How Humans are Shaping Evolution

Madelaine Stannard gives a beginner’s rundown of Darwin’s natural selection, and how humans are shaping evolution of the animal kingdom.

What is Natural Selection?

You don’t have to be a zoologist to know the name Darwin - you don’t even have to understand a word of ‘The Origin of Species’ to know that he was one of the greatest minds of the 19th century. A steady march throughout the 21st century has brought with it an enlightened understanding of sociological topics, such as race and gender, however some of Darwin's highly criticised theories contain colonial and racist subtones. But looking at Darwin’s work through an entirely scientific, evolutionary lens, biologists, naturalists, and denizens of planet Earth without an academic credit to their name can observe one of the most controversial, and yet oh-so-applicable theories we have - evolution.

It shocked the nation to its core, snatching at the heart of religious belief, but the great thought behind the ideas of Darwin, Lamarck, Wallace and others was this: all living things are related, and all have their origins, deep in the prehistoric history of the planet.

It was Darwin’s conclusion that humans, Homo sapiens, arose from an ape ancestor deep in the Miocene, that elicited the strongest reaction from the general public, and his ideas were for some time widely criticised. But for many people today, it is of firm belief that from monkey arose man, and a series of gradual changes to form and function, over the course of millennia, has resulted in the human race.

Darwin’s ideas of human evolution shocked society, but his theory of natural selection is one of natural science’s most important principles, underpinning all biological life. Image Credit: Alexas_Fotos on Pixabay.

Survival of the fittest is not just a slogan to be slung around on macho t-shirts and assorted paraphernalia. Organisms which accumulate random, and most importantly, beneficial mutations to their genetic code tend to (in the simplest terms) just do better. It is their enhanced ability to adapt with ease and plasticity to disease, climate change, never-constant environmental conditions, and cascade effects in their food webs, that inevitably means they leave more of their offspring behind with their genes.

When the next generation of organisms inherits these advantageous genes, they too fare better than individuals past, who perished in response to the challenges they failed to overcome. In this way, an entire population can be shaped as favourable genes increase, bringing forth a somewhat augmented frequency of new traits in the species.

A peppered moth with a genetic mutation for cryptic camouflage successfully evaded predators at the height of the Industrial Revolution, when the soot-covered trees and smog of the city made the moth harder to detect when it lay flat against the bark. The moth that remained white was an easier target for avian species, who snapped it up with their beaks as easily as following Hansel and Gretel’s breadcrumb trail. But, which morph left the most descendants, becoming the most frequent within the population?

If you guessed the camouflaged individual, you would be correct. With repeated selection, over time, the population of peppered moths with the beneficial colouration increased. Natural selection is one of the greatest phenomena we know of, responsible for shaping the past, present, and future of every living thing on Earth. At this very moment, species are evolving at minute levels, responding to changes all around.

How Humans Are Shaping Evolution

In some cases, the direct actions we bring forth with the anthropocene can influence evolution within the natural kingdom. Probably, in more cases than not. When humans and animals become as interlinked and woven as today, it would be naive to believe that we leave them unmarked after years of encroaching on their territories, and hunting them for our own gain.

In elephant societies, males with the largest, heaviest tusks reap the rewards of dominance. Like in Darwin’s acclaimed theory of sexual selection (the laws governing the interplay of sex and reproduction in nature), dominance is often positively correlated with size. ‘Big-tuskers’ - male elephants with tusks weighing more than forty-five kg each, gain more mates, garnering higher reproductive success. According to Joyce Poole, of Elephant Voices, there is an age-related component to tusk size. ‘Big-tuskers’ are often older, experienced breeders, whose years have allowed them to grow their tusks to the staggering size that gives them their name.

However, Poole also says, there is genetic variability in the size of tusks within an elephant population. Genes for enhanced tusk size, as well as the relatively simple factor of age, mean that some elephants are genetically predisposed to grow larger appendages than others.

Elephants are targeted in the illegal ivory trade, with those boasting larger tusks particularly vulnerable. Image Credit: Right - stevepb on Pixabay; Left - Poswiecie on Pixabay.

The dominant ‘tuskers’ not only boast higher survival odds, through using their particularly applicable tusks to a range of activities (including stripping trees of bark, and digging for water) but are also more attractive to females of the species.

A principle component of Darwin’s sexual selection is interspecific mate choice, in which, generally, females of a species select their mates based on elaborate traits (like tusks!) that indicate their potential and quality as a reproductive partner. Although sex-role reversal, with females stepping into the competitor’s ring, can be seen, in elephants, males duel with their tusks and those with more impressive, heavier ivory appendages tend to father the majority of calves in the population.

In this way, the gene pool is sustained for large tusks, and elephants of both sexes inherit advantageous genes. What happens, however, when humans are thrown into the mix?

Known as selective harvesting, years of the illegal ivory trade and trophy hunting has meant that elephants with the largest, most valuable tusks are targeted by poachers and hunters - relentless killing and sustained trade essentially leads to the removal of ‘big tuskers’ from populations, and with them, their genes. Not only can there now be a lack of aged, experienced breeders being killed for their ivory, but fewer of the resulting generations of elephants are inheriting the genetic variants that allow them to grow large tusks.

A study from 2015, conducted in southern Kenya, found that tusk length and circumference declined by up to twenty-one percent in female elephants during population recovery, after a period of severe harvesting in the 1970s and 80s. When poachers hunt elephants for their large tusks, they essentially strip the population of the gene for enhanced size.

Research in Mozambique’s Gorongosa National Park has shown how after the end of the Mozambican Civil War in 1992, elephants which were born with a rare genetic mutation (promoting tusklessness) had higher survival odds than those with tusks. The gene for tusklessness is sex-linked, inherited on the X-chromosome. If a male receives a defective copy, for him, it is usually lethal. For a female, with one more X-chromosome to inherit, the mutation is somewhat a help and a hindrance.

Using the ivory trade to finance their war efforts, the elephant population along with other large herbivores was decimated by up to 90%. But female elephants lacking their tusks had better survival odds than those with, promoting their survival and leading to an augmented frequency of the tuskless gene in the resulting female generations.

In areas ravaged by ivory trade, elephants are losing their tusks through selective harvesting. Image Credit: Right - stevepb on Pixabay; Left - RoryCurrin on Pixabay.

In 2018, as baseline tusklessness has increased, of the two-hundred known females in the park, fifty-one percent of those who survived the war were tuskless. As for their daughters, as a collective, thirty-two percent bore no tusks. Astonishingly, it is a trait that without the intense selection pressure occurring throughout the civil war, would occur naturally in only two to four percent of female African elephants.

Lacking tusks may have helped elephants survive and conquer the war, providing some advantage in today’s hostile environment of poaching and trophy hunting, but it comes at a detriment. These appendages are vital for survival, and in an effect associated with the sex-linkage of the tusk gene, female elephants without tusks seem to be significantly unable to bear male calves. Eventually, the sex ratio of the population could be entirely skewed, impacting the growth of the population in years to come.

Humans influence everything they touch - not least the natural world. This is just one example of the many ways we are leaving our mark on the animals we share the land with, by dampening their odds of survival. Yet, despite this, the incredible story of evolution in elephant societies, under intense human pressure, shows us that not only is nature in danger, it is also tenacious, resilient and adaptive. Humankind may throw its worst at the animal kingdom, but for now at least, it is indelible.

About the Author: Madelaine Stannard is a Zoology student at the University of Sheffield, with a keen interest in science communication, rewilding, carnivore ecology and endangered species. You can find her on Instagram @maddie__stannard_wild.