The formation of new species is only the tip of the hybrid iceberg. Horticulturalists have brought tens of thousands of varieties into existence by making crosses between different garden plants. And hybrids are not confined to gardens. Recently published maps identify over nine hundred different crosses between plant species living in the British countryside.8 Many of them will simply hang around for a while and then die out–but some are here to stay. Purple-flowered rhododendrons, which beautify or ruin our British landscapes (depending on your perspective), are mainly of European origin, but they contain a hint of genes from two North American rhododendron species.9 Their ‘pure’ European ancestors were apparently somewhat tricky to culture, which is not surprising, given that they come from much warmer regions in Spain and Portugal, whereas the new garden hybrids were able to cope far more effectively with the cold weather. The vast majority of the genes in the new hybrids still come from the European species, but it has been suggested that the ‘American genes’ may make the soft Europeans a little tougher (winters have become milder since the 1800s, too, which may have helped). The resulting hybrids are extremely successful, forming evergreen under-storeys to deciduous woodlands on acidic soils and spreading out to form low-growing shrublands over heaths, moors and dunes. Traditionally known by its European botanical name, Rhododendron ponticum, the wild-growing rhododendron thickets are now referred to as belonging to Rhododendron x superponticum, the x denoting its hybrid origin and the ‘super’ providing a reminder that we should perhaps disapprove of its ability to romp across the countryside. Recent headlines describe the ‘threat from aggressive rhododendron’ in England, a ‘30 years war’ against the plant in Ireland, and the ‘rhododendron menace’ in Scotland; but Wales apparently has the answer, as reported by the Welsh Daily Post: ‘Meet Snowdonia’s “Rhodocop”: Gruffydd is looking to root out alien invaders.’ While hybridization has not instantaneously generated a new species, in this case it has formed a distinct population that has the potential to evolve into a new kind of rhododendron over a much longer period of time. It is a botanical addition to the world, however much Gruffydd and his friends love to hate it.
It is not something about the air in Britain that induces plants to hybridize and form new varieties and new species–although the British obsession with gardening has helped. The same phenomenon is under way elsewhere. Moreover, it has been taking place ever since people started to cultivate plants. Our ancestors created new species of wheat by propagating large-seeded varieties of hybrid wild grasses in the Middle East,10 they brought us hybrid peanuts that contain the chromosomes of two wild relatives from South America,11 and they generated three new hybrid cabbage species in Eurasia and Africa.12 The United Nations Food and Agriculture Organization lists at least six human-created species in their inventory of the world’s most important sources of human food around the world. And these are just the completely new species. Most of the other major crops also contain at least some genes from more than one originally wild plant species. GM purists beware–every meal you eat is likely to contain a mixture of genes that did not exist before humans came along!
Away from agriculture, golden yellow and purple salsify plants from Europe were brought together in North America, and the hybrids that formed between them experienced genetic changes. They have become two or three entirely new species that live in the US states of Washington and Idaho.13 And new hybrids between European hawkweeds have been spawned where they have been introduced to New Zealand.14 The current rate at which new species are forming on Earth is starting to look as though it is the highest ever, or at least the highest since animals and plants first colonized the land.
The formation of one new creature from two is not new. Nearly every cell in our living bodies contains energy-giving structures called mitochondria, which originated as free-living microbes in a primeval world that existed before anything that we recognize today as ‘animal’ or ‘plant’ came into existence. Similarly, the green of plants comes from their chloroplasts, the descendants of ancient microbes that still live inside their leaves and stems, billions of years after two separate organisms first started to live together. This historical merger allows plants to capture the energy of sunlight and turn carbon dioxide from the atmosphere into the sugars they use to grow. The abilities of cows and plant-sucking aphids–and humans–to digest these plants also depends on the existence of different microbes within the guts and bodies of animals. In short, every single animal and plant contains a mixture of genes that originated from different species. However, these biological joint ventures with microbes are rather different from the hybridization between two closely related species of Senecio. It is these hybridization events that appear to be on the increase.
When modern humans first spilled out of Africa, we were not the first humans in residence in Europe and Asia. Neanderthals were living in Europe and Denisovans in Asia. And one thing led to another. We may never be sure whether it was rape during acts of war, abduction and enslavement, orphans reared as children of the other species, or loving, harmonious relationships. Knowing humans, it was probably all four. Whatever the details, we mated, produced some fertile offspring, and the genes of both Neanderthals and Denisovans live on in us today.
Thanks to the ability of molecular biologists to read our genes and extract DNA from ancient Neanderthal bones, we know that roughly one to a few per cent of each person who lives outside Africa comes from our Neanderthal ancestor genes (apart from more recent migrants from Africa, who contain only trace levels of Neanderthal). Perhaps even more remarkable, each of us contains slightly different genes that were derived from Neanderthals, such that 60 per cent or more of the genome from Neanderthals still lives on in the modern human species.15 So Neanderthals are less than half extinct after all–they are us! In Asia, even more Denisovan genes mixed with those of our Ethiopian-origin ancestors as they spread into eastern Asia, Australia, the Pacific and the Americas.16
Today, the mixing continues. Consider New Zealand biologist Jacqueline Beggs, whose ancestors include Europeans who bear some of the genes of Neanderthals, and Maoris who bear some of the genes of Denisovans. It does not stop there. The split of the human and chimpanzee lineages long ago seemingly took place over several million years (different chromosomes diverged at different times), which implies that hybridization between closely related ape species happened then, too. If we add in her mitochondria, and the trillions of microbes living in her body, then Jacqueline is a multiple-species and multiple-hybrid animal, as we all are.
Some might argue that the first modern humans, Denisovans and Neanderthals were just three branches of one human species rather than separate species, although there is genetic evidence that male hybrids between modern humans and Neanderthals had reduced fertility. And perhaps different plants that can still hybridize are not fully separated species. As we have seen when discussing whether Chihuahuas and wolves, or apple flies and hawthorn flies, are separate species, it is often tricky to draw a clear dividing line, because speciation represents a continuum of separation. In any event, the overall story is unaltered by such debates. Whichever view one takes, human ancestry has not been an ever dividing tree of life but a reconnecting thicket that comes back together every so often. When hybridization takes place, whether between distinct populations or closely related species, the hybrids then move on with a mixture of genes. Mixing continues. Today, modern Africans are again interbreeding with people whose ancestors have spent up to a hundred thousand years living outside Africa, and whose Denisovan and Neanderthal genes have been living outside Africa for about half a million years. Come back in a thousand years and there is a good chance that every human on the planet will contain at least some Denisovan and Neanderthal genes, as well as the genes of an unknown and extinct African species that has only ever been detected in the genetic code of present-day Africans. And this genetic diversity is likely to be beneficial to the long-term future of humanity.
Jacqueline Be
ggs has European ancestry, which includes hybridization between modern humans and Neanderthals, and Maori ancestry, which includes modern human x Neanderthal crosses with Denisovans. She and all humans also show signs of past hybridization events between a range of ape species that lived millions of years ago. Jacqueline is holding a New Zealand kakapo, the world’s heaviest and only flightless parrot. It is also nocturnal and breeds only when several species of tree produce heavy fruit crops. It is incompatible with introduced mammalian predators, but intensive captive breeding and releases on to predator-free islands–here, on Codfish Island–have brought numbers back to 125 adults and 33 first-year chicks (in 2016).
The technologies to read and interpret our genes are still in their infancy, and the only reason we know so much about the hybrid origin of people is because we are people. We have stared into our own genetic mirror more deeply than into the DNA sequences of other species. Yet, from what we already know, it seems that the human story is not unusual. Different kinds of animals and plants do sometimes mate with one another, and they do sometimes produce fertile hybrid offspring. Those fertile hybrids may then either integrate back into one of the parent species (taking some genes from the other species back with them, as in humans and rhododendrons) or go on to produce a separate species that spreads out across the world, like the hybrid English Spartina cord grass that has colonized North America, Asia and Australia in modern times. But this understanding of the importance of hybridization is very recent.
The more people look, the more it seems that hybridization is the norm. South American Heliconius butterflies have acquired wing colour genes by mating with one another, and this process has generated a new hybrid species in valleys high on the eastern flanks of the Andes.17 Red wolves from the south-eastern United States appear to be about 80 per cent coyote and 20 per cent wolf.18 Appalachian tiger swallowtail butterflies share the genes of two parent species.19 The Atlantic’s Clymene dolphin is a self-perpetuating hybrid between spinner dolphins and striped dolphins.20 Alaskan grizzlies have polar bear genes in every cell of their bodies.21 Hybridization is really quite normal, and always has been.
Ever since Darwin, we have represented the evolution of life as an ever dividing, branching tree, but this is not quite how it works. The idea of a tree of life should be replaced by the image of a more tangled mosaic of interacting lives in which closely related species, in particular, may continue to exchange some genes for millions of years after they first separate.22 The consequence is that different genes in the bodies of each one of us arrived there by slightly different routes. This is absolutely the norm for bacteria, in which ‘species’ that apparently diverged early in the history of life still exchange genes. The reality that life is a tangled thicket demands a change in attitude towards recently hybridized animals and plants. They are not damaging the tree of life–this is how the tree of life grows. We think no more or less of any other human, I hope, on account of the fraction of their genetic code that is derived from Neanderthals, Denisovans, Ethiopians, or any other lineages of humans. It is not relevant to us. My own inner Neanderthal does not alter my humanity, and anyone who nurtures racist thoughts should contemplate the reality that their own bodies contain the genes of more than one former human species, and the genes of many different pre-human species, too. So it is with plants and animals that have hybrid origins.
There are two main reasons why humans are causing such a rapid increase in the rate at which new hybrids are forming. One is the new opportunities that we provide: an artificial canal dug by engineers enabled two types of sculpin fish to meet and mate with one another in the Rhine river system in Europe; coyotes carry dog genes around rural communities in New England; and house sparrows followed human cultivators out of India, met Spanish sparrows and spawned the hybrid Italian sparrow. New habitats and connections represent opportunities for species that were previously separated to meet up and hybridize and, in some cases, turn into new species.
One such example is the Lonicera fly. In 1997, Pennsylvania State University student Dietmar Schwarz was out for a jog when he stopped off to take a look at a honeysuckle bush, which was of particular interest to him because he had previously studied insects that eat honeysuckles, back home in Germany. Dietmar spotted some maggots, collected them and so started an accidental research project. The honeysuckles growing in the area included hybrids between creamy-flowered Lonicera morrowii, a honeysuckle from Japan, Korea and parts of China, rose-flowered Lonicera tatarica from Siberia and pink-flowered Lonicera korolkowii from the dry mountains of Afghanistan and Pakistan. As in so many other cases, these reunited distant relatives had started to exchange genes, in this instance within the 250 years since their introduction. While the foreign plants and their hybrids might be defined as invasive noxious weeds by the human inhabitants of North America, the local flies were not to be put off. To them, the honeysuckles represented a new potential habitat. The insects in question were the North American blueberry fly, whose maggots eat North American blueberries, and the North American snowberry fly, the maggots of which like nothing better than to chew their way through North American snowberries. However, while blueberry flies dislike snowberries, and snowberry flies eschew blueberries (and so they do not normally meet or interbreed), both these native insects were prepared to check out the berries of introduced honeysuckles.23 It seems likely that this is where the two species of fly met up, and mated–genetic analysis reveals that the honeysuckle maggots are hybrids between blueberry and snowberry flies.24 The love child of the blueberry and snowberry flies had a liking for honeysuckle berries, and the Lonicera fly dynasty has not looked back since. Dietmar’s convenient rest stop resulted in him discovering a new hybrid species: new plant, new opportunity, new insect species.
The second and far more pervasive reason why hybridization is on the rise is because humans are moving so many species around the world directly, bringing them into contact with their distant relatives. In Scotland, red deer are starting to hybridize with imported Asian sika deer, especially on the misty Kintyre Peninsula, which is a long, thin promontory that rudely protrudes from the west coast of Scotland.25 In 1893, eleven sika deer from the southern Japanese island of Kyushu were released into the Carradale Estate, but it didn’t take long for them to escape. The sikas established a feral population and spread up the peninsula towards the Highlands, where they encountered larger numbers of red deer. Their ability to hybridize may seem surprising because the Asian and Scottish deer last shared a common ancestor about 6 million years ago, which is not so different from the time that humans separated from chimpanzees. Nonetheless, up to one in five hundred matings are thought to be between the two species of deer, rather than within their own kind, and this has been sufficient to enable their genes to mix. Given this level of genetic mixing after 120 years, it seems likely that most individuals will eventually bear at least some genes that originated in each species.
This hybridization has led to much hand-wringing among the conservation community. As the Great Britain ‘Non-native Species Secretariat’ sika deer factsheet puts it: ‘Hybrids with the native… red deer are fertile, and… hybridization… is threatening the genetic integrity of both red and sika deer.’26 This is interesting wording because it reflects the old thinking that the tree of life should have a perfect branching structure. Individuals and populations that contain a mixture of genes that originate in different species are regarded as worse than either of their ‘pure’ parents. Although some of the hybrids can be identified by their physical appearance, gamekeepers are not able to tell many of them apart from the parent species, so removing all sika genes from the red deer population is not much more practical than the prospect of removing our own Neanderthal genes. The reality is that a new Scottish and British population is being established that will contain a mixture of red deer and sika genes, as well as a touch of American wapiti; and this new population will likely evolve in new directions. I find it difficult to understand why this s
hould be regarded as a threat.
Similar anguish has been expressed in North America, although the details of the ‘beefalo’ story are rather different. It all started as a mishap back in the 1700s, when the odd mating took place between North American bison and domestic cattle, which are themselves descendants of wild European aurochs. These hybrids did not work out particularly well, and so it was not until the 1880s that serious attempts were made to produce hybrids between the two. The idea was to generate animals that were more docile than bison but hardier than cattle, which ranchers could run on rangelands where the wild bison had been hunted to extinction. These attempts were not particularly successful either (until the 1960s), but the wild plains bison population was down to a few hundred animals in the late nineteenth and early twentieth century, such that even a modest amount of hybridization was likely to leave its mark. The consequence is that most plains bison herds now contain at least a smidgen of cattle genes.27
One such population is happily munching its way through meadows on the north side of the Grand Canyon, where they are blamed for ‘destroying water sources, vegetation, soil and archaeological sites’.28 A debate has been raging as to whether they should be culled because of their impurity, spawning all sorts of panic-stricken headlines like: ‘Scheming buffalo herd roams amok at Grand Canyon’, ‘How do you solve a problem like the “beefalo”?’, ‘A beefalo invasion is causing trouble in the Grand Canyon’, ‘Failed experiment beefalo “destroying Grand Canyon” with uncouth ways’, ‘Grand Canyon: Bison hybrids trampling… sacred sites’, and, it being the US, ‘Bison problem? Let Arizona hunters deal with it’. The official response agreed by the US National Park Service is to cull the 400–600 strong herd down to 80–200 animals, and to ‘improve’ their genetics, which may involve releasing ‘pure’ bison females into the herd.29 Having lots of bison-sized animals in the landscape will certainty alter the vegetation, but it seems a tad unfair to blame the changes on the hybrid nature of the animal. The animals look like bison, and it is the bison genes that enable them to thrive in the harsh environments which they stand accused of damaging.
Inheritors of the Earth Page 20
