In the 18th century, Europe started sending boatloads of white settlers to Australia. But unbeknownst to these colonists, Australia had sent its own white contingent to set up colonies in Europe, around 450,000 years earlier. These migrants were sharks – great white sharks.
When Chrysoula Gubili from the University of Aberdeen compared the DNA of white sharks from around the world, she found a big surprise. The great white is the most genetically diverse shark studied so far but the Mediterranean fish are only distantly related to nearby populations in the North-West Atlantic, or even in South Africa. Their closest kin actually live half a world away in the Indo-Pacific waters of Australia and New Zealand.
On 7 August 1999, a lucky photographer snapped a female humpback whale frolicking off the east coast of Brazil. Two years later, on 21 September 2001, the same whale was caught on camera again, by a tourist on a whale-watching boat. But this time, she was a quarter of the world away, off the eastern coast of Madagascar. The two places where she was spotted are at least 9800 kilometres apart, making her voyage the longest of any mammal.
I’ve just flown from London to North Carolina, a trip of around 6,200km. As flights go, it’s a pathetic one, a mere jaunt in the park compared to the epic voyage of the Arctic tern. Every year, this greatest of animal travellers makes a 70,000 km round-trip, in a relentless, globe-trotting pursuit of daylight. In summer, it spends its time in the sun-soaked Arctic and in winter, it heads for the equally bright climes of Antarctica. In its 30 years of life, this champion aeronaut flies more than 2.4 million kilometres – the equivalent of three return journeys to the Moon.
The Arctic tern’s marathon flight is fairly familiar, but estimating the length of such a massive trek isn’t easy. It would be charitable to forgive scientists for getting it wrong, given that they had to rely on observations at sea and capturing banded birds at different places. But few would have predicted just how wrong the textbook figures are. They typically suggest that the tern covers 40,000km in a year. The bird should be insulted – in reality, it flies almost twice that amount.
Its true itinerary has only just been revealed through the use of tiny tracking devices. Similar machines have already exposed the travel plans of larger seabirds like albatrosses, petrels and shearwaters. But these gadgets been too large and clunky to attach to smaller fliers – strapping a 400g recorder to a 100g bird isn’t going to give you an accurate picture of its flying abilities.
Carsten Egevang from Denmark’s Aarhus University changed all of that by developing tiny geolocators, less than 1g in weight. These locators can track the movements of migrating birds by recording the amount of light falling upon it at different points in its journey, and they’ve already been baptised by recording the entire migration of songbirds. Egevang strapped them to the leg of 50 terns, and managed to retrieve 11 of them the following season, when the birds returned.
In the forests of Germany live large numbers of blackcaps, a small species of songbird. They all look very similar, but they actually belong to two genetically distinct groups that are becoming more disparate with time. For the moment, the best way to tell them apart is to wait for winter. As the cold sets in, one group of blackcaps flies southwest to Spain, while a smaller group heads northwest towards Britain.
If the prospect of spending winter in Britain rather than Spain seems odd to you, you’re not alone. Indeed, blackcaps were hardly ever ventured across these shores before the 1950s. But since then, the birds have taken advantage of the glut of food left out on bird tables by animal-loving Brits. These banquets, along with the luxury of not flying over the Alps, have made Britain an increasingly popular holiday destination for wintering blackcaps. And that has set them down the path towards becoming two separate species.
The mystery of Britain’s winter blackcaps was solved in a classic series of experiments by Peter Berthold (awesome beard) in 1992. Berthold found that chicks from the two populations (those that fly to Britain and those that fly to Spain) would always fly in the same direction as their parents even if they were raised in identical environments. This strongly suggested that their travel plans were genetically set, and Berthold proved that by breeding birds from the two groups. Amazingly, their offspring migrated in a west-northwest direction, about halfway between the routes of their parents.
Berthold went on to show that the blackcaps’ inherited itineraries were the result of a handful of genes at most. And these initial differences have become magnified over time. When spring returns, the blackcaps fly home, they select mates and they form bonds that will last until the next year. But those returning from Britain have less distance to cover so they reach Germany first and they pair up with each other. When the stragglers from Spain get there, they only have each other to mate with.
Even though all of these birds spend most of the year in each others’ company, they are actually two populations separated by barriers of time that prevent genes from flowing from one group to another. Gregor Rolshausen from the University of Freiburg has found that their genetic separation is already well underway.
He has found the Spanish migrants are genetically more distinct from the British ones than they are to individuals from more distant parts of Germany, some 800km away. These differences have arisen over just 30 generations and they’re now sizeable enough that with a bit of DNA sequencing, individuals can be assigned to the right group with an accuracy of 85%.
It’s highly unlikely that the British migrants arose because of an influx of genes from other blackcap populations. For a start, no European blackcaps had ever been found to migrate in a northwesterly direction before 1960.
Instead, Rolshausen thinks that the crucial factor was human altruism – by giving food to wintering birds, we also gave an advantage to any individuals with mutations that sent them in an unorthodox direction. Previously such birds would have simply died, but with humans around, they (and the genes they carried) flourished.
Their bodies have even changed. The British migrants have rounder wings. In general, European blackcaps with shorter migration routes tend to have rounder wings – they’re more manoeuvrable but less suited to long distances. They also have narrower and longer beaks, for they are generalists that mostly eat seeds and fat from garden feeders. Birds that arrive in Spain eat fruit and those with broader bills can eat larger fruit.
Their colours are also slightly different. British migrants have browner backs and beaks, while the Spanish migrants are greyer. It’s not clear why, but Rolshausen thinks that these changing hues could provide the birds with a way of recognising, and sticking to, their closer relatives.
This is one of the few studies to show that human activities – the provision of food to wintering birds – are powerful enough to set up reproductive barriers among animals that live in the same place. It also shows that these first few steps of speciation can happen with extraordinary pace, in just 50 years or so. As Rolshausen notes, the blackcaps are testament to the speed with which evolution can operate.
No one can say whether the blackcaps will actually split into two different species. All the conditions are right, but our activities may change the playing field once again, so that the birds experience entirely new sets of evolutionary pressures.
Reference: Current Biology 10.1016/j.cub.2009.10.061
More on speciation:
In the summer of 2007, thirty-four travellers left home with backpacks in tow to see the world. But these weren’t human students, out to
get drunk and pretentious find themselves in foreign lands – they were small songbirds, migrating to tropical climates for the winter.
Their backpacks were light-measuring devices called “geolocators”, each about the size of a small coin. By measuring rising and falling light levels, these miniature contraptions revealed the timings of sunrise and sunset wherever the birds happened to be flying. Those, in turn, revealed where they were in the world, and allowed Bridget Stutchbury from York University, Toronto to achieve a world-first – track the entire voyage of a migrating songbird, from the start of the outbound trip to the end of the return journey.
The recordings show that tiny wood thrushes and purple martins are far more capable fliers than anyone had thought. They can cover 500 kilometres in a day, flying more than three times as fast as previously expected. Previous studies had credited these tiny fliers with top migration speeds of just 150 km/day. But these had major flaws.
Songbirds are so small that they can’t be tracked by satellites, making their annual migrations difficult to track. Until now, what we knew about their journeys came from brief glimpses on radar or studies done at pit-stops along the way. One incredible study managed to track thrushes during a short part of their travels by injecting them with mildly radioactive isotopes and following them in a plane. All of these studies have provided mere glimpses of the overall migration, like piecing together a movie from still shots and trailer clips. Stutchbury’s team from the University of York, Toronto have managed to record the entire film.
The area collectively known as Austronesia covers half the globe. It stretches from South-East Asia and Taiwan, across New Guinea and New Zealand, to the hundreds of small islands dotted around the Pacific. Today, it is home to about 400 million people.
They are the descendants of early humans who spread throughout the Pacific in prehistoric times. These forebears are long dead but they left several unexpectedly important legacies that are evident in their modern descendants. The languages they used evolved and splintered into over 1,200 tongues spoken by modern Austronesians. The bacteria in their bodies did the same, giving rise to distinct strains in different parts of the region.
Two new studies have used these very different hitchhikers – one cultural and one biological – to piece together the routes of this ancient mass migration. And they have come to the same general conclusion.
The Austronesian people originated in Taiwan some 5,000 years ago. After a few centuries of settlement, they started a massive pulse of migration, spreading southwards and eastwards. They moved to the Philippines, dispersed across South-East Asia, and spread as far west as Madagascar and as far east as the Micronesian islands. They reached Fiji and other islands in Western Polynesia about 3,000 years ago and there, they paused again. About 1,500 years ago, they started a second big migration pulse that took them east across the Pacific all the way to Easter Island.