Imagine taking a course of antibiotics and suddenly finding that your sexual preferences have changed. Individuals who you once found attractive no longer have that special allure. That may sound far-fetched, but some fruit flies at Tel Aviv University have just gone through that very experience. They’re part of some fascinating experiments by Gil Sharon, who has shown that the bacteria inside the flies’ guts can actually shape their sexual choices.
The guts of all kinds of animals, from flies to humans, are laden with bacteria and other microscopic passengers. This ‘microbiome’ acts as a hidden organ. It includes trillions of genes that outnumber those of their hosts by hundreds of times. They affect our health, influencing the risk of obesity and chronic diseases. They affect our digestion, by breaking down chemicals in our food that we wouldn’t normally be able to process. And, at least in flies, they can alter sexual preferences, perhaps even contributing to the rise of new species.
Toads are an evolutionary success story. In a relatively short span of time, they diversified into around 500 species and spread to every continent except Antarctica. Now, Ines van Bocxlaer from Vrije University has uncovered the secrets of their success. By comparing the most home-bound toads with the most invasive ones, she has outlined seven qualities that enabled these amphibians to conquer the world. In a common ancestor, these seven traits came together to create an eighth – a pioneer’s skill are colonising new habitats.
Some, like the harlequin toads, are restricted to such narrow tracts of land that they are vulnerable to extinction. Others, like the infamous cane toads, are highly invasive and notoriously resistant to extinction despite the best efforts of Australians and their sporting equipment. This diversity of lifestyles allowed Bocxlaer to search for characteristics shared by the most pioneering of toad species.
She compared over 228 species, representing just under half of all the known toads, and constructed a family tree that charts their relationships. She showed, as others before have suggested, that the family’s fortunes kicked off in South America, around 35-40 million years ago. This was the start of their global invasion.
Seven qualities make for wide-ranging toads. For a start, the adults don’t have the typical amphibian dependency on constant water or humidity. They have skins that can cope with the drier side of life, giving them a chance to seek out new habitats away from the safety net of moist environments. Secondly, they tend to have fat deposits near their groin, which act as a back-up energy source when food is scarce. Thirdly, they tend to be larger (meaning at least 5 centimetres in length), which also helps to conserve water. Larger animals have larger bladders so they retain more water, and they lose less of it because they have small surface areas for their size.
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:
Walk through the rainforests of Ecuador and you might encounter a beautiful butterfly called Heliconius cydno. It’s extremely varied in its colours. Even among one subspecies, H.cydno alithea, you can find individuals with white wingbands and those with yellow. Despite their different hues, they are still the same species… but probably not for much longer.
Even though the two forms are genetically similar and live in the same area, Nicola Chamberlain from Harvard University has found that one of them – the yellow version – has developed a preference for mating with butterflies of its own colour. This fussiness has set up an invisible barrier within the butterfly population, where traits that would typically separate sister species – colour and mate preferences – have started to segregate. In time, this is the sort of change that could split the single species into two.
Heliconius butterflies defend themselves with foul chemicals and advertise their distasteful arsenal with bright warning colours on their wings. The group has a penchant for diversity, and even closely related species sport different patterns. But the butterflies are also rampant mimics. Distantly related species have evolved uncanny resemblances so that their warnings complement one another – a predator that learns to avoid one species will avoid all the ones that share the same patterns. It’s a mutual protection racket, sealed with colour.
The result of this widespread mimicry is that populations of the same species can look very different because they are imitating different models. This is the case with H.cydno – the yellow form mimics the related H.eleuchia, while the white form mimics yet another species, H.sapho.
How can we be sure that the pairs of butterflies that look alike aren’t in fact more closely related? For a start, scientists have shown that the frequencies of the yellow and white versions of alithea in the wild match those of the species they mimic. Genetic testing provides the clincher. It confirms that the two mimics are indeed more closely related to each other than they are to their models.
Genetics also tells us how alithea achieves its dual coats. Colour is determined by a single gene; if a butterfly inherits the dominant version, it’s white and if it gets two copies of the recessive one, it’s yellow. Pattern is controlled in a similar way by a second gene. These variations aside, there are no distinct genetic differences between the two alithea forms. They are still very much a single population of interbreeding butterflies.
But that may change, and fussy males could be the catalyst. Chamberlain watched over 1,600 courtship rituals performed by 115 captured males. Her voyeuristic experiments showed that yellow males strongly preferred to mate with yellow females, although white males weren’t so fussy.
This isn’t just a whimsical preference – Chamberlain thinks that the colour gene sits very closely to a gene for mate preference. The two genes may even be one and the same. Either way, their proximity on the butterfly’s genome means that their fates are intertwined and they tend to be inherited as a unit. That’s certainly plausible, for the same pigments that colour the butterflies’ wings also serve to filter light arriving into their eyes. A change in the way those pigments are produced could alter both the butterfly’s appearance and how it sees others of its kind.
To see what happens when this process goes further, you don’t have to travel far. Costa Rica is home to another H.cydno subspecies called galanthus, and a closely related species called H.pachinus. They represent a further step down the road that alithea is headed down. Galanthus and H.pachinus look very different because they mimic different models – the former has white wingbands reminiscent of H.sapho, while the latter has green bands inspired by H.hewitsoni.
Nonetheless, the two species could interbreed if they ever got the chance. Two things stand in the way. The first is geography – H.cydno galanthus stays on the eastern side of the country, while H.pachinus remains on the west. The second is, as with alithea, sex appeal. Males prefer females bearing the same wing colours as they do so even if the two sexes of the two species were to cross paths, they’d probably fly right past each other.
Genetically, these species have also diverged far further than the two forms of alithea have. They differ at no less than five genes involved in colour and pattern, two of which are practically identical to the ones that causing alithea to segregate. They also provide more evidence that the genes for colour and mate preference are closely linked, for crossbreeding the two species yields offspring with half-way colours and half-way preferences.
These butterflies are by no means the only examples of speciation in the wild. In this blog alone, I’ve discussed a beautiful case study of diversity creating itself among fruit flies and parasitic wasps, explosive bursts of diversity in cichlid fish fuelled by violent males, and a giant predatory bug that’s splitting cavefish into isolated populations.
But Heliconius butterflies may be the most illuminating of all these case studies. They’re easy to capture, breed and work with. And as Chamberlain’s study shows, they can marshal together the contribution of experts in genetics, ecology, evolution and animal behaviour in an effort to understand that most magnificent of topics – the origin of species.
Reference: Science 10.1126/science.1179141
More on speciation:
In Mexico’s Cueva del Azufre (the Sulphur Caves), a small fish called the shortfin molly (Poecilia mexicana) is on the way to becoming two separate species. Those that live in the dark interior of the caves are very different to their relatives that swim in the bright, surface waters. They have lighter colours and live more solitary lives. Their eyes are smaller, less sensitive and have lower levels of light-sensitive pigment. Instead, they rely instead on a hypersensitive pressure detector – the lateral line – to sense disturbances in the water.
Their differences aren’t just skin deep either. Michael Tobler from the Texas A&M University has been studying the mollies for years, and has shown that the surface and cave populations have started to become genetically distinct. The question is why? The caves are an open habitat with no physical barriers separating the two populations. What’s stopping them, and their genes, from mingling?
Tobler has found that one of these barriers is a living one. The mollies are hunted by an insect, the giant water-bug (Belostoma spp). It’s about the same size as the fish and lurks close to the water’s surface, waiting to stab passing prey with stiletto-like mouthparts. In the gloom of the cave, the surface fish are more vulnerable to the bug, but in the light, it’s the cavefish that are at a disadvantage. The same predator, hunting throughout the Sulphur Caves, is keeping two populations of the same species apart.
This is the second of eight posts on evolutionary research to celebrate Darwin’s bicentennial.
When new species arise, they can set off evolutionary chain reactions that cause even more new species to spring forth – fresh buds on the tree of life create conditions that encourage more budding on different branches.
Biologists have long suspected that these “cascades of speciation” exist but have struggled to test them. Enter Andrew Forbes from the University of Notre Dame – his team of has found a stunning new proof of the concept by studying a fruit fly called the apple maggot (Rhagoletis pomonella) and the parasitic wasps that use it as a host.
Contrary to its name, the apple maggot’s natural host is not apples – it’s hawthorn. The fly only developed a taste for apples about 150 years ago, when the fruit was first introduced to North America. This culinary switch has created two races of apple maggot – one that eats hawthorn and another that eats apples. Even though they are often found in the same place, the two races don’t mix and they don’t breed together. They are well on the road to becoming separate, genetically distinct species.
And so are their parasites. A wasp called Diachasma alloeum specialises in attacking apple maggots. It lays its eggs inside the fly larvae, and its grubs eat the victim from the inside out. Forbes found that the wasp has also started to form separate races that don’t crossbreed with one another, even though they have overlapping ranges. By adapting to new host plants, the flies inadvertently set up barriers that separated their respective parasites from one another. Now, the wasp, like its hosts, are also on the way to becoming separate species. It’s a fantastic example of diversity bringing itself about.
Humans have been blamed for the disappearance of species before but never quite like this. Scientists at the University of Oxford have found evidence that two species of bacteria are merging into one. The two species are swapping genetic material at such a high rate that they are on the road to sharing a single, common genome. Their genetic merger is probably the result of being thrust into a new environment – the intestines of heavily farmed chickens, cattle and other domesticated livestock.
The two bacteria in question – Campylobacter jejuni and Campylobacter coli - are two of the most common causes of gastroenteritis, or food poisoning, in the world. They infect a broad range of animals including mammals, birds and even insects. The two shared a common ancestor and their housekeeping genes – essential genes that are always switched on – are about 87% identical. They are currently recognised as separate species but it looks like some populations are headed back towards a unified direction.
To swap or not to swap
Classifying bacterial species and building family trees for them is never straightforward. Unlike multi-celled creatures, which must content themselves with passing genes on to their offspring, bacteria can happily trade genetic material with their neighbours through a process called conjugation.
Earlier experiments from the lab of Martin Maiden showed that C.jejuni and C.coli are indeed exchanging genetic material. Samuel Sheppard, working in Maiden’s lab, carried on the work by comparing the sequences of seven housekeeping genes in almost 3,000 samples from the two species, taken from a wide range of locations.