The rate at which species came back together and fuelled new episodes of diversification in the global archipelago operated so slowly, relative to the timescales of human generations and civilizations, that we could be forgiven for thinking of these global-scale patterns of biological diversity as unchanging. But this is no longer the case. Today, humans have become that long-distance glue that has turned isolated lands into a network of increasingly connected nodes. East Asian raccoon dogs, North American mink and muskrat and South American coypu now find themselves living alongside European badgers and roe deer. Previously separated by uncrossable distances of land and ocean, these species all now live together in Europe. We have converted separate continents and distant locations within the same continent into archipelagos of partially connected regions.
This flow of species is akin to the movement of finches and mockingbirds between islands in the Galapagos. Having arrived in Europe, raccoon dogs, mink, muskrat and coypu will already be starting to take different evolutionary paths from those in their homelands, as they experience new conditions and begin to live alongside European species, which they had not previously met. Critically, while the rates at which humans are moving animals and plants are enabling species to reach many more parts of the world, they are not, in most instances, sufficient to stop them evolving separately in their new locations. There must be many millions of individuals that belong to introduced mammal species living wild in Europe today, but only a handful of further individuals will be deliberately released or accidentally escape each year. In other words, new arrivals from the homelands of these species will be only a drop in the genetic bucket–insufficient to stop American-origin animals becoming distinctly European. The influx is fuelling a massive increase in evolutionary diversification of the species that manage to make these new journeys.
Of course, new connections also result in the extinction of incompatible species, and particularly of forms that lived only on remote islands–as I have already discussed. This is a sad consequence of the increased globalization of biology. Still, extinction associated with immigration is rather rare, and increased connections between continents usually result in a net increase rather than a net decrease in diversity.14 Even on islands in the middle of the Pacific Ocean, where some of the original species died out, the total number of species on each island is far higher after contact than before. With increasing transport, each continent, island or sea is likely to hold more species in total, each with the potential to evolve into a new form. In the case of the South American deer, rats and lupins, we can see that animals and plants are liable to evolve into tens, and in some cases hundreds, of distinct species associated with different climates, habitats and geographic locations–an evolutionary radiation–when they arrive in a different continent. Once they have evolved in their new homes, the daughter species will then have the potential to spread back into their former homelands and further increase the total number of species on Earth, like a giant, worldwide Galapagos archipelago.
The biological coming together of the previously separated continents of South and North America is referred to as the ‘Great American Interchange’, one of the most significant biological events of the last 10 million years. It caused some species to die out, others to thrive and new species to be born. Today we are centre stage in a new Great Global Interchange–a far, far grander thing. We are watching the formation of a New Pangea, conceivably the greatest spur to evolution for a hundred or more million years. While some might think of the New Pangea as a single human-connected megacontinent, this new world is more akin to a Pangean archipelago. Each continent, and each region with a continent, and each true island, represents a node in a global network of islands. Each species that arrives in a fresh location experiences the physical conditions there, meets species it has not previously encountered, and starts to evolve into something a bit different. This new global archipelago has the potential to deliver a torrent of evolutionary changes.
Far to the north-west of the Galapagos, the evolutionary consequences of species coming into contact with each other for the first time can be seen. No longer can the human inhabitants of Kauai in the Hawaiian Islands lounge in a hammock after a tiring day in the tropical sun and enjoy the sound of male crickets–a joyful series of long and short chirps–emanating from their shrubberies. The crickets have gone silent, all because of a small fly that has travelled the world.
Kauai’s crickets are themselves recent arrivals, although it is not certain whether they accompanied Polynesian colonists or stowed aboard with later merchant traders. It seems that these Pacific crickets initially set off from Australia, most likely in a series of steps in which they established new colonies on various intervening islands, and eventually journeyed as far as the Hawaiian islands, where populations can now be found on Oahu, the Big Island, and on Kauai. Coming the other way was a fly called Ormia ochracea that is extremely dangerous to crickets. These flies originated in continental North America and at some point travelled westwards to Hawaii and met the Pacific crickets. Welcome to Anthropocene biology. Two species, one from Australia, another from North America, meet up in an idyllic third location. In case this sounds romantic, it isn’t; nature can be a nasty business. Ormia ochracea likes to eat flesh–cricket flesh, to be precise. Ormia lay small fly larvae (rather than eggs), and the larvae proceed to burrow into their cricket victims, eating them from the inside out.
Each female fly hunts for her victims by sound. She sneaks up on the male crickets when they are engaged in their lustful songs before leaving behind her deadly grubs. The female fly has to be able to tell exactly the direction a cricket is calling from if she is to locate it, which is very difficult for an insect whose ears are too close together to be able to detect differences in the time it takes sound waves to arrive at each ear. It has overcome this by evolving an internal physical connection between its ears that is so effective, and small, that it has inspired the development of a new generation of potentially much smaller hearing aids for humans.
In the early 1990s, Marlene Zuk, a professor of biology at the University of Minnesota, noticed that the Hawaiian crickets had dropped their melodious Australian accent and that their songs had become shorter.15 Short songs could be expected to reduce the chances that the crickets would be located by the flies, she reasoned. But the male crickets were still singing and the female flies were finding enough of them in which to leave behind their ravenous larvae. Then it all went quiet. Somewhere, a genetic mutant had arisen in the cricket population that gave the males smoother wings, and smooth-winged males do not call–it is the rubbing of the normally roughened wing surfaces that makes the sound. This could have been a disaster. Males that do not sing will find it difficult to attract females. Yet it turned out that the quiet males still had some ability to find mates, and this was less of a problem than being rasped to death by fly larvae (30 per cent of calling males at any one time harboured fly larvae).
Marlene and her fellow researchers discovered that the frequency of silent flatwings grew steadily–90 per cent fell silent in less than twenty cricket generations. That is rapid evolution. It was far harder for the flies to find smooth-winged crickets, and so enough crickets survived in each generation for the population to recover.16 Now, people are more likely to swing in hammocks to avoid the crickets that are crawling on the floor than to listen to their song. Whether the fly is evolving new ways to find silent crickets is not yet known, but a huge advantage is awaiting any Ormia fly that can home in on silent males. The contest is not yet over. The evolutionary game never is.
Exposure to flesh-eating maggots has generated differences between the Kauai crickets and those in Australia and Moorea in the South Pacific, starting the process of evolutionary separation in the global archipelago as a consequence of species meeting up for the first time. Since each species of cricket has a distinct song that enables the females to recognize their mates, this is exactly the sort of change that could eventually lead t
o the Hawaiian crickets becoming a new species. It could even happen on other Hawaiian islands, if the crickets develop different acoustic means of evading the flies on each island. The process of separation and evolution in partial isolation is starting already.
For the Pangean archipelago to generate increases in the number of species in the world requires the transported populations that now live in separate continents, separate habitats within each continent, and separate islands to become so different from one another that they form separate daughter species. New species are usually thought to take hundreds of thousands of years to form, if not a million, so this potential for novel diversity might seem more relevant to our descendants thirty thousand to forty thousand human generations hence than to those of us alive today.
So perhaps it is reasonable for gloom-merchants to dwell solely on the losses. On the other hand, enough examples of rapid evolution are coming to light to suggest that we have initiated a great new evolutionary acceleration: hybrid Italian sparrows have come into existence; house sparrows have evolved characteristic beak and body dimensions in different regions; the size of fruit flies and the arrangement of their chromosomes have diverged in different places to which they have been introduced;17 and crickets have become silent. Another example is that Argentine fire ants became more aggressive once they arrived in the USA, to such an extent that the native fence lizards had to evolve new ways to escape them.18 The question is whether any of these changes are fast enough that ex-pat populations are starting to turn into distinct species in their new homes. If so, how long will it take?
California is a hotbed of immigration, so it is an excellent place to start. Are Californians turning into new species? Foreign species are not necessarily welcomed to this part of the world, and they stand accused of ousting the natives. Among the arrivals are Spanish plants belonging to the genus Centaurea. California enjoys a Mediterranean-style climate, with cool and relatively moist winters and dry, hot summers, so it is not surprising that the European yellow star-thistle Centaurea solstitialis and its relative the sulphur star-thistle Centaurea sulphurea established wild populations there. The yellow star-thistle, in particular, has become so successful that it is regarded as a noxious weed–despite the fact that its spiky golden-yellow flowers supply nectar to butterflies and bees (Californian star-thistle honey is a new culinary favourite) and it mainly grows on disturbed ground where native wildflowers are rare. In any event, there is no getting rid of it now.
Long established in California, there have been plenty of generations available for the two plants to evolve in isolation from their Spanish ancestors–the sulphur star-thistle was introduced to California around 1923, allowing the Spanish and Californian populations to develop in isolation for up to eighty-six generations.19 But could they actually have become that different after such a short period of time? No one would really have expected this to be the case, and University of Montana researchers Daniel Montesinos, Gilberto Santiago and Ray Callaway were no exceptions–ecologists and evolutionary biologists have been brought up on the ‘knowledge’ that it takes a very long time for new species to form. In fact, they were not thinking about it at all. The main goal of their experiment was to obtain ‘pure’ seeds of each population and species to use in the rest of their research. However, just to amuse himself, Montesinos, who is now at the Universidade de Coimbra in Portugal, in his own words ‘playfully decided’ to transfer pollen from Spanish to Californian plants ‘just to see what happened’.
The results were very surprising.20 Californian plants produced 44 per cent fewer seeds per flower when they were fertilized using Spanish pollen than when they were supplied with Californian pollen. Over the period since the plants were introduced to California, the compatibility of the Spanish with the Californian sulphur star-thistle has declined. Isolation in the yellow star-thistle is even greater, at around 52 per cent reduction in fertility. However, this is over a larger number of generations. The yellow star-thistle was first found growing in California in 1824, but its journey was an indirect one, via Chile, so the chances are that the Spanish and Californian yellow star-thistles last interbred 350 or so generations ago. Nonetheless, this is still exceptionally fast. The Californian and Spanish star-thistles seem to be losing the ability to mate with one another. They are on the path towards becoming separate species.
Yellow star-thistles that are growing in California (foreground and throughout the meadow) are already partly incompatible with their European ancestors. They are rapidly turning into a new American species. The plants are prickly–note the spines at the base of each flower–and regarded as an invasive weed, but they also represent an important nectar and pollen source for beekeepers and for native insects.
Because closely related species can sometimes mate with one another and produce hybrid offspring (the topic of the following chapter), the benchmark for the Californian plants to be regarded as different species is not a full 100 per cent reduction in fertility. Knowing this, Montesinos and his colleagues decided to find out what the fertility might be when you cross different wild star-thistle species with one another. They tried to fertilize yellow star-thistles with the pollen of sulphur star-thistles, and also with the pollen of yet another related species. The answer was a 65–88 per cent reduction in the number of seeds produced when crosses were made using pollen from different species. This suggests that the Californian plants, at 44 per cent and 52 per cent reduction in fertility, are probably not yet fully-fledged species, but are well on the way towards it, a mere 86 to 350 years after they separated from their Spanish ancestors. If they continue to diverge at the same rate, then they might well be quite distinct ‘human-created’ species within a few more centuries. Will Californians, at this point, put aside their hatred of these ‘alien’ plants and treat them as natives?
The arrival and establishment of star-thistles and thousands of other plants provide opportunities for insects to eat them, diseases to infect them and for birds and lizards to seek meals among their leaves and stems. As we have seen, Edith’s checkerspot took to eating white-man’s footprints plantains when these plants started to grow near Carson City on the eastern slopes of the Sierra Nevada range, and Taylor’s checkerspot is almost entirely reliant on the new plant. There is no suggestion that this butterfly is about to become a new species; many populations have their own characteristics without turning into different species. Yet out of the many millions of populations of ‘native’ insects that start to eat ‘alien’ plants, some do change in a way that separates them from their ancestors. Then, new species start to come into existence.
The apple fly, which began to infest domestic apples in North America only a century and a half ago, is a case in point. They are charming flies, albeit not the all-time favourite of fruit farmers and those of us who like unblemished apples. Their larvae, otherwise known as maggots, or ‘worms’, burrow into the flesh of the apple, although they will not do you any harm if you do consume them. The adult flies are delightful, however. They have F-shaped black marks on their otherwise transparent wings, and when they perceive that they might be under attack, they twist them around and walk sideways, waggling their wings up and down. Miraculously, this behaviour seemingly transforms them into spiders, the blackened wing-marks doubling up as pretend legs. Presumably, predatory insects or other spiders are tricked into thinking that they are dangerous, and avoid eating them.
The apple flies mate with one another on the apple trees and lay their eggs in the apples; the maggots develop inside the apples and, once they are fully grown, they drop to the ground, usually still inside the apple, then crawl out and form a pupa in the soil beneath the apple tree. Come the next generation, the pupal case bursts open, and a new adult pops out and flies off in search of the next year’s developing apples. I have used the word ‘apple’ a lot here. Everything the apple fly does is very closely linked to apples. In contrast, their ancestors mated on wild hawthorns and laid their eggs in developing hawthorn be
rries, and their grubs bore into hawthorn fruits before dropping to the ground and pupating there. Hawthorn flies still exist today, eating hawthorns, much as they ever did.
Biologist Jeffrey Feder first became interested in trying to work out what was going on during his doctoral research back in the 1980s. At the time, he was working with Guy Bush, an evolutionary biologist whose research focused on the processes by which one species might become two. Jeff and his colleagues at the University of Notre Dame, in Indiana, continued this work, and they have uncovered a remarkable story.21 The apple flies have evolved to like the smell of apples, while hawthorn flies like the smell of hawthorn. This makes sense. Each species of plant has its own characteristic odour, and the specific chemicals in these odours can be a reliable way for an insect to find its food. This keeps them apart. The apple flies, when they emerge in spring, sniff out apples and then mate on or near developing apples, while hawthorn flies go wild for the odour of hawthorns and mate on or near developing hawthorn fruits. In other words, apple flies and hawthorn flies hardly ever meet or mate with one another because they live on different trees. They have also evolved to become active at different times during the spring, because apples flower slightly earlier than hawthorns. Female apple flies need to be ready to lay their eggs earlier in the season and this early mating further reduces the likelihood that crosses will take place between the two types of fly. By and large, they are already genetically isolated from one another, which is the principal criterion to decide whether two different animals belong to the same species or not. The apple fly is turning into a new species: it might not quite be there yet, but it is well on its way. Like the star-thistles, these flies are on the road to becoming completely separate species, in this instance in somewhere between 150 and 200 years.22
Inheritors of the Earth Page 18
