The hawthorn fly (on downy hawthorn) has larvae that develop inside the fruits of its North American host plants.
Since domestic apple trees were introduced to North America, this fly has begun to evolve into a new species, the apple fly.
The strange markings on its wings impersonate the legs of a spider, and deter would-be predators. Three species of tiny wasp ( Utetes canaliculatus on a native snowberry) have also made the journey onto apple, where they seek out apple fly larvae to parasitize.
It does not stop there. Little flies have littler wasps inside their bodies to eat them, and the poor apple flies have three.23 Small parasitic wasps sting the fly maggots, and the wasp grubs develop inside the bodies of the fly maggots. Grubs within maggots. Parasitic wasps often find their prey by smelling out the plants first (a whole apple tree can be smelled from further away than a maggot), before turning their attention to finding the maggots inside the developing fruits. Historically, however, the wasps would have been attracted by the smell of hawthorn. Amazingly, three quite different little wasps now find the sweet smell of apples irresistible and, even more remarkably, they are repelled by the smell of hawthorn–it sends them off in the opposite direction. These three species of apple-associated wasps also mate on or around the apples, and they emerge earlier in the year to coincide with the time when the apple-fly maggots are available for them to sting. Being attracted to apples, mating near apples, and mating earlier in the season mean that the three species of apple wasp are already becoming genetically distinct from their hawthorn wasp ancestors. Four insect species–a fly and three minute wasps–are in the process of turning into eight on a timescale of centuries, and this is all associated with just one introduced plant.
There has been some debate about whether the apple flies should already be considered new species;24 of course, there is no absolute dividing line. As two populations of one species become less and less alike, there is a continuum of divergence. Nonetheless, we have to look twice before deciding that apple flies are still the same species as their hawthorn ancestors. Most of the evolutionary biologists I have asked do not think these flies have quite crossed that line yet, in the same way that Chihuahuas and Irish wolfhounds are usually regarded as belonging to a single species. Genes can still pass from hawthorn to apple flies through occasional matings, so it is reasonable to conclude that they have not yet achieved complete reproductive isolation.25
On the other hand, I can’t help wondering whether their opinions might be coloured by how recently apples were imported to North America. Imagine we did not know this, and then consider Jeff Feder’s work again–there are differences in fly behaviour as they home in on the scents of different plants, and they eat different plants, mate on different plants, exhibit developmental and physiological differences so that they become active at the right time of year to exploit each plant and they have genetic differences associated with each plant; and then specialist wasps concentrate their attacks on apple flies. Ignorant of the historical introduction of apples to North America, we might conclude that the apple and hawthorn flies are separate species. It is the indecent speed of one species turning into two, as much as anything, that is making us reluctant to acknowledge their separation.
As species travel across the world, they are starting to diverge, establishing geographic separation from their ancestors and from other introduced populations of the same species. And resident species are already evolving in response to the influx of new arrivals, becoming separated into new ecological forms, as we have seen in the case of the flies and wasps. This geographic and ecological isolation is sufficient that these animals and plants will continue to become genetically distinct until, in the fullness of time, they become new species. Speciation seems to be unfolding in front of us at a pace that is sufficient to generate entirely new species on a timescale of a few thousand years, if not sooner.
The real avalanche of new species will, however, come tens and hundreds of millennia from now. Thousands upon thousands of plants have been introduced to new localities around the world; they already account for a third of Europe’s wild-growing plants. Each one of these now-European plants is likely to start diverging from their American, Asian, African and Australasian ancestors, in a manner similar to that of the star-thistles in California. First, they will become distinct populations, then species, and, a million or more years hence, each introduced plant may form dozens of new species, associated with different habitats or geographic locations, akin to the diversification of lupins in the Andes. Animal diversity will follow. Just as checkerspot butterflies now eat introduced plantains and hawthorn flies have taken to apples in North America, European insects, diseases and fungi are all starting the process of becoming adapted to these new introduced plants. They are being joined by other plant-feeding insects, pathogens and fungi imported from distant continents, which will separate into European species in due course. And these insects will all have their own enemies. Tales like that of the hawthorn fly wasps will become the norm. Meanwhile, introduced birds and mammals are joining the throng. Stand back and a great increase in European diversity will ensue.
As humans, we have set ourselves up to keep the biological life on this planet as unchanged as possible, and part of our strategy is to repel invaders so that we can protect biodiversity ‘just the way it is’. Yet our failure to control the global transport of species has a silver lining. Although the arrival of additional species speeds up ecological change (which is occasionally inconvenient), letting more species in immediately accelerates the rate of evolution and subsequently increases the rate at which new species come into existence on Earth. In other words, the biological processes of evolutionary divergence and speciation have not been broken in the Anthropocene. They have gone into overdrive. We have created a global archipelago, a species generator, which will give rise to considerably more species on Earth than existed before humans started to spill out of Africa. Come back in a million years and we might be looking at several million new species whose existence can be attributed to humans.
9
Hybrid
Yellow-flowered Senecio eboracensis, Yorkwort, germinated in my front garden in the summer of 2014, the first time this plant has grown in its native Yorkshire for over a decade. Gordon Eastham, the grounds maintenance manager at the University of York, also sowed it in the middle of a university campus roundabout–it is a plant that is not too bothered by human disturbance. And the City of York planted it out in one of its formal gardens for the first time since the City authorities unknowingly weeded and sprayed it out of existence a decade before. This is by now a familiar story: a species dies out, then there are attempts to bring it back.
But why does York possess a rare plant species that appears nowhere else in the world in the first place? At 54 degrees north, York is further north than Newfoundland, and at a similar latitude to the frigid Aleutian Islands, the Kamchatka Peninsula and Lake Baikal. York–not that it existed–was cowering beneath a massive block of ice a mere twenty thousand years ago and, today, much of the city is perched on top of glacial moraine, a pile of stones and sand dumped by the melting glacier at the end of the last ice age. Yorkwort can’t have originated under the ice. Species usually take a million or more years to evolve, so where did it come from? And when did it spring into existence? Bizarrely, the answers to these questions are: in York, and during the last quarter of the twentieth century, before it was obliterated again by a human desire for neat, tidy pavements. Fortunately, renowned plant geneticist Richard Abbott had researched the plant’s origins and had had the foresight to keep some seed.
The Yorkwort story starts two thousand kilometres away and several centuries earlier, with botanists hunting the slopes for new curiosities on Mount Etna in Sicily, in southern Italy. The botanists in question were Francesco Cupani, the first director of the botanic garden at Misilmeri in Sicily, and William Sherard, a Briton of modest origins who rose to become British Consul at Smyrna, in
Turkey, from 1703 to 1716, where he established a wonderful garden and made his fortune, the latter enabling him to devote much of the remainder of his life to his true love–botany. The first step was to collect some unusual bright-yellow-flowered Senecios from the middle elevations of Etna’s volcanic slopes. It later transpired that these were natural hybrids between two wild species of ragwort, one of which, Senecio aethnensis, naturally grows high on the mountain, whereas the other, Senecio chrysanthemifolius, is a denizen of the lower slopes. Shipped to Oxford in the early 1700s, this mixed-up Senecio behaved for a while like any other self-respecting garden plant, growing where it was put in the city’s botanic garden, until its wind-blown seeds fetched up elsewhere in the developing city.1 Escaping the fertile soils of the botanic garden flower beds, it found a home in a more desolate environment–on the volcano-like walls and gravels of Oxford University’s colleges and libraries. Within a century, its yellow flowers adorned the city. It acquired its name, Oxford ragwort, and the less fortunate scientific binomial Senecio squalidis. The plant was genetically distinct from its parents, and it did not breed with them–to all intents and purposes, it was a new species.2
And so things might have stayed, with one new plant added to the world’s list. While the seeds of Oxford ragwort can be blown in the air from one of Oxford’s dreamy spires to another, there were limits to how far they were likely to go. If they landed in the surrounding pastures and woodlands, they would be unlikely to find suitable places to grow. That is, had it not been for the opening of Oxford railway station in 1844. Industrialists, enthusiastic to transport goods and people the length and breadth of Britain, inadvertently set about constructing an Oxford ragwort conveyor. The long strips of lava-like gravels accompanying every new railway line represented a habitat to which the inhabitants of Etna were considerably better adapted than were the flowers of British meadows. Thank you, Isambard Kingdom Brunel. And then, kindness itself, the industrialists built a system of seed-suction devices–trains–which could hardly have been better designed to generate aerial vortices and move the seeds along these artificial ‘lava flows’, delivering them to one city after another, where they found man-made cliffscapes just waiting to be colonized. Thank you, George Stephenson. Were its history unknown, Oxford ragwort could be renamed after any other town or city connected to the railway system; it is one of Britain’s most familiar flowers.
Senecio plants are not best known for their chastity. When they arrived in York, the related native common groundsel was already there. Groundsel, Senecio vulgaris, is less showy than the Oxford variant, lacking proper petals. When I say native, groundsel mainly grows in flower beds, building sites, unkempt pavements, yards and supermarket car parks. It is not quite clear when groundsel arrived, but it has been around for a long time. Its Anglo-Saxon name in the latter half of the first millennium was groundeswelge, meaning ground-swallow, referring to the plant’s habit of invading previously bare ground. In any case, it was the Senecio in occupation. And when the Oxford ragwort pitched up at York railway station, introductions and a little bit more were made. A new hybrid came into existence in 1979, give or take a few years, and the hybrids bred true.
The ability of the parents to cross at all was quite impressive, because the groundsel had forty chromosomes–the structures in every cell that host each species’ genetic code–whereas the Oxford ragwort only had twenty. The York hybrid version opted for forty, perhaps explaining why it can interbreed more readily with its groundsel parent than with the incomer from Oxford. Nonetheless, the York plants showed the capacity to establish a genetically distinct and self-perpetuating population in the 1980s and 1990s. Richard Abbott was happy to recognize the York plants as a new species, which seems reasonable, given that many other plant species are also a bit promiscuous.3 So, Senecio eboracensis was born, Eboracum being the Roman name for York, which itself refers back to Iburakon, meaning place of the yews in a former Celtic language. Species number two. By the 1990s, eboracensis could be found growing in some profusion between the cracks of the pavements on the banks of the River Ouse, a short seed-blow from York’s railway station. However, it disappeared again when the City of York authorities decided to clean up its pavements. Minus one species. Or, it would have been but for the collection of seeds that Abbott and his colleagues still had, and donated so that it could again be grown in the city of its origin.
So, we are still at plus two species: Oxford ragwort and Yorkwort. And it did not stop there: Oxford ragwort’s tour of Britain has been extensive, hybridizing with common groundsel elsewhere, often spawning a plant called Senecio baxteri. Because thirty-chromosome baxteri is sterile, that doesn’t count as a new species. However, a few individual baxteri plants must, by chance, have experienced a developmental ‘error’ somewhere in North Wales, and a fertile sixty-chromosome version was born. Hence Senecio cambrensis, Welsh groundsel, arrived on the scene4–speciation in an instant. Species number three. The same thing happened in Edinburgh, but they were very like cambrensis and died out again, so perhaps we should not add the Edinburgh hybrids to the credits.
Yorkwort, Senecio eboracensis, a new hybrid species that came into existence in the City of York at the end of the 1970s. Yorkwort formed as a consequence of humans moving its ancestors to Britain and providing suitable disturbed and stony ground for the plants to grow in.
Three new species and an array of other hybrid forms have all been generated in three hundred years as a direct consequence of human actions: transporting a plant across the world from Sicily to Britain, building artificial cliffs and beds of gravel, creating trains that could move the seeds great distances and thereby allowing Oxford ragwort to meet up with groundsel, which was itself thriving in human-created habitats. This spawning of around one new species per century is about ten thousand times faster than the conventional rate at which new species are expected to form. And human actions are responsible.
Are these Senecio plants a one-off, or are we initiating a mass diversification of new species on Earth, at the same time as others are being driven extinct? Californian weeds are diverging from their European ancestors, apple flies have come into existence, wasps that kill apple flies are thriving, Italian sparrows formed when house sparrows hooked up with Spanish sparrows, and now Senecio plants are turning into new species at immodest speed. Senecio hybrids are likely to be spawned wherever Oxford ragworts and their relatives meet up with groundsel and other Senecios throughout Europe. Give it a few millennia and each major European city might have acquired its own unique type of Senecio, many of which could be distinct enough to be regarded as a separate species.
As far as we know, no British plant species has become globally extinct over the same period that the Senecio species were diversifying. Some species have died out in Britain, but still survive elsewhere, in continental Europe. This means that the contribution of human actions within the geographic confines of Britain over the last three centuries has been to increase the number of plant species on the planet. The new hybrid Senecios are nowhere near as distinctive as some of the world’s extinct species, however, so they should not be regarded as like-for-like replacements. The great auk, for example, was a flightless seabird that used to nest on offshore islands surrounding the British mainland and on other rocky islets in the North Atlantic; it became globally extinct in the middle of the nineteenth century because we killed them all for their meat, eggs and feathers. It was unique. The great auk’s ancestors split from those of the razorbill some 25 million years ago (the razorbill is a smaller and flying version of the great auk, and survives today–just as we saw in New Zealand that the flying pukeko survived while the pedestrian takahe is flirting with extinction) and so it was genetically one of a kind. The different Senecio species are genetically much more similar to one another than the great auk was to the razorbill.
On the other hand, new Senecio species are living organisms. The Anthropocene epoch is indeed the regrettable end of the evolutionary story for many
animals and plants, but it also represents the beginnings of many new stories. Evolutionary acceleration is starting to gather pace.
Britain is perhaps the only part of the world where we can begin to estimate how fast new species are being added to the world list, given our long interest in gardening and natural history, as well as it being the birthplace of evolutionary biology. Two different species of monkey-flower, one from South America and one from North America, escaped from our gardens and, on two separate occasions, their hybrid offspring have doubled their chromosomes and turned into a new species that now lives along Scottish stream-sides. This presents a real dilemma for conservationists–should they kill the new alien plant or celebrate its arrival on Earth? A marsh-dwelling cord grass originated when North American Spartina grasses hybridized with British Spartina on the intertidal expanses of mud close to Southampton. A new hybrid species was born, Spartina anglica, proudly bearing its country of origin in its name, before spreading around the coastline of Britain, and then setting off to create its own English cord-grass empire in estuaries across the world.5
In a more domestic setting, we have the Kew primrose, which first came into existence in 1898 after plant hunters imported one of its parents from the Himalayas, and the other from North Africa or Arabia. Together at last in the Royal Botanic Gardens at Kew on the south bank of the River Thames in London, pollen could meet stigma. And their offspring, the Kew primrose, was born.
All told, seven or eight new plant species have come into existence in Britain since 1700. Two to three species per century in Great Britain might not seem like much, but this rate of formation of new species is about a hundred times faster than new plants have come into existence in the geological past.6 If the same rate of hybrid plant speciation is replicated over the rest of the world’s land surface (which may not be true yet but could well be the case within the next century, given the acceleration in transport of species between the continents), then–very roughly–a thousand new plant species will be generated per century.7
Inheritors of the Earth Page 19