Tag Archives: Evolution

Evo-Devo – a parody music video

Evo-devo (evolutionary developmental biology) is an area of study which helps to illuminate how evolution works, yet even people who have a bit of an interest in evolutionary theory seem to overlook it (and deniers often seem to have little to no knowledge of its existence). For me, reading about it in Sean B. Carroll’s Endless Forms Most Beautiful was eye-opening and made sense of so much (couple it with Neil Shubin’s Your Inner Fish). A Capella Science has done an excellent parody of this year’s summer smash/annoyance, Despacito, packing it full of fascinating science which fits the rhythms and rhymes of the original song brilliantly. Check it out and, as the video also recommends, check out Carroll’s book too if you haven’t already.



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October 8, 2017 · 12:00 pm

Ediacaran ecosystem engineers – the Savannah hypothesis and our Skynet-type origins

Image from Wikipedia.

Out on the savannah, it is easy to find certain resources as they tend to be concentrated in limited areas. Trees, termite mounds, changes in terrain, all contribute to this concentration of resources. It is this sort of environment which has been hypothesised as resulting in our own bipedality which enabled human ancestors to move efficiently between resource hotspots. In a recent review, Budd and Jensen have proposed a ‘savannah’ hypothesis as an explanation for the evolution of bilaterian animals and consequently the Cambrian explosion.

Bilaterian animals are widely thought to have driven the Cambrian explosion, particularly as they function as ecosystem engineers altering the environment around them. The burrowing activities of bilateral organisms altered ocean chemistry and the nature of the sediment, opening up more resources to be exploited and resulting in a cascade of diversification. But why did bilaterians evolve in the first place? Ecological causes for the Cambrian explosion tend to presuppose features they should be explaining, such as the ability to burrow or the presence of predation (both likely contributed enormously to the diversification, but were also caused by it). Environmental causes tend to suggest limiting factors such as a lack of oxygen, which may actually be mistaking cause for effect.

The savannah hypothesis suggests that the Ediacaran biota also functioned as ecosystem engineers, causing carbon hotspots in the sediment and water around the organisms which were exploited by bilaterian animals which went on to diversify, eventually displacing their Ediacaran providers. Dissolved organic carbon in Ediacaran seas would not likely have clumped together, instead being spread out through the water column – not an economical resource for active organisms. Burrowing is highly energetic and would require dissolved carbon to be concentrated; without the Ediacaran organisms it would have been too diluted and sequestered away by the abundant microbial mats. Just like trees on the savannah, the Ediacaran biota concentrated dissolved organic carbon, providing sufficient resources for active burrowing and the need for motility.

In their thorough review, Budd and Jensen challenge the view that Ediacaran organisms went extinct by the start of the Cambrian period having been outcompeted and devoured by bilateral organisms. Instead, they survey putative evidence that shows that bilaterians first appeared towards the end of the Ediacaran period and that Ediacaran-type organisms persisted well into the Cambrian (and perhaps longer). At first, bilaterians would have been dependent on the Ediacaran ecosystem engineering, but went on to evolve their own sessile forms, such as crown-group sponges, and predatory habits which made them a threat to the Ediacaran biota – comparable perhaps to humans in the Terminator franchise creating Skynet and setting up their own demise.

They also reviewed the phylogeny of basal animals and take the view that sponges form a single clade which is the sister group to all other animals. They coined the term “Apoikozoa” which encompasses all animals and their sister group the choanoflagellates. And they made a case for Ediacaran organisms being early animals, albeit hugely problematic, whilst being highly critical of some of the optimistic interpretations. It is a paper which has provided a lot to mull over.


Budd, GE. and Jensen, S. 2015. The origin of the animals and a ‘Savannah’ hypothesis for early bilaterian evolution. Biological Reviews. doi: 10.1111/brv.12239

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Filed under Cambrian, Cambrian Explosion, Ediacarans, Evolution, Palaeontology

An optimistic approach to the Ediacaran biota?

There’s a bit of a problem with the Ediacaran fossil record – it’s not what was originally expected and the organisms we do find are problematic. Based on the complex and recognisable fossils of the Cambrian, it was anticipated that more primitive forms would be found in the Precambrian, and, in a sense, they were. When they were first recognised, Precambrian organisms appeared to fit what was predicted; amongst them, palaeontologists recognised possible sponges, jellyfish, assorted worm-like creatures, putative arthropods and echinoderms. The more they were studied, however, the more problems in classifying them arose.

At some point along the line, near enough every Ediacaran fossil which had been linked to modern phyla have been reassessed and their connections found wanting. There is a handful which can still tenuously be linked to modern groups, but there is an apparent dearth of expected animal fossils, especially when the molecular clock data is taken into account. There appears to be an evolutionary gulf between the Ediacaran biota and the Cambrian explosion fauna.

Part of the problem is preservation – comparing the fossils of the Ediacaran and the Cambrian is difficult considering that they are mostly preserved in very different ways; the fossils of the Ediacaran are soft-bodied organisms preserved mostly as moulds, the fossils of the early Cambrian are mostly tiny bits of shell and other hard parts, and then there are the exceptionally preserved organisms from deposits such as Chengjiang.

One approach we can take to link the Ediacaran and the Cambrian is to avoid trying to fit them into recognisable taxonomic groups, and instead focus on the attributes they share with modern animals, particularly their ecology. This was the process adopted by Mary Droser and Jim Gehling in a paper earlier this year, titled The advent of animals: The view from the Ediacaran. We can look at the Ediacaran period and see things which are usually associated with animals, even if we cannot properly classify the fossils in question.


One thing which clearly sets animals apart is movement – worms wriggle through sediment, fish swim about, and, of course, us humans find as many different ways to move as possible. Many animals don’t move about for most of their lives, not least sponges and corals, both of which we might expect in the Ediacaran in some form, but movement on or in the sediment would potentially be evidence for bilateral animals milling around. Most of what we see from the Ediacaran are stationary organisms, attached to the sediment by a holdfast or resting on the surface. The earliest animal traces are from 565 Ma and are most similar to traces by the polyps of anemones, providing evidence of muscular contraction, evidence of which also comes in the form of the body fossil Haootia quadriformis which possessed bundles of muscle fibres and is a possible cnidarian. The most common trace fossils in later Ediacaran rocks are in the form of grooves and levees, called Helminthoidichnites, and are interpreted as being caused by an animal too small to be preserved and limited in size by the chemical conditions of the sediment. They appear to have been mining the microbial mats, also showing evidence of avoidance behaviour, and are likely to have been created by bilaterian animals.

A few Ediacaran body fossils are associated with traces as well, lending to their interpretation as bilaterian in nature. Kimberella is a box-shaped body fossil which is often associated with scratch marks (Kimberichnus) that has been commonly seen as bilateral and has even been considered to be a possible mollusc. The associated traces have been interpreted as evidence of mat grazing though there are differences between the grazing habits of Kimberella and those of molluscs. The likely related Dickinsonia and Yorgia have been found associated with faint casts of their bodies, which appear to be resting or feeding traces where they sat ingesting the microbial mat before moving on to another patch. They often also have possible muscular contraction marks, though this interpretation depends somewhat on their phylogenetic affinity. The advent of mobility is therefore not confined to the Cambrian period though it does see an increase in the number of modes of mobility, as it is a behavioural trait of bilateral animals found in the Ediacaran.


Sexual reproduction is another trait associated with modern animals found in the Ediacaran period. The puzzling organism Funisia is a collection of tube-like structures which were previously not even recognised as body fossils. They demonstrate branching patterns which are potential evidence of asexual budding, whilst their distribution appears to be due to the production of spats, a form of reproduction mostly found in sexual organisms. Though their phylogenetic affinity is puzzling, the likely sexual reproduction of Funisia highlights another metazoan trait found in the Ediacaran period.


The Cambrian explosion was first recognised in the fossil record due to the geologically sudden appearance of skeletal parts. The evolution of hard parts appears to have been a key stage in the evolution of Metazoa though it is not restricted to the Cambrian. Droser and Gehling discussed the example of Coronacollina, a cone-shaped organism with long, straight spicules radiating outwards, interpreted as a sponge-grade organism which is important for being the oldest known multi-element organism. Other Ediacaran shelled organisms include Cloudina and Namapoikia which are possibly cnidarian-grade organisms but had their study been released more recently they would likely have included the latest interpretation of Namacalathus as a lophophore. Even if we cannot place them phylogenetically, the appearance of skeletal parts, particularly multi-element organisms, is a key step in metazoan evolution found in the Ediacaran.


Ediacaran fossils tend to have been preserved in the places they lived, as opposed to having been transported and dumped elsewhere. This allows them to be studied as communities and permits insight into their ecological nature. The Flinders Ranges of Australia contain a succession of beds which are characterised by a range of organisms in shallow marine settings. The same organisms tend to appear on each bed but with different abundances, suggesting a level of sophistication in communities similar to that in the Phanerozoic despite there being a lack of predation, organisms living in the sediment, and widespread skeletonisation.


Setting aside phylogenetic affinities, traits of modern animals are found in the Ediacaran period. An optimistic approach to the Ediacarans allows us to see signs of mobility and the presence of muscles, skeletonisation, sexual reproduction, and the beginning of complex ecosystems – all possible links to the animals found in the Cambrian, suggesting that poriferans, cnidarians and bilaterians were all found in the late Precambrian.


Droser, M.L. and Gehling, J.G. 2015. The advent of animals: The view from the Ediacaran. Proceedings of the National Academy of Sciences 112: 16. [Link]



Filed under Cambrian, Cambrian Explosion, Ediacarans, Evolution, Palaeontology

The Cambrian Explosion – An Enigma of Deep Time

Over half a billion years ago the biological world was a more peaceful place, or so the story goes. Simple multicellular organisms abounded, doing little more than filtering out food particles from the ocean waters. Then, in a geological blink of an eye, animals began to diversify, irreversibly changing the prevailing ecosystems, paving the way for modern life. They began eating each other, causing some to burrow into the sediment deeper and deeper, disturbing the slimy microbial matgrounds which had been key in the fading Ediacaran environs. Some developed armour, sporting hard parts which protected their soft flesh, leaving behind more distinct traces in the fossil record. Yet others became more mobile, learning to swim around to evade predators, or to seek out their own prey.

This event is often referred to as the Cambrian Explosion and has earned the nickname “biology’s Big Bang”. It is one of the most important events in the history of life on Earth and, due to its singular nature, is one of the most misunderstood. Misinformation proliferates on the internet, not least due to anti-evolution websites considering it to be a weapon in their arsenal, but also because it is a very mysterious time, where datasets often support multiple interpretations and cause and effect are difficult to untangle.

Image credit: Brest van Kempen

When Charles Darwin first pondered the Cambrian mystery, he was facing a fossil record where no Precambrian fossils were known – the lowest rocks contained the surprisingly complicated trilobites, not at all what he had hoped to see. Darwin invoked the incompleteness and imperfection of the fossil record as his explanation, in the hope that someday new fossils would elucidate this remarkable absence. It wasn’t until 1958 when Precambrian fossils were first recognised, but even they did little to diminish the seeming leap in complexity in the Cambrian. The Burgess Shale fossils, found by Charles Walcott in 1909, revealed a menagerie of bizarre creatures, many of which clearly had modern body plans, but many others seemed to have unique body plans, as though this Cambrian diversification was even bigger than expected.

The Burgess Shale was properly studied from the 1970s onwards and by the 1990s the diversification had been established as one of the key mysteries of palaeontology. By this point, a picture of the Cambrian Explosion had emerged which provided quite the conundrum. Those Precambrian fossils had originally been interpreted as belonging to modern animal groups but had since been questioned and even proposed as belonging to their own kingdom. Then, in perhaps as little as four million years, every modern phylum arrived on the scene, along with potentially three times as many more. It was an unprecedented diversification, expounded beautifully by Stephen Jay Gould in his book Wonderful Life. This is a picture which crops up repeatedly, despite twenty years of advances which show it to be wrong. In future posts, I will explain what has changed in our understanding of the Cambrian Explosion, which questions have been answered, and the mysteries that lie ahead.


Filed under Cambrian, Cambrian Explosion, Ediacarans, Evolution, Palaeontology

What’s That Swimming Towards Me? – Reblog

I recently took the decision to share some of my older blog posts in order to pad out this iteration of the blog a bit and get me back into the swing of things. It beats ranting about Jurassic World and provides some hopefully readable content. Here is a piece I wrote in August 2010 as a bit of pop. science writing. 

With the vast size of the Earth’s oceans, it is not unlikely that many of us will swim in them from time to time. Whether you are going for an innocent paddle, catching waves on your surfboard, or sailing the seas on a fishing trawler or pirate ship, you will be sharing the waters with myriad different animals, some of which are dangerous. If you were to find yourself swimming under water with a big, moving animal coming ever closer, how do you know whether you have a friendly dolphin swimming towards you or whether it might be a hungry shark? Well, the easiest way can be done at a distance and is a simple observation with an interesting explanation – evolution.

So, with that large, looming shadow swimming towards you, what do you look out for? If you can see its tail, simply look at the orientation of the tail fluke, is it horizontal or vertical? Failing that, look at how it moves, does it undulate its body up and down, or does it move its body from side to side? Dolphins and whales have a horizontal tail fluke, which means they have to undulate their body up and down in order to propel themselves through water, so if you see either the horizontal tail fluke or the undulating movement, you have a dolphin or whale coming towards you.

Sharks, on the other hand, have a vertical tail fluke and so must flex their body from side to side for propulsion. If you see the vertical tail fluke or side to side motion, then what is coming towards you is a fish and so might be a shark.

As our fishy ancestors used a side by side motion to propel themselves through water, so did our earliest terrestrial ancestors and so do reptiles today. Snakes are an extreme example of this sort of movement, but the side to side motion is still there. During the Mesozoic era things began to change, as our ancestors (and convergently in dinosaurs too) developed a more upright posture, instead of the sprawling gait of reptiles. With an erect posture the more effective way to rapidly move is to flex the spine up and down whilst running, rather than side by side.

Many vertebrates have some of their vertebrae fused to facilitate particular movements, so future evolution can often be restricted to working within the confines of that movement. As dolphins evolved from terrestrial mammals, their semi-aquatic ancestors also used this up and down movement and so adapted this to movement in the water. Side to side motion, like that of a shark, would require a larger number of changes when there was the simpler solution of up and down movement (though note that evolution does not have the foresight, it simply uses what is available – quick fix solutions often work in evolution). The motion of whales and dolphins is testament to their ancestry, having descended from active land mammals.

During the Mesozoic, another group secondarily took to the waters and adopted the torpedo shape of dolphins and sharks. These were the ichthyosaurs, descending directly from terrestrial reptiles. As their ancestors used the side to side motion, so did the ichthyosaurs when they swam, also possessing a vertical tail fluke. They had some unusual traits for reptiles, giving live birth and being warm blooded, but their swimming motion gives away their reptilian status. This also quite ably demonstrates that they are not dinosaurs as many laymen mistakenly think, for dinosaurs did not have a sprawling posture which uses the side to side motion. So, if you are somehow in Mesozoic waters with a shape swimming towards you, it may be too late before you can discern whether or not it is an ichthyosaur or a shark.
Ichthyosaurus anningae

Artist: James McKay


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Metro + Piltdown = Head-desk

The Metro newspaper, which is available free to commuters in the UK, recently ran a piece on the Piltdown hoax. I recently had a rant about some of the science reporting in said newspaper, but this one was a lot better. I could quibble about some small details in the post, but those pale in comparison to some of the responses which were published in the “talk” section today. The gist of the article was that Piltdown is being desperately used as an example of scientific misconduct, as though it tarnishes the reputation of the whole field, when in reality it does nothing of the sort. I didn’t get to read the newspaper itself, but thankfully a friend of mine has sent me a photo of the ridiculous responses, which I will share below. It is notable that many of the claims being made in those responses completely ignore what has been said in the article, but some raise issues which were either not properly addressed in the article or are rather irrelevant (I hope that it is readable).

Metro responses

We can start by looking at Paul from Bristol’s diatribe. He makes the claim that Piltdown Man was forged to support evolutionary theory, but he has clearly not engaged with either the article or any of the historical data. Firstly, the article gives reasons why the Piltdown hoax may have been believed for so long and gives us part of the answer: Piltdown Man was British. National pride was key in its acceptance, but so too was the fact that very few were allowed to see the find and scrutinise it. The Bristolian critic makes the bizarre claim that nobody declared that it was a joke, so therefore it must have been perpetrated to establish evolution. It does not take much searching at all to find out that the main suspect, Charles Dawson, died decades before it was exposed as a hoax. With that little bit of searching, dear Paul would have found that Dawson had carried out many hoaxes in his time. Many of his hoaxes had nothing to do with evolution and went along with a rather simple notion: if you give people what they want to see they are less likely to question it; it is called confirmation bias. He didn’t care about establishing evolution, but he could easily trick people who were interested in providing fossil evidence, particularly if it was from the United Kingdom.

As the article notes, many did question Piltdown from the beginning, especially as it clashed with legitimate finds (which the majority of these responses ignore). When it was finally exposed as a fraud it was by the same scientific community which creationists seem to think were involved in a conspiracy. Frauds being exposed are a sign of the corrective nature of science. Perhaps Mr Fulcher should check out some of the creationist hoaxes which have been trotted out from time to time (they have a fondness for man and dinosaurs as contemporaries).

The creationist argument about “kinds” is hardly worth addressing, partly because they cannot define it consistently without having to invoke some large scale evolution (which gets brushed under the table). Fulcher has clearly never had a look into the fossil history of those groups he mentions, otherwise he would find that they blur together a fair bit, as a testament to evolution. When we trace his dog and cat families back, for example, we end up looking at miacids, which were rather primitive carnivorans, roughly the size of a weasel, which look nothing like the modern groups yet have the key features. Where do they fit into these supposed kinds?

So cute, yet there’s no “kind” for him.

But really all we need to do is help people to understand how evolution works. Paul rejects the notion of “one type of creature… turning into a completely different creature” and so do I. What evolution produces is groups within groups, with each new group remaining in its original grouping, but being sufficiently different to other members of that group to warrant its own group (do a shot every time I say “group” or “kind” if you are getting bored). The result is what is known as a nested hierarchy, something which has been recognised in nature for centuries and was the foundation of Linnaean systematics. If we arbitrarily label these groupings “kinds” then we find that evolution does not show one kind becoming another, but “kinds” become more derived and form those groups within groups. We can look at our own species, Homo sapiens, and note that we never stopped being the hominin kind, which is a group within the hominids and we still fit there, which in turn is a group within Hominoidea and we have all the defining features for that group, and you can keep going through Catarrhini, Simiiformes, Haplorhini, Primata, Primatomorpha, Euarchonta, Euarchontoglires, Boroeutheria, Epitheria, and we still fit into those groups. The next larger grouping is Eutheria, which contains all of the placental mammals; we never stopped being the placental mammal kind. Ask any creationist if we are mammals and they will be revealing that we fit into nested hierarchies if they say yes. Creationist “baraminology” ends up producing nested hierarchies (because evolution is true) but they arbitrarily draw a line and declare that it cannot be crossed, contrary to what the fossil record shows.

So anyway, onto the next response…

The second letter, which appears to be from John Clease at first glance, talks about gappiness in the fossil record. Of course the fossil record is incredibly patchy, but that does not mean that it does not support evolution. What it shows is not just consistent with evolution, but utterly inconsistent with even the most optimistic creationist ideas. The next comment along, from an S. Barber, makes some claims along similar lines, so I will address them together.

What does the fossil record show? Well, if Barber is right, then it should show a big jumble (unless they are an old earth creationist, in which case it appears that God created things to look as though they evolved, following an evolutionary logic). What we actually see is what I like to label “a gradual increase in the spread of complexity over time”. If you start from the oldest rocks containing life you find the conspicuous absence of, well, anything with a nucleus. We find only bacteria and their colonial structures for the first 2,000,000,000 years or so of Earth’s history, which then leads into very simply eukaryotes. It is not until around 600,000,000 years ago that we start seeing multicellular grades of organisation, a step up in complexity from the aforementioned early eukaryotes, which are more complex than the earlier bacteria (it should be noted that these do not appear in a strict stepwise manner, but simpler forms often persist alongside the more complex forms, comfortable exploring niches of different scales). We don’t start seeing simple mineralised parts until just before the Cambrian period (which started 542 million years ago) and those gradually become more complex. I’m sure you all know the anthropocentric story from there onwards, with fishy organisms making their début in the Cambrian, developing jaws later along the line, leading eventually into primitive tetrapods during the Devonian, going through the whole amphibian to reptile to mammals and birds sequence (can you tell that I am more interested in the earlier stuff?). At this large scale we clearly find something which practically screams evolution, showing a nested hierarchy pattern and that increase in the spread of complexity. But what if we look closer?

It is when we start looking closer that creationists like to claim that gaps exist, and some really do (we find transitional bat forms, but not for their origins). But gaps are constantly being filled and not all grab the headlines. People want to see gaps between major groups, they want to see human ancestral forms, they want to see whale transitions, fish giving rise to tetrapods, dinosaurs giving rise to birds, and we have examples for all of those. Many of these forms have been found relatively recently too, but some are as old as Darwin’s seminal work. Creationist like to criticise Archaeopteryx yet they have to ignore that all of the supposedly bird features have now been found in dinosaurs (not least feathers) or that working out where these things fit on their family tree is difficult precisely because of their transitional nature; it comes out either nestling with the more familiar birds or being more closely related to dinosaurs such as Velociraptor. (I intend to do a detailed post on all of this relatively soon, so I must apologise for any brevity.) So we find that when we zoom in a bit we end up finding transitional forms, though we still have many more to find. But what if we look closer?

These must not exist…

Looking even closer is difficult, not least because it requires a lot of effort to collect samples and study them for hours on end, taking lots of measurements etc. But this sort of work has been done for decades, with palaeontologists going into a lot of detail, particularly with microfossils, and elucidating different modes of evolution. What this requires is a deposit which covers a large stretch of unbroken geological time and is chock full of fossils. Naturally such deposits are rare, so the fossil record can only show us so much. What we do see is that evolution, over geological time, follows a number of patterns. Amongst these is the gradualism which anti-evolutionists like to claim is absent, but the main pattern appears to be a jerky one, where species often change little, then change rather rapidly (on a geological time scale, and when these “rapid” changes can be found in the fossil record they tend to last tens to hundreds of thousands of years, which is practically nothing to a geologist). Darwin actually did predict such patterns of evolution, so it can hardly be claimed as evidence against his ideas. (Again, expect more on this in the near future.) Clease mentioned Dawkins explaining away gaps in the fossil record as migrationary events, yet it is this very fine-scale evolution where this can be applied. Species migrate, this is undeniable, yet the fossil sequences we require for this sort of study have to be from the same locality. What will migration look like in the fossil record? Well, if you cannot trace the migration through other localities then it will look like an abrupt appearance or disappearance.

So what we see, at all scales, is consistent with evolution. Patchiness is no excuse to ignore the fossils, as what we do have is highly informative. Barber goes on to mention that evolution cannot explain abiogenesis, but that is not what evolution is meant to explain. It applies only to life which already exists and they are separate fields of study. The appearance of design is precisely one of the things which evolution does explain, that’s part of what made Darwin’s ideas so brilliant (although I surmise that they would baulk at my choice of authors, they could do with reading Dawkins’ The Blind Watchmaker).

Finally onto the last one. I could have lumped Jeremy Craxford’s claims in with the last two, but he raises a couple of other issues. He makes the statement that “Tens of millions of fossils have been dug up and still there is not a single clear, undisputed case of a ‘missing link’ between species.” Well, I have already briefly touched on there being transitional forms at multiple levels, but there are two points I want to make. First is that the term “missing link” is a pointless tautology which really should never be used; a missing link is, by definition, missing, so when it is found it simply creates a couple more gaps which need links. We instead tend to refer to transitional forms, of which many are known and are undisputed (note that any disputes come from those who a priori deny evolution).

The second point I want to make is that we currently know of around 8.7 million species of living eukaryotes. The fossil record for eukaryotes shows that they have existed for roughly two billion years, yet the last estimate I saw of known fossil diversity was around 250,000 identified species. That’s close to 3% of the diversity we see around us, spread out over two billion years. If we conservatively start at the first unequivocal bilaterians in the Cambrian period, then we have 542 million years that these fossil species are spread out over. If we calculated it so that each species was, for its duration, the only species on the planet, they would each have 2,168 years alone. I mention that just to give insight into how little we do have. Naturally species tend to last millions of years and it is difficult to estimate how many there are due to difficulties with classification. When each of these species was living there were likely millions more which we will never get to see, particularly those which have soft bodies, or live in environments which are not conducive to preservation. Anyone who thinks we should have a fossil record which perfectly shows evolutionary history is talking out of ignorance. What we do have shows, rather clearly, that evolution is the name of the game and has been for nearly four billion years.

This was a bit of an off the cuff rant (blame Dean Lomax if you found it tedious, or the Metro for publishing those inane ramblings). It saddens me that the public seriously does not understand what the fossil record shows. Someone needs to buy them each a copy of Donald Prothero’s Evolution: What the Fossils Say and Why it Matters. I will be expanding on many of these points in detail in the future, so if I left you unsatisfied you can always come back for more, I promise it will be better.


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Exciting Progress in Modes of Evolution

In the last few days a piece of research has been published which could potentially change our approach to studying evolutionary patterns in the fossil record. So far I have only managed to read the press release and the abstract, so much of what I say could be a complete misunderstanding pulled from my backside. Since the 1970s there has been a debate in evolutionary palaeobiology with regards to the ways in which a lineage evolves. At times this debate has seemed like it has reached its peak, but then something changes the game. On my old blog I did a poorly formatted post explaining punctuated equilibrium, which may be some decent background reading, but I intend to do an update sometime soon anyway. Here is an attempt at a brief explanation of what happened in the debate:

  • Evolutionary biologists tended to assume that a species evolved with most change taking place within the lineage (often labelled phyletic gradualism). 
  • In the 1970s, Eldredge and Gould came along and pointed out that the fossil record shows that a species appears geologically abruptly and changes little during their duration. Most morphological evolution takes place at speciation. This is called punctuated equilibria. 

A comparison of the two concepts.

  • The introduction of punctuated equilibria caused rich debate, resulting in a lot of study into the patterns of evolution in the fossil record (and some additional concepts which will not be discussed here).
  • Examples of phyletic gradualism were found in the fossil record, as was another pattern, punctuated anagenesis, however, punctuated equilibria appeared to be the most common pattern.

One of the issues has always been the techniques used to study these patterns, not to mention, there is a degree of subjectivity when trying to determine which best fits the data (it is often not as unambiguous as the conclusions would have you think). The earliest studies, including those by Eldredge and Gould, tend not to be discussed, as techniques have been refined and many old examples are thrown out or surpassed. The new study claims to have used a technique which was previously unavailable and has some results which may have us scrambling back to the rocks for more samples, or reassessing old studies to see if our conclusions were wrong.

Old studies on evolutionary mode appear to have largely focussed on a single morphological trait, whereas this study looks at a suite of traits of a single lineage, and does so using several different species. They found widespread evidence for mosaic evolution, where characters change within a species at different rates. With this taken into account, analysing a single trait cannot give us enough information as to what is going on with the species; one particular trait might show a pattern which looks punctuated, another may show gradualism, yet neither show what is going on in the bigger picture. And, as is mentioned in the abstract, a single trait may show one particular mode of evolution, whilst the rest of the characters may show another, so the overall change does not fit with the first character.

I’d need to read the paper before I could say much more, but if what it claims is accurate, then this debate may be reignited. This could get interesting…

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