"If they were really separate species  they would not produce fertile offspring"

Fertile offspring resulting from species crosses are not particularly uncommon.  In fact there is even a named rule for such occasions.  The rule is in species crosses when one sex is rare, absent or sterile that sex will be the one that is heterozygous at the sex chromosome.  This rule is named Haldane's rule.  Even female mules are occasionally fertile.  Species crosses in birds are quite often fully or partially fertile.  That is how red canaries were made. They are the result of a cross between the Venezuelan red siskin and canaries followed by back crosses to canaries to move the genes to make red carotene pigments from the siskin to canaries.  Cattle crossed to American Bison are also produce fertile offspring as do a number of other cattle crosses to other species.  Wolves crossed to coyotes produce fertile offspring.  The average eastern US coyotes today are roughly 25% wolf.

Dick

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