The world’s ecosystems are in danger – leading evolutionary biologist Christopher Wills suggests their complexity could help save them
The world’s ecosystems, the complex living communities on which we all depend, are in grave danger. I am vividly aware of this, because I have watched their decline for longer than most of us. I began SCUBA diving in 1957, only four years after the publication of Jacques Cousteau’s wondrous book The Silent World. Since then, I have dived repeatedly on reefs and other underwater ecosystems throughout the world and have watched their gradual and heartbreaking deterioration.
As an evolutionary biologist, I have also been honoured to join forces with many of the world’s forest ecologists to study remote rainforests. These forest ecosystems, too, are starting to show damage.
How irrevocable is this damage? Can evolution reverse it? Surely not, for the slow and hesitant process of evolution should take hundreds of thousands or millions of years to restore ecosystems to health and complexity.
Perhaps not. We are learning some startling things about evolution. It may not be the achingly slow process that we had supposed.
Darwin was perhaps the first to glimpse this possibility. During his round-the-world voyage on the Beagle, Darwin had encountered many complicated ecosystems. He had realised that such complexities, with their myriad opportunities for both competition and cooperation among species, must be powerful generators of evolutionary change.
At the end of The Origin of Species, in a famous passage, Darwin encapsulated the importance of ecosystems to evolution by leading his readers on a tour of a thriving, fiercely competing ‘entangled bank’ of animals and plants. It now appears that the ‘bank’ may have been a real place of a type familiar to his readers, the rich and densely wooded Orchis Bank that lies not far from Darwin’s house in Kent. But as he wrote those words, he was undoubtedly also thinking about the coral reefs and rainforests that he had seen.
At the same time, Darwin was forced to assume that evolution, in spite of its power, must be slow, with any substantial changes requiring millions of years. This is because he, along with most other biologists of the time, thought that the ‘palette’ of inherited variants within a species that evolution would be able to draw on must be severely limited. These variants, he thought, must mostly be the small changes that happen to organisms during their lifetimes. Such a limited set of variants must surely put severe limits on the speed of evolution.
We now know that the real ‘palette’ of variants possessed by a species consists of mutational changes that take place at random in the DNA of its cells. Such changes can have small or large effects, which can lead to simple or to really complicated results. Because these changes happen in the DNA, some of them can also persist for many generations and accumulate in the “gene pool” of the species. This accumulation means that even the palette of genetic variations that is carried by a population of simple creatures such as bacteria can be both vast and rich.
Because the gene pool of each of the world’s species has a gigantic store of these mutations, shuffled every generation, evolutionary changes can actually be amazingly rapid. We now know that the swiftest changes take place in populations of microbes and viruses, aided by their short generation times and huge population sizes. Indeed, the ability of these tiny organisms to evolve new biochemical and organisational capacities has led to some of life’s most planet-altering events, from the appearance of multicellular organisms to the evolution of the ability to photosynthesise.
And the very complexity of ecosystems, as Darwin had intuited, plays a huge role. Even now, the evolutionary pressures acting on the thousands of species of bacteria and viruses in every ecosystem are unimaginably intense. One-third of the bacteria of the open ocean fall victim to a legion of bacterial viruses every day. These bacteria are, In effect, suffering the equivalent of a Black Death every twenty-four hours! Strains of bacteria that cannot adapt to this slaughter soon disappear, and the survivors must keep defending themselves against an ever-changing legion of enemies.
Other startling features of these palettes of genetic diversity explain why Darwin’s teeming tangled banks are turning out to be much faster and more powerful generators of evolutionary change than he had supposed. Bacteria and viruses can exchange genes with other bacterial and viral species, even very distantly related ones, through many kinds of genetic transfer that can cross the genetic gulf between the species. Similar transfers have also taken place in our own ancestry, but much less often.
Enjoying this article: Check out related articles:
Experiments from the laboratories of Michael MacDonald and others have shown that mixing together different strains of bacteria and viruses in the laboratory can cause the evolution of each of the species to speed up. As I suggest in my recent book Why Ecosystems Matter (Oxford University Press), all of the entangled banks that Darwin visited during his around-the-world voyage are actually, at the genetic level, simmering and bubbling cauldrons of evolutionary change.
These bubbling cauldrons are driving the evolution of bacteria and also the evolution of larger, more complex organisms. Bacteria are, for the most part, tiny little single cells. But we multicellular organisms are far more complex. Change is slower, because big creatures’ populations are smaller and accumulate fewer new mutations each generation. But the palette of variations that is available to big creatures is unimaginably larger. By drawing on their palettes, aquatic four-footed animals were able to evolve into whales. Tiny dinosaurs evolved into birds. And small, big-eyed, tree-dwelling insectivores evolved into us.
As usual, Darwin was right. The complexity of ecosystems, and the myriad ways in which the species living in an ecosystem interact with each other, are indeed powerful drivers of evolutionary change.
The powerful juxtaposition of our new genetic and evolutionary knowledge with our growing understanding of the true diversity of the world’s ecosystems allows us to measure the health of these ecosystems in new ways.
For example, how quickly are endangered ecosystems losing their precious genetic diversity, and thus their ability to adapt? How can we safely restore these evolutionary cauldrons to their former, briskly bubbling, state? Which of a damaged ecosystem’s myriad variation-filled gene pools are most effective at driving the system’s recovery? These are questions that we can now answer in astonishing detail. And they are questions that we must, as stewards of the health of our planet, learn to answer — and soon.
Christopher Wills is an evolutionary biologist at the University of California San Diego and the author of Why Ecosystems Matter. published by Oxford University Press
