Is There a Compelling Reason to Bring This Species Back?
Probably—and hopefully—the first question that comes to the minds of most people when contemplating de-extinction is “Why?” Why this species? Why now? Why here? As I noted before, most people advocate for species that they know, beyond reasonable doubt, are extinct because of something that humans have done. Bringing these species back goes some way to mitigate the guilty conscience of an ecological savvy human. But mitigation of guilt is not a compelling reason to bring something back to life. I may feel some guilt by association with my Native American ancestors, who probably participated in hunting short-faced bears, and with my European ancestors, who probably were in some way involved with the extinction of the Neandertals. This does not mean I want to bring back short-faced bears and Neandertals. Indeed, in both of these cases, de-extinction for the purposes of alleviating guilt seems remarkably selfish; what kind of existence would either of these have in the world today?1
Compelling reasons to bring something back to life are more likely to relate to the species themselves and the roles these species are likely to play in the environment of the present day. For example, if the species filled a particularly important niche within its ecosystem, then its loss is likely to have resulted in chaotic destabilization of that ecosystem. Bringing it back might restore lost interactions between species and restabilize the ecosystem, in turn saving other species from extinction. The kangaroo rats that I mentioned earlier are a good example of keystone species that play important, stabilizing roles in their ecosystems. The Cascade Mountains wolf—an additional species suggested for de-extinction by a student in my class—is another. Importantly, both of these are very recent extinctions, and their ecosystems may not yet have adapted to accommodate their loss.
The potential role of the Cascade Mountains wolf in maintaining ecological balance can be extrapolated from work in Yellowstone National Park over the last two decades. When wolves were reintroduced to Yellowstone National Park in 1995, many people were convinced this was going to lead to disaster. The cause for concern was that wolves are predators and, as such, would likely depredate livestock from local farms, much to the dismay of the ranchers who depend on the livestock. This was an appropriate concern. As wolf populations have grown, there have been many instances of wolves taking livestock. However, the main source of the wolves’ diet is local wildlife, and elk in particular. By 2006, the elk population in Yellowstone had shrunk to 50 percent of the size it had been when wolves were first reintroduced to the park. Today, elk are no longer overgrazing the plants and young trees that grow along the meadows and valley bottoms, and consequently, woody plants are making a comeback throughout the park. The increase in woody plants provides a greater diversity of habitat for small mammals, whose populations are also on the rebound. Wolves are outcompeting coyotes, whose populations had become much larger after the disappearance of wolves. Fewer coyotes is good news for the animals that coyotes like to eat, including red foxes, pronghorn, and sheep.
Certainly, wolves are predators. Wolves will take livestock when they have the opportunity to do so. However, it seems clear that restoring wolves to Yellowstone National Park has played a critical role in stabilizing the Yellowstone ecosystem.
The Cascade Mountains wolf is a subspecies of gray wolf that lived in the mountains of Washington, Oregon, and British Columbia until around 1940. Based on the positive results of the Yellowstone wolf reintroduction, there is compelling ecological reason to bring back the Cascade Mountains wolf and reintroduce it to its former range.
The case of the Cascade Mountains wolf touches on another intriguing issue. This wolf was a subspecies of gray wolf, and not its own, distinct species. This raises a different question: is it appropriate to select a subspecies for de-extinction?
Before attempting to answer that question, I should first clarify what it means to be a subspecies as opposed to a species or population. From an ecological perspective, a population is a group of individuals of the same species that live together in the same place. The individuals interbreed, compete with each other for resources, and share the same geographic space. A species tends to be defined as an evolutionary lineage that is reproductively isolated from all other evolutionary lineages. Individuals of the same species can move between populations with little consequence to their ability to find a mate and reproduce. Individuals belonging to different species cannot mate. Or if they do, the offspring that are born either do not survive into adulthood or cannot have offspring themselves.
This species-as-reproductively-isolated-lineages concept, known as the biological species concept, was formally described by Ernst Mayr in 1942. The concept turns out to have some flaws. Specifically, some lineages that we strongly believe are separate species are not strictly reproductively isolated. Polar bears and brown bears, for example, are commonly considered to be two different species. But bears born from crosses between brown bears and polar bears survive and can continue to mate and produce offspring. Dogs, wolves, and coyotes can and do interbreed frequently. Cows and bison and yaks can all interbreed and produce fertile offspring. And ancient DNA from Neandertal bones revealed that our species can (and did) mate with Neandertals and that, as a result of this hybridization, Neandertal genes survive in all living humans with Asian or European ancestry.
Why do biologists hold on to this confusing system? As humans, we are compelled to categorize. When we see chaos, we desire to transform that chaos into something ordered so that our brains can make sense of it. Clearly, evolution does not work in absolutes. An animal is not born one day as an entirely new species, incapable of reproducing with anyone in its parents’ species. Instead, speciation is a long process involving many underlying genetic and behavioral changes. Populations become geographically isolated and evolve along independent trajectories. Eventually, enough changes will have evolved so that individuals are incapable of breeding between populations. As we see with brown bears and polar bears and with humans and Neandertals, however, what common sense would call species-level differences will sometimes evolve before the two lineages are completely reproductively isolated.
To impose order on the disorder that is biology, Carl Linnaeus, an eighteenth-century Swedish biologist and physician, devised a taxonomic system to describe and categorize all forms of life. His system provides a hierarchical classification of everything according to its relationship with everything else. The biggest bins classify organisms into kingdoms: Animalia, Plantae, Fungi, Protista, Eubacteria, and Archaeobacteria (although the latter two are sometimes grouped into one kingdom, the Monera). Wolves, dogs, bears, snakes, and rabbits are all animals, so they all belong to the kingdom Animalia. Within that, wolves, coyotes, bears, and rabbits are mammals (class Mammalia). Wolves, coyotes, and bears are carnivores (order Carnivora). Wolves and coyotes are canids (family Canidae). Both belong to the genus Canis, but wolves are Canis lupus and coyotes are Canis latrans, where lupus and latrans are the official Latin names for the two different species.
After that, it gets messy. Sometimes species are subdivided into subspecies. But this is tricky. Some taxonomists will refer to a population that seems to be particularly isolated from other populations as a subspecies, while a different taxonomist might look at the same population and decide that it is not sufficiently different to merit subspecific status. Unlike a species, there’s really no rule to go by to decide whether a subspecies is real or not.
What does all of this have to do with de-extinction? A lot. If a subspecies is not real, or if it is just a slightly different version of something that is not extinct, should time and energy be spent to bring that subspecies back to life?
Sometimes subspecies are defined geographically. This means that, while there may be no physical or genetic barrier to interbreeding, they are simply too far apart in geographic terms for interbreeding to take place. For example, it is not particularly likely that the Iberian wolf will breed with the Mexican wolf. In the absenc
e of interbreeding, the two populations will accumulate genetic differences that make them look and act differently from each other. There is no doubt, however, that both of these are wolves. So, if either Mexican wolves or Iberian wolves were to go extinct, would it be reasonable to use de-extinction technology to bring them back?
Consider a hypothetical scenario in which there are two subspecies that are ecologically very important in their ecosystems—keystone species—and one of these goes extinct, destabilizing the ecosystem in which it lived. The two subspecies are very closely related to each other. In fact, the only things that differentiate them are that they lived in different places and had some small morphological difference—let’s say they had slightly differently shaped ears. To restabilize the ecosystem, we plan to reintroduce the extinct keystone species. Is it better to bring it back to life using de-extinction science or to introduce the close relative? In other words, how different does an extinct lineage have to be from a living lineage to justify its de-extinction?
From a technical perspective, de-extinction of a subspecies like the Cascade Mountains wolf would be much simpler than de-extinction of a distinct species. As I will discuss, assembling the genome sequence of an extinct organism can be extremely challenging and requires a guide genome to act as a scaffold onto which the short, damaged fragments of ancient DNA can be mapped. The Cascade Mountains wolf genome could be assembled using another gray wolf genome as a guide, simplifying this process greatly. Cascade Mountains wolf embryos could be implanted into a mother gray wolf, and families of gray wolves could rear the pups in established gray wolf packs. This then raises the question, how would the purportedly unextinct Cascade Mountains wolf differ from the wolf subspecies into which it is born? Would it be preferable simply to introduce another gray wolf subspecies into the Cascade Mountains?
While some species or subspecies seem too similar to living species to justify their de-extinction, other extinct species have no evolutionarily close living relatives. This argues both for their de-extinction, because bringing them back will restore more evolutionary novelty than would bringing back something that has a close, living relative, and against their de-extinction, because bringing them back will be much more costly to achieve.
Moa, for example, were subdivided into three extinct families within the order Dinornithiformes, which has no living representative. The closest living relative to the moa is the tinamou, and the common ancestor of moa and tinamou lived around 50 million years ago. The moa represents a long history of independent evolution, and bringing it back would restore many unique traits to the world. However, with no close relative, it would be extremely hard to assemble the broken bits of DNA recovered from moa bones into a reasonably accurate moa genome. In this case, the guide genome would be more than 100 million years diverged—twice the evolutionary distance to the common ancestor—from the ancient genome. The same is true for any extinct species that lacks an evolutionarily close living relative. Identifying appropriate surrogate mothers or eggs within which the embryos could develop would be extremely challenging for species that lack close living relatives. We would also have little way to know what the native behaviors of such species should be, how much parental care would be required to rear them, or how to mimic this parental care or other important social interactions. In a sense, these individuals might be too different from anything that is alive for de-extinction to succeed.
The ideal candidate for de-extinction has both sufficiently closely related living relatives to make its de-extinction feasible and unique traits or adaptations to a particular habitat. The golden toad, for example, was last seen in the cloud forests of Costa Rica in 1989. It was such a peculiar, bright orange color that Jay Savage, the herpetologist who described it, had trouble believing it was real and not some trick. The golden toad was tiny—adult males measured a bit more than five centimeters in length—and is a good candidate for de-extinction. It belongs to the genus Bufo, which is species-rich and diverse and therefore has many close relatives that are not extinct. However, among its many close relatives, the golden toad was the only Bufo to display such a striking orange color. What if the proteins that made the orange color had some undiscovered medical purpose, or psychoactive properties? We’ll never know until somebody licks one, and for that we’ll have to bring it back.
Finally, the ideal candidate for de-extinction may be one that has only recently gone extinct. Ecosystems are constantly in flux. They are influenced by abiotic changes, such as how much rain falls or how cold the winters are, and biotic changes, including both species extinctions and introductions. When a species goes extinct, the ecosystem in which it lived changes to adapt to its disappearance. If the extinction took place many thousands or possibly even only several hundred years ago, it may be that reintroducing that species would actually destabilize whatever new equilibrium that ecosystem had achieved. This does not mean that the only acceptable de-extinctions will be of recently extinct species. Certainly, some species, such as large herbivores, played roles in ancient ecosystems that have not been filled in their absence. As I discuss below, it may be that careful evaluation of the relative risks and rewards of their de-extinction will lead to the conclusion that they, too, should be brought back on the grounds of their potential beneficial impact on the environment of the present day.
Why Did This Species Go Extinct the First Time?
People tend to be most interested in bringing back species that went extinct because of human activities. Given that proposition, asking the question “Why did they go extinct?” seems a bit silly. In fact, it is not at all silly. Humans are remarkably creative when it comes to killing things.
We killed many species using brute force. In the nineteenth century, we netted and shot billions of passenger pigeons, eventually leading to their extinction (figure 3). When Europeans arrived in North America, passenger pigeons were thought to make up 25–40 percent of the bird population in the eastern United States. In 1866, a report described a single flock of more than 3.5 billion passenger pigeons flying across the Ohio skies, taking more than fourteen hours to pass overhead. At one o’clock in the afternoon on the first of September 1914, the last remaining passenger pigeon, named Martha, died in captivity in the Cincinnati Zoo (plate 1).
Figure 3. A flock of migrating passenger pigeons. Drawing from The Illustrated Shooting and Dramatic News, July 3, 1875. ©The Archives and Manuscripts Department, John B. Cade Library, Southern University and A&M College.
Extinction due to overexploitation is a common theme in human history, and a human tendency that we are still grappling to overcome. Steller’s sea cow was a large—nine meters long and up to ten tons in weight—marine mammal whose closest living relative is the dugong (figure 4). Steller’s sea cows were once abundant throughout the North Pacific but were hunted to death after their discovery in the eighteenth century. Overexploitation is also blamed for the extinction of the great auk, which was taken for its fat, feathers, meat, and oil. We continue to overexploit the species we rely on today. In 2012, the State of the World Fisheries and Aquaculture Report reported that 30 percent of the world’s fisheries were overexploited and would require strict management to be sustainable into the future.
Figure 4. Steller’s sea cow, Hydrodamalis gigas. Illustration by J. F. Brandt, 1846. This image was published in Extinct Animals, by E. R. Lankester (London: A. Constable, 1905).
But we don’t only kill things with brute force. Indirect effects of human population growth—including the conversion of wild habitat for cities, towns, and agricultural land; deforestation; monoculture; and the construction of roads and highways to connect all of these things—result in changes to habitats that disrupt and destabilize ecosystems, leading to extinctions. Bird species are particularly susceptible to habitat destruction; the gradual clearing of forests in order to make way for people and agriculture on the islands of the Pacific alone has been blamed for the extinction of thousands of bird species. Indeed, habitat destructio
n accounts for more than half of the endangered birds of the world today.
We also move around the world, and as we move we bring stuff with us, both accidentally and intentionally. We introduce parasites, predators, and competitors to ecosystems in which these organisms did not exist previously, resulting in extinctions. Again, birds on islands are particularly susceptible to introduced predators, especially rats, cats, and snakes, which are keenly adept at finding and consuming eggs. Rats and cats are blamed for the extinction of the Tahitian sandpiper and Society parakeet in the Society Islands; the dodo, solitaire, Reunion pigeon, Rodrigues starling, and red rail in the Mascarenes; and the Raiatea parakeet, white-winged sandpiper, and Maupiti monarch in French Polynesia, to name only a few. In addition to introduced predators, diseases brought to these new locations spread from domestic to wild species, also causing extinctions.
And, with the byproducts of agriculture and industry, we pollute the world around us. The Yangtze River dolphin is an example of how pollution, combined with habitat destruction, can lead to extinction. The Madeiran large white butterfly was declared extinct recently, its extinction blamed on a combination of habitat destruction and pollution from agricultural fertilizers.
In some cases, even when it is clear that humans are ultimately at fault for an extinction, it is very difficult to know what the proximate causes of that extinction were. If the cause of the extinction is not completely understood, the species is most likely not a good candidate for de-extinction. If we don’t know what caused a species to go extinct the first time, how can we know that it won’t quickly become extinct again? Equally, some species for which we do know the most proximate cause of extinction are poor candidates for de-extinction. For example, despite the emotional allure of dodo de-extinction, if we were to bring dodos back and reintroduce them to Mauritius, their eggs would be promptly gobbled up by the large rat and cat populations that thrive on the island today.
How to Clone a Mammoth Page 4