Reducibly complex bombardier beetles.

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The classic creationist argument regarding “irreducible complexity” is that many multi-part biological systems cannot function without all of their parts present together in the current configuration, and thus that there is no possibility that such systems evolved through gradual intermediates. In the past, the most common examples included features at the organ level such as eyes, but with the rise of intelligent design this has shifted to more esoteric cases like bacterial flagella, the immune system, and the blood clotting cascade.

The argument is false in any case, as it overlooks the commonplace occurrence of shifts in function in evolution, in which subsets of the current system still function, but in a different way. For example, one can remove large numbers of components from bacterial flagellar systems rendering it useless for locomotion, yet the structure still functions in secretion. The point is that such complex structures could evolve through functional intermediates, but that the functions of those intermediates may have been quite different from the function of the system as it is observed now.

In other cases, the concept of irreducible complexity is refuted by simply finding cases in particular species where one or more components are missing but the system still functions in the same capacity. Some bacteria exhibit only a subset of the flagellar components found in other species but still swim. Whales lack Factor XII (Hageman factor) but their blood still clots1 (and, in fact, blood in humans with Hageman factor deficiency also still clots, though more slowly). Plenty of species lack major components of eyes found in vertebrates and cephalopod molluscs, but they see to an extent sufficient for their niches.

Bombardier beetles represent another classic creationist example of irreducible complexity that supposedly would be totally non-functional if any of the numerous components were absent. These beetles employ a mixture of hydroquinone and hydrogen peroxide which, when catalyzed, generates a strongly exothermic reaction in which a hot, repellent liquid is ejected at predators. The argument is that without all of the components present together, the defence mechanism of bombardier beetles would be useless. A typical example is given in this video, which gives an interesting overview of the complex system before concluding that the system could not work without all of the parts. In the beetle shown in this video, the liquid is heated to the boiling point of water, can be aimed with some accuracy, and is released in a series of high-velocity pulses.

It should be noted at this point that there are about 500 bombardier beetle species, divided into several tribes within the ground beetles. These are generally classified into two groups, the brachinoids and the paussoids. It is not entirely clear whether these are monophyletic, meaning that they all evolved from a common ancestor, or whether bombardier-like defence mechanisms evolved independently at least twice. For the purposes of this discussion, the details of their relationships are not particularly relevant, and instead what matters is the diversity they now display. Most of what is known about bombardier beetles comes from studies of the brachinoids (e.g., Brachinus), but this reflects only a portion of bombardier beetle diversity.

Metrius contractus is a species in the paussoid lineage that differs from other bombardier beetles in both lineages in important respects (Eisner et al. 2000). Notably, in this species the mixture of reagents is not ejected in a jet, it oozes out in a froth. M. contractus lacks the elytral flanges found in its relatives that allow it to direct the fluid forward in a jet. It does not aim the abdomen to direct the liquid, rather the excreted foam may be channeled anteriorly along grooves in the elytra (wing covers). The mixture continues to bubble after it is released, indicating that the reaction is not completed before being ejected and the solution is not boiling hot but is ejected at only about 55 degrees celcius. But the beetle survives.

Images from Eisner et al. (2000).

Crepidogaster ambraena and C. atrata are two African species of bombardier beetles in the brachinoid lineage but part of a tribe (Crepidogastrini) different from that of the brachinoids that have been studied in detail (Eisner et al. 2001). These species are similar to other brachinoids in morphological structure of the defensive system and in aiming the excretion by directing their abdomens at predators. However, they differ from the commonly described system in that their excretion is not boiling hot (about 64 degrees celcius), is more mist than jet, and is not pulsed when ejected. As a result, they are less effective than other brachinoids in terms of accuracy, the efficient ejection of liquid at high velocity, and repetitive cooling of the reaction chamber. In other words, slight changes in the structure or function of valves within the system would provide significant advantages, as they did in other brachinoids. And yet, this beetle survives as it is.

The beetles discussed above are all modern species, and none is suggested to be ancestral to any others. As such, these comparisons do not in themselves provide information on the historical sequence of changes in the evolution of the most complex bombardier beetle defences. However, the continued existence of some species lacking one or more of the features found in other species clearly refutes the argument that the complete system must arise all together in order to be functional. Much work remains to be done in sorting out how their remarkable features arose, but there is no reason to believe that they are not the product of gradual evolutionary change.

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Notes

1. It is worth pointing out that whales do have the gene for Factor XII but it is pseudogenized, and that the amino acid sequence of the protein it would produce is more similar to artiodactyls than to other mammals (Semba et al. 1998).

References

Eisner, T., D.J. Aneshansley, M. Eisner, A.B. Attygalle, D.W. Alsop, and J. Meinwald. 2000. Spray mechanism of the most primitive bombardier beetle (Metrius contractus). Journal of Experimental Biology 203: 1265-1275.

Eisner, T., D.J. Aneshansley, J. Yack, A.B. Attygalle, and M. Eisner. 2001. Spray mechanism of crepidogastrine bombardier beetles (Carabidae; Crepidogastrini). Chemoecology 11: 209-219.

Semba, U., Y. Shibuya, H. Okabe, and T. Yamamoto. 1998. Whale Hageman factor (Factor XII): prevented production due to pseudogene conversion. Thrombosis Research 90: 31-37.


What is a just-so story?

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As I and others have noted many times, facts, theories, and hypotheses are independent elements in the scientific process. Contrary to their vernacular meanings, they are not ranks indicating differential degrees of certainty in some claim.

Evolution is scientific fact, meaning that the numerous types of evidence point so overwhelmingly to shared ancestry that scientists have accepted it as true about the world, in the same provisional-but-extremely-likely sense that they accept other facts like gravity or the existence of atoms.

Evolution is also a theory, meaning that there is a cohesive body of mechanistic explanations that seeks to explain the historical fact of common descent. This includes, but is not limited to, random processes like genetic drift, quasi-random ones such as mutation, and absolutely non-random ones like natural selection.

In addition to being a fact (that species are related through common ancestry) and a theory (well supported mechanisms that explain how evolutionary change happens), evolution represents the unique historical path that living lineages have followed. Whereas there is no longer any real disagreement in biology (or indeed, in science in general) over the historical factuality of common relatedness, evolutionary biology is rife with heated debate regarding the mechanisms and their relative importance, the specific historical relationships linking related groups, and the intermediate steps that occurred in the origin of particular features. As such, hypotheses are also important in evolutionary biology, because they represent testable statements that are used to support or refute specific details of theory or path.

One of the more stinging criticisms that evolutionary biologists level at each other when they argue about evolution as path is to call a proposed account a “just-so story”. The phrase itself comes from Rudyard Kipling’s Just So Stories from the early 1900s, many of which included fantastical accounts of the origins of particular features, like the elephant’s trunk, the leopard’s spots, and the giraffe’s neck. According to Wikipedia,

A just-so story, also called the ad hoc fallacy, is a term used in academic anthropology, biological sciences, and social sciences. It describes an unverifiable and unfalsifiable narrative explanation for a cultural practice or a biological trait or behavior of humans or other animals. The use of the term is an implicit criticism that reminds the hearer of the essentially fictional and unprovable nature of such an explanation. Such tales are common in folklore and mythology.

The important criterion in whether something represents a just-so story rather than a hypothesis is a lack of testability and hence unfalsifiability. A lack of complete supporting data in itself is not an indicator that something is a just-so story, because many hypotheses also lack these prior to being subjected to testing. In other words, what matters is whether it can be tested, not whether it has already been tested, though obviously some supporting data must be provided eventually if the hypothesis is to be considered seriously.

Creationists, who seem unequipped with irony detectors, tend to dismiss all hypotheses about evolutionary path, even ones for which there is substantial supporting evidence, as just-so stories. In fact, they often demand an absurd amount of evidence and detail, such as an observable, repeatable, mutation-by-mutation demonstration of some feature evolving. When this obviously cannot be delivered (nor could its counterpart in any science), they believe this supports their unobservable, unrepeatable, vague explanation for the feature’s origins. Theirs is the ultimate just-so story, but that does not prevent them from projecting onto scientists.

As a case in point, consider the infamous bombardier beetles, which I mentioned briefly in a recent post. Along with eyes, blood clotting, the immune system, and bacterial flagella, the defence mechanism of these intriguing beetles supposedly represents an un-evolvable feature due to its irreducible complexity.

The claims made by creationists about these beetles relate to both fact and path. In terms of fact, they often suggest that the reagents used in its defensive system will explode when mixed together, and thus that their mechanism of storing them separately must have arisen fully formed. This is demonstrably false. A catalyst is required, as Dawkins showed by mixing the liquids.

With regard to path, they argue that a series of functional intermediates in the gradual evolution of the defensive apparatus is impossible in principle. This claim is also easily refuted by the presentation of plausible, testable hypotheses showing how functional intermediates could have occurred. One example is given in this video.

Now, is this the answer to how the spectacular defence system of bombardier beetles arose? I do not know, but I suspect probably not. A similar point of view is presented by Mark Isaak, who authored the Talk.Origins article Bombardier beetles and the argument of design:

The scenario above is hypothetical; the actual evolution of bombardier beetles probably did not happen exactly like that … Determining the actual sequence of development would require a great deal more research into the genetics, comparative anatomy, and paleontology of beetles. The scenario does show, however, that the evolution of a complex structure is far from impossible. The existence of alternative scenarios only strengthens that conclusion.

Are such scenarios just-so stories? No, they are hypotheses, and they are testable. For example, are the hypothetical intermediate stages found to be functional in any other species? Are the chemicals used in combination in the defence system also functional on their own elsewhere in the beetle’s body? Are there genetic differences between bombardier beetles and other Carabidae related to this system?

Without clear answers to these and other questions, the path of bombardier beetle evolution will remain an open and interesting question. However, biologists do not assume that these beetles did not evolve simply because the specific path has yet to be elucidated. And they certainly would not assume that an untestable, supernatural just-so story of cosmic proportions is the null hypothesis.

That, of course, is the difference between science and pseudoscience.


Monty Python guards Expelled.

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Apparently PZ Myers was refused admission to Expelled, the pro-ID movie about how, you know, people are prevented from engaging in debate about the subject. Anyway, they decided that PZ, who is interviewed in the movie, should not see it, but they neglected to stop his guest — some guy named Richard Dawkins — from entering. I imagine the scene played out something like this as the guards were given instructions on who to let in…


235 years in Canada.

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Some of you may recall my answer to the question Am I a MacGregor? was

What’s the point? I am not Scottish, I am Canadian, and I am perfectly happy with that identity.

I still do not have an answer to the MacGregor question (nor have I been looking for one), but I did receive some rather interesting information regarding the past 250 years of Gregory family history thanks to some detective work done by one of my father’s cousins.

It appears that one Phillip Gregory, who was born in Cheshire, England, in 1751 was a member of the British military and arrived in Quebec in 1773. He was eventually given some land near St. Catherine’s, Ontario, on which he settled down with his wife Margaret. The Gregory clan remained in southern Ontario, mostly as farmers, for the next 5 generations (Phillip -> Barnabus -> William -> Richard -> Joseph -> Clarence). My grandfather, Clarence (who went by “Pete”), began as a farmer but later worked in a factory in St. Marys, Ontario, where my father grew up.

In short, I am a 7th-generation Canadian in a family that has lived here since nearly 100 years before Canada was Canada. That’s good enough for me.

Bombardier beetles.

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Last night, after Larry Moran‘s interesting and informative talk, a student (not one who had taken my evolution course, I should say) brought up irreducible complexity. Not flagella, as I was expecting, but the bombardier beetle. How could this evolve when you need all the chemicals and they explode when you combine them? You know the story. Larry responded graciously (note: that is not a typo) and explained about shifts in function and used an example from his own area of study, biochemistry.

For those who think that the bombardier beetle is a problem to explain, here are some resources you may find helpful.

Bombardier beetles and the argument from design (Talk.Origins)


The great headline mismatch.

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So, on ScienceDaily there is a story (adapted from a press release by the University of Bath) about an interesting study in PNAS regarding patterns of macroevolution in crustaceans. In particular, it seems that there is often an increase in morphological complexity over time in different lineages within this group, which makes sense because complexity in this case relates to specialization of limbs and so on. (Lineages necessarily begin simple, with repeated segments that are the same, then some limbs evolve to become specialized for feeding, walking, swimming, and so on — it’s probably a driven trend in that it is adaptive to have specialized features, but one also can really only increase in complexity when the beginning is around some minimum level of simplicity, as one of the authors notes in the story).

Here is the abstract:

The prospect of finding macroevolutionary trends and rules in the history of life is tremendously appealing, but very few pervasive trends have been found. Here, we demonstrate a parallel increase in the morphological complexity of most of the deep lineages within a major clade. We focus on the Crustacea, measuring the morphological differentiation of limbs. First, we show a clear trend of increasing complexity among 66 free-living, ordinal-level taxa from the Phanerozoic fossil record. We next demonstrate that this trend is pervasive, occurring in 10 or 11 of 12 matched-pair comparisons (across five morphological diversity indices) between extinct Paleozoic and related Recent taxa. This clearly differentiates the pattern from the effects of lineage sorting. Furthermore, newly appearing taxa tend to have had more types of limbs and a higher degree of limb differentiation than the contemporaneous average, whereas those going extinct showed higher-than-average limb redundancy. Patterns of contemporary species diversity partially reflect the paleontological trend. These results provide a rare demonstration of a large-scale and probably driven trend occurring across multiple independent lineages and influencing both the form and number of species through deep time and in the present day.

The news release itself is interesting, and includes an excellent quote from the study’s lead author Sarah Adamowicz:

Our results apply to a group of animals with bodies made of repeated units. We must not forget that bacteria – very simple organisms – are among the most successful living things. Therefore, the trend towards complexity is compelling but does not describe the history of all life.

And yet, the headline of the piece is…

First ‘Rule’ Of Evolution Suggests That Life Is Destined To Become More Complex

Do the people who determine headlines not even read the stories?


Who pays for DNA barcoding?

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John Wilkins, who apparently didn’t learn the first time, has repeated his ill-informed claim that “Barcoding syphons off money and resources from real systematics.” I have said it before: DNA barcoding has brought in money from sources that never supported systematics (a prime example being Genome Canada). I co-authored some of these grants. I have been in the field with some of the taxonomists who are supported by them. I am around a five minute walk from the world’s major barcoding facility, which hosts systematist postdocs and collaborators and processes material from a large network of taxonomists. Wilkins, on the other hand, gets all his information from critics of barcoding.


Orgel’s Second Rule and toxicity — again.

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Some time ago, I posted about a story involving frogs and snakes in which an author of the study was quoted as saying that:

In evolutionary terms, the snake’s strategy of ‘bite, release, and wait’ is unbeatable by the frogs. Although prey often evolve ways of overcoming predator tactics, the frogs can’t do so in this case – because the snake’s strategy only becomes effective after the frog has died. Natural selection ceases to operate on an individual after that individual’s death, so frogs will probably never evolve toxins that last longer in response to the snake’s tactic. Thus, this waiting strategy is likely to be stable and unbeatable over evolutionary time.

I mentioned Orgel’s Second Rule — “evolution is cleverer than you are” — and provided a list of seven possible scenarios which would refute the claim that a response could never evolve in frogs.

Today in ScienceDaily there is another example of a toxicologist who does not get Orgel’s Second Rule. In this case, it is toxic newts and garter snakes. The newts are extremely poisonous in some regions, which is a result of an arms race between them and snakes that feed on them. While there is often an escalating interaction with no net gain for either side in an arms race, in some locales a particular mutation has made snakes completely resistant. Here is what the story notes:

In most locations, the snakes’ level of resistance closely matched newt toxicity. In such cases, the poison temporarily slows the snakes down but isn’t enough to kill them. This supports “arms race” theories explaining how toxicity and resistance co-evolve.

But in some areas where newt toxicity was relatively high, the poison had no measurable affect on snake mobility.

The team found that resistant snakes had a single genetic mutation on TTX receptor sites on their neural and muscle cells, which prevented the toxin from binding. It made snakes with this mutation “untouchable”.

“It is pretty much biologically impossible for the newts to ever catch up,” Hanifin says.

Compare this with what is stated in the abstract of the actual peer-reviewed paper:

This coadaptation proceeds until the evolution of extreme phenotypes by predators, through genes of large effect, allows snakes to, at least temporarily, escape the arms race.

I wonder if a reviewer, knowing Orgel’s Second Rule, made them insert the “at least temporarily” caveat.