We tend to fixate on the drastic changes caused by evolution over huge timescales. It’s easy to ignore the small variations between generations that add together over time to form the big evolutionary changes we focus on. This unintentional side-lining of small adaptations can blind us to the ways in which humans are directly affecting the evolutionary processes of nature. From tuskless elephants to fish that can’t smell, animals are developing specialized adaptations to allow them to live in ecosystems that have been disrupted and altered by mankind. These adaptations are one step in the evolutionary process that already bears the unmistakable marks of humanity’s influence.
Just as humans are changing the planet, they’re changing the fauna that inhabit it. Here are some examples of how.
The Peppered Moth
One of the most poignant examples of unintended evolutionary consequences, the peppered moth (Biston betularia), is found in most of the world, including Asia, North America, and Europe. One place that’s especially relevant to a discussion of its evolution, however, is England. That’s because it was in England that the peppered moth adapted to its dynamic surroundings by changing its color, not once but twice!
Prior to roughly 1811, only the light-coloured form of the peppered moth (called Biston betularia morpha typica) was common to see in England. However, in the following half a century the dark-coloured form (called Biston betularia morpha carbonaria) came to dominate peppered moth sightings. By 1864, researcher R.S. Edleston noted that it was more common to see the dark carbonaria than the light typica.
Image source: Wikipedia
Image source : Flickr
To thank for this melanistic modification we have the industrial revolution! Due to new coal-powered factories, the English countryside became blanketed with soot. Many of the lichens and mosses that traditionally covered trees were killed by sulphur dioxide emissions, leaving trees much darker than they once were. In this new environment, the carbonaria form of the peppered moth thrived. While its inky coloration had made it stand out against the light-coloured trees and lichen, it now gave it exceptional camouflage capabilities. The light color of the typica form, once a useful protective feature against predators, now made them an easy target as they contrasted sharply with their surroundings.
Image source: Wikipedia
While the carbonaria ascendancy was almost comprehensive (by 1895 it was reported that 98 percent of peppered moths seen in Manchester were of the carbonaria form), its reign was destined to come to a close. The end of the industrial revolution around 1840, and the subsequent years since, have brought cleaner factories, air, and environments to England. With trees no longer covered in soot, and mosses and lichens thriving as before, the tables again turned, making carbonaria the odd-moth-out, and typica the camouflage king once more. In the twenty-first century, it is once again uncommon to see a carbonaria type peppered moth in England.
While a scientific paper detailing the changes in moth colorations wasn’t published until 1896 by J.W. Tutt, over a decade after Charles Darwin died, the peppered moth’s colorful evolutionary history would have provided evidence to support Darwin’s theory of natural selection. Interestingly, it seems a British entomologist named Albert Brydges Farn wrote to Darwin in 1878 telling him about a type of moth that he had observed be white, black, grey, brown, and red depending on its environments. Unfortunately, it seems that Darwin never replied to this letter. For all we know he never even read it. A real shame, given that, as H.B.D Kettelwell wrote in 2009, “had Darwin observed industrial melanism he would have seen evolution occurring not in thousands of years but in thousands of days.”
The Red Fox
A study published on June 3, 2020, looked at 111 red fox (Vulpes vulpes) skulls collected in London and its surrounding boroughs and found something really interesting. When the skulls were categorized according to their collection site, rural versus urban, a trend emerged. Urban foxes had shorter, wider muzzles than their wild counterparts.
The researchers believe that the changes were “likely driven by differing biomechanical demands of feeding … between habitats.” Put more simply, because an urban fox’s diet can be made of more than 37 percent scavenged human food, which is found, accessed, and eaten in a very different way to traditional prey, their jaws are adapting. A shorter snout reduces how quickly a fox can snap their mouths shut, something that’s important to catch a rabbit, but not so important to catch McDonald’s leftovers. The shorter snout does, however, improve the mechanical strength a fox can exert with their jaw.
Besides their muzzles, urban foxes also showed differences in their brain size and sexual dimorphism (differences between males and females). These changes are consistent with something called “domestication syndrome,” which describes a set of behavioral and physiological traits that domesticated animals exhibit but their wild ancestors don’t. Notably, previous experiments in domesticating foxes based on their behavioral traits (specifically their friendliness toward humans) also resulted in reduced muzzle sizes.
While domesticating a wild animal is an obvious and extreme way for humans to influence the genotypes and phenotypes of animals, these changes in urban foxes highlight the steps that come before domestication. Though the humans that live around urban fox populations are (presumably) making no distinct attempts to domesticate them, their morphologies are changing none the less. Humans are influencing the traits of foxes without even trying to, simply by living near them.
The African Elephant
If I ask you what the difference between a male and female elephant is, you’re liable to tell me that only the males have tusks. That, however, is only partly correct. It’s true that in the Asian elephant (Elephas maximus) only the males grow tusks, but in African elephants (the African bush elephant, Loxodonta Africana, and the African forest elephant, Loxodonta cyclotis) both sexes have the iconic extended teeth. That is unless they’re elephants from Gorongosa National Park in Mozambique.
Elephants have been hunted for their ivory for centuries, but between 1977 and 1992, during Mozambique’s fifteen-year civil war, 90 percent of the elephants in Gorongosa National Park were killed to fund the fight. Males were targeted first because their tusks are larger and provide more ivory, but females eventually fell victim to the ivory trade too.
Naturally, only 2–4 percent of female elephants do not develop tusks. Today, however, of the female elephants that survived the civil war, 51 percent are tuskless. Their offspring show a similar decreased interest in tusk-growing, with 32 percent of the females being tuskless.
These genotypic and phenotypic effects of the ivory trade can also be seen in other countries. At Ruaha National Park in Tanzania, 35 percent of the female elephants who survived poaching in the 1970s and 1980s are tusk-free as well as 13 percent of their female offspring.
Image source: National Geographic
The implications of this tuskless trend reach beyond just elephants and their progeny. While not having tusks will impede elephants’ abilities to dig for water, strip bark off trees and fend off both rivals and predators, it will also affect other flora and fauna that live alongside elephants. For example, the holes elephants dig provide water to those who come after them, and the trees they knock down provide shelter. It’s almost certain that we haven’t fully understood all the ecological consequences of elephants losing their tusks.
The European Bass
As the last example of how humans are influencing the evolution of animals, let’s take a trip to the ocean. The European sea bass (Dicentrarchus labrax) is native to the waters west and south of Europe. Like many fish, the European bass migrates with the seasons, relying largely on its sense of smell to guide its swimming. Unfortunately, human activities in the next eighty years could severely impede their ability to do that.
As carbon dioxide (CO2) emissions continue to deposit themselves in bodies of water, ocean pH continues to drop. This increase in acidity causes coral bleaching and is generally bad for aquatic species. By the end of the century, ocean CO2 levels are predicted to be 800–1,000 µatm. This is more than double the current CO2 levels of 400 µatm and seems to be more than high enough to influence the olfactory senses of the European bass.
In tests, when exposed to CO2 levels of 1,000 µatm bass needed to be 42 percent closer to a source of an odor to detect it. They also seemed less likely to recognize the smell of a predator. Sea bass kept at 400 µatm “reduced their activity by 50% in the presence of the predator odour, whereas sea bass exposed to elevated CO2 levels reduced their activity by only 20–27%.”
These findings do not bode well for the future of the European bass. Assuming that humans do not curb their CO2 emissions, these fish will be forced to adapt and evolve in new ways to overcome their loss of smell. It may be by increasing the sensitivity of their other senses, changing habitats, or developing new methods of olfaction. That’s assuming that they don’t just die out and go extinct. We can’t really predict what will happen to them, and that’s kind of the point. Humans barely understand the complex ways our actions are affecting organisms in the present, never mind how we’re changing their evolutionary futures.
Thankfully, it is possible to avoid disrupting the olfaction of the European bass, and we certainly should try. While one type of fish needing to be 42 percent closer to a source of an odor to detect it doesn’t seem like a big deal, the general rule of ecology is that everything is connected. Even if humans made peace with never ordering the sea bass main at a restaurant again, changes to bass behavior such as a decreased ability to sense and avoid predators will impact their ecosystems, and therefore all the flora and fauna that call it home. While the European sea bass hasn’t lost its sense of smell yet, this study reminds us how these fish’s evolution could be affected if action is not taken now.
Evolution takes place over millions of years, but that doesn’t mean that it isn’t also happening all the time. Humans are changing animal’s evolutionary courses whether they mean to or not and have a responsibility to ensure that these changes aren’t for the worse. If you ever wished you could actually see evolution happening, the good news is that you can. We just need to make sure that this human-guided evolution doesn’t spell the end of the species we know and love.