Life on Earth evolves (changes) in response to a variety natural forces. These forces that can drive long-term evolution in species are numerous (and still being worked out by biologists). For the record, the most well-known and controversial of them is natural selection, trumpeted to fame and infamy in the 19th century by a British naturalist from Shropshire named Charles Robert Darwin.
Many biologists study evolution directly, thinking constantly about natural selection (and other evolutionary forces) and how these forces can shape a population, a species, or an entire ecosystem over many generations. However, there is another evolutionary force afoot that (in this scientist’s opinion) is rarely discussed within earshot of the general public: us. Human beings (Homo sapiens). After all, we affect life on this planet. Think about all we can do, and have done, in our short history. With big brains, opposable thumbs, and dashing good looks, humankind has done much: felled forests, sowed crops, drained swamps, built and bombed cities, mowed the grass, trimmed the hedge, obliterated both smallpox and the dodo, migrated to nearly all of the habitable landmasses, and (to my delight) also found time to domesticate the cat. Just five hours ago, I got my annual influenza vaccine. So, of course our presence on Earth, equipped with modern technology and in numbers exceeding 7 billion, affects how life evolves on Earth. It’s a phenomenon that’s easy to fathom once you start fathoming. But, the details (exactly how we affect the evolution of other lifeforms) have not yet been precisely worked out.
Today, in a ritual perusal of scientific articles, I stumbled upon three studies illustrating new ways in which human beings are affecting the evolution life on Earth. All three are rather small in scope. Two studies from Canadian research teams investigate the role of human activity on large species of deer. The third study concerns a virus that infects chickens. But, in all three cases, the results are clear: humans are influencing evolution in other organisms.
The first two studies are concerned with behavior in two large deer species. In one case, whether deer behavior causes individuals to be killed by human hunters, and in another case whether deer behavioral responses to human activity affect the survival of their offspring. Both studies make use of global positioning satellite (GPS) technology to tag individuals within a particular population, track their movements, analyze how individual movements effected survival (in one study) or offspring survival (in the other), and correlate those movements and survival rates with the types of environments individuals chose to inhabit.
Scientists from the University of Alberta, the University of Calgary, and Alberta Innovates Technology Futures (Vegreville, Alberta) studied human predation of elk (Cervus elaphus). They wanted to know whether personality traits that within an elk population affected survival rate against human hunters. Elk, like humans and many vertebrate (backbone-bearing) animals, show a range of personality traits, from painfully shy to big and bold. These scientists wanted to know who is more likely to survive a hunting season: bold or shy elk? They used GPS collars to track male and female elk across their Rocky Mountain range in Alberta, British Columbia, and Montana. Bold elk moved frequently, and didn’t hesitate to venture into areas where they were more likely to encounter humans (such as roads or flat open spaces). Shy elk avoided open areas, and in general made slower, less aggressive moves. All the males elk they tagged and tracked were the same age (approximately 2.5 years), making them relatively young and inexperienced adults. Among these males, bold individuals (based on their GPS-tracked movements) were much more likely to be killed by human hunters than their shy counterparts.
Scientists also tracked female elk, though their ages ranged from 2 to 19 years. All the females that were killed by hunters were 9 years old or younger, and displayed behavioral traits similar to the bold males: they didn’t avoid open areas or roads, and were more likely to run or make sudden movements instead of staying put, slowing down, or hiding. All the older females survived, and behaved more like the shy males. They avoided open areas and moved slowly, making themselves less likely to be noticed. This may indicate the influence of social learning on behavioral patterns. That is, older elk may have learned from experience to stay put and keep quiet. For Rocky Mountain elk, boldness will cost you: at the trigger of a hunter’s rifle. But with age comes wisdom, as older elk avoid areas that make them more likely to encounter a bullet.
From eastern Canada comes a study that also concerns predation, but not from humans. Scientists from the Quebec Ministry of Natural Resources and Wildlife, the University of Quebec, and Laval University joined forces to investigate how human activity affected survival in another large deer species, the woodland caribou (Rangifer tarandus caribou). Unlike the Alberta study which focused on survival of adults against human predation, the Québécois researchers surveyed survival of young calves against natural predators based on the habitats their mothers chose. Over a period of almost three years, they tracked caribou mother-offspring pairs in the Charlevoix, a forested region approximately 60 miles north of Quebec City. The Charlevoix region offers a variety of habitat options for mothers: steep hills, deep valleys, thick forest, and open roads. Charlevoix is also timber country. Over one-quarter of the land has been harvested for timber within the past three decades. Thus, this large region offers an ideal mix of habitats (both human-influenced and natural) to investigate how caribou and their predators interact.
Previous ecological studies indicated that young caribou in the Charlevoix often fell victim to two predators: grey wolves and black bears. But, human activity (logging, road construction, etc.) affects the range of these predators. Could caribou mothers protect their offspring from natural predators in an environment where humans were reshaping the landscape? The survey results were striking in their answers: yes and no. On the one hand, caribou mothers and calves tended to avoid low-lying areas and human-built roads, both areas frequented by grey wolves. Thus, only a single calf from this study (out of 38 total) fell victim to grey wolves. But, clinging to higher elevations and more heavily forested portions of the Charlevoix made caribou calves more vulnerable to black bears, as 21 out of 38 calves (55%) were killed by these “occasional” caribou predators. There was a silver lining to this grisly finding: while highly-forested regions in general were bad news for caribou calf survival, caribou mothers and calves that chose “young” forested regions (which had been cleared by logging activity and only recovered for several years) fared relatively well. These recently-cleared regions are less dense and lack tall trees. They may have offered sufficient protection to mother-calf caribou pairs while still offering the nutrients they needed to survive, while lacking the more rich vegetation that could draw black bears. Thus, while human activity may have sent caribou out of grey wolf-infested areas and right into the paws of higher-elevation and forest-loving black bears, recent human logging activity at these elevations may have spared some of the 14 of 38 calves (37%) that survived this three year survey.
The third study I encountered offers (as they’d say in Monty Python) “something completely different.” Shifting from large deer in Canada to domesticated poultry, the question becomes: what effect does human activity have on a bird virus? That is the question scientists from the United States, the United Kingdom, and Australia sought to address through their analyses of previously collected veterinary data from domesticated chickens. Their target was a virus called Marek’s disease virus (MDV), a pathogen that has evolved into a rather nasty poultry disease in the past 60 years.
MDV spreads among chickens through the air, though it does not infect humans. Before the 1950s, MDV-infected chickens suffered from a mild form of paralysis caused by tumors. Rarely, they’d die. After the 1950s, MDV evolved rapidly into a chicken-killing machine. It causes tumor growth all over the chicken’s body, in some cases causing all infected birds in a farm to die. Why did MDV evolve so rapidly into this deadly virus? Scientists investigated two innovations in chicken rearing that may have influenced MDV evolution: vaccination and nutrition. Decades ago, veterinarians developed vaccines to mediate the symptoms of MDV. Critically, while these vaccines prevented disease among infected individuals, these vaccines did not stop MDV from passing from chicken to chicken. Poultry farmers left and right scrambled to vaccinate their birds against MDV, which is now standard practice for any large-volume chicken coop. During the same period, farmers also increased the nutritional content of chicken feed and the quality of their rearing facilities. The result of these improvements was a dramatic reduction of the amount of time it takes to rear a chicken for the market. What once took over 70 days to produce a fully grown broiler chicken now takes half that time. In 35 days or less, a chicken is off to the grocery store. So, in attempting to ascertain why MDV evolved into such a nasty virus in as little as six decades, scientists tried to dissect which matters more to the virus: a widely-used vaccine which eliminates symptoms but does not affect transmission, or a shorter lifespan for its domesticated host? Mathematical models pointed toward a slight preference for the former phenomenon: vaccination. Thus, it appears that this imperfect (but widely used) vaccine, which permits viral transmission without the nasty symptoms, may have pushed MDV to evolve into a highly-virulent strain which gave the un-vaccinated birds a rather punishing and deadly version of the disease.
These studies (and their thousands of other seldom-acknowledged counterparts) are additional lines of evidence that humans are affecting the evolution of life on this planet. From Rocky Mountain elk and Québécois caribou to bird tumor viruses, our fingerprints are widespread and here to stay. I’m not using these studies to heap blanket criticisms on our roles as ecological masters of this planet (though it is clear that we’ve much to learn when it comes to being wise caretakers of the only habitable pearl of rock we’ve encountered to date). I merely highlight these studies so we can at least acknowledge the effects we have, from climate change to viral evolution, and every level in between. Our ancestors let the genie of human meddling out of the bottle. There’s absolutely no way to put that genie back, and we should at least be fully aware of these fact and our abilities as we decide what we’re going to do with this authority. Humanity is not the only driving force for evolution of life on this planet. But, we’re certainly in the driver’s seat. We should at least keep this in mind as we decide which road to follow.
- Ciuti S, Muhly TB, Paton DG, McDevitt AD, Musiani M, Boyce MS. 2012. “Human selection of elk behavioural traits in a landscape of fear.” Proceedings of the Royal Society, Series B. doi: 10.1098/rspb.2012.1483. [Subscription required to access full text.]
- Dussault C, Pinard V, Ouellet JP, Courtois R, Fortin D. 2012. “Avoidance of roads and selection for recent cutovers by threatened caribou: fitness-rewarding or maladaptive behaviour?” Proceedings of the Royal Society, Series B. doi: 10.1098/rspb.2012.1700. [Subscription required to access full text.]
- Atkins KE, Read AF, Savill NJ, Renz KG, Fakhrul Islam AFM, Walkden-Brown SW, Woolhouse MEJ. 2012. “Vaccination and Reduced Cohort Duration Can Drive Virulence Evolution: Marke’d Disease Virus and Industrialized Agriculture.” Evolution. doi: 10.1111/j.1558-5646.2012.01803.x. [Subscription required to access full text.]
- Sunset Country, Ontario (“The Great Canadian Experience”): Woodland caribou cow and calf.