As we continue to change the planet, animals are forced to adapt. Already, researchers are observing species shifting their ranges in response to warming temperatures; moving to higher ground on mountains, deeper in the ocean and to cooler latitudes. Overfishing of the largest specimens has led to some species maturing at smaller sizes.
Now, a study shows that as well as changing the behaviour of animals, we are also altering their genetics, forcing evolution by unnatural selection.
Jon Waters, an evolutionary geneticist at the University of Otago, cites a famous earlier example of human-driven evolution: the peppered moth. In the early 1900s, a darker version of this moth started appearing in England, likely in response to industrial pollution which meant that lighter-coloured individuals were no longer blending in as they rested on increasingly soot-covered tree trunks. Over decades, the darker and better-camouflaged form increased rapidly, then decreased as pollution declined.
“This is a great example of colour change over time, but the agents of selection have never been clear in this case,” Waters says.
Some researchers have suggested that the melanin production that produced the darker colouring influenced the moths’ immune response and helped individuals deal with environmental toxins.
Waters’ new research also focused on insects and colour change – stoneflies living in forests –but this time, the team was able to show conclusively that the shift in colour was a response to environmental change and was caused by a genetic mutation. They even pinpointed the gene.
Among stoneflies is a forest-dwelling genus known as Austroperla, which has a species native to New Zealand. To avoid being eaten by birds, these stoneflies produce cyanide and advertise their toxicity by black and yellow markings. A native, non-toxic species of stonefly has evolved a clever strategy to avoid predation by birds: it mimics the appearance of its toxic cousins, tricking predators into assuming it, too, is poisonous. Waters says this mimicry has evolved over millions of years.
When people began burning forests, they wiped out populations of forest birds as well as the toxic, forest-dependent stonefly. This was a game-changer for the non-toxic mimic. As many of its predators and the genuinely poisonous stonefly species vanished, it no longer needed its colourful warning signals. Populations in deforested habitats quickly lost their colouration because it “was costly, and when no longer essential, evolution rapidly removed it”.
Waters says a comparison between populations across several parts of the South Island showed a consistent picture. In areas where forests had disappeared, so did the stonefly’s colour warning.
The team then used stonefly models with different colours to show that birds had driven the rapid change – they only avoided feasting on stoneflies with the warning colouration in forested areas.
The next challenge was to show that the colour shift was due to DNA changes. Scouring the genetic variations across the harmless species’ genome, Waters says it became clear that one gene – known as ebony – was responsible. “We were able to tie together the whole system, from the ecology [including bird predation and the interactions among insect species and their environment] to the genetics that control the changing insect colour.”
What to make of the confirmation that humans drive the evolution of other species? The knowledge that adaptation is possible and can occur over short timescales to allow species to change with their environment is positive, Waters says. “On the negative side is the finding that humans have so quickly disrupted ecological interactions that had evolved over millions of years.”
