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Hybrids Weave Darwin’s Tree into a Web


We may be on the third wave of a scientific revolution in biology. It may be so big, the story “no doubt has Ernst Mayr hyperventilating in his grave,” thinks evolutionary biologist Nora Besansky of the University of Notre Dame. Mayr influenced a generation of evolutionists. Is one of his core Darwinian concepts unraveling? In Science Magazine, Elizabeth Pennisi sets the stage:

Most of those who studied animals had instead bought into the argument by the famous mid-20th century evolutionary biologist Ernst Mayr that the formation of a new species requires reproductive isolation. Mayr and his contemporaries thought that the offspring of any hybrids would be less fit or even infertile, and would not persist. To be sure, captive animals could be interbred: Breeders crossed the African serval cat with domestic cats to produce the Savannah cat, and the Asian leopard cat with domestic breeds to produce the Bengal cat. There’s even a “liger,” the result of a zoo mating of a tiger and a lion. But like male mules, male ligers are sterile, supporting the notion that in nature, hybridization is mostly a dead end. [Emphasis added.]

Indeed, the biological concept of “species” practically requires reproductive isolation. Hybridization, while known since ancient civilizations bred mules, seems unnatural and rare. It played little role in classical Darwinian theory, which relies on emergent variation and selection for the origin of species. According to hybridization specialist Eugene M. McCarthy in “Darwin’s Assessment of Hybridization,” “Darwin did come to attribute more significance to hybridization in his later years,” but it never gained significant traction in any edition of the Origin, his most widely read book. “Certainly such ideas were never canonized among the dogmas of neo-Darwinian theory.”

For Darwin’s branching tree-of-life diagram to work, innovations must be passed along in ancestor-descendent relationships, moving vertically up the branches over time by inheritance of chance mutations. Hybrids interfere with this picture by allowing branches to share genetic information horizontally all at once. And if the branches can re-join by back-crossing, the tree metaphor becomes more like a net. Pennisi understands the challenge to Darwinism in her title, “Shaking Up the Tree of Life,” when she says, “Species were once thought to keep to themselves. Now, hybrids are turning up everywhere, challenging evolutionary theory.”

The revolution has come in three waves. The first involved microbes, when horizontal gene transfer (HGT), sometimes called lateral gene transfer (LGT), was found to be common (see Denyse O’Leary’s article last year, “Horizontal Gene Transfer: Sorry, Darwin, It’s Not Your Evolution Any More“). HGT doesn’t just complicate efforts to construct phylogenetic trees, she says; “because where HGT is in play, there just isn’t a tree of life.” In another Evolution News article, Paul Nelson cites Woese, Koonin and other evolutionists going out on a limb to dispute the existence of a universal tree of life — at least when it comes to the origin of the three kingdoms of microbes.

The second wave involved plants. As far back as 1949, Pennisi says, it was a radical idea to suggest that plant species shared genes via hybridization. Botanists grew to accept the idea, but zoologists resisted it:

In 1949, botanist Edgar Anderson suggested that plants could take on genes from other species through hybridization and back crosses, where the hybrid mates with the parent species. He based this then-radical proposal on genetic crosses and morphological studies of flowering plants and ferns suggesting mixtures of genes from different species in individual genomes. Five years later, with fellow botanist G. Ledyard Stebbins, he argued such gene exchange could lead to new plant species. Their ideas quickly hit home with other plant researchers, but not with zoologists. “There was a very different conventional view in botany than in zoology,” Rieseberg says.

Now, the third wave is encompassing the rest of biology: animals. (This wave hits close to home, involving as it does the human lineage.) Starting in the 1990s, zoologists began seeing hybridization as more than a breeder’s trick. Pennisi gives three examples of the growing realization that natural hybridization contributes to speciation in animals, too.

  1. Darwin’s finches: Peter and Rosemary Grant witnessed a hybrid finch establishing its own population, with its own phenotype, in its own ecological niche. Pennisi tells the story of “Big Bird” in a separate Science Magazine article.
  2. Butterflies: James Mallet’s work on Ecuadorian butterflies a decade ago, borrowing on earlier work by Larry Gilbert, proved that more than 30% of Heliconius species formed hybrids, “swapping wing patterns and sometimes generating entirely new ones.”
  3. Neandertals: “In 2010, a comparison between the genomes of a Neandertal and people today settled what anthropologists and geneticists had debated for decades: Our ancestors had indeed mated with their archaic cousins, producing hybrid children,” Pennisi says in the lead story. “They, in turn, had mated with other modern humans, leaving their distant descendants — us — with a permanent Neandertal legacy. Not long afterward, DNA from another archaic human population, the Denisovans, also showed up in the modern human genome, telling a similar story.”

Finding hybridization in the human lineage “created a shock wave,” Pennisi says. She quotes Malcolm Arnold whose imagination was captured by this important but long overlooked aspect of inheritance. “That genomic information overturned the assumption that everyone had.” Pennisi helps us consider the implications for evolutionary theory:

The techniques that revealed the Neandertal and Denisovan legacy in our own genome are now making it possible to peer into the genomic histories of many organisms to check for interbreeding. The result: “Almost every genome study where people use sensitive techniques for detecting hybridization, we find [it] — we are finding hybridization events where no one expected them,” says Loren Reiseberg, an evolutionary biologist at the University of British Columbia in Vancouver, Canada.

All these data belie the common idea that animal species can’t hybridize or, if they do, will produce inferior or infertile offspring — think mules. Such reproductive isolation is part of the classic definition of a species. But many animals, it is now clear, violate that rule: Not only do they mate with related species, but hybrid descendants are fertile enough to contribute DNA back to a parental species — a process called introgression.

The revolution was slow in coming till rapid genomic sequencing techniques became available. Now, with a plenitude of sequences published, what biologists had come to accept in microbes is forcing them to reconsider what they thought they knew about evolution for the entire tree of life. Pennisi all but announces the revolution:

Biologists long ago accepted that microbes can swap DNA, and they are now coming to terms with rampant gene flow among more complex creatures.A large percent of the genome is free to move around,” notes Chris Jiggins, an evolutionary biologist at the University of Cambridge in the United Kingdom. This “really challenges our concept of what a species is.” As a result, where biologists once envisioned a tree of life, its branches forever distinct, many now see an interconnected web.

Hybridization, says Mallet, “has become big news and there’s no escaping it.”

The tree metaphor is being replaced with a net or web. That’s the point where Pennisi describes Ernst Mayr, Darwin’s paramount tree gardener, hyperventilating in his grave. In a new world of rampant hybridization and introgression, what is to become of neo-Darwinism? Pennisi gives a glimpse of the implications, hinting at a revolutionary new view of the origin of species. Putting a happy face on the revolution, she ends this way:

The Grants believe that complete reproductive isolation is outdated as a definition of a species. They have speculated that when a species is no longer capable of exchanging genes with any other species, it loses evolutionary potential and may become more prone to extinction.

This idea has yet to be proven, and even Mallet concedes that biologists don’t fully understand how hybridization and introgression drive evolution — or how to reconcile these processes with the traditional picture of species diversifying and diverging over time. Yet for him and for others, these are heady times. “It’s the world of hybrids,” Rieseberg says. “And that’s wonderful.”

It will certainly be wonderful for intelligent design theorists, but it’s hard to see how Darwinians will cope with the revolution. Why? Because HGT and hybridization involve the shuffling of pre-existing genetic information, not the origin of new genetic information. Information isn’t emerging by accidental mutations; it is being shared in a biological World Wide Web! Pennisi suggests this may be advantageous:

As examples of hybridization have multiplied, so has evidence that, at least in nature, swapping DNA has its advantages. When one toxic butterfly species acquires a gene for warning coloration from another toxic species, both species benefit, as a single encounter with either species is now enough to teach predators to avoid both. Among canids, interbreeding with domestic dogs has given wolves in North America a variant of the gene for an immune protein called Β-defensin. The variant gives wolf-dog hybrids and their descendants a distinctive black pelt and better resistance to canine distemper, Wayne says. In Asia, wolf-dog matings may have helped Tibetan mastiffs cope with the thin air at high altitudes. And interspecies gene flow has apparently allowed insecticide resistance to spread among malaria-carrying mosquitoes and the black flies that transmit river blindness.

In each case, the beneficial genetic changes unfolded faster than they would have by the normal process of mutation, which often changes DNA just one base at a time. Given the ability of hybridization and introgression to speed adaptive changes, says Baird, “closing that door [with reproductive isolation] is not necessarily going to be a good thing for your long-term survival.”

Think of the possibilities for design theorists. We can see strategies for robustness with information sharing, allowing animals to survive environmental perturbations or recharge damaged genomes, for instance. Indeed, all kinds of “wonderful” possibilities open up for exploring design when information sharing is available in the explanatory toolkit. New vistas for explaining symbioses, ecosystems, and variability emerge. Could some apparent “innovations” be loans from other species? How can the information-sharing biosphere inform practical applications for medicine?

In the wonderful new “world of hybrids,” ID advocates can take the lead, breathing new life into biological explanations, while the neo-Darwinists hyperventilate to delay the inevitable.

Photo: Mule train, Grand Canyon, by dnak [CC BY 2.0], via Wikimedia Commons.