158. How evolution works

If life evolves steadily from one species to another, then why do homo sapiens and chimpanzees still co-exist ? That’s a classic question, and one which goes right to the heart of evolution.

It’s important to our understanding of how all life forms develop, and to reconstructing the the evolution of early man (thanks to Ella for the link).

ammonites2.jpg

The point is that whilst evolution is a slow process, the mechanism which allows change to happen is not a gradual one at all. We might see Darwin’s drawings of Galapagos finches as a continuous spectrum of evolutionary development, but perhaps that sketch gives quite a false impression of how evolution really works.

When evolutionary change takes place, it does so rapidly and abruptly.

The qualities of any large and connected gene pool drift more or less together – all things being equal, then most adaptations will typically spread only very slowly. The gene pool is in equilibrium, most of the time.

Whilst variation between individuals is inherent, and random mutations are constantly occurring, systematic drift within a species will most likely be ruthlessly denied by constant re-equilibration with the wider population.

That’s clear to everyone – if you stable your sleek and shiny thoroughbred with a humble carthorse, then the offspring will never win the Derby. Likewise let your pedigree poodle razzle with a mongrel from down the road, and the puppies will never show at Cruft’s. Those analogies serve to prove the point – under ordinary circumstances, the scope for quite advanced stages of adaptation to survive contact with the wider gene pool is surprisingly limited.

So how is evolutionary change possible at all ?

The answer is that evolution proceeds much more effectively within smaller populations. We all know that breeding strains of beans, racehorses, fruit flies or pedigree dogs works best in tightly-controlled conditions. Keep your poodle firmly on a lead at breeding time.

Within the natural world, subsets of the gene pool will develop in any setting where the breeding population is restricted. In geographically disconnected communities within remote valleys, impenetrable forest and on isolated islands, then mutationary adaptations which are found to be favourable within that setting have a far greater chance of establishing themselves.

Eldredge and Gould termed such environmental subsets ‘peripheral isolates’.

Over time, the successful spread of mutations within a peripheral isolate can (and inevitably will) lead to the emergence of a population which really is genetically different from the main gene pool. This is especially true where the environmental conditions begin to differ from those experienced by the larger population – for example a wetter climate on a windward island might foster a substantially different vegetation and food supply for the animals living there.

Depending upon the length of time, the degree of environmental separation, and the size of the population, this process can lead progressively through the development of different races and subspecies, eventually to the evolution of new and entirely distinct species and genera.

This is the process which is reflected today within the distinct faunas of isolated islands – both on the small scale of an individual Galapagos island, and on the relatively larger scale of Australia.

It was the observation and mapping of such faunal differences across the island chains of Indonesia which led Alfred Russel Wallace to propose the theory of natural selection, in turn encouraging Darwin belatedly to publish his own ideas within The Origin of Species.

But isolation does not always last for ever. Onshore, sea level falls may reconnect communities through the emergence of new land and ice bridges, whilst forest fires and flash floods may link once separate savannahs and lakes. Offshore, plate tectonics is expert at fragmenting and re-linking isolated marine shelves. The eternal dance of the continents around the globe has efficiently driven the formation and destruction of new environments for three billion years – including 2 700 million of them before life first dared to grace the land.

Following reconnection of distinct communities, there will be competition between the different populations as they mix and spread across the reconnected area. If divergence has proceeded far enough, beyond the point of dangerous competition, then the two communities may be able to co-exist successfully in parallel (as was the case, at least until recently, with chimpanzees and humans).

Alternatively, if the gene pools have not separated too far, then the different populations may still be able to re-equilibrate once more (as is happening in our cities today between the once-isolated Chinese, Japanese, African, Caucasian and aboriginal Australian peoples).

Often, divergence has gone far enough to prevent inter-breeding, but not far enough to prevent competition for food and living space. Reconnection may see one population now facing a selective disadvantage. The new adaptations may die out, proving to offer only a short-lived evolutionary dead-end. But sometimes, the newcomers will succeed in taking over the wider realm. In the blink of an eye, long-established lifeforms can be obliterated, with the new species replacing them seeming to have appeared from nowhere.

That begs another famous question – if evolution occurs gradually, over many generations, why do we rarely find intermediate forms ?

And now we can appreciate the answer. New forms typically evolve within geographically isolated communities, often across tiny areal distributions and populations which are numerically almost insignificant compared to the whole.

It’s the small size and local scale of isolated populations which allows them to diverge. Equally it’s those same factors which limit their preservation potential within the fossil record to almost nil. And it’s the sudden reconnection of isolated communities which inevitably results in rapid (geologically instantaneous) bursts of evolutionary change – since discrete evolutionary ‘jumps’ are just as certain to be recorded whether the new population supplants the old, or even if both forms can distinctly co-exist.

And far from being a slow process, taking place via slow drift across the gene pool, now we can envisage evolution as a fundamentally episodic process of rapid change. Evolution progresses not strictly through gradualism, but through an inexorable series of punctuated equilibria. Put simply, there are long periods when nothing much happens in evolutionary terms, and then suddenly new forms will appear and old ones disappear, all within a short space of time.

So that’s how evolution works.

It’s a simple but devastatingly effective mechanism which has operated on this planet, continuously and relentlessly for over three billion years.

A conventional view is that biology, genetics and zoology tell us how evolution proceeds today. Some would say that geology and palaeontology merely augment that story, with the rocks and fossils recording how evolution progressed throughout the past.

They’re right, of course. And yet, I’d like to go much further still. Because an appreciation of geological and plate tectonic processes really does provide the key to how evolution ever worked at all.

Related articles:
144. East of Eden – evolution and enlightenment
43. A sense of time – Earth history and the London Marathon
129. Tenerife – 1: the light at the end of the world
140. The Great Global Warming Swindle
41. A Lincolnshire legend – Sir Isaac Newton

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17 responses to “158. How evolution works

  1. There’s a lot to take and and digest there, and even then, I think the precise mechanisms of evolution and the triggering factors are bound to remain obscure and a matter of speculation.

    I’m not sure why the initial question you ask is a problem, ie why do homo sapiens and chimpanzees still coexist. Merely asking that question seems to presuppose that there’s a direct linear descent of man from monkey. However that’s not the case: man and monkey merely shared a common ancestor. It’s that ancestor that is no longer around, a missing link, where one has to rely on an incomplete fossil record, full of missing links, to trace the path of evolution.

    Perhaps I could use this box to float an idea of my own. It’s to do with the unusual fauna one finds on isolated islands.

    Flightless birds are usually found only on islands. Why is that ? I’ve seen very little speculation. An obvious point is that if there are no natural enemies of that bird on the island, it no longer “needs” to fly, so loses the ability to fly through lack of use. That’s sounds a touch Lamarkian, however. The inheritance of acquired characteristics is thoroughly discredited.

    I think there’s another factor: if a bird carrying eggs is blown far out to sea by a gale and has the good fortune to reach an island, then there’s a new situation for the fledglings that develop from the eggs. There will be inbreeding obviously that favours survival of mutations. But a mutation that reduces the ability to fly is surely one that aids survival if on an island that is far from land. Birds that can fly may try to escape from the island, and then die in the process, if having insufficient strength to turn back in the event of not finding land. But a mutant bird unable to fly would not even attempt the perilous voyage, and would stay put, perpetuating its genes. Bit by bit, birds still capable of flying would do just that – fly off, never to return – and all the remaining survivors in time would all be flightless birds and gradually become a distinct species.

    Any takers ? BTW, I don’t claim to be the first to suggest that idea , but it’s not one I’ve come across in my admittedly sporadic reading.

  2. Thanks very much for your comment, Colin.

    The initial question is not a problem for the evolution scientist, except in the sense that it is typical of the objections to evolution put forward by creationists.

    Darwin’s finches, and the famous drawings showing progressive evolution of man from crawling ape to upright man – those images give a misguided impression of evolution as a gradualistic process.

    The reality is very different – at least from the geological perspective. Now we can look on evolution as being characterised by long periods of calm and stasis, followed by instantly bloody revolution. That’s exactly what the fossil record shows, and it’s why we can correlate different strata all around the world by their time-unique assemblage of fossils.

    If evolution really were a strictly gradual process, then dating rocks would become a much greyer art, depending on opinion and shades of shape and form. In fact, it’s not – palaeontology is a razor-sharp science, capable of dividing most of the the last 600 million years of Earth history into globally recognisable time horizons just 250 000 years long. That’s a fantastic achievement, but perhaps the most impressive aspect is really not that the intervals concerned are relatively short, rather that there are so many of them – unique assemblages of life developed hundreds of times over hundreds of millions of years. The processes that led to the formation and destruction of such assemblages were one-off events, and yet they were repeated over and over and over again.

    By contrast, the reporting of anthropology sometimes seems problematic to a geologist like me, maybe because the development of homo sapiens is interpreted so easily in the press as having an almost sacrosanct significance, somehow lifting our species above all the rest. The article I link to above is a perfect example where the anthropological science almost fades into the philosophical and semi-religious quagmire surrounding the development of man. In short, it becomes a contest of ideology rather than a deductive analysis of facts.

    Whilst anthropology’s time span just scratches the geological surface, it’s a fascinating area which is critical to understanding how we got here. More rigorous analysis would go far to solving the mysteries here, I feel.

    I like your model for the evolution of flightless birds. Very elegant – and perhaps it serves as an illustration of just how important the isolation of small populations can be to evolution, as well as the pivotal role throughout the history of life on Earth of physical obstacles (such as oceans, rivers, lakes) which owe their origin to geological processes.

    If we lived on a static planet, we wouldn’t have progressed far from amoebae. And you only have to watch the marvellous Guinness adverts to appreciate that observation perfectly.

    Thanks again – that’s thought-provoking stuff, and much appreciated.

  3. When evolutionary change takes place, it does so rapidly and abruptly.

    Sometimes. Slow evolution doesn’t falsify evolution theory. Rapid evolution doesn’t falsify it, either. Evolution theory allows evolution to proceed at its own paces.

  4. Flightless birds are usually found only on islands. Why is that ? I’ve seen very little speculation. An obvious point is that if there are no natural enemies of that bird on the island, it no longer “needs” to fly, so loses the ability to fly through lack of use. That’s sounds a touch Lamarkian, however. The inheritance of acquired characteristics is thoroughly discredited.

    Darwin speculated on that, and Gould had at least one great column on it.

    Basically, it takes a lot of energy to fly, and it takes a lot of energy to grow wings to fly. If there is no place to fly to, nothing to fly away from, then that energy may be better spent growing a bigger body, or a different beak, or making eggs, and so on. This sort of selection is observed in several places, on different species — dodos being just one (they were descended from pigeons, by the way).

    Darwin was a beetle expert, and he noted that on some Atlantic islands, beetles had no wings. He correctly (so far as we know) pointed to the high winds, noting that any beetle spreading its wings would be blown out to sea — that could easily work for birds, too (and might be another reason for the size of the dodos; too big to blow away).

    In any case, look for Gould’s essays.

    This isn’t Lamarckian, by the way. It’s not that the bird desires to lose its flight; it’s not that one generation strives to lose its flight and then it’s passed on. It is that those who are flightless compete better, for food, for mates, for nesting space, against the elements (wind, for example), and so are selected.

  5. Punctuated evolution would be less likely to leave a complete fossil record than evolution that occurred at a steady even pace.

    One would like some assurance that punctuated evolution is not being proposed merely as an handy “excuse” for having huge gaps in one’s record, or worse still, as an attempt to counter objections from the creationists about perceived deficiencies in the fossil record.

    Does anyone know any good a priori reasons for thinking that evolution proceeded in fits and starts ?

    The only one I can think of is as follows: just as nature abhors a vacuum, so the living world abhors unexploited niches and habitats. So if there were a series of geological events, say, that created new habitats, like fertile volcanic terrain, or nutrient-laden oceans etc there could be an associated spurt in evolution of new life forms capable of exploiting the new niches.

    Anyone know if the fossil record supports the idea of evolution occurring in concert with environmental changes, whether due to volcanoes, plate tectonics and other geological events, climate change, asteroid impacts (apart from those dinosaurs) etc etc ?

  6. Welcome, Ed, and thank you for your comments.

    “Evolution theory allows evolution to proceed at its own paces.” I’d absolutely agree with you.

    There’s evolution which is ‘slow’ in terms of a zoological timescale, and yet can still appear rapid on a geological timescale. The perspectives of the sciences are slightly different.

    But my point is simply this – that relatively rapid and abrupt change is a common occurrence, much more so than is widely imagined as a result of those classic cartoons featuring gradualistic progressions.

    From a geological viewpoint there are some long-term trends discernible, but in detail these are always characterised as a series of discrete steps along the way.

    Colin:
    The story of evolutionary development is extraordinarily closely linked with plate tectonics.

    As one example, the end of the Permian saw the drifting together of several continents to form a huge supercontinent which geologists call Pangaea.

    Each continent had previously had its own marine shelf environments, separated from all the others. Those continents had differing climates, too.

    Once the continents combined, all their shelves were suddenly linked, and the range of environments was largely equalised as well.

    That time just happened to see one of the most profound crises in the entire history of life on Earth. Vast swathes of life’s diversity as it existed then were simply wiped out.

    Soon afterwards, within Triassic times around 200 million years or so ago, Pangaea began progressively to split apart.

    Once again, new environmentally distinct and geographically isolated shelves and land environments began to form.

    That time saw an almost explosive emergence of many new kinds of life, both onshore and in the sea.

    For more details, you could research some references on ‘mass extinctions’ and ‘adaptive radiations’ in Google and Wikipedia.

    There’s little dispute that such events are geologically controlled, on the macro-scale at least.

  7. That’s fine, as long as the reconstruction of how continents drifted about in the past is based purely on non-biological data. It would take only one reference to the “fossil record” to render the arguments circular, laying oneself open to the charge from creationist snipers that one was playing fast and loose with the so-called facts.

    We scientists (or in my case, retired scientists) have to be scrupulously careful that we did not appear to be true believers ourselves, pretending that our position is watertight, allowing no argument.

  8. That’s agreed. Ideas about continental drift go back to Alfred Wegener and beyond. Wegener postulated the fitting back together of Africa and Brazil, as any more than occasional viewer of the school atlas will instantly appreciate.

    But there was no mechanism understood then to explain about how continents could move around the globe. It was not until the 1960s that Vine and Matthews successfully interpreted magnetic stripes from the ocean floor as clear evidence that new oceanic crust was constantly being created at Mid Ocean Ridges, and destroyed by subduction in the deep ocean trenches.

    Whilst continents are built of rocks up to 3 800 million years old, the oceanic crust preserved within the oceans is much younger. The oldest, in the Pacific, is of Jurassic age, less than 200 million years old – since all the older oceanic crust has been lost back into the mantle.

    The proof of seafloor spreading at last provided a mechanism for continents to be translated around the globe – essentially we are living on mobile rafts which are in constant (although almost infinitessimally slow) relative motion.

    Reconstruction of tectonic plate movements is primarily achieved through the study of magnetic polar wandering curves from different continents.

    The foundations of plate tectonics are firmly based in geophysics, but the implications of the theory seamlessly explain the history of the Earth, and have much to say about the development of biological diversity as well.

    Plate tectonics is a wonderfully beautiful and unifying piece of science, and as a geologist and a humble inhabitant of such a brief moment in the history of this planet, it continues to amaze and enthrall me, each and every day.

  9. I agree with you entirely about plate tectonic theory. I had never heard of it until listening in to a coffee time discussion inthe mid 70s from someone not long out of school, and was instantly hooked to learn about sea-floor spreading, zones of subduction etc.

    It then fell to me to teach it as part of the Earth Science module in GCSE science. Where the resource materials were strangely quiet was on the subject of what drives the conveyor belt. Oh, it’s convection currents in the magma, they said. Yes, I know, I said, but what drives them ? Oh, the Earth’s core is very hot, didn’t you know ? Yes, I did, but why is it hot ? Er, well it’s because the Earth was once a molten blob, and is still cooling down, or because of the crushing pressure at the centre creating friction.

    Well, I didn’t buy either of those ideas, and asked around some more. That’s when I learned for the first time the theory that there are heavy radioactive elements at the Earth’s core that are decaying, releasing a steady stream of heat that keeps our lives exciting up on this fractured crust of ours, what with volcanoes, earthquakes, tsunamis etc.

    There’s surprisingly little debate about that radioactive core theory, and even less popular knowledge of it, which has one wondering if it’s maybe a wild-eyed theory that is not yet received wisdom. It would help if we knew more about tectonic processes on other planets, or maybe moons as well, to see whether they are the norm, or are the result of unusual conditions.

  10. Don’t worry – the radioactive generation of heat within the Earth’s core is accepted science, Colin.

    There has been some recent investigation into the possibilities of plate tectonic processes operating on Mars and Venus.

    If made up of similar solar system components, which presumably they are, then mantle convection and hence surface seismic activity are very likely to have taken place on those planets, too.

  11. Ah, it’s so refreshing to encounter thoughtful discussions about evolution, a subject near and dear to my heart as a biologist. I’m in the camp that believes that our scientific understanding of Life and all Creation remains a mere gap in our ignorance…although I’m thrilled to see that our understanding of the natural world is growing by leaps and bounds and is ever accelerating. I’m struck by the fact that much of what I was taught in college more than three decades ago has been proven false or modified extensively by subsequent findings/observations. That’s the splendor of science. No doubt, the mechanisms impacting speciation will be studied in perpetuity and our collective understanding will grow more sophisticated over time. I find that incredibly exciting. As we map genome after genome, as we unearth more and more fossils (I read somewhere that we discover new fossilized species every week), our insights into the evolutionary process will expand exponentially. Of one thing I am sure, we’ll never grasp it all. And that’s OK. Despite the natterings of the Creationists and their wily cousins, the “Intelligent Design” proponents, the case for evolution through natural selection is overwhelming. All that’s missing are the myriad details. I’m sure forthcoming discoveries will amaze and confound us, but the overall case for evolution has already been irrefutably made. And the beauty of it all is that it all makes sense on the macro-level. Our grasp of the earth’s geologic history, fossil record and the principles of genetics complement each other elegantly. Those individuals who wish to force fit observable facts into some theological construct are the ones who are hopelessly lost. The fact that they are given voice at all disturbs me. I’m stunned that a Creationist Museum has opened in Ohio, in order to “prove” that God created all life forms approximately 6000 years ago (over the course of six days, no less). Putting such blatant scientific ignorance on public display and giving it ANY measure of credibility is absolutely appalling and is a disservice to humanity.

  12. Thanks as ever for your thoughtful comment and insights, Jonas.

    It’s a great shame that the incredible and beautiful insights which evolution gives us seem to cause some ‘religious’ movements such trouble.

    To me, the complexities and wonders which creation (in it widest sense) produces can only serve to enhance my sense of awe and humility.

    Narrow-mindedness does great discredit to religion. Unfortunately, those two things are all too often found together, all around the world – in Baghdad, Birmingham, Baltimore and Belfast alike.

  13. Holding to the theory that life-form evolved on this planet is no problem for those with a scientific bent, who appreciate that so much of what happens in the natural world happens as a consequence of random events, or even collisions at the sub-atomic, atomic or molecular level.
    However, there are still huge conceptual gaps to be filled, like understanding how the first self-replicating lifeforms came about, and whether this happened on Earth (statistically improbable we are told, on account of the relatively short time scales – a mere 3 billion years, give or take) or whether it developed elsewhere, and we were seeded from outer space ( as proposed by Hoyle and Wikramasingh). As yet, we do not as yet have even the crudest test-tube model for a self-replicating chemical, so it would be premature to shed a degree of humility about how we come to be here.

    While we have a theory to explain the plethora of species on this planet, we still do not understand the plethora of subatomic particles. Even if we accept the idea that it all began with a Big Bang, there is no understanding about how the different particles, up-quarks, down quarks, charm quarks, strange quarks etc come to be there, where they came from, how they are related etc. Did they too arise by a process of evolution?

    Neo-Darwinist theory, plate tectonics are the easy bit . Understanding the first few milliseconds after the Big Bang is a lot more tricky. As for understanding the nature of a singularity 1 micro-second before the Big Bang is going to be a whole lot more difficult, if not impossible. One does not have to be a creationist to believe there may still be room for God, even if he/she/it is merely an expression of a fact, difficult for the human mind to grasp, that absolute nothingness is impossible or, to put it another way, as Hoyle did, the Universe has always existed, and obeyed its own rules.

  14. Well put, indeed.

  15. “To me, the complexities and wonders which creation (in it widest sense) produces can only serve to enhance my sense of awe and humility.”

    YES! YES! YES! The majesty, the wonder of it all, the Awe I feel as I contemplate what we know…and what we may never know…serves as the foundation for my personal theology/spirituality and, yes, humbles me.

  16. Here’s another enthusiast for plate tectonics.

    Wombats puts it very well – and I simply couldn’t have packed more wonderment and amazement and excitement in there myself.

  17. See the comments on this article for an interesting discussion on Evolution: fact or theory ?

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