On May 15, 1985, trainers at Hawaii Sea Life Park were stunned when a 400- pound gray female bottlenose dolphin named Punahele gave birth to a dark-skinned calf that partly resembled the 2,000-pound male false killer whale with whom she shared a pool. The calf was a wholphin, a hybrid that was intermediate to its parents in some characteristics, like having 66 teeth compared with the bottlenose's 44 and the 88 of the false killer whale, a much larger member of the dolphin family.
In 2006, a hunter in the Canadian Arctic shot a bear that had white fur like a polar bear's but had brown patches, long claws and a hump like a grizzly bear's. DNA analysis confirmed the animal was a hybrid of the two species.
While one might think that these oddities are examples of some kind of moral breakdown in the animal kingdom, it turns out that hybridization among distinct species is not so rare. Some biologists estimate that as many as 10 percent of animal species and up to 25 percent of plant species may occasionally breed with another species. The more important issue is not whether such liaisons occasionally produce offspring, but the vitality of the hybrid and whether two species might combine to give rise to a third, distinct species.
While several examples of human-bred animal hybrids are well known and can thrive in captivity including zorses (zebra-horse), beefalo (bison-beef cattle) and, of course, mules (donkey-horse), naturally occurring animal hybrids have many factors working against their longer-term success.
One of the main obstacles is that, even if members of different species might mate, when the two species are too distant genetically or carry different numbers of chromosomes, the offspring are usually inviable or infertile (like zorses and mules), and are therefore evolutionary dead ends. A second problem is that any hybrid will usually be vastly outnumbered and outcompeted by one or both parent species.
But because species hybrids create new combinations of genes, it is possible that some combinations might enable hybrids to adapt to conditions in which neither parent may fare as well. Several such examples are now known from nature. Furthermore, DNA analysis is now allowing biologists to better decipher the histories of species and to detect past hybridization events that have contributed new genes and capabilities to various kinds of organisms including, it now appears, ourselves.
The familiar sunflower has provided great examples of adaptation by hybrids. Loren H. Rieseberg of the University of British Columbia and colleagues have found that two widespread species, the common sunflower and prairie sunflower, have combined at least three times to give rise to three hybrid species: the sand sunflower, the desert sunflower, and the puzzle sunflower.
The parental species thrive on moist soils in the central and Western states, but the hybrids are restricted to more extreme habitats. The sand sunflower, for instance, is limited to sand dunes in Utah and northern Arizona and the puzzle sunflower to brackish salt marshes in West Texas and New Mexico.
The species distributions suggest that the hybrids thrive where the parents cannot. Indeed, recent field tests that examined the relative ability of the parental species to thrive in the hybrids' habitat, and vice versa, found that the sand sunflower was better able than its parents to germinate, grow and survive in its dune habitat but fared relatively poorly in parental habitats. Similarly, the puzzle sunflower was much better at growing in salty conditions than its parents.
One lesson from the sunflowers appears to be that hybrids may succeed if they can exploit a different niche from their parents. The same phenomenon has been discovered in animal hybrids.
In the past 250 years, various forms of honeysuckle have been introduced to the Northeastern states. In the late 1990s, researchers led by Bruce McPheron of Pennsylvania State University discovered that this invasive honeysuckle was infested by a particular fruit fly species they called the Lonicera fly. When they analyzed DNA to determine its relationship to others, they were stunned to find that it was a hybrid of two closely related flies, the blueberry maggot and the snowberry maggot.
In laboratory experiments, the researchers found that the Lonicera hybrid preferred its honeysuckle host plant over its parent species' host plants and that each parent species preferred its own host plant over the other's. However, both parents also accepted honeysuckle. The researchers suggest that since the two parental species were thus more likely to encounter each other on honeysuckle in the wild, the newly invasive weed served as a catalyst for matings between the species and the formation of the hybrid species that now prefers honeysuckle.
The sunflower and Lonicera fly examples raise the question of whether hybridization between species has been more frequent than biologists once assumed. The most provocative report of possible hybridization came from the recent analysis of more than 60 percent of the Neanderthal genome sequence, which raised the specter of our ancestors commingling their genes with a long-diverged cousin.
Analyses of the overall genetic distance between Neanderthals and modern humans reveal that our DNA is 99.84 percent identical to that of Neanderthals. This small divergence indicates that the two lines split off from each other about 270,000 to 440,000 years ago. The fossil evidence shows that Neanderthals were restricted to Europe and Asia, whereas Homo sapiens originated in Africa. Various kinds of evidence indicate that modern humans migrated out of Africa and reached the Middle East more than 100,000 years ago and Europe by about 45,000 years ago, and would have or could have encountered Neanderthals for some time in each locale. The crucial question for paleontology, archaeology, and paleogenetics has been what transpired between the two species. To put it a little more crudely, did we date them or kill them, or perhaps both?
If the former, then there could be a bit of Neanderthal in some or all of us. The first comparisons of small sections of Neanderthal DNA did not indicate any hybridization, and the lack of interbreeding became a widely accepted conclusion. That remained the case until this year, when a much greater portion of the Neanderthal genome was obtained by Svante Paabo and colleagues at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. It now appears that 1 percent to 4 percent of the DNA sequence of Europeans and Asians, but not Africans, was contributed by Neanderthals mixing with Homo sapiens, perhaps in the Middle East 50,000 to 80,000 years ago. It is possible that some Neanderthal versions of genes enabled modern humans to adapt to new climates and habitats.
The discovery of hybrid species and the detection of past hybridizations are forcing biologists to reshape their picture of species as independent units. The barriers between species are not necessarily vast, unbridgeable chasms; sometimes they get crossed with marvelous results.
Sean B. Carroll is a molecular biologist and geneticist at the University of Wisconsin.
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