Edible Frog

Normally, a hybrid between two species is either sterile or its offspring blend back into one parent species over generations, but the edible frog persists as a stable hybrid because of an unusual reproductive system called hybridogenesis. An edible frog carries one set of chromosomes from each of its two parent species, the pool frog and the marsh frog, but when it makes eggs or sperm, it deletes one parent species' entire chromosome set from its germ cells and passes on only the other, unrecombined. It then mates with a member of the species whose genome it deleted, which restores the full hybrid combination in the next generation, so edible frog populations can continue indefinitely as long as one parent species is present to breed with.

Because of hybridogenesis, an edible frog cannot simply breed with another edible frog and reliably produce more edible frogs, since both parents would be contributing the same single unrecombined genome and the offspring would often be non-viable or would not be hybrids. To keep producing edible frogs, the hybrid must mate with the parent species whose genome it discards in its own reproductive cells, so functional edible frog populations almost always exist as mixed communities living alongside pool frogs, marsh frogs, or sometimes both, with the different frog types coexisting in a pond in ratios that can remain remarkably stable over long periods.

Yes, the edible frog, along with its parent species the marsh frog and pool frog, which are all closely related green water frogs of the genus Pelophylax, is the group of frogs traditionally harvested in Europe for the dish of frogs' legs, most strongly associated with French cuisine but eaten in several countries. The common name edible frog comes directly from this long culinary history. Large-scale historical harvesting for food, along with habitat loss, contributed to declines of these water frogs in some regions, and much of the frog leg meat sold commercially today is now imported from farmed or wild-caught frogs of other species in Asia.

It is genuinely difficult, which is part of why the whole group is often just called green frogs or water frogs in casual use, and reliable identification frequently requires genetic testing, call analysis, or measurements of specific body proportions and the metatarsal tubercle on the hind foot. In general the edible frog is intermediate in size and appearance between the smaller pool frog and the larger marsh frog, its parent species, which is expected given its hybrid origin, but the overlap is extensive and a single frog in the hand often cannot be confidently assigned to one of the three forms by eye alone.