Thoughts on “The Greatest Show On Earth” — chapter 5 — Before Our Very Eyes
In chapter 5, Dawkins continues with a point that he began in chapters 2 and 3: we can see evolution in action even on a short time-scale; if such changes can occur on such a short time-scale, then an ancient earth gives evolution the time it needs to work its wonders. Dawkins presents several cases of change within a short time (just a few decades). He includes the famous Lenski long-term E. coli experiments.
Data from the Uganda Game Department (published in 1962) shows a declining trend in tusk weight in Ugandan elephants. Between 1925 and 1958, average tusk weight declined from approximately 54 pounds down to 38 pounds. Dawkins is hesitant to definitively interpret it as an evolutionary change, since as he says, it could be due to other environmental factors. But as he says:
We must seriously entertain the possibility that it is a true evolutionary trend, in which case it is a remarkably rapid one. We must be cautious before concluding too much. It could be that we are observing strong natural selection, which is highly likely to result in changes in gene frequencies in the population, but such genetic effects have not so far been demonstrated. It could be that the difference between large-tusked and small-tusked elephants is a non-genetic difference. Nevertheless, I am inclined to take seriously the possibility that it is a true evolutionary trend.
It seems reasonable to be cautious. But if it is a genuine evolutionary change, Dawkins says he would expect natural selection to result in changes in gene frequencies in the population. As I’ve said in my response to previous chapters, changes in a population’s gene frequencies can produce rapid change in the short term, yet that alone is not sufficient to drive the large-scale changes that evolution proposes to explain the history of life. For several chapters, Dawkins has demonstrated the ability of natural selection to select the more favourable genes existing in the population. For evolution to explain life’s history, more than that is required—namely, mutations that produce viable alternative or additional genes. If natural selection is only varying the frequency of a population’s existing genes, then it can’t create something genuinely new.
The next logical step in the explanation of evolution, as far as I can tell, would be to demonstrate the role and effectiveness of mutations as a key mechanism to drive evolution. The Lenski experiments, coming up, finally make some inroads into that topic, but first Dawkins offers another case study. Yet again, his main point seems to be simply “evolutionary change can happen quickly”, without providing further insight into evolutionary mechanisms beyond adjusting a population’s gene frequencies.
The case study is of lizards on a Croatian islet called Pod Kopiste. The neighbouring Pod Mrcaru had none, until experimenters in 1971 introduced 5 pairs of Pod Kopiste lizards onto Pod Mrcaru. In 2008, scientists visited the islands to compare the two lizard populations on these islands.
Pod Mrcaru had a healthy population of the lizards, presumably descendants of the 5 originally introduced pairs. The scientists observed marked differences between the two lizard populations. The Pod Mrcaru lizards had a more vegetarian diet, and accordingly showed stronger skeletal and muscular features to support that diet.
But what seems most dramatic is the appearance of a “caecal valve” in the Pod Mrcaru lizards. Caecal valves are apparently useful in vegetarian digestive systems.
Now, the fascinating thing is that, although caecal valves don’t normally occur in Podarcis sicula and are rare in the family to which it belongs, those valves have actually started to evolve in the population of P. sicula on Pod Mrcaru, the population that has, for only the past thirty-seven years, been evolving towards herbivory.
This evolution of a caecal valve seems to be a most significant event, which raises some big questions. Dawkins speaks as though this appearance of a caecal valve was an inventive process. But it seems too convenient, too much to be coincidence, that the feature so quickly arises which is recognisable from many other species. There could be a variety of biological mechanisms that could explain the change. What genetic changes were needed to produce the caecal valve in these lizards? Were mutations needed? Of all the genetic “recipe” that is needed to make a caecal valve, maybe it was already present in some latent state in the original 5 lizard pairs’ DNA, and natural selection was able to select the genes for that capability when the environment demanded it. Were genetic changes even necessary? Perhaps no genetic changes were necessary, but rather it could be explained purely by gene expression during development varying in response to environmental conditions.
Even from an evolutionary perspective, there are several possible scenarios, with different implications for the “ease” at which a caecal valve can evolve. It could be explained as a vestigial ancestral feature, which could be fortuitously “re-enabled” when the environmental conditions favoured it. In that case, that scenario might be relatively easy and less remarkable. On the other hand, if the lizards previously possessed no latent capacity for caecal valves, then their advent is much more remarkable, and much more favourable evidence for evolution as the explanation of life’s history.
One creationist explanation is that creatures have been given certain in-built capacity for variation in response to the environment, bestowed by the providence of a wise Creator. Yes, natural selection would act to drive the variation, but the scope of change would be limited to the designed genetic capacity for latent features.
In the end, the tale of the lizards of Pod Mrcaru raises more questions than answers. I hope scientists can study the DNA of the lizard population, compared to the ancestral Pod Kopiste lizards, and gain more insight into the genetic differences that explain their physical differences.
Lenski Experiments and Mutations
Dawkins tells the tale of the famous Lenski experiments next. I thought this was the highlight of the chapter, with the really significant data to ponder. I won’t try to recap the technical details here, since it is too much to cover briefly. So you will need to read it yourself, and I will just record my own commentary here.
The Lenski experiments make a compelling case for useful mutations—mutations that don’t kill an organism, but give it improved fitness. Previously, I had been skeptical that mutations could be beneficial, since in human populations they usually seem to endow a person with cancer, if anything, and practically nothing else noteworthy. But this presentation of the data from the experiments convinces me that mutations can be beneficial. Having said that, I think it simultaneously points to some problems with evolution.
Dawkins indulges in a bit of gloating at creationists:
As we shall see, the Lenski experiments are distressing to creationists, and for a very good reason. They are a beautiful demonstration of evolution in action, something it is hard to laugh off even when your motivation to do so is very strong. And the motivation for dyed-in-the-wool creationists is very strong indeed.
Even young-earth creationists agree that “evolution” happens. They make a distinction between “microevolution” and “macroevolution”, saying that microevolution undeniably occurs (since we can observe it as Dawkins has described), but it doesn’t extrapolate to explaining the entire history of life. Old-earth creationists make a similar distinction between small-scale and large-scale evolutionary change, although differ on technical details of the micro/macro distinction. So why does Dawkins say the Lenski experiments are distressing to creationists? Surely he is not unaware of their stance.
As I said, the new detail that the Lenski experiments bring to the topic is the efficacy of mutations. Young-earth creationists talk about the ineffectiveness and destructive nature of mutations. My previous writings have echoed this, though I now have to reconsider it. So perhaps Dawkins means that the Lenski experiments drop a bomb-shell on creationists’ “mutations are bad” argument. For me, this is one point that the book has forced me to reconsider—I cannot deny that in this experiment, mutations have given positive benefits to the organisms.
Firstly, experimenters observed a general increase in “fitness”, as measured by comparing bacteria to frozen samples of an earlier generation of bacteria. In terms of observable changes, an increase in average body size was seen in all the bacteria populations.
What does this tell us about the efficacy of mutations? For one thing, it demonstrates that at least some percentage of mutations were beneficial. But not only that:
But perhaps even more interesting is that sometimes a pair of tribes seem to have independently discovered the same way of getting bigger. Lenski and a different set of colleagues investigated this phenomenon by taking two of the tribes, called Ara+1 and Ara-1, which seemed, over 20,000 generations, to have followed the same evolutionary trajectory, and looking at their DNA. The astonishing result they found was that 59 genes had changed their levels of expression in both tribes, and all 59 had changed in the same direction. Were it not for natural selection, such independent parallelism, in 59 genes independently, would completely beggar belief. The odds against its happening by chance are stupefyingly large. This is exactly the kind of thing creationists say cannot happen, because they think it is too improbable to have happened by chance. Yet it actually happened. And the explanation, of course, is that it did not happen by chance, but because gradual, step-by-step, cumulative natural selection favoured the same—literally the same—beneficial changes in both lines independently.
It shows that the 59 changes could occur in a gradual fashion, each individual change providing a significant enough increase in fitness to warrant the change “sticking around”. I really want to know: Does the data indicate that the changes happened in the same order in both populations? Or, maybe a small number of independent sub-sequences of changes in the same order? If so, that indicates there was one (or a small number of) very specific path of improvement by which the 59 genes could change in the same way. It also implies that the individual changes weren’t independently compellingly advantageous—that could be verified by comparing with the DNA of the other tribes, to see how many of the 59 changes they also exhibited.
So this demonstrates that
- Mutations can be beneficial.
- Practical pathways of gradual improvement could be demonstrated.
- A specific pathway or set of changes could be compelling enough (according to fitness) to happen twice in two tribes.
Secondly, Dawkins describes a remarkable event in one tribe, in which it suddenly developed the ability to digest citrate, a nutrient present in abundance but which the bacteria could not normally process. This mutant bacteria was able to grow explosively given its new ability to utilise this food source.
Dawkins says of Lenski’s reasoning:
Was it, in other words, just another mutational step, like the ones that seemed to be demonstrated in the small steps of the fitness graph on page 125? This seemed to Lenski unlikely, for an interesting reason. Knowing the average mutation rate of each gene in the genome of these bacteria, he calculated that 30,000 generations was long enough for every gene to have mutated at least once in each of the twelve lines. So it seemed unlikely that it was the rarity of the mutation that singled Ara-3 out. It should have been ‘discovered’ by several other tribes.
Further experimentation indicated that this ability was more difficult to “achieve” because it relied on two mutations to work—but the first mutation wasn’t beneficial on its own. The first mutation had to occur purely by chance, with apparently no immediate fitness benefit. But then the bacteria was “primed” for the second mutation to deliver the beneficial change.
The magic moment turned out to be approximately generation 20,000. Thawed-out clones of Ara-3 that dated from after generation 20,000 in the ‘fossil record’ showed increased probability of subsequently evolving citrate capability. No clones that dated from before generation 20,000 did.
Dawkins is thrilled to describe the experimental work that deduced the circumstances of this two-mutation change. I also admire the detective work they used to achieve understanding, but I don’t see it as marvellous evidence for evolutionary theory. Dawkins says it “...undermines [creationists’] central dogma of ‘irreducible complexity’.” It does? What did I miss? On the contrary, it seems to highlight a significant limitation. Unlike the previously observed improvements, this one lacked a path of gradual stepwise improvement, so faced a more significant barrier to improvement. It happened, but with a significantly reduced probability—only happening once out of the 12 tribes. Isn’t this a demonstration of the reality of irreducible complexity? That is, if any feature requires more than one mutation—say n mutations—in order to succeed, the probability rapidly drops as n increases. It succeeded for n=2, but what would the chances be for n=3 or higher?
The Lenski experiment demonstrates the scope for beneficial mutations, something that I had not previously thought feasible. On the other hand, it also seems to demonstrate two key limitations of evolution.
One limitation is ‘irreducible complexity’, despite Dawkins’ claim to the contrary. Was the jump to citrate utilisation a dramatic break-through that smashes the ‘irreducible complexity’ theory? Initially it sounds dramatic and impressive, but really the step was a fairly small step, yet still difficult. The bacteria apparently already had some potential to process citrate, but only if it was in a low-oxygen environment. I’ve heard that the barrier to processing it is not a barrier of metabolisation—the metabolisation capacity is already there in the cell—but a difficulty of simply transporting the citrate across the cell membrane. So the bacteria was already so close to being “nearly there”, and yet it was held up by a two-mutation barrier. Irreducible complexity made a two-mutation development necessary, but statistically unlikely.
So I agree evolution can work, to the degree that life’s complexity can come about entirely by gradual change. So the key question now is: Can all of the complexity of life be achieved by paths of gradual change? How can we qualify and quantify the answer to that question? I suppose geneticists would have to understand the number of genetic changes required to achieve any new “feature” of a critter, and the size of the barriers that could be encountered along the crucially necessary path of “gradual change”. Doesn’t science have a long way to go to quantify that? And so evolution involves faith. Faith that gradualism works, that every creature that’s ever lived can exist along a plausible path of gradualism, including all the amazing far-out ones. But of course, evolution is the only game in town. The only naturalistic explanation, that is. Meanwhile, my gut feel is of incredulity. Yes, a gut feel, but at some point we have to be persuaded for or against, and it’s not looking so great from they way I’m seeing it.
Of course, let’s not forget what is surely the biggest ever case of ‘irreducible complexity’—the origin of life itself. For evolution cannot even begin to work any magic until mutating, self-replicating life exists. (Yes, I have heard, “But evolution doesn’t claim to explain the origin of life”. Yet, we’re not just talking about evolution. We’re also talking about the origin, meaning and purpose of life, as given by Christianity on one hand, and secular humanism on the other.)
Time, give us more time
The second limitation exposed by the Lenski experiments, is time. Dawkins titles the section “Forty-five thousand generations of evolution in the lab”. He says at the start of the section:
The average generation turnover of those lizards is about two years, so the evolutionary change observed on Pod Mrcaru represents only about eighteen or nineteen generations. Just think what you might see in three or four decades if you followed the evolution of bacteria, whose generations are measured in hours or even minutes, rather than years!
And then later:
If we assume that the probability of a gene mutating during any one act of bacterial reproduction is as low as one in a billion, the numbers of bacteria are so colossal that just about every gene in the genome will have mutated somewhere in the world, every day. As Richard Lenski says, ‘That’s a lot of opportunity for evolution.’
In that case, my question is: given how little the bacteria changed in 45,000 generations and colossal numbers, this seems to thoroughly demonstrate the infeasibility of evolution. In 45,000 generations, the bacteria were still essentially E. coli, with variations, and a demonstrated difficulty in accomplishing a fairly rudimentary mutation to modify its capability to process nutrients.
Compare that with the fossil record: In the so-called Cambrian explosion, dozens of never-before-seen complex life forms simultaneously burst onto the scene. Trilobites appeared, with complex eyes. They must have also had the necessary visual processing in their brains to make sense of the signals coming from those eyes. Given the evolution that E. coli have demonstrated they can achieve in 45,000 generations and colossal numbers, how many generations would it take for trilobites to “evolve” from whatever their imagined ancestor might be? In the previous chapter, Dawkins talked about ancient time—over 4 billion years. Surely, even that isn’t nearly time enough.