(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?
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.