Tag: evolution

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You’re grounded!

journal.pone.0002271.g009

Pterosaurs are the largest animals to have ever flown. Some species had wingspans exceeding 10 metres dwarfing the largest avian challenger. It must have been quite a sight to see one of these things blocking out the Mesozoic sun. But there have been niggling doubts about the ability of the larger representatives to fly. Will we have to re-evaluate our mental image of the Mesozoic and ground our pterosaurs?

Flight is no easy thing for an animal. It makes all sorts of demands on the physiology, morphology and ecology of the creature trying to take to the air for a living. With every added kilo a bit more lift has to be generated, for every extra wing flap more energy is required. Still, most pterosaurs look like they fit the bill. Their skeletons were heavily pneumatized and they had a hyper-elongated fourth finger from which they could support a membranous wing.

The problem arises when we look at the giant pterosaurs especially the Azhdarchidae family which houses the biggest species like Quetzalcoatlus northropi and Arambourgiania philadelphiae. One analysis gave a mass estimate of half a tonne for Quetzalcoatlus n., which would almost certainly render it flightless. Other researchers point to the terrestrial adaptations seen in this family and of course we can see many instances of birds who have become secondarily flightless. A size gap was pointed out where there exist small pterosaurs and giant ones but no intermediates which was said to mirror the pattern of flying birds and flightless ratites. Then there is the taphonomic bias seen in the fossil record whereby most of the Azhdarchid skeletons are found in terrestrial environments.

But not all palaeontologists are convinced by these arguments, pterosaur specialists Mark Witton and Michael Habib have taken each one of these lines of evidence to task and found them wanting.

Firstly, while most of the fossils have been found on land this doesn’t mean the animals were terrestrial, many bird species fly exclusively over land, so that bias is neither for nor against.

Secondly, the suggested size gap looks like an artefact in the fossil record which has been filled with intermediate forms.

Perhaps the most convincing piece of evidence in favour of flightlessness are the huge mass estimates. A half tonne reptile is going to struggle to get airborne. But this figure is beginning to look like an overestimate, the result of distorted fossils and inappropriate scaling techniques. A more lightweight figure of 240 kg looks to be more realistic when these biases are accounted for.

What of the terrestrial adaptations? Well, there is no issue with the animals being adept on the land while still being able to fly. Indeed the authors above argue that large Azhdarchids occupied the niche of modern day ground horn bills or storks both of which are well adapted to the land while still being able to fly.

In the end it looks like giant pterosaurs did take to the skies. Piecing together the mode of life of long extinct species is never easy but it’s not impossible.

Author

Adam Kane: kanead[at]tcd.ie

Photo credit

Witton MP, Naish D (2008) A Reappraisal of Azhdarchid Pterosaur Functional Morphology and Paleoecology. PLoS ONE 3(5): e2271. doi:10.1371/journal.pone.0002271

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No animal is an island

800px-Impala_mutualim_with_birds_wide

No man is an island; the same could be said for the millions of life forms that populate our planet. Think of all the ways in which organisms interact with each other through predation, parasitism and the countless symbioses. Sometimes a pair of interacting partners can become inextricably linked such is their mutual dependence. Each one may provide the other with a resource it’s unable to obtain on its own.

A recent collaboration explored instances when these interactions lead to the loss of a trait and showed the fragility of this situation. One of the examples the authors use is an ant species that farms fungus. The fungus provides the ants with all the arginine (an amino acid) they need so they have lost the ability to synthesise it themselves. Thus anytime an ecological interaction involves some provision of a resource by one partner to another the evolutionary pressure is removed and the trait can be lost in the species receiving the goods.  In other words we end up getting ‘compensated trait loss’ due to the ecological interaction. This can tighten a symbiosis from a facultative to an obligatory one.

But the fragility of compensated trait loss should be obvious now. In the ant example, were the fungus to go extinct the ant would disappear along with it. It’s like the ecological interaction is undermining all the good work done by natural selection in providing the ancestral ants with all the traits they need. The authors reckon that trait loss is “grossly underestimated” which puts many species in a precarious position in this age of mass extinction. Although there have been some instances where the trait was recovered, in flagrant disregard for Dollo’s law. Some of these law breakers include parasitic insects who regained their ability to synthesise lipids once the provision was lost.

A difficulty in studying these systems is how to detect when trait loss is taking place. A decreased expression of some gene in some members of a population would probably be reported as natural variation. But with ever improving molecular techniques we will be able to get a better estimate of the number of compensated trait loss interactions.

Author

Adam Kane: kanead[at]tcd.ie

Photo credit

wikimedia commons

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Friends with benefits

White-winged Choughs
White-winged Choughs

With the apocalypse come and gone we can still theorise about how our downfall will come about. E. O. Wilson wrote that ‘‘sex is an antisocial force in evolution’’. Charlie Cornwallis, his colleagues at Oxford and their promiscuous birds illustrated why this is so. As is so often the case in evolutionary theory the question centres on sociality, in this instance it takes the form of cooperative breeding. Why would an individual help someone else raise their young rather than having progeny of their own?

The authors ask us to consider the case in birds of a mother and her offspring. If the offspring remains at his natal site and his mother is monogamous he can help in rearing his siblings who are all as related to him as he would be to any of his own potential children. This makes evolutionary sense in terms of relatedness.  However, if his mother is promiscuous his level of relatedness to the resultant other birds will be less than that of his own offspring. The sensible thing for him to do here is to disperse and start a family rather than helping out his half siblings.
The authors of the study collected data on almost 300 bird species recording levels of promiscuity and degree of cooperative breeding to test the monogamy hypothesis. This states that species with high levels of promiscuity will tend towards societies with low levels of cooperative breeding and vice versa.

They were able to show that highly promiscuous birds tend not to have cooperative breeding whereas species like the White-winged Chough, which are completely dependent on others for help in raising their young, are faithful.

But more than this their results demonstrated that, over evolutionary time, transitions to cooperative breeding were associated with low promiscuity. The reverse is also true and echoes Wilson’s statement, because highly promiscuous groups saw a breakdown in cooperation.

There are some exceptions to the rule in that birds can be promiscuous and still have a society of cooperative breeders but this is offset by kin discrimination which involves “directing aid preferentially towards relatives.” So there you have it, sex has the power to destroy societies.

Author

Adam Kane: kanead[at]tcd.ie

Photo credit

wikimedia commons

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World’s ending: enough time to read this?

Asteroid_explosion

What will happen today on the last day of the world (21st)? Will some giant asteroid hit the Earth? Will massive tsunamis ravage all the coast lines? Will climate suddenly be way to warm for life? Will methane bubble out of the oceans and asphyxiate everybody? Or are aliens going to take over our planet?

We could spend a long time discussing the causes (or not !) of the end of the world. However people usually ignore the timing of this type of inevitable (or not !!) catastrophe. We all have in mind the asteroid that Alvarez and his fellows discovered – the one that wiped out in a blink of an eye the ferocious Mr. T-Rex. But what people tend to ignore/forget is the timing of such events…

When studying the history of life, the timing and the scale of the timing is always very important! Did the asteroid wiped out the dinosaurs? If we could watch the impact of this asteroid, would we see a herd of Triceratops peacefully grazing on the ground and then, in a blink of an eye… Nothing ?! Same for the even more massive end-Permian event, would we have seen the trilobites bubbling in the sea and then the day after a desolated planet? No.

As this biological crisis appears really swift and savage in the fossil record, it does not mean that they were quick in reality. The fossil record is a random and imperfect record of time. What might look as quick as a blink of an eye could also be something as smooth and long as several million years !

As a French guy, I’m not putting the Catastrophism vs. Uniformitarianism debate back on the table. Obviously these crises were real, loads of species disappeared and in a small amount of time. But a small amount of time relative to  the fossil record, not according to the Mayan calendar !

So I’ll say no worries, if the Mayans were right we still will have time to enjoy Christmas turkey as well as the next couple of million years to go !

 

Author

Thomas Guillerme: thom.g[at]free.fr

Photo credit

wikimedia commons

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Sex on the beach?

Mr Garrison taught  South Park Elementary children (season 10, episode 12) the good old fashion way of seeing evolution; “we are the retarded children of some retarded frog-fish-squirrel…”. This is the gradualist way of seeing evolutionary processes; leading from uninteresting jelly fishes to the mighty Arnold Shwarzenegger.

Many scientists reject this gradualist view in favour of Darwin’s idea that “there is no innate tendency leading to progress in evolution”. But do they really gave up this idea? A few gradualistic events in the history of life remains firmly accepted such as how the vertebrates went out of the water. . This theory refers to  fossil evidence from the Late Devonian period (that’s about 350 million years ago), suggesting that early tetrapods used their legs to go out of the water and conquer the brave new terrestrial world.

However, during this last decade, many paleontologists (yes, they don’t just look  for dinos) pointed out some disturbing facts: the early tetrapods, such as the iconic Ichthyostega, were in fact not able to walk out of the water at all ! So why the hell did they develop legs?

This question is an example of my favourite part of macroevolution: exaptations (when the function of a trait shifts during evolution). The French palaeontologist, S. Steyer, pointed out that early legs may have been used in many way before being useful to Usain Bolt: for swimming, for hunting (used to hold onto the seabed) or even… for sex ! S. Steyer suggested that legs could have been used to grab the female during sexy time as frogs  do today.

Other interesting aspects of this  gradual transition from water to land are the timing and the possible reasons why  early tetrapods left the water to go conquer the new terrestrial world – which was not new at all by the way; arthropods had ventured out of the water since the early Devonian. Recent tracks discovered in Poland suggest that a 50cm long “retarded fish-frog” walked on the beach in the Early-Middle Devonian, that’s 20 million years before Mr. Ichthyostega was groping his lovely wife. Niedzwiedzki and his team interpreted these tracks (and some other “younger” ones like the Irish Valentia Island trackway) as belonging to some tetrapods that eventually went out of the sea to scavenge  on the beach on low tide.

So was there really a graduale evolution from water to land in vertebrates? Who really made these tracks if it wasn’t Ichthyostega who  actually just “walked ” like a walrus?  Even though legs probably didn’t evolve for walking, some tetrapods may have thought that it would be fun to go on the beach for a walk and get yummy carcasses. Were their other tetrapod colleagues  using their legs for less adventurous but still cool purposes such as the possibilities listed above? As an answer to these questions, let’s just say that it’s complicated and it takes more than one big step on the shore to go from the “retarded fish-frogs”…

 

Author

Thomas Guillerme: thom.g[at]free.fr

Photo credit

South Park – season 10, episode 12

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War of the worms

A battlefield

Some of the most successful animals on earth live in societies characterised by a division of labour between reproducing and non-reproducing castes.  One role non-reproducing members may undertake is defence. Spectacular examples include the heavily armoured termites and ants. Recently a soldier caste was discovered in an entirely new and unexpected battleground, inside the bodies of snails. The soldiers? Tiny parasitic flatworms.

Flatworms, or trematodes, have complicated life cycles, involving several different stages infecting a variety of host species. In one host, often a snail, a single trematode undergoes repeated clonal reproduction. Clones produce more clones or go on to produce the next infective stage, which leaves the snail to infect the final host. While working with trematode colonies of Himasthla sp. infecting the Californian horn snail Cerithidea californica, researchers at the University California Santa Barbara observed that the trematode occurred in two distinct morphological forms. There was a large reproducing primary morph, which appeared to be the form typically described in the literature, and a secondary smaller, thinner morph.

These secondary morphs had a number of other features which set them apart. They rarely showed any signs of reproduction and were far more active. They also had huge muscular pharynxes and guts relative to their larger sisters. When researchers preformed behavioural tests, they discovered just what those large mouth parts were for. The secondary morphs attacked and killed other trematode species and unrelated conspecifics. This behaviour is not unknown in trematodes; a number of species attack and kill other trematodes. What was novel was that the smaller morphs appeared to be doing all attacking. The behaviour was rarely observed in the primary morphs. There was also a spatial segregation of morphs. Primary morphs were located in the visceral mass, mainly in the region of the gonads. The secondary morphs were more widely distributed though mainly found within the mantle. The snail mantle is the main entry point for trematodes, a strategic area to defend against invading armies. Finally, the researcher found very few intermediate morphs, suggesting that the smaller morphs were not simply juvenile stages of the primary morphs. They were a distinct, permanent caste whose function appeared to be defence – soldiers. The researchers had discovered eusociality in a completely new taxonomic group.  Previously, eusocial systems consisting of morphologically distinct, specialised reproductive and non-reproductive castes had only been recognised in insects, snapping shrimp, a sea anemone and mole rats. The researchers have already suggested a further five species of trematodes that may have soldier castes.

Work from New Zealand, published this year, on another species (Philophthalmus sp.) has expanded the list of trematodes with soldier castes. The authors also showed that interspecific competition has a heavy impact on colony numbers. This is just the sort of pressure that favours adaptive strategies to reduce competition, such as a permanent soldier caste. However, competition may not be the only selective pressure driving or maintaining caste differentiation in trematodes. In the absence of competition, the presence non-reproducing morphs were found to provide a benefit to the colony, as measured by the number of infective stages produced. Precisely how this benefit comes about is not yet known. The authors suggest some form of communication or nutrient exchange may be taking place between the two morphs. This gives tantalising hints that these colonies are even more complex and interactive than previously thought.

Not only has the discovery of the eusociality in trematodes widened the taxonomic range of this phenomenon, it has also provided researchers with an exciting new tool to study its evolution. The Trematoda class contains at least 20, 000 species with a wide variety of life-histories and ecologies. The discovery is also a great example of how new and unexpected results can still come from well-studied animals. The Himasthla sp. /Californian horn snail system had been studied for over 65 years.

Author

Karen Loxton: loxtonk[at]tcd.ie

Photo credits

wikimedia commons

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The language of evolution on trial

Humans are purpose seeking beings. Such a fact is nowhere more apparent than in our language. Some scientists argue that this tendency is a cause of confusion in their subject, especially when it comes to descriptions of evolution. The teleological turn of phrase is so tempting because of how much easier it is to read and understand than a dry purposeless, but more accurate, expression.  ‘Wings evolved for flight’ isn’t quite right but we understand the message. I remember my chemistry teacher’s classes were replete with teleology, ions wanted to gain or lose electrons so they could balance their charge. But of course, none of us believed for a second that the atoms intended to do this. All there was to it were the blind forces of the atomic world. So it goes for evolution as our current understanding of the process is teleology free.

Richard Dawkins, who was put on this Earth to popularize evolution, is always quick to correct himself when his tongue slips to purpose. But I would argue that our linguistic short-cuts are not the primary cause of the public misunderstanding of evolution. It was Eugenie Scott who said for many people the problem behind evolution is not one of confusion, rather it’s a full understanding and disgust at the implications of it. Some of us don’t like the idea of being a ‘mere’ animal. Of course language matters but it would be a shame for us to avoid using language which can convey an idea so succinctly when it’s not to blame. Perhaps I’m being overly naïve here and we’re adding to the confusion with our lack of precision. So I’m open to debate on this one. What do you think?

Author

Adam Kane: kanead[at]tcd.ie

Photo credit

wikimedia commons

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Tenrec tales

Tenrecs are one of the most interesting and fascinating mammal groups yet many people have never heard of them. They are one of only four mammalian groups to have colonised Madagascar, a land filled with evolutionary curiosities.

Tenrecs are a striking example of convergent evolution. From a single colonising ancestor, tenrecs have evolved into incredibly diverse species which resemble moles, shrews, hedgehogs and even otters! Contrary to appearances, tenrecs’ closest relatives are actually the golden moles and elephant shrews (Chrysochloridae). However, physical convergences are so strong that early taxonomists didn’t recognise tenrecs as being closely related to each other, an easy mistake to make when you look at this picture from Richard Dawkin’s 1996 book “Climbing Mount Improbable”.

Convergent evolution in tenrecs. The Algerian hedgehog, Erinaceus algirus (a), is a close cousin of the shrew hedgehog, Neotetracus sinensis (b). The greater hedgehog tenrec, Setifer setosus (c), is a close cousin of the long-tailed tenrec, Microgale melanorrhachis (d)

In addition to being great species for studying convergent evolution, the tenrec family includes a whole host of quirky traits. For example, the common tenrec (Tenrec ecaudatus), an animal which is only around 30 cm and 2kg, holds the record for the largest litter size of any mammal at an astounding 32 babies!

My personal favourite tenrec oddity is the unusual means of communication found in the lowland streaked tenrecs (Hemicentetes semispinosus).  These cute critters are covered in spiny quills, a special set of which are used as a stridulating organ. Reminiscent of grasshoppers or crickets and uniquely among mammals, these tenrecs rub the quills together to produce sound which then allows them to keep in contact with their family group. This clip from the David Attenborough BBC series on Madagascar shows the stridulating tenrecs in action.

In short, tenrecs are an awesome family filled with evolutionary oddities yet they remain relatively understudied and poorly understood.

In my PhD work, I’m particularly interested in measuring the extent of convergent evolution in tenrecs and figuring out the reasons why they have evolved to be so similar to unrelated species. I’m also intrigued by early behavioural experiments which showed that 3 species of tenrec use echolocation. I want to test whether other tenrec species also echolocate and hopefully link this behavioural convergence to genetic similarities in “echolocating genes” which are conserved in whales and bats.

In the meantime, a charming children’s book gives the perfect excuse for some light, PhD-related extra reading!

Author

Sive Finlay: sfinlay[at]tcd.ie

Photo credits

wikimedia commons

Richard Dawkins

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Bamboo systematics: less swaying in the wind

Young shoots of Phuphanochloa Sungkaew & Teerawat.: a new species and genus of woody bamboo discovered by TCD botanists.

The bamboos are an extraordinary group of plants and the only large group of grasses to diversify in forests. They represent a major radiation in the angiosperms with nearly 1,500 species. The Bamboo Phylogeny Working Group (including TCD botanists Sarawood Sungkaew and Trevor Hodkinson) have recently used molecular, anatomical and morphological characters to update the tribal and subtribal classification of bamboos including the new genus (Phuphanochloa) shown in the photo.

Authors

Sarawood Sungkaew

Trevor Hodkinson: hodkinst[at]tcd.ie

Photo credit

Trevor Hodkinson

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A dose of Darwin

One of the more irksome aspects of life is getting sick, I always think we should be beyond this. When it comes to medicine I suppose we can ourselves lucky to be alive at a time when doctors and butchers are no longer interchangeable. But even in the 21st century we’re still engaged in arms races with bacteria and chemical warfare with cancer (military metaphors are never in short supply) . Bacteria evolve resistant strains to our best antibiotics and tumours do likewise against our attempted cures. Less grave but still illustrative is the perennial persistence of the common cold. Illness comes in many forms, wellness only one. The main problems we face in trying to create cures is that all pathogens are evolving and to realise that we too have evolved. We often end up using static measures against dynamic problems.

That said, I think this century will see some considerable advances in medicine owing to the increasing use of evolutionary insights to combat disease. This is the science of evolutionary medicine.

Some of the symptoms we most associate with being sick like fever, pain, nausea and vomiting aren’t from the disease per se, but the body’s defences in action. Randolph Nesse, who wrote the book on evolutionary medicine, says that medical doctors would do well to recognise our evolutionary origins when treating their patients.

Our biology is playing catch up to our cultural advances and this lag period can explain many modern afflictions. Nobody reading this suffers from prolonged periods of hunger but we still stuff our faces when we get the chance. Of course, before modern civilization our next meal wasn’t always a sure thing, so gorging when the going is good was the way to go. The upshot of this is a significant proportion of the developed world is obese.

Some of the ideas of evolutionary medicine seem to run counter to our regular approach and our intuition. For instance, when treating a patient with chemotherapy, we use the most potent chemicals and this seems perfectly reasonable. We want to hit the tumour hard and fast. In fact, all we do then is put a strong selective pressure on the tumour cells to evolve resistance, ultimately shortening the patients’ lives. Instead, we should only give the minimum dose necessary to keep the cancer under control.

Fleming’s discovery of penicillin is one of those instances of serendipity that turned out to be a great boon for medicine. But I’d prefer not to rely on serendipity if you don’t mind! It would be far preferable to know why how and why diseases arise. Bernard Crespi is doing this with his research on schizophrenia and autism. Crespi argues that these disorders are diametrically opposed, autism is the result of an underdeveloped social brain whereas schizophrenia comes from a hyper-developed one. They are caused by the abnormal expression of imprinted genes, i.e. ones expressed according to the parent they came from. If true, then we can use opposing treatments to treat these twinned disorders.

Unfortunately, as Richard Dawkins puts it, we evolved to be fecund not necessarily to be healthy. Any competition between the two and the latter won out. But now at least we know that’s the case and we can do something about it.

Author

Adam Kane: kanead[at]tcd.ie

Photo credit

Dr. Barbara Natterson-Horowitz