Randomly on the topic of what defines a species, they found that two species of cichlid fish will interbreed and produce viable offspring if you use a filtered color of light that makes them look identical. .

But in natural conditions, they will refuse to even approach each other because they look too different.

A similar study was done on two species of grasshoppers. The females breed with the males that sound right. And each species female has a different note that means 'this male is my species'.

But the notes the males sing at depends on their temperature! The put heaters on the heads of grasshopper A and made them sing at the note for Grasshopper B species... And the females accepted the warmed up males. I don't remember if the offspring were viable though

(Also they found where the two grasshopper species overlap, there was a bigger difference between their songs. But further apart the songs actually sounded more similar, where the risk of cross breeding was smaller)

Species are weird, fluid, and artificial. And also what is 'natural conditions '? In the wild, the cichlids and grasshoppers would probably never cross breed unless a mutation broke down the barrier.

It makes you think what is a 'natural' environment for a human. We have interned with Neanderthals, so we cannot be a distinct species from them... Or did we put ourselves and them into artificial, unnatural conditions when we broke down language and cultural barriers? Yes there could have been force involved... But iirc no other species forces itself on other species (except perhaps dogs and amorous pet cats, which can be because of a lack of natural releases or abnormal breeding causing these issues)

It can become a controversial and even DANGEROUS thing especially when you apply such discussions to humans, who are ever the exception to the rule of nature.