There is a lot of debate and misunderstanding about hybridization in fish. Many feel it is wrong for a number of reasons including ‘playing God’, ‘messing with nature’, or unknown consequences.
Hybridization is generally defined as a cross between two different species. This usually creates offspring of a variety not found in nature. Many people feel this is an atrocity. They feel it is playing God or messing with nature. But in fact hybridization is a very common act in nature, even being the cause of speciation of many species. Some species take part in it so often it is hard to distinguish them as a unique species. To properly understand hybridization and its consequences, people need to understand exactly what a species is, what hybridization is, its role in nature, and the process of speciation.
The most common definition of a species is ‘a group of organisms that can interbreed and produce fertile offspring’. Many people will further specify that definition by adding ‘in nature’. In his textbook Evolution, Futuyma defines a species as ‘the members of a group of populations that interbreed or potentially interbreed with one another under natural conditions’ (552). This definition follows the biological species concept, that a species must be reproductively isolated or that it cannot interbreed with other species to be classified as a separate species (Lawrey). Many species are extremely challenging to observe and monitor in the wild, so it is hard to determine which individuals are interbreeding so we cannot be sure what all does and does not take place in the wild. In addition, there are many cases of natural hybridization between two or more long-standing species.
Hybridization is so common that there is currently a debate as to whether the red wolf, Canis rufus, is even a unique species or a hybrid between the gray wolf, C. lupus, and the coyote, C. latrans (Lawrey). Genetic analysis has revealed that there is substantial cross-breeding between these two or three species. Recently a hybrid between a polar bear, Ursus maritimus, and a grizzly or brown bear, U. arctos, was shot and killed in Canada (Associated Press, “Wild”). Based on the biological species concept this would make the two species of bears actually only one since they can successfully reproduce in the wild. Phylogenetic evidence supports that polar bears are no more than a subspecies of grizzly bear. Many people are now familiar with ligers, the resulting offspring between a male lion, Panthera leo, and a female tiger, P. tigris. The offspring are fertile. Another example is that of macaw parrots. When the blue-and-gold macaw, Ara ararauna, is crossed with the green-winged macaw, A. chloroptera, the result is the harlequin macaw, a relatively common captive pet (see Figure A). Many species of the Swordtail genus Xiphophorus spp. of fish can interbreed with one another (Lawrey). In addition, many species were formed via hybridization. Many plant groups contain many hybrids and potentially hybridizing species (Lawrey). This has obvious evolutionary value to the organisms with that combination.
The classic situation of speciation involves a single species that for some reason begins to become two species. There are many ways this can occur. In some cases a new trait appears within a population that can allow for sexual selection, such as some males developing a unique coloration. In other cases a physical barrier can separate populations of the species, such as a shallow sea or a mountain range. When this happens the subsequent two populations are then genetically isolated. This means that any change in the genome in a population due to mutation, genetic drift, etc. becomes unique to that population. As more and more genetic differences build up, which happens more quickly in smaller populations, the two populations become more divergent, and less like the original species or each other. Over time this can generate entirely new species.
An example of this type of speciation is the speciation of two mosquito species in London. One, Culex pipiens, is a common urban mosquito species found in London. However, in the London Underground there is a genetically distinct and reproductively isolated species, C. molestus (Lawrey). In this situation the construction of the London Underground created a new, not yet exploited niche for a mosquito. A population was formed by individuals of C. pipiens that populated the London Underground. Over time this newly isolated population developed genetic changes that made it unique from the parent species. Eventually this new population became unique enough from the parent species that it became genetically distinct and reproductively isolated and therefore a new species.
In many cases the isolating barriers are later removed. If the populations have not fully speciated and are not reproductively isolated, then they remain as populations of one species, even if they are morphologically very different. Because they can appear to be so different morphologically they can easily be classified as different species by scientists who do not realize all the details of the situation. As more information is gathered scientists discover that many species are actually hybrids or commonly hybridize with each other.
Hybridization is actually one method of speciation. Research done with sunflowers showed how the hybridization of two closely related species could yield super-combinations that were better adapted and therefore more fit (Indiana). This can result in diploid species (the genomes of the two species are combined into one, the size of the two genomes put together). This is actually a natural process, common in plants but rare in animals, although not non-existent. (Futuyma)
Hybridization is a natural process. It is a natural form of speciation and species preservation. Hybridizing in captivity is not harmful when certain things are avoided. It is possible to produce hybrids that have physical deformities such as mouths that are unable to close, or bodies so misshapen that even swimming is a challenge. These crosses should be avoided, already produced broods entirely culled, and the cross not made again. But just because something is a hybrid does not make it inherently wrong. If natural limitations are bypassed then the person is jumping hurdles that nature has set in place that have limited that cross. In these cases the two species are separate enough that hybridization should be avoided. Cases like this would be situations where breeding was done artificially (artificial insemination) and humans forced the cross even though the fish followed nature’s limitations.
If two species cross under their own will and produce healthy, fertile offspring, then by the biological species concept’s definition of a species they are actually the same species. They may look different or be currently classified as separate species, but those may be false classifications. This is such as tricky subject in science that many scientists do not even believe in the idea of a species. There are no definitive barriers in most cases so we as humans cannot make up barriers and definitions just so that things are easier for us to understand and we cannot pigeonhole them just to suit our expectations of simplicity. Evolution is not done. Things are constantly changing and species are still in the process of evolution, still changing. Some are definitely too different to hybridize. Others are barely isolated enough to be separate populations and subspecies, but since they look different (something that can change very quickly under natural selection) we decided to classify them as separate species. So if hybrids are healthy and fertile then the two populations are not actually different species, but different subspecies.
What about artificial selection? It has produced some extremely deformed breeds. Goldfish are an excellent example. They have been domesticated long enough to produce major alterations from the natural form. Some of the breeds and individuals are so deformed that they cannot or can barely swim, breathe, have proper digestive flow, see, or maintain buoyancy. Yet because these are not hybrids it is okay? This is okay but a perfectly healthy animal that simply has a different coloration but happens to be a hybrid is wrong?
One major argument is that by allowing hybridization to take place, or to let it out of the breeder’s own tanks, we are harming the species as a whole. But as shown here, if they are that closely related they are not actually different species in the first place. The preservation of a natural ‘species’ in captivity is an oxymoron. We are not going to collect the individuals of a species from hobbyists’ tanks and release them into the wild to repopulate. They are in the hobby because we enjoy them. If we enjoy a certain type of fish and it is healthy then there is no reason for some people to decide it is wrong simply because it isn’t currently classified as a natural species.
We really need to step back and look at what the actual consequences of what we do are. If there is no actual harm then it is not bad. If we produce fish that have special needs, deformities, or limitations (whether they are hybrids or not) then we have done something wrong.
Works cited and consulted:
Alaska Department of Fish and Game, “Alaska’s Bears” 2008 retrieved April 30, 2008 http://www.wildlife.alaska.gov/index…dfg=bears.main
Alcock, J., Animal Behavior (8th ed.) Massachusetts, Sinauer, (2005)
-Associated Press, “Wild find: Half grizzly, half polar bear” 2008 retrieved April 30, 2008 http://www.msnbc.msn.com/id/12738644/GT1/8199/
-Futuyma, D. J., Evolution Massachusetts, Sinauer, (2005)
-Indiana University. “Cross-species Mating May Be Evolutionarily Important And Lead To Rapid Change, Say Indiana University Researchers” ScienceDaily (August 8, 2003) retrieved April 3, 2008 http://www.sciencedaily.com/releases…0808081854.htm
-Lawrey, J. D., GMU BIOL 471 Evolution Lecture notes/slides (Spring 2008)
Pages, M., et al. “Combined analysis of fourteen nuclear genes refines the Ursidae phylogeny” Molecular Phylogenetics and Evolution vol.47 (2008): 73-83
Wauer, L., “Parrots we’ve seen during our travels” retrieved April 30, 2008