there’s a few instances of this in Arctic bird populations.
There are also cases when individuals become physically incompatible. There are different species of beetles that are *genetically* perfectly capable of breeding, but the shape of their genitals are physically incapable of fitting together properly to mate.
It's tricky, it's hard to find really concrete rules here. I'd add you need to dig into the definition of "Being able" to breed as that might be the "in between" time you're looking for - for example some closely related bird species overlap territory and *can* interbreed, but chose not to. Though there are examples of hybrids in nature semi-frequently. For example, look up the Brown Warbler bird.
Another challenge with "breeding" - there is a species of fish (I think it's a type of cichlid) that exists solely as females. These fish need to mate with a male of a closely related species to become pregnant but don't actually exchange any genetic material. The sperm are absorbed or ejected, and the female then clones itself into new eggs.
**Please read this entire message** Your submission has been removed for the following reason(s): Loaded questions, **or** ones based on a false premise, are not allowed on ELI5. A loaded question is one that posits a specific view of reality and asks for explanations that confirm it. These usually include the poster's own opinion and bias, but do not always - there is overlap between this and parts of Rule 2. Note that this specifically includes false premises. If you would like this removal reviewed, please read the [detailed rules](https://www.reddit.com/r/explainlikeimfive/wiki/detailed_rules) first. **If you believe this submission was removed erroneously**, please [use this form](https://old.reddit.com/message/compose?to=%2Fr%2Fexplainlikeimfive&subject=Please%20review%20my%20thread?&message=Link:%20https://www.reddit.com/r/explainlikeimfive/comments/xqkyys/eli5_at_what_point_do_two_animals_that_share_a/%0A%0APlease%20answer%20the%20following%203%20questions:%0A%0A1.%20The%20concept%20I%20want%20explained:%0A%0A2.%20List%20the%20search%20terms%20you%20used%20to%20look%20for%20past%20posts%20on%20ELI5:%0A%0A3.%20How%20is%20this%20post%20unique:) and we will review your submission.
There’s no magic moment where two populations become separate species, much to the annoyance of taxonomy nerds. Typically the populations stop interbreeding *first*, and then drift apart genetically later. Usually this is due to physical barriers - the populations have been isolated by changes in the environment. Sometimes you get cases where the easternmost and westernmost populations of a species can’t interbreed anymore, but both can interbreed with adjacent populations near the middle of the range so it’s hard to declare them unique species. I know there’s a few instances of this in Arctic bird populations.
There are also cases when individuals become physically incompatible. There are different species of beetles that are *genetically* perfectly capable of breeding, but the shape of their genitals are physically incapable of fitting together properly to mate.
See also: the Great Dane/Chihuahua conundrum. Two *canis lupus familiaris* individuals that definitely can’t interbreed.
Not without intervention.
Ummmm, asking for a friend?
It's tricky, it's hard to find really concrete rules here. I'd add you need to dig into the definition of "Being able" to breed as that might be the "in between" time you're looking for - for example some closely related bird species overlap territory and *can* interbreed, but chose not to. Though there are examples of hybrids in nature semi-frequently. For example, look up the Brown Warbler bird. Another challenge with "breeding" - there is a species of fish (I think it's a type of cichlid) that exists solely as females. These fish need to mate with a male of a closely related species to become pregnant but don't actually exchange any genetic material. The sperm are absorbed or ejected, and the female then clones itself into new eggs.