In the third week of this blog, I am going to finish my (short) series on Community Change by talking about freak events that can dramatically change the course of a community – mass extinctions and, the lesser known event called an “interchange”.
Now, mass extinctions are a point in time (ranging from a few hundred years to a few thousand years) where the extinction rate is much higher than the background rate. At these points in time, hundreds of families can go extinct.
The most well known of these events is an event known as the K-T Extinction (or the Cretaceous-Teritary Mass Extinction, or the Cretaceous-Palaeogene boundary) which happened around 65 Ma – the extinction which saw the end of the dinosaurs, the ammonites, the pterosaurs and the large marine reptiles. After this extinction (which is the penultimate circle on the graph above) the “Age of Mammals” started and, eventually, we evolved.
This extinction is the most famous (considering how famous the dinosaurs are) however was not the largest. The End-Permian (or Permian-Triassic Mass Extinction which occurred around 252 Ma) was the largest one in Earth’s history so far – 96% of marine species became extinct at this point and 70% of terrestrial vertebrate species also went extinct. The “Big 5″ – the largest mass extinctions over the planet’s history – are shown above the graph and here is when they happened;
- Ordovician-Silurian Extinction: Happened around 440 Ma and resulted in the extinction of 27% of all families.
- End-Devonian Extinction: Happened around 370 Ma and resulted in 19% of all families going extinct.
- End-Permian Extinction: Happened around 252 Ma and resulted in the loss of 57% of all families.
- Triassic-Jurassic Extinction: Happened around 200 Ma and resulted in 23% of all families going extinct.
- Cretaceous-Palaeogene Extinction: Happened around 65 Ma and resulted in 17% of all families going extinct.
But what causes such a dramatic decrease in biodiversity in such a (relatively) short time?
There are a lot of answers to this question. The first thing that springs to anyone’s mind when thinking of what wiped out the dinosaurs in the K-T extinction is “a meteor did it” – but there are many more hypothesises that are floating around. A quick Google search reveals the following theories;
- Meteor Impact: With a crater (Chicxulub) found dated to 66 Ma and the rock record showing unusually high levels of iridium (a rare metal to find at the surface) the impact theory was long thought of as the main reason as to why the dinosaurs went extinct. After this layer, and the crater, we find no more dinosaurs – right? The meteor would have thrown dust into the air to block photosynthesis and cause widespread death of organisms as a knock-on effect. To my knowledge, such a dust/ash layer has not been found, and scientists are still arguing over whether or not this was the sole cause of the extinction.
- The Deccan Traps: The Deccan Traps are a series of volcanic basaltic eruptions that occurred during this time, and have long been associated with the idea of climate change and, therefore, the idea of a gradual mass extinction of the dinosaurs. This theory, although a nice one, is widely unaccepted due to the more recent thinking that dinosaurs were, on the whole, not undergoing a gradual mass extinction.
- Supernova Hypothesis: A slightly more elaborate response, this suggests that a nearby supernova explosion caused cosmic radiation to wipe out life on Earth and would result in the Iridium anomaly. So far, no other evidence can support this theory and thus it is pretty much a dead theory.
- The more silly explanations: Yes, there are some people out there who point to explanations ranging from “aliens” to “they lost their sex drive.” I am not going to discuss these.
Now, obviously mass extinctions are a global disaster, wiping life clean for it to struggle back upwards again but, at the level of the community, these might not be a bad thing at all. If you are one of the organisms that survive the mass extinction, and your competition is wiped out then you can diversify and colonise to your heart’s content. If it wasn’t for the K-T boundary, for example, then reptiles (dinosaurs) might still be the dominant land organisms of today instead of mammals (and instead of us).
[Disclaimer: when I mention dinosaurs I am referring, of course, to non-avian dinosaurs. Obviously, birds are still with us today and are, of course, dinosaurs. Let's just assume that I am talking about dinosaurs are a paraphyletic clade - a group of organisms which does not include all the descendants of the earliest common ancestor.]
The real reason is that, for the larger mass extinctions, we can never be sure exactly why the organisms went extinct – we just know what went extinct. Often mass extinctions can be related to the actual amount of fossil material we have available and, so, estimations of the exact scale of extinctions are still a good estimate at most.
Now, you have probably been sitting there skimming through my post and thinking “I still don’t know what an interchange is…” – do not fret! I will get to them now. First, let me show you a relevant picture from the book Biogeography by Cox and Moore.
The picture shows the similarity of organisms between different regions (mammals only are shown). As you would expect, there is no similarity between places like the Americas and Africa, but there is quite a bit between places like Russia and India. What does this graph show? The effects of Interchange (or lack of it).
Interchanges are events where two distinct biomes are allowed to mix and interact when previously they would have been apart. Often these are large scale events, which is what I will mainly be discussing, but they can be as small as a flood allowing two ponds to mix together.
The larger scale events are normally caused by land bridges (such as can be formed in a large ice age) and the collision of continents – such as the case when North and South America collided. When the two continents came together (and, oddly, a few hundred thousand years before) families of animals who had never had the chance to interact were finally put together in the same space. For a community, this can be either devastating or the complete opposite.
The marsupials, never before seen in North America, this was the ample opportunity to colonise a new area and, now, we see animals like Opossums in North America where previously they were unknown. For other animals, this is bad news. When humans first arrived in North America, there was a devastating effect on the bird and mammal populations through hunting, causing widespread extinction as man colonised the area. In general, the effect on a community or interchange is normally extinction and replacement, with a new species out-competing the native species, causing them to go extinct and then taking up the niche for themselves.
The three aspects I have discussed over the last few weeks have been a brief introduction to why we see such variation across the planet today. Evolution causes new species to emerge from older ones, causing a variety of animals over a period of time if the conditions are right. Persistence is the mechanism by which these organisms survive, and Mass Extinctions are what results in the end of a lineage, allowing new species to adapt and survive in the space left behind. Finally, Interchange is how organisms move across the planet, and the effect they have on each other as they interact. These processed can take any amount of time to happen, from a few months (in the case of some interchanges) to millions of years (in the case of long term evolution) and have helped shape the life of this planet since the early days over 500 million years ago.
With that, my mini-series on community change is over, and I will be writing my first stand-alone post next week. I hope it has been informative and worth sticking around to finish!
Any book on fossils or dinosaurs will have to mention mass extinctions somewhere in it. Michael Benton’s book When Life Nearly Died is worth a glance – Benton is a well respected palaeontologist working in Bristol who is the current head of the Palaeontological Society, so his work is always worth a read.
As for the more academic reading, the textbook Biogeography: An ecological and Evolutionary Approach is what I have used for a lot of my writing on interchange and the paper by Webb in 1991 titled Ecogeography and the Great American Interchange is a fantastic read on interchanges.
Last week I began to answer the question “How, and why, do we see such a large variety of different species on the planet today?” by talking about how evolution can cause splits in a lineage and result in different species over time.
Today, I am going to talk about Persistence – what it is and how it affects a community over time.
Now, the dictionary definition for the word persistence is as follows:
Persistence, NOUN; The fact of continuing in an opinion or course of action in spite of difficulty or opposition.
However, I shall be using a slightly modified definition when discussing the fossil record. The definition I will be using is “persistence is the apparent lack of change in a fossil community over time.” This is where, if you were to track a community over a geological period, you would see no or very little change to those organisms.
This is seen in the fossil record quite often, a study done by the palaeontologist DiMichele in 2005 showed that plant fossils in the Carboniferous (about 360-300 Ma) showed no change for long periods of time, with sudden changes over short periods of time. This is called Punctuated Equilibrium – a theory devised by Niles Eldredge and Stephen Jay Gould in the 70s – and is a theory to account for these sudden bursts of change in the fossil record.
The theory of punctuated equilibrium is that change in the fossil record is rare and rapid, taking place in events called cladogenesis in which new species are formed (note, that it does not say that new orders and classes are being created out of the blue). In between these events is a period of stasis, where little evolution takes place. However, is punctuated equilibrium a reasonable theory for explaining this persistence?
The problem with the theory is that it does not allow for any change from one generation to the next – which there is, of course – it is observable. Last week, when I talked about rabbits (who have a very quick generation time relative to the larger mammals) who can show this (in the words of Kim Sterelny) “wobble about the phenotypic mean” (which just means, variation in a species over generations) very quickly. Even humans show this “wobble” – think of how subtly different you look to your parents, then compare that to your grandparents and so on. There will be similarities, but also quite a few differences from one generation to the next.
When I defined the word “persistence”, I deliberately added the word “apparent” in to the definition. Remember that the fossil record is extremely bias in what it shows us. If you were to walk outside, fall into a ditch and die – what would happen to you, assuming that no human beings were around to find you?
Chances are, you would either be eaten by wild animals or just decompose – only leaving your bones remaining. Now, the chances of them being buried and then surviving compaction and fossilisation are very low – and even still, we only have your bones. We cannot see what your face looked like, what your organs looked like or what you ate for breakfast that morning. In a similar way, we have no idea what the soft bodied parts of an organism looked like (but we can make a very good guess at it!) or how they were used. When I say the “apparent lack of change”, we cannot account for changes in ecology (where and how the organism lived) or body plan (what the organism looked like compared to another fossil in terms of soft tissue). The phyetic gradualism theory could, therefore, work perfectly – it’s just that we cannot see it working.Eventually, smaller changes could lead to a visible change that we see in the fossil record.
Smaller scale instances of persistence can also be explained by evolved ecological relationships. The relationship, for example, between “Darwin’s Moth” and “Darwin’s Orchid” is a perfect example of this. The moth has evolved a particularly long proboscis in order to feed off the plant, which has developed a particularly long spur. The moth is perfectly evolved to feed off this plant, the the plant has evolved in such a way to ensure that it is getting pollinated. As long as one member of the evolutionary relationship survives (does not go extinct for any reason) then the other will also flourish and persist.
DiMichele also mentioned in his 2005 study about the “Theory of Large Numbers” – a theory that literally says that the more prevalent organisms will remain the most common due to their higher offspring count. This has links to another theory devised by the biologist Hubbell in 2001 – the Unified Theory of Biodiversity (UTB). The UTB says that any given community will have a constant number of individuals and new individuals can only join the community when an old one has died. There are three different ways of replacement of the old member;
- A new member of the same species is born – a reproduction event
- A member of the metacommunity joins the community – an incorporation event
- A new birth results in a new species – a speciation event
The theory takes the probability of any of the events happening, as well as the probability of an individual dying, to be equal. Species will persist due to the chance of a new species surviving being very low, allowing the numerically superior species (through the Theory of Large Numbers) to survive.
Persistence can explain why we see stasis in fossil communities over time. The evolution of life on Earth is not as simple as what we can see from the fossil record, and we need to consider these periods of “stasis” for all the possibilities that they might be.
Next week, in the final week of this mini-series on Community Change, I will be discussing what happens when everything goes wrong – Mass Extinctions and Interchange.
Until then, have a wonderful week.
Persistence is not a topic which is commonly written about in literature, however some of the ideas can be read up upon. If you want to know more about punctuated equilibrium and the minds behind it, picking up any of Goulds books wouldn’t be a bad idea. Wonderful Life, although not directly about the theory, is a good read and (although controversial) can help with ideas of evolution on an Earth were life was just taking a hold. Other than this, any books on evolution might mention some of the ideas in this post.
If you are looking into more academic papers, then the one I have heavily used in this article is the one by DiMichele et al. titled LONG-TERM STASIS IN ECOLOGICAL ASSEMBLAGES: Evidence from the Fossil Record – which is one I would highly recommend. Another good read is a paper by Abrams, P written in 2000 titled THE EVOLUTION OF PREDATOR-PREY INTERACTIONS: Theory and Evidence.
One of the biggest questions that floats around in a lot of ecology is “How, and why, do we see such a large variety of different species on the planet today?” With new species being discovered all the time – this is an ever present and relevant question and, therefore, a good one to theme my first few post on.
I am going to attempt to answer this question over the next three posts; with one on Evolution, one on Persistence and one on Interchange and Mass Extinction.
Now, the obvious answer is just to say “evolution”, but before we delve into this, let me define some terms;
- SPECIES: There are many different definitions of the word “species” – the most common being the strict biological definition – If they can breed and produce offspring which are fertile, then they are the same species. Now, that isn’t much help when you are looking at a fossil – they are pretty incapable of breeding. In this case, we are more inclined to use a morphological definition – If they look pretty much identical (from the fossilised material) they are the same species. This, of course, has its own problems. Where do you draw the line? What changes denote actual species differences and what changes are just variations? For now, though, it’s all we can base it off.
- EVOLUTION: Whole books have been written defining what evolution is, but I am going to summarise it as simply The gradual change in a population over time, both genetically and behaviourally. The important thing to note here is that evolution doesn’t have to be a physical change – but physical changes are pretty much the only thing we can (easily) get from fossil activities. (NOTE: We can infer some behavioural activities from fossils but, for now, I am going to ignore these. Sorry!)
- FITNESS: When the term “survival of the fittest” is thrown around, it helps to define what biological “fitness” means. No, it doesn’t mean that an organism will survive if it goes for 10km runs every morning and then does 80 one handed press ups – biological fitness is How many genes from a particularorganism (or species) are passed down to the next generation or, alternatively, How well a particular organism can reproduce. Now, the term “survival of the fittest” makes sense using this definition – an organism that has 20 offspring is incredibly fit and the species as a whole is more likely to survive.
- COMMUNITY: The palaeontologist DiMichele defined a community as A recurring, recognisable assemblage of organisms. Or, in other words, A group of organisms that both persists through time and can be traced through time.
Now, how and why do communities change over time?
Well, evolution plays a big role in this. Evolution “favours” (although, remember, that organisms don’t choose their own evolution) fitter organisms. Fitter organisms will be the ones who have many offspring, and many offspring results in plenty of your genes, the things that define who you are (and what species you are), being passed down.
But how does this work? Let’s take two theoretical examples that show this in action.
Imagine you are a rabbit living in a very cold environment. Now, let’s assume the colour of your coat is determined by one set of genes that give you either a brown or a white coat. We will assume that the gene for having a brown coat is dominant and the gene for having a white coat is recessive. All this means is that, at any given time, there will many more brown rabbits than white rabbits. (We will also assume all mating is random, and that brown/white rabbits will happily breed with each other as well as similarly-coated rabbits.)
Now, let’s say that the environment changes a bit and has a particularly snowy winter. When the foxes are out, any rabbit with a white coat will be much less likely to be caught and eaten. In this way, the community changes
A community that started off with 15 Brown rabbits and 5 white rabbits might now have 4 white rabbits and 4 brown rabbits – The proportion of brown:white rabbits is now much more in favour of white rabbits - what was a population where 1/4 were white rabbits is now a population where 1/2 are white rabbits. They breed, and the next winter comes. Over time, if the weather stays consistently snowy, white rabbits will tend to increase their numbers (relative to the brown rabbits) over time.
For another example, let’s look at something that focuses more on behaviour.
Imagine you are a peacock (male). Your genes (a number of them, this time) will determine how impressive your tail is – how long it is, how bright the colours are and so forth. We will mark your tail on a scale from 1 to 10, based on these aspects.
Now, the peahens (female) you are trying to impressive are very picky. They will only mate with a peacock with a tail which is at least 7/10 on the impressiveness scale – the mating is not random. As such, only the most impressive peacocks will mate, and pass their genes on – increasing their fitness. Their offspring will, on the whole, have more impressive tails (the “impressive” genes will be passed down).
Overtime, peacocks will grow to have more impressive tails. This is an example that shows evolution that, at first glance, may not make much sense. Having a larger tail makes you more of an obvious target to predators – it slows you down and makes you clumsy. Looking at change over time in a peafowl community would probably reflect this.
In our example, you needed a tail which scored at least 7/10 to have a chance of mating. However, let’s say that a score of 9 or 10 out of 10 resulted in you being too vulnerable to escape from predators. Overtime, the community would change to mainly have tails which score 7 or 8, with very few scoring above this (reduced chance to survive) and very few scoring below this (reduced chance to mate).
There are, therefore, two major things that affect change through evolution: the environment and behaviour. Both are equally important and both can radically change the outcome of a species. But how does evolution change a species over vast periods?
The idea to take into mind is the idea of separation and mutation. Let’s take the rabbit example again from above;
Let’s say that, at some point in time, the community of rabbits becomes separated. One group of rabbits, during their travels, goes down one route on a path and the other group goes another – and they end up completely separate – They are unable to interbreed. This separation could take place through a geographical separation (the type I have described – where there is a physical barrier such as a body of water or a mountain range in the way) but could also be a mechanical separation (where the two groups are in close proximity but unable to breed with each other, perhaps due to physical size differences).
Now, let’s say one group ends up where a white coat is more advantageous, and the other ends up where a brown coat is more advantageous. Give it a few million years and, with no other interference, one group will end up predominantly white coated and the other mainly brown coated.
The other thing that will happen in this time is genetic mutation - where a random mutation during the birth or conception of an individual rabbit results in a change to the organism. It could be anything from a slightly better coat colour (a green coat, perhaps?) or the eyes being slightly closer together (or anything more radical than that). If this is a significant enough advantage, that rabbit may pass it on to their offspring – resulting in (eventually) a change in the species. Your brown coated rabbit colony may end up being a green-brown colour if the environment favours it and the mutation happens.
If the changes are vast enough, and are given enough time, then in a few million years the two original rabbit colonies may be different enough that they now are unable to breed – they have become separate species.
Over time, colonies can change quite substantially – but it does take time. You can’t expect a fish to one day grow lungs and walk out onto land – it takes millions of years for things like that to develop. This is one of the reasons we see so much variety today – separation on different continents has resulted in a plethora of different organisms for us to look at and study.
But what happens if the mutations don’t happen, and the species don’t change? In my next post, I shall be discussing Persistence in the fossil record and how evolution can show apparent stasis – the lack of change – in a community over longer periods of time.
Until then, have a wonderful week.
For those who are interested, there are many, many, books written about evolution. On the Origin of Species by Charles Darwin is a classic, and was the original book on the subject, but can be quite dry and has aged. Collin Paterson wrote an introduction to evolution just called Evolution which is a good starting material for evolution and genetics – and is written in a very clear way. A quick google search for books on evolution will show you several hundred results – and a trip to the local library will probably give you the same.
(Another good book to pick up is Dinosaur Hunters by Deborah Cadbury – which gives a bit of insight into early ideas about evolution and geology. it is written less as a science textbook and more as a story, and the evolution bits are quite hidden in there. The Selfish Gene by Dawkins is also meant to be quite good on the subject, but I have not read that yet so cannot comment on it’s accuracy.)
When I was five years old (or so) and standing (being carried) on the Northern Irish coast, I (my dad) came across a strange looking shape in the rocks. Although I don’t have an image of the original rock we found, it would have looked something like this:
It was an ammonite – a Paltechioceras specimen – not perfect but it was mine. I (my dad) found it and I was the first (second) pair of eyes to gaze upon it in 190 million years. Since this moment, I had set aside my future to finding out everything I could about the life of the past. I moved to England, studied Geology and Biology at A-level and now, I am in my second year at university studying the grand sounding course of “Palaeobiology and Palaeoenvironments”.
Or, as I like to introduce it as: Palaeontology.
Now, as soon as this is mentioned one of the following four questions is either asked or goes through the head of the listener (or reader, as the case may be).
- “Palaeontology? Like Ross from friends, right?”
- “Why are you studying *that*? Surely your job prospects are limited to, what, teaching and museum curation?”
- “Yeah… I don’t know what Palaeontology is – sorry.”
- “Jurassic Park was great, right? Dinosaurs are so cool.”
Depending who I am talking to, the answers vary from between one word answers and answers which would make my GCSE English teacher very happy indeed. It’s the second one that has caused me bother, though.
Obviously, every child’s (boy or girl) dream is to go out and dig up a dinosaur skeleton when they are older. it was my dream at one point, but more recently I have moved away from that idea and on to more achievable goals. First I was toying with the idea of going into research, and helping shape the future of palaeontological ideas. Then I wanted to go into teaching – but the idea of doing a lot of work for it puts me off. Oil exploration is another option open to me, but not one that appeals to me as much.
My current career goal (and one I hope I see out to the end) comes when I started hearing the third question more and more. “If ‘Palaeontology’ isn’t as household a name as ‘Physics’, ‘Biology’, ‘Astronomy’ or ‘Chemistry’,” I thought to myself, “maybe I should help it to be!”
And here I am.
A career in media and journalism is a long way off – as is any teaching and education work I may end up doing. But I can start here. This blog aims to bring Palaeontology to a new audience, however small it might be, and show that it needn’t be as alien a subject as it is at the moment. It will be scientific, but not anything that requires any highly academic knowledge to understand, and for those interested I will post additional reading with each topic.
I might even try and find a suitable enough Friends scene to show you all.
And yes, Jurassic Park was pretty cool.
Until next time,
~Matt Kerr (But you can call me “Mattchu”)