Species
There is no clear universally accepted definition of species. A commonly cited definition is that two organisms are in the same species if they can produce fertile offspring.[1]
There are a number of problems with this definition
There are cases of Galapagous
Some animals are genetically compatible but may not be anatomically compatible. The smallest and largest dogs might find sex to be impractical. Humans might be able to artifically fertile an ovum and produce fertile offspring.
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