So I learned today from this that, homo sapiens( humans) cannot interbreed with salmo salzar( salmon)There is a very long answer to this (Obviously) and It can get quite complicated, but briefly;
But there is still a running debate on what actually constitute a species (especially now with modern genetics), a very simple definition is "a group of living organisms consisting of similar individuals capable of exchanging genes or interbreeding."
So obviously that means Homo Sapiens and Salmo Salar are two distinct species from each other as they are distinct and unable to interbreed.
Many people include a caveat of " interbreed and produce fertile offspring" Which then allows for Tigers lions ect to still be distinct from each other as liger are not fertile.
Now this definition also allows for geographic limits to breeding. A Cichlid in South America for example might be able to breed with one in Africa, but never will because of geography. They may have the ability to do so if brought together, but naturally would never be able to do so.
Then it can be broken down further to behavioural, if one species breeds at night, and the other in the day, they may have the ability to interbreed but are isolated due to their behaviours. So if you as a human strip the fish at night and hold the gametes until you strip the day fish you could cross breed these two together. But they never would naturally.
Similarly where do you draw the line? would some fish we have isolated and bred in captivity for 50 years be a distinct species? could they interbreed with their wild counterparts if place back into that environment?
It's an interesting topic in Atlantic Salmon where researchers have to define which strain of Atlantic Salmon they use in trials because they are so removed from the wild populations and from each company which produces Salmon. Could they breed if they were placed back into the wild? Maybe (Salmon in pens are Tripliod, so no) but the broodstock? maybe? who knows?
I learn the most interesting things on this board