Macroevolution Airlines

(This post has been mostly written for a long time but I never got round to publishing it. It’s kind of my darling baby, and I never felt quite ready to let it out into the world. Well, every parent has to let go at some point…)

In the creation vs. evolution section of Christian Forums, “macroevolution” is a common topic of name-calling discussion. At some point in what seems like every other thread, a creationist demands “proof” of macroevolution. The common reaction from the evolution side is that the creationist doesn’t understand evolution, and macroevolution is just lots of microevolution, and here is a list of observed speciation events anyway. While the first point is true more often than not, I have been increasingly uncomfortable with the second lately. To my mind, and I think to anyone interested in palaeontology and/or evo-devo, it’s not at all obvious that macroevolution must be fundamentally similar to the everyday adaptations and driftings we commonly observe real-time.

(Image from the UCMP Understanding Evolution site)

What exactly is macroevolution?

Before I continue my musings, I must first clarify what I mean by micro- and macroevolution. I see two interpretations in use in the scientific community, and I don’t think they are entirely equivalent. The “rigorous” interpretation defines microevolution as anything that happens this side of speciation. Populations adapting to short-term environmental change, individuals and their genes migrating back and forth between neighbouring populations, ordinary everyday genetic drift, etc. are microevolutionary phenomena. Macroevolution starts with the formation of new species. The “wishy-washy” interpretation defines macroevolution as “evolution on the large scale”, or “big change”. This is the one I think many palaeontologists would prefer, and many students of evo-devo as well. This is also the one most creationists seem to have in mind. Most – if not all – of the examples in the well-worn speciation lists I’m guilty of pulling out myself are only macroevolution in the first sense. This is something people often seem unaware of: speciation and big change do not go hand in hand.

The definition I prefer (and I changed my mind on this fairly recently) is the second, because despite its vagueness, it gives us a word for something vitally important, all the things that are (usually) bigger than the evolutionary processes we can readily observe on human timescales. How did something resembling a sausage on legs give rise to the mind-boggling diversity of arthropods? How did our own ancestors end up with legs instead of fins? Why did dinosaurs grow into giants and rule the land while the ancestors of mammals retreated to the shadows? This is what macroevolution means to me. As far as I’m concerned, the population geneticists’ kind of macroevolution already has a perfectly good word for it, and that word is speciation.

The question: what is the question?

With that in mind, is macroevolution something different? This is actually at least two questions. One can ask whether the external forces that set out the path of evolution act in the same way on all scales. Did the environment always exert the same kinds of pressures on living things? The answer to this is probably no – from the appearance of oxygen in the atmosphere to the arrival of predators in animal communities, both non-living and living factors have changed the rules of ecosystems many times in earth history. Do the same sorts of pressures that determine the fate of single populations also affect whole lineages? Does selection operate on more than one level? Do the same traits that natural selection favours in ordinary times also help you in extraordinary times? (Another “no”, if David Jablonski can be believed.)

Alternatively, one can also ask whether small and large changes in the properties of organisms are governed by different intrinsic rules. Do, say, new body parts originate through the same kinds of mutations as new hair colours? Are major changes and small adjustments associated with different developmental stages (Arthur, 2008)? Did the nature of variation itself change over evolutionary time (Gould, 1989; Erwin, 2011)? That last one especially intrigues me, and it may yet return in future meanderings. (It’ll return in force if I ever muster the fortitude to discuss the Cambrian explosion ;))

The way to America

In the aforementioned creation vs. evolution debates, physical distance is a commonly used analogy for evolutionary distance. If you believe in centimetres, the argument goes, how can you not believe in kilometres? If you can walk to the kitchen, why can’t you walk a mile?

I think this analogy is worth examining a little further, because it turns out to be great parallel to the micro vs. macro issue. It is true that anyone who can walk can walk a mile. It may take long and it may tire you out, depending on your physique, but it is possible. However, it isn’t very hard to think of destinations that are simply impossible to reach by walking. I live in Europe. Barring ice ages and Bering land bridges, no amount of steps would take me to America. It is still possible for me to go there, but I have to take a flight or perhaps hop on a ship. Is macroevolution like a mile, or is it more like the distance between Europe and the New World? Does a velvet worm-like creature evolve into an arthropod by lots of tiny steps of its chubby legs, or does it take a ride with Macroevolution Airlines?

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References:

Arthur W (2008) Conflicting hypotheses on the nature of mega-evolution. In: Minelli A & Fusco G (eds.) Evolving Pathways: Key Themes in Evolutionary Developmental Biology. Cambridge University Press, pp. 50-61

Erwin DH (2011) Evolutionary uniformitarianism. Developmental Biology 357:27-34

Gould SJ (1989) Wonderful Life: The Burgess Shale and the Nature of History. W. W. Norton & Co.

Distinguished company

It hasn’t been long since my friend published her awesome ghost Hox paper, and it has already earned her a great honour. It seems a creationist found it and tried to wrestle it into supporting creationism. (Judging by the comments at Marmotism, he failed big time, in most of the usual creationist ways.)

And with that, young Olivia enters the distinguished company of evolutionary biologists whose works got misrepresented by creationists. Dunno about everyone else, but I’d be pretty proud to belong with the likes of Stephen Jay Gould, Jerry Coyne and Doug Erwin even in such a crooked way…(Sorry, I’ll get off gloating and back to science any time now.)

What use is (not even) half a leg?

 

(I’m even further behind on things than usual, so this is not that “hot” off the press, but the walking lungfish can’t not be posted on.)

The evolution of new traits serving new functions is always a bit of a chicken and egg problem. Why would you need wings if you don’t fly, and how could you start flying without them? Why would you need legs if you don’t walk, and how would you walk without legs?

Often, as in the case of wings, the most likely answer is that the trait originally had a different function that didn’t necessitate a “perfect” version of it. Wings that are no good for flying could be anything from egg-warmers/shades through mate attraction devices to balancing organs for prey-wrestling predatory dinosaurs (latter idea from Fowler et al., 2011, which by now has probably gone as viral as scientific papers can).

With legs, though, it seems that the chicken really did come first. We’ve known for a long time that coelacanths (which are somewhat distantly related to vertebrates with legs) sometimes move their pectoral and pelvic fins in an alternating rhythm that resembles walking. (IIRC you can find a fair few YouTube videos in which they are filmed doing that.) Nonetheless, coelacanths use this movement for swimming. They don’t actually get down and plod along the bottom.

Lungfish, however, do. King et al. (2011) videoed them doing it.

Just to be clear, the animal in question is the West African lungfish (Protopterus annectens). Unlike the respectable paddles of the Australian species, its spindly paired appendages barely even deserve to be called fins, let alone legs. (Drawing below from King et al., 2011)

Yet this creature uses its pelvic fins to propel itself along the bottom in a variety of ways. It can walk with alternating “steps”, it can bound by moving both fins at once, and sometimes it just ambles along in a slightly irregular way (videos here). If there’s no traction on the bottom of the tank, it slips and can’t get anywhere, which indicates that it does indeed propel itself by pushing against the bottom with its hind fins. And sometimes, when the fins push off, you can see part of the body come clear off the ground.

(Interestingly, the lungfish walks and bounds only with its hind fins. Meanwhile, the pectorals flail around doing other things, but they don’t engage with the floor. The diagram above gives a clue why: the animal has huge, air-filled lungs – the grey blob – that help its front half float. It doesn’t need its forefins to stroll around.)

Given how un-leglike the fins of African lungfish are, it is obvious that walking underwater doesn’t require anything as sophisticated as ankles or toes or, heck, even proper fins. Just about any ancient lobe-finned fish we know could have been capable of it. Could this be how our ancestors took their first unknowing steps towards land? Were they bottom-dwelling fish that patrolled their territories in a stately fin-walk? Did increasingly leg-like fins just help them do that better rather than breaking new ground? As the authors remind us, we already know that many of the earliest tetrapods – creatures with true legs – lived in water. If less tetrapod-like creatures could walk, then the picture fits quite nicely together.

And speaking of chickens and eggs, once again nature proves how much human incredulity is worth. Just because you don’t know what to do with half a wing, just because you don’t think X is possible without Y, doesn’t mean solutions don’t exist. Studying nature is a life-long lesson in humility in that way.

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References:

Fowler DW et al. (2011) The predatory ecology of Deinonychus and the origin of flapping in birds. PLoS ONE 6:e28964

King HM et al. (2011) Behavioral evidence for the evolution of walking and bounding before terrestriality in sacropterygian fishes. PNAS 108:21146-21151

“That innocent little word ‘to'”

It seems the editor-in-chief of the review journal BioEssays shares my concerns about using teleological language when discussing evolution. He calls it anthropomorphic rather than teleological, which is probably a better way of putting it – likely, the reason we think of evolution in teleological terms in the first place is because we do things for purposes, and we are biased to think of everything as if it had a human mind. The word “anthropomorphism”, I think, gets to the root of the problem.

In his short opinion piece “We need a new language for evolution… everywhere” (BioEssays 33:237), Andrew Moore discusses how language (including “that innocent little word”) can subtly lure scientist and layperson alike into this dangerous trap.

I especially like this bit:

Another concept that arises from the ‘anthropomorphisation’ of evolution is the ‘problem’: in other words, an organism or system evolves towards what we, retrospectively, identify as a barrier, or ‘problem’ that had to be ‘solved’, and we wonder how it was overcome. Nature doesn’t solve anything.

(It’s a point I’ve never quite articulated to myself, so these remarks were something of a lightbulb moment for me.)

Moore doesn’t just tear down the old language. He provides a table listing some common turns of phrase that give entirely the wrong impression – and offers alternatives that, for the most part, aren’t clumsy at all. (I’ve got to say, though, that calling the alternatives “new-speak” is a tad ironic in an article about using the wrong language, IMO.) Interestingly, one of his “replacements” still sounds teleological/anthropomorphic to me. Instead of “innovation of evolution”, he suggests using “product”, and my first thought upon hearing that word tends to involve factories. Just goes to show how difficult it is to get rid of deeply ingrained thought patterns.

Still, kudos to Moore. For bringing the problem to the fore, and for suggesting specific ways of solving it.

The folly of hindsight

Recently, I’ve been re-reading Life on a Young Planet. As I said before, it’s an excellent book. It is beautifully written, cleverly structured, and the author is obviously knowledgeable about the subject (which, sadly, isn’t always true in popular science). Most importantly, it emphasises the process of science, as opposed to the actual knowledge gained through that process. “How do we know what we know?” is a question at least as important to Andrew Knoll as “What do we know?” As he so eloquently puts it, “[t]extbooks may portray science as a codification of facts, but it is really a disciplined way of asking about the unknown.” This is an attitude I share with him, and probably a big part of the reason the book has such a special place in my heart.

So, I was surprised to discover on this re-read that Knoll falls into one of the most common traps of talking about evolution: teleological thinking. In Chapter 11, “Cambrian Redux”, he writes that “[f]orty million years after the Cambrian began, evolutionary way stations still played a major role in the ecology of marine environments.” He is discussing the Cambrian explosion, of course, and here he is talking about stem groups of living phyla living alongside the crown groups [1]. I don’t think he means to convey a sense of goal-orientation, but the wording does exactly that. It sounds as if, say, Anomalocaris was just something evolution had to pass through to get to arthropods, not a successful animal in its own right. It suggests that the eventual supplanting of these now-extinct lineages was meant to happen.

Richard Dawkins called this “the conceit of hindsight” and complained about it at length in the introduction to his (also really good) book The Ancestor’s Tale. Dawkins characterises such thinking as “seeing the past as aimed at our own time, as though the characters in history’s play had nothing better to do with their time than foreshadow us.” (In this particular case, he’s talking about ordinary history, as a prelude to introducing the same problem in evolutionary history.) It’s a very common way of thinking about evolution (just look at any of the traditionalmarch of progressimages), and it’s also totally wrong.

If you’ve been in prolonged contact with creationists, you’ve almost certainly encountered conspicuous examples of this common misconception. Types of questions I’ve personally seen include “what use is half a wing/[insert transitional feature here]?”, “why didn’t all X evolve into Y?”, and “how did X know they were evolving into Y?” At the heart of each lurks the idea that evolution works towards goals. That it doesn’t seems to be one of the most difficult aspects of evolutionary theory to grasp, and it’s especially hard to escape when we are looking at the past.

Simply put, evolution has no foresight. Rather than working towards something, the process always reacts to something. Rather than looking ahead, it constantly lives in the present, though it’s often saddled with the baggage of the past. The kinds of things that cause mutation (such as replication errors, radiation and chemical damage) have random effects [2]. Moreover, the processes that sort among mutations, such as natural selection, are similarly blind. Because the mechanisms of evolution are not thinking entities, the only traits that get passed on are traits that help their owners reproduce in the here and now. Any long-term trend is the outcome of repeated rounds of selection on the same traits. Evolution has no goal in the same way a snowflake doesn’t aim for your nose, though in retrospect you can perhaps reconstruct the path it took to get there.

That’s the problem with history: we are looking back on processes whose outcomes we already know. It’s so tempting to view the preceding events as mere stages in a journey aimed at those outcomes. After all, we humans work with goals in mind all the time (ironically, nowadays we might use evolutionary principles to attain those goals!). Unfortunately, viewing evolution in this way can lose sight of the process by focusing on the endpoint – and then people start asking about half wings.

It’s important to remember that the ancestor of the wing was not “half a wing”. It was just a modified arm that had some advantage over its ancestor, e.g. large feathers to help a dinosaur keep her eggs warm, or (closer to “wingness”) glide from tree to tree. These animals weren’t half-functional fliers, they were fully functional at whatever they were doing. If an alien scientist looked around in a Middle Jurassic forest, it might have marvelled at the exquisite gliding adaptations of small dinosaurs much like Microraptor [3], but it surely wouldn’t have focused on how bad they were at flying.

(Also, always remember that when you are the only one who can do something, by definition you’re the best at it!)

I wish we could just drop the teleological language altogether. It’s surprisingly difficult even when you actively try, though. It could be something about the way language works (at least the two I know well). Somehow, it seems much easier to say things like “X evolved to do Y” in them than to give a more accurate description of the evolutionary process. I’m sure that says something profound about human minds…

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[1] In systematic jargon, a crown group is the last common ancestor of all living members of a group, and all of its descendants (including extinct ones). The corresponding stem group (stem groups are always relative to a crown) includes anything extinct that’s more closely related to the crown group in question than to any other living lineage. For example, all non-avian dinosaurs were stem birds.

[2] We have to be precise about the meaning of “random” here. Some mutagens cause very specific mutations. “Random” refers to their fitness effects, not the chemical changes that happen or even the places where they happen (though the latter is largely random, except for trivial constraints). The same mutation in different parts of the genome can be beneficial, harmful or have no effect at all, and conversely, the same is true for different mutations at the same spot – and all of this is uncontrollable. If you keep your study organisms in a hot environment, they won’t suddenly start producing more mutations that make them heat-resistant. That’s the main thing we mean when we say mutations are random.

[3] Microraptor itself is Early Cretaceous – birds were already around when these guys inhabited the forests of China. The first part of the Jurassic – i.e. the time between early dinosaurs and Archaeopteryx – doesn’t have a great record of dinosaur fossils, so most of what we know of the origin of birds comes from relatives of birds that persisted alongside birds later on. However, a few very bird-like fossils are contemporaneous with, or older than, Archaeopteryx. Like Microraptor, some of these creatures have long leg feathers (unlike Microraptor‘s, theirs aren’t very aerodynamic) , so that may be something ancestral for the “birdy” lineage.