It is not a normal occurance in the wild. There are many triggers in nature
to get animlas to breed. Many times animals that are in the location
(stream/pond) have slightly different triggers (temperature for example). If
the temp is even a couple degrees different to induce spawning, that would
essentially keep two species from hybridizing. Physical separation is one of
the greatest reasons there are not more hybrids. Though it may happen in the
wild, it is not "normal".
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