Caution: species crossing; hybridization often results in deformation and death, but sometimes it gives birth to new species

Natural History, Sept, 2002 by Menno Schilthuizen

What do you get if you cross a carrier pigeon with a woodpecker? Or a bear with a vampire? Riddles like these can be heard in schoolyards and at children's parties all over the world. Science fiction, too, employs hybrids--offspring from the crossing of two different species. Think of the movie The Fly, in which the accidental hybridization of Jeff Goldblum (or, in the original, David Hedison) with a housefly has disastrous consequences. In both riddle and sci-fi, hybrids are often portrayed as ridiculous or grotesque--and sometimes dangerous.

In nature, hybridization is usually less dramatic, occurring between taxonomic neighbors: horses and donkeys, lions and tigers. But even at this level--members of the same genus or family--hybrids have long had a bad reputation, as suggested in the very word. The Latin hybrida derives from the Greek hubris, meaning "arrogance or insolence against the gods." Many naturalists and scientists still implicitly hold to the Platonic idea that every species has an "essence" that can be maintained only if members of the species mate solely among themselves and thus keep genetic pollution at bay.

Hybridization upsets such a nicely pigeonholed view of the natural world. Traditionally, museum curators have often ignored hybrid specimens or put them in a box marked Unidentified, because their collections followed the Linnaean binomial naming system, with every species given a genus and a species name--for example, Canis familiaris for domestic dog. Hybrids have no place in such a system. Botanical gardens and zoos, as well as museums, prefer to display the "pure" species rather than confusing mixtures. Interestingly, in the terrarium trade, the "unnaturalness" of hybrid lizards and snakes makes them special in the eyes of many people, who are willing to pay high prices for them.

Their ambiguous taxonomic status also means that hybrids do not enjoy much protection under environmental law. The U.S. Endangered Species Act of 1973 (ESA) did not mention hybrids. In fact, protection of hybrids was actively discouraged, the concern being that hybridization with a more common species might actually edge an endangered one closer to extinction. Later, the Fish and Wildlife Service and the National Marine Fisheries Service took pity on hybrids, noting that hybridization is often a natural phenomenon and that the existing "rigid standards ... should be revisited." A new hybrids policy was proposed in 1996 (the proposal sought to substitute "intercross" for the loaded term "hybrid"), but it was never officially adopted. As a result, there still are no official guidelines for dealing with hybrids under the ESA--which is bad news for animals such as the red wolf. According to some genetic studies, this endangered canid is the result of hybridization between the gray wolf and the coyote and hence may not qualify for the federal captive breeding and reintroduction program from which it currently benefits.

What is it about hybrids that has made people so dismissive of them? One source of the disrespect is that they appear to flout the near-sacred boundaries between species. Another is that hybrids, in fact, often are inferior. A mule may inherit "its size and strength from the horse, and its surefootedness and supreme sense of self-preservation from the donkey," as Paul and Betsy Hutchins wrote in the authoritative Modern Mule, but in many cases, a hybrid is a very troubled creature. Problems often begin before birth. In a hybrid embryo, the genetic instructions from one parent will steer the developing organism in certain directions, while the gene set from the other may steer it in quite different, even contradictory, directions. Often the instructions are so incompatible that embryonic development can't go forward. If the incompatibility is less severe, the hybrid may be born with a form somewhere between the two parents. Sometimes this is harmless enough. For instance, the Greek land snail Albinaria spratti has coarse, wavy ribs on its shell, whereas A. hippolyti has straight, threadlike lines. Hybrids of the two have shells adorned with riblets that are not really coarse but also not very thin, and only a little bit wavy. Oblivious to their unorthodox appearance, the hybrids appear to go about their business normally (though there is no evidence that they breed).

But sometimes disaster ensues from hybridization. In large, permanent bodies of water in the lowlands of eastern Europe, for example, lives Bombina bombina, the European fire-bellied toad, so called because of the red splotches on its belly. Western and southern Europe, as well as the mountains of Hungary, Poland, and Slovakia, are home to a close relative, the yellow-bellied toad, Bombina variegata. Along a central European front, the two species hybridize, but the hybrids suffer from a whole range of defects, many of which are lethal at the embryonic stage. Those that survive to become tadpoles often have misshapen mouths and are unable to feed properly. Those that make it to adulthood suffer from skeletal abnormalities: ribs fused to vertebrae, reduced numbers of vertebrae, and asymmetry in the sacral region. All in all, these hybrids are often eliminated by a form of natural selection that is termed "endogenous selection"--it's not the outer environment, but the animals' own inner environment, so to speak, that does them in.