Inheritors of the Earth
Page 16
It is not just checkerspots that are experimenting. The caterpillars of one third of all of the 236 native butterfly species that live in California include newly arrived exotic plants in their diet.8 That is astonishing, given that most of these introduced plants have been growing in California for less than two hundred years. American butterflies are seemingly rushing to exploit foreign plants. In Europe, where fundamental habitat change has a longer history, nearly all butterflies make use of human-modified land. In the picturesque Dordogne valley in France, five species of fritillary, all distant relatives of North American checkerspots, can be found gliding on ochre wings across traditional meadows that, without humans, would be forest. Not only are they living in a human-created habitat but, like their North American cousins, the caterpillars of three of the five consume the same kind of plantains.9 The plants would not be there without people to maintain the meadows. Perhaps these European checkerspots evolved to make use of them thousands of years ago, when the forests were first cleared.
It is the same in England. Continuing my stroll, I notice a brown argus butterfly. This insect was concentrated in the south of England in the 1970s and has spread northwards only in recent years. The males perch in sheltered corners, glistening in the sun, hinting at an ultraviolet sheen I cannot see. They lustfully intercept passing females and fly out pugnaciously if a rival intrudes. They, too, have had a change of diet. Working with my brother, Jeremy, butterfly conservationist Nigel Bourn established that the caterpillars of this insect eat juicy rockrose plants, which mainly grow on hot slopes–on south-facing hillsides–on the chalk-and-limestone hills of southern England.10 Hot caterpillars grow fast, allowing the butterfly to complete its lifecycle–from egg to caterpillar to chrysalis to adult butterfly–twice each year. Then, as the climate began to warm, it became possible for them to complete their lifecycle on flat terrain. The only catch was that rockroses were few and far between, away from the chalk hills. They were stuck. And then the brown argus did something very similar to the checkerspots. In fact, it was even more impressive because it allowed the butterflies to double their British range in less than twenty years.
Rather than place their eggs on the usual rockroses, some of the female brown argus butterflies took to laying on small wild geranium plants. In particular, they liked one called Geranium molle, known by the inconveniently cumbersome name of dove’s-foot cranesbill. Laying eggs on this plant proved to be an enormous advantage because, unlike the rockroses, these particular geraniums are common in the British landscape. From the 1980s onwards, the butterfly expanded away from its traditional rockrose-containing suntraps on to road verges, field margins, country parks, rough pastures and even building sites, spreading northwards across landscapes that would previously have been regarded as too cold and lacking in suitable habitats.11 In the early 2000s, they appeared in my meadow, 150 or more kilometres from where they used to live, not only laying eggs on wild geraniums that grow where the ponies churn up the ground but actually shunning the rockroses I have planted. The geranium-liking brown argus butterflies that live over large parts of northern England today are genetically different from those that used to live in chalk downland habitats in the south.12 Just as some species are thriving under the new conditions of the Anthropocene epoch, some genetically distinct populations and individual genes are becoming increasingly successful. European butterflies did not evolve to make use of human-altered habitats thousands of years ago and then ‘bask in their achievement’. They are still changing.
It is not just butterflies. Rapid evolution must be taking place in nearly all animals, plants, fungi and microbes. Almost all populations and species contain genetic variation, and some variants will survive better than others under the novel physical and biological conditions that now exist. The atmospheric concentration of carbon dioxide is already higher than it has been for some 3 million years, and may well end up higher than for 20 million years. This alters the capacity of plants to carry out photosynthesis, acidifies the oceans and warms the climate. Greenhouse gases have already raised the world’s average temperature by 1°C and, writing in 2016, we are on course to reach an average global temperature that will be the highest for 3 million years,13 and possibly for 10 million. Needless to say, temperature directly alters the physiology and growth of almost all organisms, hence their survival and reproduction. Humans have also changed the amounts of nitrogen, sulphur and atmospheric dust in circulation. And as we have seen, we have removed most of the largest vertebrates, transformed the land for agriculture, set species on the move as a result of climate change and generated a modern version of Pangea.
Faced with these novel conditions, it follows that nearly all wild animals, plants and microbes must be evolving in response. Genetically distinct individuals and populations differ, for example, in how fast they grow at different temperatures, their size, and in their interactions with other species (including the choices female butterflies make when selecting plants that will be good for their offspring to eat). There is variation in almost everything that anyone can imagine. When the environment changes, some of these variants are almost bound to survive at least slightly better than others, such that the characteristics of the next generation will differ from those that went before. Given how much and how fast humans have changed the world, it is entirely credible that we are now living through the most rapid period of evolution since the aftermath of the extinction of the dinosaurs 66 million years ago.
Of course, evolution is not a get-out clause that will enable every population or species to survive–populations of flightless birds do not contain any individuals that can soar off into the sky and escape from invading hordes of carnivorous mammals. Failed evolution will be commonplace. However, the populations and species that do survive will be somewhat genetically different from those that went before. This is the normal stuff of Darwin and Wallace’s theory of evolution by natural selection. The difference now is that humans are spurring evolution on. When Charles Darwin first distinguished between evolution by natural selection (in nature) and by artificial selection (guided by human intent), he could not possibly have imagined that humans would change the world so much that we would influence the evolution of nearly every population in the world. It is worth considering artificial selection, then, to gain insight into the speed at which evolution can take place when humans are at the helm.
With one simple extension of his tongue, Rex could lick the entire surface of the kitchen table. Many was the time I would return to a smear across the table and a chewed cheese wrapper on the floor. Rex was our Irish wolfhound, a sighthound bred for strength and speed, thought to have been developed originally for hunting wolves and larger animals, and possibly for war. He was 85 centimetres tall on all fours, 2 metres on his back legs, weighed about 55 kilograms and ate a lot of cheese. He was usually extremely friendly, but his immensely deep growl and enormous teeth were sufficient to turn our soot-covered chimney sweep distinctly pale. While Rex was the only Irish wolfhound in the village, there were several Yorkshire terriers. At 3 kilos apiece and 22 centimetres high, ‘Yorkies’ are better suited to tackling mice and rats than wolves. Chihuahuas are even smaller, a mere kilo in weight and 18 centimetres high, perfectly adapted to living in celebrity handbags.
No one doubts for a moment that they are all dogs, or that they are derived from domesticated wolves. On the other hand, one wonders what palaeontologists would make of their fossils in the distant future. It is not so easy to decide whether any two types of (closely related) animal belong to the same or different species because the process of speciation is a continuous one of increasing evolutionary separation. We expect there to be situations where it is hard to call, one way or the other. However, a greatly reduced ability of one animal to reproduce with another is integral to the concept of a species. For this reason, fossil hunters have assigned the 4.3-metre-high giant mammoths on the North American mainland and the 1.7-metre ‘micro-versions’ from the Californian Channe
l Islands to different species. This is quite reasonable. The island mammoths were not only geographically separated but likely morphologically incapable of mating with their continental relatives, which were 2.5 times taller and about 12 times heavier. If we apply the same reasoning, a 4.7 times taller and 50 times heavier male wolfhound would require advanced yoga skills to reach down to a female Chihuahua, and even then probably not manage to get the appropriate part of his anatomy in the required location; if impregnated, half-wolfhound pups developing inside a Chihuahua bitch would probably prove fatal. Similarly, a male Chihuahua would need to levitate to reach a female wolfhound, unless she lay down. This is not completely impossible, but unlikely. Chihuahuas and wolfhounds are as different as many species of deer are different from one another, or lions and tigers, or cattle and bison. In many respects, they are even more different, despite the fact that dogs started to separate from wolves only some fifteen thousand years ago.
It is quite reasonable to argue, therefore, that Chihuahuas and Yorkshire terriers represent a different species from the original wolf. There are plenty of intermediate-sized breeds, however, and so it would be possible for the genes of Chihuahuas to make it into wolfhounds over the course of several generations and thence, perhaps, into wolves.14 However, this does not normally happen, and the clubs that maintain pedigrees deliberately ensure that each breed remains as ‘pure’ as possible. Whether you take the view that some or all domestic dogs represent a species that is distinct from the original wolf is not the issue. The important point is that very substantial evolutionary changes can take place in short periods of time and can generate differences that are as large as those between species.
The pace of change is still quite slow, relative to the lifetimes and memories of individual humans, so we don’t usually notice. Yet paintings and photographs of dogs from a hundred or two hundred years ago are often surprising. The bulldog used not to be nearly as squat, the pug not so snub-nosed and the Alsatian used not to have such a crouching gait. Once thick-set, snub-nosed and crouching were specified as the ideal standards of a particular breed, dog owners ensured that they selected individuals with these characteristics for their future breeding programmes. And so, generation after generation, the breeds have increasingly become caricatures of those original standards, success at dog shows having taken over from hunting ability as the agent of selection. Within a few hundred years–perhaps fifty dog generations–breeds have changed radically under the influence of selection. Over the full period of domestication, the largest have remained similar in size to wolves, yet Chihuahuas have become so small that their dimensions are comparable to those of fennec foxes, the smallest of all wild canids. The entire size range across all the wild species of wolves, dogs and foxes, which separated from one another over 20 million years ago, has been replicated in a few thousand generations of human-influenced evolution. Both the speed and the magnitude of change in modern dogs are remarkable. Evolution is not necessarily a slow process.
Dogs are but one example. Cattle, pigs and horses also show great variation in their size, while sheep and llama breeds differ in the character of their wool. Darwin particularly loved the varieties of the domesticated rock dove (the same species as the feral pigeon): appearance, behaviour and flight differ so much among them that the different forms could easily be taken for dozens of different wild-pigeon species. Similar changes can be seen in plants. One species of cabbage, Brassica rapa, has been bred into turnips (a root vegetable), Napa cabbage (Chinese cabbage), rapini (a broccoli-like vegetable), field mustard (an oil seed) and mizuna (a peppery salad). Different varieties of Capsicum annuum produce bell or sweet peppers that range from green and yellow to red and purple; other varieties are chilli peppers and jalapenos that vary in the concentration of heat-producing capsaicin chemicals (some associated with medicinal as well as culinary uses); while small, roundish-red ‘Bolivian rainbow’ and the dark-leaved and black-fruited ‘Black pearl’ are ornamental garden varieties that are not intended to be eaten at all. Without prior knowledge of their histories and genetic relationships, a botanist could easily assign these varieties to many different species, with the assumption that they evolved millions of years ago.
Development of an increasingly squat body shape in the bulldog between 1790 and the present day. Modern breeders try to obtain pups from animals that best meet the breed specification. The UK Kennel Club 2010 ‘Breed Standard’ specifies that the bulldog should possess the following attributes: ‘Skull relatively large in circumference. Muzzle short, broad, turned upwards and deep. Flews (skin on the cheeks) thick, broad and deep, covering lower jaws at sides. Ears small and thin. Teeth not visible. Teeth large and strong. Neck thick, deep and strong. Shoulders broad, sloping and deep, very powerful and muscular. Forelegs very stout and strong, well developed, set wide apart, thick, muscular and straight, bones of legs large and straight. Chest wide, prominent and deep. Back short, strong, broad at shoulders. Hind legs large and muscular, slightly longer in proportion than forelegs.’ The breeders have been rather successful, although it escapes me why anyone would want a dog that shape.
Darwin called all this artificial selection, but the processes are just the same as in any other form of evolution. Impressing a human admirer, and thereby gaining the opportunity to reproduce, is not fundamentally different from a peacock with an especially large and brightly eyed tail impressing an admiring peahen and thereby obtaining the matings that will pass on his tail genes. Nor is it fundamentally different from the butterfly that has passed on more copies of its genes because it laid its eggs on a different plant. There is no clear dividing line. The development of different varieties of animals and plants is simply a consequence of some individuals surviving and reproducing better than others, and thereby becoming highly successful in the Anthropocene. They succeed because they have characteristics that result in their genes being propagated. The fact that humans have played such a major role in this propagation is hardly surprising, given the worldwide abundance of humans and our impact on the Earth. Widespread animals will always affect the evolution of many other species, and humans have simply taken this to a new level.
Many of these relationships are mutually beneficial, just as they are between bees and flowers, and between fruits and birds. By visiting flowers, bees receive rewards of sugary nectar and nutritious pollen; while the flowers benefit from the bees acting as mobile reproductive organs, transferring some of their pollen from one plant to the next. Of course, there are costs. The flowers have to produce nectar and lose most of their pollen, and the bees expend energy transporting the pollen. Nonetheless, it is mutually advantageous because the genes of both plants and bees are passed on to subsequent generations at increased frequency as a consequence of the partnership. Similarly, birds benefit from eating berries, while the seeds of those plants are deposited in places where they may grow (often encased in a bag of bird-dropping fertilizer). Given enough time, and strong enough selection, extremely complex interrelationships can develop.
Mistletoes, for example, need their seeds to end up on the branches of the trees they will subsequently parasitize, which is why mistletoe berries have evolved to be sticky. A bird will swallow the fruit, digest the nutritious outer parts of the berry (benefitting the bird), then regurgitate it. The stickiness of the fruit requires the bird to wipe the seed off its bill, and where better to do this than on a small branch or twig, to which the seed sticks and then grows into a new plant (benefitting the mistletoe). Some Bornean mistletoes and flowerpecker birds have gone even further, opting for a rear-exit strategy.15 These mistletoe seeds have tadpole-like tails and pass completely through the gut of the birds. The seeds then emerge at the bird’s backside and get stuck, but once the birds wipe their bottoms on a convenient branch, the extremely sticky tail glues the seed in position. The gardener bird has planted the seed. The bird took advantage of the plant, and the plant took advantage of the bird.
This is no different to the relation
ship between us and our crops, or our livestock. We plant; they grow. Any genes our livestock or crops possess that cause humans to propagate them more effectively will increase in frequency; any variants of our genes that enable humans to make better use of these plants and animals will also increase within the human population. Take the ability to digest milk. Mammals do not need lactase enzymes after they have been weaned of their mother’s milk, so they stop producing them. It requires energy to produce lactase, and thus it would be a disadvantage for adults to keep secreting it. This is how all humans used to be. Once our ancestors started to keep cattle and had access to milk, however, the benefits of being able to digest milk outweighed the metabolic costs of producing lactase. The adaptation to keep the genes for lactase ‘switched on’ in adults has subsequently spread through a third or more of the world’s human population. Cattle have evolved in the presence of humans to produce more milk and meat, which has been a great evolutionary success for them (the worldwide cattle population numbers 1.5 billion); and humans have evolved an increased capacity to digest the dairy products we get in return. Successful cow genes to produce more milk and successful human genes to digest it have increased in tandem.
There are plenty of similar examples. People from populations with more starch (from crops) in their diets usually have more copies of a gene that produces salivary amylase to digest it; many other genes associated with the digestion of carbohydrates and fats have also changed since humans adopted agriculture. Even our tooth enamel and the way we sense flavour have altered as a consequence of mutualistic relationships between humans, our crops and our livestock.16