Why are horses and donkeys considered different species, even though they can produce offspring?

Donkess and Horses are considered a different species even though they can produce offspring if they mate. Because biology has defined that the species concept can only appply to organisms that can produce fertile offspring.

Explanation:

Biology species concept sustains that a species is only considered when two different organisms can conceive a fertile offspring that can have offspring. So even though they can share many characteristics, mate, and produce offspring even if they come from different continents. They are not considered from the same species because they can't produce together a fertile offspring, The phenomenon that doesn't let them create fertile offspring is called prezygotic and postzygotic barriers.

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explanation:

the answer will be a) proteins i believe