A RAFT full of elephants and rats gets stranded on a remote island. The animals survive and reproduce. But as the generations pass, something odd happens: the elephants shrink to the size of Shetland ponies and the rats grow to the size of cats. They have found themselves at the mercy of one of evolution’s weird rules.
Most of us are familiar with evolution by natural selection, in which species change and diverge over time as those that successfully adapt to their environment pass on the genes that helped them flourish. What you might not be aware of, however, is that evolution’s work is in some places governed by a handful of rules that can have some pretty surprising results.
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Near the poles, for instance, animals tend to grow larger than you might expect. In the tropics, meanwhile, birds often have strikingly big beaks, while their feathers may be unusually dark. And on islands, evolution gets very peculiar indeed – which explains why just a metre tall and why rats in New Zealand are about .
Many of the biological “rules” behind these patterns were proposed in the 19th century and it hasn’t been entirely clear whether they stand up to modern scrutiny. In the past decade, however, biologists have not only confirmed that many of these rules hold true, but also revealed the intriguing details of how and why they work. In some cases, researchers have even begun to use the rules to predict how species will evolve as the world warms.
Cope’s rule: The bigger, the better
Edward Drinker Cope knew a thing or two about big beasts. A palaeontologist famed largely for his role in discovering a host of huge dinosaurs in the late 19th century, Cope also analysed North America’s ancient mammals and spotted a trend in the fossil record: mammals evolve larger body sizes over geological time.
The idea has since been broadened out to cover all sorts of life on land and in the sea, but it has been controversial. The late palaeontologist Stephen Jay Gould because of our tendency to fixate on big things and ignore little ones. But in 2015, a team led by at Stanford University in California put it to the test using a vast data set of 17,000 different types of marine animal that existed over the past 542 million years. The upshot was that the rule is real, in the marine realm at least: all manner of as the aeons have passed.
Why remains a mystery. There appears to be some sort of inherent advantage to being large, but we don’t know exactly what it is. “My sense is it’s more about there being empty ecological space for large species,” says Payne. “There are things you can do when you’re big that no one else is doing.” Some whales, for instance, can hunt cephalopods that are too large for other animals to eat, so they face less competition.
Cope’s rule, then, suggests the oceans may continue to be a good place for giants to live far into the future. And while predicting what those giants will look like isn’t easy, Payne suggests it may be no coincidence that the very largest marine creatures – from whales to marine reptiles – are or were air breathers. “There is just so much more oxygen in air than in water, and air is less viscous so it’s easier to breathe,” he says. “If you can get oxygen to your tissues easily, it makes it easier to get bigger.”
Allen’s rule: Shape-shifting to keep cool
In the 1870s, zoologist Joel Asalph Allen noted something unusual about the birds and mammals of North America. Roughly speaking, as you travel north, hares and foxes have ever-shorter ears, while crows and woodpeckers have ever-shorter beaks. We now know that Allen had stumbled on a global rule linking the size of bodily appendages to temperature. “Just look at the toucans,” says at Deakin University in Australia. The toco toucan of the Amazon has an enormous beak. “But a mountain toucan from the cooler Andes has a [relatively] little, stubby beak,” he says.
The explanation for Allen’s rule is that tropical animals, which are at risk of overheating, evolve big appendages with a high surface-area-to-volume ratio and use them to more efficiently dump body heat into the environment. Polar species, by contrast, need to conserve body heat, so their appendages are smaller.
But there are other ways for animals to lose or retain body heat. In the 1840s, biologist Carl Bergmann noted that species tend to have smaller bodies in warmer climates and larger ones in colder climates – a pattern that has been suggested to hold true for all manner of animals, . Intriguingly, Bergmann’s rule has a similar explanation to Allen’s rule. A tropical animal that needs to lose body heat can boost its surface-area-to-volume ratio by simply evolving to be smaller.
Earlier this year, Symonds and his colleagues published an analysis of the interplay between both rules across 99.7 per cent of the world’s bird species. “The two rules interact,” says Symonds. For instance, Bergmann’s rule predicts that species should shrink as the temperature rises. But large birds can stay large, violating Bergmann’s rule, if they to enable heat loss.
And there is a final twist. As the world warms, Allen’s rule may cause animals to grow larger appendages to stay cool. But eventually, air temperatures in some regions will exceed animal body temperatures – whereupon the rule no longer applies. “At that point, having a big beak becomes a liability because it actually absorbs energy from the environment,” says Symonds. So, while appendages may grow in the short term, they may shrink again in the longer term.
Foster’s rule: The strange power of islands
The time Charles Darwin spent in the Galapagos Islands was crucial in the development of his theory of evolution by natural selection. But there was something about islands that he didn’t notice: animals living on them often evolve to be unusually large or small.
There is a good reason why Darwin was unable to spot what is now known as the island rule or Foster’s rule, named after J. Bristol Foster, who wrote about it in the 1960s. It only becomes obvious once you can reconstruct the evolutionary relationships between species on islands and on continents. Doing so suggests that members of large animal species that become isolated on islands tend to shrink, while small animal species tend to grow. The kakapo, for instance, is a large flightless parrot in New Zealand that evolved from smaller ancestors – while some Indonesian islands are home to Burmese pythons that are half the size of their 5-metre-long relatives on the mainland.
However, critics argue that , meaning the island rule only exists if you cherry-pick the data. “That’s what intrigued me and made me want to investigate,” says at the Doñana Biological Station in Spain. For a paper published in 2021, she and her colleagues analysed body size data for more than 1000 species of island-dwelling animals and a roughly similar number of their continent-dwelling relatives to assess the level of support for the rule. They concluded that . What’s more, the effect is more pronounced on smaller, more remote islands.
There are probably several reasons why the rule exists, says Benítez-LÓpez. A small island might not have enough food to sustain a population of large animals, for instance, making it more beneficial for them to shrink. Islands may also be free of predators, however, and some ecologists argue that this reduces the pressure on small animals to hide – so they grow larger, as this helps them compete better for food or mating partners.
Benítez-LÓpez also points out that the mysterious island rule is just one part of a broader phenomenon called the island syndrome. In the absence of land predators, island birds, for example, tend to evolve into flightless forms and often lose their fear of predators, rendering them tame. This helps explain why island species can be particularly vulnerable to extinction by hunting. “The dodo couldn’t fly, and it was easy for humans to catch,” says Benítez-LÓpez.
Van Valen’s law: Inescapable extinction
In the 1970s, the late biologist Leigh Van Valen reached a surprising conclusion: a species that has been on Earth for a few million years, proving itself to be a successful survivor, is just as likely to go extinct as a species that appeared just a few thousand years ago. Van Valen came up with an explanation for this “law of constant extinction”, or Van Valen’s law. He argued that a species can never improve its survival odds because it is always in competition with other species. Cheetahs, say, might evolve to run faster, but because the antelopes they hunt are also evolving to run faster, cheetahs don’t become more likely to catch the prey they need to survive.
The rule might seem even more surprising given that, although a new species is typically confined to a handful of individuals living in a small area, its population then expands such that it might comprise millions of individuals across an entire continent. How can it always remain just as likely to vanish?
A few years ago, at the University of Helsinki, Finland, and her colleagues sought to address this question. They built a computer model that shows extinction rates remain the same because of an interplay between biological competition and non-biological factors – natural disasters or climate change, for instance.
A newly evolved species might carry a new innovation in the way it feeds or moves that means it faces relatively little competition. But because the population is small and confined to a tiny geographical area, it is at heightened risk of extinction from local events, such as a volcanic eruption. In contrast, a mature species spread across a wide area is at less risk from a localised natural disaster. Then again, if that species is no longer largely isolated, as time passes it faces an ever-greater risk of extinction through biological competition as new species evolve. The threat can even emerge from related species: for instance, as modern humans spread around the world, they may have contributed to the extinction of many ancient human species.
Žliobaitė and her colleagues suspect their work might even offer us insights into society. They recently began to explore whether their computer models can explain the rise and fall of industrial companies, human languages or even music genres. “We want to know how entities replace one another,” she says.
Gloger’s rule: A duller future?
Species at the equator tend to be darker in colour than their relatives nearer to the poles. For instance, . Constantin Gloger was one of the first biologists to notice this, in the 1830s. But quite why this pattern exists is still up for debate.
A 2019 study reviewed the evidence and suggested it may be a . The idea is that, since parasites and pathogens are more numerous in warm and humid habitats, animals living nearer the equator evolve to have stronger immune systems than those living elsewhere. By chance, the immune system genes are linked to those coding for darker-coloured bodies.
But that isn’t the only reason why colour might vary with latitude. The tropics also typically receive more ultraviolet radiation from the sun than areas nearer the poles, says at Clemson University, South Carolina. In principle, then, some species nearer the equator might be darker because they carry UV-absorbing pigments that protect biological tissue from radiation damage.
In 2015, Koski and his colleague at the University of Pittsburgh, Pennsylvania, revealed that this may be the case for a plant called silverweed (Argentina anserina). Although its flowers appear yellow to our eyes, under UV light they have a black “bullseye” due to the presence of UV-absorbing pigments – and .
This clear latitudinal pattern might break down in future because shifts in the pattern of cloud cover induced by climate change could alter the levels of UV radiation reaching Earth’s surface. A 2020 study offers evidence that plants in different regions . The worry is that this might confuse insect pollinators as they forage.
Already, Koski and Ashman have demonstrated that some insects are less likely to visit flowers with larger bullseyes, suggesting such flowering plants may go unfertilised. “That’s not great for plant reproduction,” says Koski. More broadly, it might be an ominous sign for the ecosystems across the world that ultimately rely on the success of plants.
Predicting evolution
Can we use the patterns of evolution described by "biological rules" (see main story) to predict the appearance and behaviour of animals in the future?
Some argue that it could be relatively easy. "If climates become warmer by this or that amount, we can predict something about how certain groups of animals will evolve," says at the University of Bristol, UK. In 2020, he and at the China University of Geosciences, Wuhan, used a relatively straight reading of the rules to suggest in the future.
Matthew Symonds at Deakin University in Australia, however, has reservations. "Does changing its appearance help an animal cope with climate change? Or does it screw up the rest of its internal biology and lead the animal into a decline?" For instance, a bird may have adapted to forage for insects using a small, delicate beak. If, in a warmer world, it follows Allen's rule – which says hotter conditions favour bigger appendages – and evolves a larger beak, it may have trouble finding food.
"One thing we're trying to do at the moment is look at the way animals are changing and see how that translates into population numbers," says Symonds. "Even within a species, there may not be a uniform way of responding to climate change."
Colin Barras is a science writer based in Ann Arbor, Michigan
