Ebola and evolution
The Ebola outbreak in West Africa has international medical organizations on high alert and people all around the world antsy — even those who live in the Americas and Australia, oceans away from the disease's epicenter. The disease is normally carried by animals like fruit bats, but occasionally makes the jump to humans, and when it does, it is deadly, killing more than half of those infected. However, because it is only spread by direct contact with bodily fluids, most of the world need not fear for their lives. In recent months, some media outlets, and even a scientist or two, have begun to wonder aloud whether the Ebola virus could "mutate" and become airborne — but of course, what is actually meant is whether the virus can evolve in ways that allow it to be passed along more easily, just as the flu can be spread by a sneeze. Here we'll unpack the question of Ebola's evolution a little further and see why this outcome is unlikely.
Where's the evolution?
First, it's important to be clear about the difference between mutation and long term evolution. Mutation is a normal process that occurs every generation whenever an organism reproduces. Ebola incurs mutations whenever it is copied, just as humans, plants, and other organisms introduce new mutations to their genomes each time they reproduce. Evolution occurs when these mutations are passed down from generation to generation and spread through a population, often via natural selection. The idea here is that if an Ebola virus particle were to wind up with mutations that allowed it to be transmitted through the air, that viral lineage (i.e., the original particle's descendants) might be favored and spread because those viruses could infect new hosts more rapidly than other Ebola viruses could. However, mutations are random, that is, they are not biased to produce "good" or "bad" effects. In the case of Ebola, this means that the virus is not mutating "in order to" spread faster and that mutations allowing this occur are not more likely than mutations with any other effect.
Nevertheless, we do know that Ebola is experiencing plenty of mutations. Ebola has RNA, not DNA, as its genetic material. When RNA is copied, many more mistakes are made than when DNA is copied. This gives viruses like Ebola a particularly high mutation rate when compared to DNA-based viruses like smallpox or chickenpox — though not as high as the rates at which HIV and the flu accumulate new mutations.
Ebola's high mutation rate and rapid rate of replication combine to allow it to evolve quickly. High mutation rates generate lots of genetic variation for evolutionary processes to work on. And short generation times mean that processes like natural selection and genetic drift can sort through that variation and cause evolutionary change on timescales that are short in human terms.
A recent study of the current Ebola outbreak — the worst in recorded history — found that the virus is evolving much more quickly than in the past. Why would Ebola's evolution have sped up so much? One possibility is that we are observing this pathogen spread through a relatively novel host: humans. Ebola normally lives in wild animals — bats are the most likely candidate — and has become well adapted to that host. As Ebola replicated and was passed from bat to bat, any random mutation that made life in bats easier would have been favored and spread. This process of adaptation would have occurred over and over again until the virus was fairly well adapted to its host. At that point, random mutation would only rarely generate a variant that actually improved the situation for the virus, and so its evolution would have slowed — that is, until Ebola's environment changed, as it has now. Ebola (at least the strains that we are most interested in) now live in humans, which have many physiological differences from bats. The quick pace of evolution in the recent Ebola outbreak may in part reflect the initial stages of the virus' adaptation to humans, as natural selection favors mutations that make the virus more successful in its new host.
However, the fact that the virus is evolving does not mean that killer sneezes are just around the corner. From tracking the evolution of other viruses, we know that there are some traits that viruses are much more likely to evolve than others. Viruses sometimes evolve traits that make them more or less virulent. The virus that causes dengue fever, for example, seems to be evolving characteristics that cause worse symptoms. We know that many viruses rapidly evolve in response to medications. For instance, HIV can quickly evolve resistance to antiretroviral drugs. And, of course, we have observed that viruses can adapt to new hosts. HIV came to humans from chimpanzees, SARS came from bats, and now it seems that Ebola has moved to human hosts from bats as well. However, evolving a new mode of transmission — e.g., a viral lineage switching from being transmitted by blood to being transmitted by air — seems to be much, much rarer. HIV has been infecting humans for more than 100 years, reached epidemic levels in the last 30 — and yet remains a virus that is transmitted only by bodily fluids. We haven't ever seen a switch in transmission mode occur in any of the viruses that cause serious human disease today. In fact, the viral family to which Ebola belongs (the Filoviridae) seems be about 10,000 years old and, as far as we know now, is entirely made up of viruses that are transmitted by body fluids. Though it's not completely impossible that an unusual series of mutations, combined with natural selection or genetic drift, could result in airborne Ebola, based on the available evidence, it is extremely unlikely.
So should we be worried about Ebola's evolution at all? Yes. Researchers studying the recent epidemic found that the virus is accumulating many mutations in the region of its genome that diagnostic tests use to identify the disease. Too many mutations of this sort could render our standard tests inaccurate, making it much more difficult to diagnose Ebola infections and hampering the medical community's ability to treat and contain the virus. Such changes could also impact vaccine development and therapies for treating the disease. While Ebola's ongoing evolution is unlikely to lead to an airborne virus, it is likely to lead to other changes that will affect how we fight this deadly outbreak.
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Understanding Evolution © 2020 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California