Inheritors of the Earth

by Chris D. Thomas

  While these direct evolutionary relationships between humans and our domestic animals and plants apply only to a minority of all the species that live on Earth, they have reshaped our planet. Over a third of the world’s land surface is covered by human-selected crops, pastures and forestry plantations,17 and many of these plants bear little resemblance to their ancestors. Approximately 97 per cent of the total weight of all mammals added together is dominated by these newly evolved relationships. The genes of animals and plants that are favoured by humans have been successful, and the genes of humans that have enabled us to make a success of these partnerships have also grown in number.

  Everything we are doing to the world is forcing evolution into overdrive.18 The butterflies are evolving when their habitats are changed and when new plants arrive from the other side of the world. Our agricultural systems are dominated by human-altered animals and plants, as we have just seen. Whenever we kill individuals of wild species–whether for food or to eradicate pests and diseases–the survivors nearly always have characteristics that enable them to survive better than those which died. When we fish, evolution favours individuals that can escape through the mesh of fishing nets and breed before they are killed. Thus, individual fish have become smaller and breed faster (offsetting fishing deaths) in the great majority of populations where fishing takes place.19 This evolutionary response may not be sufficient to prevent populations from declining, but at least it makes it somewhat harder for us to exterminate fish stocks entirely. Evolution can be good news, even if we would prefer our fish to be larger.

  However, recovery of the survivors becomes a problem whenever we attempt to exterminate a species deliberately. ‘Diseases of the past’ are evolving resistance to antibiotics, while human hair and body lice, bedbugs, disease-transmitting mosquitos, sand flies and blackflies are increasingly difficult to kill with insecticides.20 Our crop pests typically evolve resistance to new insecticides in a decade,21 and over 200 different weeds have overcome at least 150 herbicides.22 As for mammals, brown rats have repeatedly evolved their resistance to anticoagulant poisons, and mice are not far behind.23 Each gene for resistance in all these pathogens, plants, insects and mammals is its own biological success story.

  Evolutionary responses to climate change are probably even more widespread. In Finland, the pale grey plumage of owls that used to provide camouflage in snowy weather has been replaced by brown-coloured individuals that are hidden in darker, rainy forests;24 and German blackcap warblers have abandoned their migration to the Mediterranean to take advantage of warmer British winters (and a nation of people who put out food for birds in the winter). Butterflies have grown large muscly thoraxes (facilitating flight) and bush crickets have developed long and powerful wings to help them speed towards the North Pole.25 When it comes to biological invasions, the cane toads that are invading Australia have evolved longer legs that enable them to walk faster, accelerating their spread across the continent.26

  Even pollution has been influential. Peppered moths evolved blackened forms that were camouflaged against the trunks of trees in polluted industrial Europe (they are now evolving to look like lichens again, since the introduction of clean air legislation),27 while some species of plants have adapted to grow in soils that have been polluted by heavy metals.28 And so it goes on. Innumerable evolutionary changes have taken place in response to climate change, to introduced species, to habitat change, to new poisonous chemicals, and to harvesting, and in most cases these changes have taken place on a timescale of decades. Human-influenced evolution ‘in the wild’ seems to be happening just about as fast as evolution by ‘artificial selection’.

  A truly global episode of rapid evolutionary transformation has been unleashed, as particular genes enable the individuals, populations and species that bear them to succeed. This should be no surprise. There are bound to be successful (and unsuccessful) genes, just as there are successful (and unsuccessful) species. Oddly, though, while the declines and increases of species are widely recognized, each case of evolutionary change is still reported with surprise. It is as if, ever since Darwin, we have swallowed the story that evolution is so slow that we can’t usually see it. This suits science writers and natural historians, who can report each new example with a sense of shock and amazement; it suits scientists, who milk it for publicity; and it suits ecologists and conservationists, who find it convenient to treat species as if they are fixed entities. In the absence of national and global schemes that track evolutionary change–in contrast to the Audubon bird counts and other programmes that document changes to the abundances and distributions of species–each new instance can be revealed as a surprising event.

  We all know perfectly well that evolution is not unusual, however. It is how life works. Genes are passed from every generation to the next in every population of every species in the world. Successful genes inevitably increase and inherit the human-dominated world, hence evolutionary responses to human-caused environmental change must already be virtually universal. Moreover, these changes are virtually instantaneous on a geological timescale. We live in a time of rapid and accelerating evolutionary change.

  8

  The Pangean archipelago

  Sea lions lolloped on the beach while frigate birds soared overhead. The Diamante swayed on the gentle waves, anchored in the shallows while we came ashore to visit the island of Española. A family of primates–my wife, sisters, brother and sundry in-laws–relaxed under the tropical sun. I was sitting on a log that had washed up on a lonely beach a thousand kilometres away from the nearest continent, drinking water from a plastic bottle. Suddenly, a speckled-grey bird with an elongated tail reminiscent of a mockingbird hopped up beside me. It seemed to be eyeing my water bottle, and then it started to drink from my hand, its darting tongue tickling my palm. It was the tamest wild bird I had ever met, though not stupid–fresh water is in short supply on Española, a cactus-dotted patch of brown earth adrift in the Pacific Ocean.

  Later that day, we left Española, the Diamante parting flocks of ocean-going red-necked phalaropes as we set sail into the evening light, white-vented storm-petrels dancing in our wake, seeking out the plankton that had been disoriented by the churning water. We woke the following morning to a furore of diving boobies, circling upwards, folding their wings and plummeting in shuddering beak-first dives into shoals of hapless fish, perhaps corralled under water by the sea lions, penguins and flightless cormorants that patrol the rich but chilly waters that flow between the islands of Isabela and Fernandina. Seas of plenty surround lava-strewn islands where life is harsh. Alighting on Isabela, we were again met by mockingbirds, but they were far less tame, darker around the face and back and had fewer speckles than those on Española. They are also genetically distinct. The Isabela mockingbirds are not the same species as those on Española.

  Our journey continued in Darwin’s footsteps, travelling from one Galapagos island to another, seeing new variations on a theme. Each island had mockingbirds that are obviously quite similar to one another, yet also a bit different. Some, it transpires, are slightly different forms of one species, some are entirely separate species, and others are hybrids whose ancestors had come from two or more other islands.1 The differences exhibited by the mockingbirds across the islands impressed Darwin,2 as did the differences between the giant tortoises: animals from different islands are distinguished by the shapes of their shells. On the drier islands, their carapaces form neck-accommodating arches, enabling them to reach up for food during times of drought; on the moister islands, their shell is conventionally tortoise-shaped, as green vegetation is always available on the ground.

  Darwin was beginning to formulate the idea that animals would become adapted to new conditions whenever they colonized fresh locations. They would start to look different and, in the case of mockingbirds, behave differently from one another. This is not surprising. Soils in the region differ according to the type of lava from which each Galapagos island was formed and
its geological age. Rainfall varies, too. The younger islands in the west have high volcanic peaks that force the moist air upwards towards the summits of the mountains, where the water condenses into clouds and rain falls. In contrast, the peaks have long since eroded on some of the more ancient, lower-lying islands, and they are prone to drought. And it is not just the climate. The small offshore rocky islands are more likely to be covered by nesting seabirds, their nutrient-rich guano peppering the ground. Then each island has a separate set of animals and plants, altering the food that is available and the shelter provided by the vegetation. These distinctions between the islands are sufficient to set evolution off on slightly different tracks: partly predictable because the characteristics of the islands differ, and partly governed by the chance genetic mutations that arise on each island.

  Originally, a single species would have colonized one of the islands from a distant continent, Darwin reasoned, and then have spread from island to island.3 The populations of this one species will then become somewhat isolated and experience varying conditions on each island,4 where they evolve different characteristics and eventually become two, three, or even as many separate ‘daughter’ species as there are islands. This is the point the Galapagos mockingbirds have reached, with separate species on some islands, and described varieties (subspecies) on others. One of the daughter species might, however, become so different from its ancestors–in the foods it eats, in the habitats it occupies, or in the way it recognizes its mates–that it could potentially now live alongside other daughter species. All it needs to do is colonize another island.

  Rowing ashore from the Diamante, we were first met by sea lions guarding the beach access to the island of Española.

  Once on the beach, extremely tame Darwin's mockingbirds hopped up and begged for water and scraps (juvenile, right; with parent).

  Portrait of an adult.

  We know that the gap between the islands is not insurmountable–otherwise, the birds would never have flown to them in the first place–so the chances are that this will eventually happen. At some point, one of the daughter species starts colonizing the other islands, so that there are now two species living on each island, both descended from the same ancestor. This can potentially go on, again and again. The daughter species that has successfully colonized all the islands can now start to become a bit different on each island, just as before. And then they can separate into separate ‘granddaughter’ species, and the most distinctive ones may once again be able to spread back across the islands of the archipelago.

  This is what happened in the case of the Darwin finches, the birds for which he became particularly famous; although, at the time of his visit, the young naturalist was more intrigued by the mockingbirds. Larger- and smaller-beaked species that predominantly eat larger and smaller seeds, respectively, can live together, having, it seems, originated separately on islands where the types of seeds that were available differed in size (depending on how dry the island is). Over the two–perhaps three–million years that finches have been living on the Galapagos, this process of separation and coming together, and of evolving to consume different foods and live in different habitats, has happened many times. The ground finches that greet you on Santa Cruz island search for seeds that have fallen on to bare earth, yet they live alongside small tree finches that dangle from bushes and the branches of trees, while the woodpecker finch can manipulate thorns and thin twigs to find insect food the other species cannot reach.5 Naturally, the detailed ancestry of each species is complicated,6 but the basic pattern is clear: diverge on different islands and come together. This is the way of archipelagos.

  The Galapagos are not that unusual. Darwin would have been equally amazed if he had arrived in the Hawaiian islands. Stout-billed relatives of present-day Asian rosefinches colonized these islands about 6 or 7 million years ago and encountered an archipelago that was far wetter than the Galapagos.7 The word ‘colonized’ makes this process sound purposeful but, more likely, a small flock was caught up in a violent storm, struggled on and finally collapsed, exhausted, on a distant beach. In any event, they arrived, found food and water, survived and started to breed. As they spread along the island chain, they diversified. Some became thick-set finches, others dainty honeycreepers. This diversification went on and on, until the original general-purpose finch had descendants with an enormous diversity of bill shapes and sizes suited to consuming nectar, fruits, insects and even snails. They became so different in their habitats, foods and behaviours that many species could live alongside one another on the same island. And the story is much the same for other Hawaiian animals and plants: the dramatic silversword plants and dancing fruit flies evolved unique forms on different islands and in different habitats, as did the happy-faced and web-building spiders, and Hawaiian geese, among many others. Another archipelago, another species generator.

  Nowhere is separation followed by speciation easier than in archipelagos:8 remarkably, 10 per cent of all land bird species were confined to Pacific islands prior to the arrival of land mammals, even though the total area of these islands represents just one quarter of 1 per cent of the Earth’s land surface.9 Archipelagos have been the perfect place for one species to turn into many, and for Darwin to develop his revolutionary thoughts.

  This diversity-generating process is not limited to oceanic archipelagos. Lakes surrounded by land are islands for water-dwelling animals and plants–the wet archipelagos of lakes in the African Rift Valley are filled with a spectacular diversity of fish. Mountaintops surrounded by lower lands represent islands to animals and plants that are unable to survive in the hot lowlands. Similarly, base-rich calcareous soils may exist as isolated outcrops, and heavy-metal-containing serpentine rocks are likely to be surrounded by less toxic geologies. The story is similar. Diverge in different locations, and then there is the potential to come back together.

  On longer timescales, this same process takes place at a planetary scale. After South and North America had gradually joined together and eventually become one continent, species that had evolved separately over very long periods of time came into contact. Although South American marsupial lions and some other species died out as a result of the contact, the total number of species increased.10 First, this was simply because the number of immigrants into each continent was larger than the number of species that died out when the two biological worlds met. And then the number of species on Earth started to increase. Deer that had spread from North America evolved into fourteen species that are restricted to different habitats and geographic regions within South America (this is twice as many species of deer as exist in North and Central America put together).11 Likewise, camel relatives that moved southwards evolved into vicuña and guanaco.

  More dramatically, over 375 species in more than 80 genera of New World rats and mice12 evolved after they negotiated the then archipelago of Central American islands and arrived in South America around 5 million years ago; and perhaps 80 species of pea-like lupin plants have come into existence within the last one and a half million years in different parts of the Andes, a mountain range that can be thought of as an archipelago of cool, open habitats at the highest elevations, above an ocean of lowland trees and deserts.13 Going in the opposite direction, the Virginia opossum, North American porcupine and armadillo came into existence, their predecessors migrating from the south to the north.

  As we saw in Chapter 6, continents are not entirely isolated, even when there is a gulf of water between them. Some movements are quite regular. Dust-like bacteria, fungal spores and the minute seeds of orchids can be blown huge distances, and tiny insects such as thrips and aphids can be transported as aerial plankton in the wind. Migrating birds today regularly cross the Caribbean Sea, with the odd seed, spore and louse on board, and they must have passed back and forth between North and South America long before the two continents merged. For most animals and plants, however, the transport of species between continents is exceptional. Nonetheless, ju
st as freak storms may very occasionally deliver exhausted birds, seeds or reptiles clasping mats of floating vegetation or logs to remote islands, they have the potential to deliver individuals from one continent to another. Cattle egrets seemingly did manage to fly from Africa to South America in the 1870s. Although the movement of individuals may be rare, those that survive the ordeal will eventually evolve into new species in their new home. A few tens of millions of years later, their descendants might reverse the journey.

  Distant parts of the same continent can also be connected, isolated and then connected again. Asia and Europe form a single landmass, Eurasia, but the oak trees and beech trees that grow in eastern Asia and in Europe are not the same as one another. A single species of beech tree forms cathedral-like forests carpeted in golden autumnal leaves in Europe, but different beech species grow in China and elsewhere in eastern Asia, some with trunks that soar towards the canopy, while others branch close to the ground and jostle for space with bamboo thickets on mountain slopes. The climate of central Asia is too harsh, and these trees survive best in the more moderate oceanic regions that exist towards each end of the Eurasian landmass. Isolated, they have become separate species. European oaks also differ from those in eastern Asia, as though the two regions were giant islands separated by the frigid aridity of the continent’s centre. North American beeches and oaks differ again from those in Europe and Asia. Perhaps 55 million years ago, a new kind of tree evolved: the first oak. It originated, spread, colonized different continents, evolved into different species in different regions and climates, and at least some of them came back together again; acting like a giant, slow-acting global archipelago.