Two different groups of aliens from the same planet come visit Earth. They don’…

Two different groups of aliens from the same planet come visit Earth. They don’t have
evolutionary biologists on their planet so they are not sure if they are the same species
of different species. Biologists encourage them to mate and they find that the mating
does not allow their offspring to be successfully born. What species concept are the
biologists on Earth using and what type of reproductive isolation did they observe?
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