Over on Why Evolution is True, Jerry Coyne asks “What would disprove evolution?”. The basic idea of his article is to a) summarize what is claimed by “the theory of evolution” (a phrase I dislike nearly as much as “Darwinism”), and b) what observations could falsify it.
Here’s how he summarizes “the theory of evolution”:
First, let’s reprise what I see as the major components of the theory of evolution.
1. Evolution occurs, that is, there is gene frequency change in populations over generations.
2. Significant evolution takes time—that is, it usually (though not always) requires hundreds to thousands of generations to occur. It is not instantaneous, and it is the population and species rather than the individual that evolves.
3. Lineages of organisms split, or speciate, so that the single lineage that gave rise to life 3.5 billion years ago has undergone numerous splitting events to produce the millions of species alive today (and also the even more millions that went extinct).
4. The converse of #3: any pair of living species has a common ancestral species some time in the past. That is, if you trace any pair of twigs on the tree of life, you will find a node where the line from the trunk bifurcates to produce them.
5. The process producing the appearance of design in organisms is blind, purposeless natural selection. (There are, of course, evolutionary forces other than selection, including genetic drift, but they don’t produce the marvelous design that was once seen as the prime evidence for the hand of God.)
I, and others, have pointed out that there are three aspects of evolution: evolution as fact, evolution as theory, and evolution as path. Evolution as fact refers to the historical reality that species are related through common ancestry. This is supported by a massive amount of evidence from a wide array of independent sources. Evolution as theory refers to the proposed explanations for how “descent with modification” occurs — mutation, natural selection, genetic drift, etc. Evolution as path refers to the actual patterns that have occurred during the history of life, such as when certain events (e.g., branching points, extinctions, etc.) took place, how lineages are related, when and how many times certain traits evolved, and such.
The important point is that these three components are largely independent. We can all accept the fact of evolution (i.e., species are related by common descent) while disagreeing about mechanisms (e.g., is genetic drift more important than natural selection?), or we can agree on mechanisms (e.g., adaptations are the product of natural selection) but argue about historical patterns (e.g., are birds descended from dinosaurs or some other lineage of reptilian ancestors?). Darwin himself recognized this distinction:
“Whether the naturalist believes in the views given … by Mr. Wallace and myself [i.e., the theory of natural selection], or in any other such view, signifies extremely little in comparison with the admission that species have descended from other species and have not been created immutable [i.e., the fact of evolution].”
– Charles Darwin, 1863
“I had two distinct objects in view [in The Origin of Species]; firstly, to show that species had not been separately created [i.e., the fact of evolution], and secondly, that natural selection had been the chief agent of change.”
– Charles Darwin, 1871
Not only is it possible for people to accept the fact of evolution and yet reject particular theories that endeavour to explain it, but this is what actually did happen with The Origin of Species. Darwin’s arguments in support of the fact of common descent were very effective, and served to convince most of the contemporary scientific community of the historical reality of descent with modification. However, natural selection was not widely accepted at first, and competed with various alternative theories (e.g., orthogenesis, neo-Lamarckism, mutationism) until the Modern Synthesis in the 1930s and 1940s.
With this critical distinction between fact, theory, and path in mind, let’s have another look at Jerry’s list.
“1. Evolution occurs, that is, there is gene frequency change in populations over generations.”
First, this refers only to microevolution. Second, this is an observation that has been made thousands of times. So, this falls under evolution as fact, and only a very weak version of evolution as fact at that.
“2. Significant evolution takes time—that is, it usually (though not always) requires hundreds to thousands of generations to occur. It is not instantaneous, and it is the population and species rather than the individual that evolves.”
This is a combined claim about path (how long something takes) and theory (it is a population process, not a matter of individual organisms changing). “Usually” is a claim about relative frequency — sometimes it can be rapid, most of the time it’s slow. But this has nothing to do with whether it occurs. It’s a detail, and it’s not a detail about the fact or the theory of evolution, but about how it happens to have actually played out. In other words, settling this issue one way (it’s usually slow) or another (it isn’t usually slow) does not, by itself, have any major consequences for the sort of thing we mean by “could evolution be disproved?”. And, indeed, it already was proposed in the past, by De Vries and others, that evolution occurrs rapidly most of the time (mutationism). Likewise, the second claim has already been dealt with within evolutionary theory. Lamarck based his theory on the widespread assumption that acquired characters could be passed on to offspring. That is, that individual organisms could change and that this would be evolutionarily relevant. That turned out not to be the case, but if it had, we’d still have evolution as fact and we’d have evolution as theory (albeit a different version) and evolution as path.
“3. Lineages of organisms split, or speciate, so that the single lineage that gave rise to life 3.5 billion years ago has undergone numerous splitting events to produce the millions of species alive today (and also the even more millions that went extinct).”
This is a reiteration of the simple statement that evolution is a historical reality — species share common ancestors. However, there is also an element of evolution as path thrown in, for example that there was a single ancestral lineage, that there have been millions of splitting events, etc. What if there had been two early lineages and they both had left modern representatives? We’d still have evolution as fact, evolution as theory, and a slightly different version of evolution as path. Also, it is entirely possible in principle that most evolutionary change occurs within a single lineage (anagenesis) rather than via branching (cladogenesis). Once again, these are merely historical details, not do-or-die predictions of evolutionary biology.
“4. The converse of #3: any pair of living species has a common ancestral species some time in the past. That is, if you trace any pair of twigs on the tree of life, you will find a node where the line from the trunk bifurcates to produce them.”
Again, just a restating of the fact that species share common ancestry.
“5. The process producing the appearance of design in organisms is blind, purposeless natural selection.”
This is a claim about evolution as theory, the same one that Darwin made separate from his argument about the fact of evolution. What would happen if this claim turned out not to be true? Say, there was some self-organizing principle at work, or some developmental feedback, or what if Lamarck had been right? Only the specific evolutionary theory that we know today would be different. The fact and path would still be evaluated independently.
So, on this basis, I don’t think this particular list is very useful because it obviously conflates evolution as fact, theory, and path. Coyne does note that these elements in his list are independent — but he still lumps them together under “the theory of evolution”. If each one can be refuted or confirmed independently, then some could be refuted and some could be supported. If that happens, then have we falsified “the theory of evolution”? Is it best 3 out of 5? You can see why this kind of mixing of independent aspects is unhelpful.
In any case, Coyne proposes some observations that could, in principle, falsify the claims listed above.
“I give a list of seven observations that, if repeated and confirmed, would disprove parts of the theory of evolution described above. This shows that it is a scientific theory in the Popperian sense of being falsifiable. Here are some of those conceivable observations:”
“1. Fossils in the wrong place (e.g., mammals in the Devonian). If the fossil record were all out of order like this (a single anomalous fossil might not overturn everything, of course, since it could be in the wrong place for other reasons), we’d have to seriously question the occurrence of evolution.”
This is a pretty common argument. J.B.S. Haldane is reported to have responded curtly to a question about evolution and falsifiability that “fossil rabbits in the Precambrian” would falsify evolution. Richard Dawkins has said something similar, as in this image that is floating around on Facebook currently:
And now Coyne suggests that mammals in the Devonian would do the trick. Now, depending on which fossils were in which place, this may only change some details about path. It’s extremely unlikely that mammals occurred in the Devonian and discovering fossils of mammals in those layers of rock would be surprising, but how far back does the origin of mammals have to be pushed to count as disproving “the theory of evolution”? Molecular data suggest an earlier divergence time than we see in the fossil record for many groups. And new fossils are found relatively often that push times of origin back. Let’s imagine that we do find lots of mammals in the Devonian. Would we really reject evolution as fact? Or would we simply have to rethink our understanding of evolution as path? Evolution as fact is, indeed, falsifiable — it could have been fundamentally inconsistent with all sorts of observations that have been made. But I think that looking for a single Popperian falsification (seriously? Popper?), especially one that deals primarily with path rather than fact, is misguided.
Apparently, I am not alone in this. Larry Moran wrote this on Facebook in response to the Dawkins quote mentioned above:
The statement is untrue. If we discover that a given species is older than we thought then we will just revise our view of the history of life on Earth. It will not disprove the fact of evolution and it will have no effect on evolutionary theory. It is a mistake to link the truth of evolution to our current inderstanding of the history of life. That history can be easily changed without threatening evolution.
“2. Adaptations in one species good only for a second species. There are plenty of adaptations in species that are good for other species, but also help members of the first species: these are the basis of mutualisms. (Lichens, for instance, are a mutualism between algae and fungi.) But we don’t expect to see—and don’t see—adaptations in one species that evolved solely for the benefit of another species.”
If an example of adaptations purely for the benefit of another species were shown, I doubt anyone would reject “the theory of evolution” as a whole. Rather, we’d start re-thinking our understanding of how some behaviours evolve. And how do we define “adaptations” anyway? I recently taught a graduate course in evolutionary biology in which we explored this question all semester. Believe me, it’s not a simple issue. Could we imagine traits that evolve in one species that happen to benefit only another species? Sure. Would we call them adaptations? Maybe not. (Actually, probably not by definition). So, this is not a particularly clear test, and even if it were, I don’t think it would have that much of an effect on evolution as theory, let alone evolution as fact or path.
“3. A general lack of genetic variation in species. Evolution depends on genetic variation. If most species had none, they couldn’t evolve. However, the universal efficacy of artificial selection (I’m aware of only three lab experiments that failed to show a response to such breeding experiments), shows that genetic variation is ubiquitous in nearly all species.”
If we looked around and saw very little variation in modern species, this would not, in itself, negate the fact of common descent nor necessarily alter many details about evolution as path. Let’s say all bird species today have little or no standing genetic variation. Does this mean they are not related to crocodilians and not descended from dinosaurs? Or might it mean that, for whatever reason, genetic variation has been depleted (e.g., due to bottlenecks, low mutation rates, intense natural selection, or whatever). It would definitely make it harder to find support for natural selection in terms of showing that all the requirements for it (variation being one of them) are met today. But it would not automatically mean that natural selection never happened in the past.
“4. Adaptations that could not have evolved by a step-by-step process of ever-increasing fitness. This is of course the contention of advocates of Intelligent Design like Michael Behe. But adaptations like the flagellum, which Behe and other IDers cite as features that couldn’t have arisen by a step-by-step process of increasing adaptation, have been shown to plausibly arise by just that process. We don’t need to completely reconstruct the evolution of things like flagella, but simply show that their evolution by a stepwise adaptive process was plausible.”
What is the important claim here? That it occurs step-by-step? Or that fitness increases throughout the process? These are two different issues. One, the evolution of complex organs is, itself, complex, often involving multiple shifts in function, co-option of parts that were doing very different things, and so on. So, could the eye have evolved bit by bit from eyespot to camera-type eye the way Dawkins portrays it? Maybe, but it didn’t. It was far more complex than that. So, I would say that refuting a simple step-by-step model would not (and, indeed, hasn’t) undermined the major aspects of evolutionary biology. How about the notion of ever-increasing fitness? Well, again, it’s not that hard to imagine that some parts that ended up being co-opted into a new function as part of a complex adaptation were, at one time, neutral or even detrimental. It’s also easy to imagine that the current state of a particular complex adaptation is suboptimal because of historical contingency, genetic constraints, the influence of population bottlenecks, or what have you. So, how much deviation from “step-by-step”, “ever-increasing fitness” does it take to refute “the theory of evolution”? The main point is valid, in that it would be significant if there were obvious features of organisms that just could not be explained by evolutionary processes. However, that might mean that our understanding of evolutionary mechanisms is simply incomplete.
“5. The observation that most adaptations of individuals are inimical for individuals or their genes but good for populations/species. Such adaptations aren’t expected to evolve often because they would require the inefficient process of group or species selection rather than genic, individual, or kin selection. And indeed, we see very few features of organisms that seem inimical to organisms or their genes but useful for the population or species. One possible exception is sexual reproduction.”
Once again, this relates to a specific question about evolutionary mechanisms, and it’s a debate that has actually occurred during the development of evolutionary theory. If “naive group selectionism” had turned out to be correct, would we have rejected evolution as fact or path, or would we simply have a different understanding of how certain traits evolve? There is another key distinction here that Coyne seems to be missing regarding trait evolution: 1) how and why a trait evolved in the first place (origin), 2) why it became common and continues to exist today (persistence), and 3) why it is found in the species in which it is found and not others (distribution). These can have three different explanations. Coyne lists sexual reproduction here. Well, sex may have evolved initially as a mechanism for spreading transposable elements (selection at the subgenomic level), become common because it provides some benefit in co-evolution with parasites (Red Queen effect), and be common across eukaryotes because it leads to more rapid speciation and/or lower extinction rates (interlineage selection). That would be three levels of selection to explain the evolution of sexual reproduction. The latter question, “why is sex so common?”, may very well have more to do with differential survival of lineages than with individual organism benefits. This is a detail of evolutionary theory to be worked out, not a test of the validity of evolution in general.
“6. Evolved “true” altruistic behavior among non-relatives in non-social animals. What I mean by “true” altruistic behavior is the observation of an individual sacrificing its reproductive output for the benefit of individuals to which it is either unrelated or from whom it does not expect to receive return benefits. In this “true” altruism your genes give benefits to others and get nothing back, and this shouldn’t evolve under natural selection. And, indeed, we don’t see such altruism in nature. There are reports that vampire bats regurgitate blood to other individuals in the colony to whom they’re unrelated, but those need confirmation, and there may also be reciprocal altruism, so that individuals regurgitate blood to those from whom, one day, they expect a return meal. Such cooperation can evolve by normal natural selection.”
How are non-social animals supposed to engage in altruistic behaviour? By definition, this is something that occurs when animals interact with each other. In any case, once again, this would only challenge our understanding of how a very specific set of behaviours evolves. Say true altruism does exist in some animals, and it can’t be explained by inclusive fitness or reciprocal altruism (the latter of which definitely can only happen in social species). Would this mean that bats don’t share a common ancestor? Or that echolocation did not evolve by natural selection? Once again, this is a detail and is only an issue if one thinks that our current understanding of natural selection is absolutely central to “the theory of evolution”. Not to mention that we don’t need to posit radical new mechanisms of evolution to explain apparently maladaptive traits. Maybe a behaviour is not good for the individual or his genes, but it is fixed in the population because of a bottleneck, or it arises from a trade-off with a behaviour that is beneficial, and so on. Natural selection is not the only mechanism by which traits can evolve.
“7. Complete discordance between phylogenies based on morphology/fossils and on DNA. While individual genes can show discordance by lateral transfer—rotifers, for example, have incorporated into their genome from DNA from very unrelated organisms, and this is also common for bacteria. But lateral transfer of genes, as opposed to their direct descent from parent to offspring, is relatively uncommon. So, for example, if we sequenced the genome of a blue whale and found that on the whole the species was more closely related to fish than to mammals, we’d have a serious problem for the theory of evolution.”
There are various examples in which molecular phylogenetics has given us surprising results that appear to be at odds with expectation based on morphology: crocodiles are more closely related to birds than to lizards; hippos are more closely related to whales than to cows; sea stars are more closely related to humans than to lobsters. So, it’s not clear how extreme this discrepancy would need to be in order to refute evolution as a whole according to Coyne’s argument. If you accept the three domain view, then you would also probably argue that archaea are more closely related to eukaryotes than to bacteria. But let’s say archaea are actually closer to bacteria. Or bacteria are closer to eukaryotes to the exclusion of archaea. Or that there aren’t three domains at all. Do we reject “the theory of evolution” in any of those circumstances? Or do we just update our understanding of evolution as path?
In any case, if we observed widespread discordance between molecular data and expected evolutionary relationships as inferred using other data (fossils, comparative morphology, etc.), we would have to revise our understanding of those relationships (if we favoured the DNA evidence) or our understanding of molecular evolution (if we favoured the other types of data). There are various reasons why molecular evolution and morphological evolution can be decoupled, or why there can be convergence across distantly related species, or how genes can be transferred across taxa. And there may be additional processes of molecular evolution of which we are not yet aware. It would be extremely surprising if molecular phylogenies always disagreed with other data, but again I question whether this would lead us to reject “the theory of evolution” rather than inspiring us to work toward a better understanding of mechanism and/or path within evolutionary theory.
To sum up, any particular aspect of evolution — the fact of common descent, the mechanisms of evolutionary change, the paths of descent and relatedness — can be tested. This means that they can also be modified as necessary according to new evidence. This is exactly how the history of evolutionary theory has unfolded, with the fact of evolution now supported by mountains of evidence from a wide array of sources. Basic mechanisms like natural selection and genetic drift are also well established, though which one is at work in which case remains an interesting detail to sort out. And the major details of evolutionary path are emerging thanks to fossil and molecular data, but there continue to be substantial debates about details here as well.