The question of the tempo of evolution cuts right to the heart of evolutionary theory. Emeritus professor in evolutionary biology (and a list of other disciplines) Philip D. Gingerich here takes an empirical stab at quantifying how fast evolution happens, something which has not been done very often. The resulting Rates of Evolution is a technical monograph for an academic audience that contains thought-provoking ideas.
The disagreement over how quickly evolution proceeds goes right back to the days of Darwin. His idea of slow and gradual evolution has its roots in geology, specifically in uniformitarianism. Darwin’s mentor, the famous geologist Charles Lyell, was a fierce proponent of this idea, which sees geological change as slow and gradual, to the exclusion of cataclysmic explanations. Lyell heavily influenced Darwin’s thinking . One alternative to this notion of gradualism is that of punctuated equilibrium: periods of stasis punctuated by short bursts of rapid evolution. Niles Eldredge and Stephen Jay Gould coined this term in 1972, though, interestingly, the concept was not new – Gingerich here briefly documents its long history.
Now, theorising is one thing, but actually quantifying rates of evolution is something else altogether. Gingerich breaks it down into three parts: an introduction to his methods, an analysis of three groups of datasets, and an extended discussion and interpretation.
The first six chapters are vital to understanding the rest of the book. Because how, indeed, can you compare long-term rates of evolution taken from the fossil record with short-term changes observed in field studies or in the lab? How do you even compare studies that focus on radically different traits? One study might look at evolutionary rates by measuring beak size of birds over many generations. The next might look at changes in the number of fin rays and dorsal spines in fossil fish.
“Niles Eldredge and Stephen Jay Gould coined the term [punctuated equilibrium] in 1972 […] though, interestingly, the concept was not new.”
Without going into the details, Gingerich explains that it is not the trait values that matter, but the rate with which these change. Specifically, the rate of change per generation. And a rate not expressed in millimetres or number of fin rays, but in standard deviations (assuming, of course, a normal distribution of trait values). Other fun concepts introduced are random walks and Brownian diffusion, and how you can decide whether observed patterns of change in time represent stabilizing, random, or directional selection.
Yes, dear reader, prepare yourself for plenty of formulas and diagrams. I will be frank that though I read this section carefully, some of this went a bit over my head. Especially once authors start juggling with equations (“Look, I can express this term like that, and then simplify these equations by dividing by this other term, and if we now log-transform…”) my response is usually one of “I am sorry, I am just going to have to trust you on this one, because you are speaking a language that I do not comprehend”. So, if there are holes to be picked in his logic here, I am afraid I will not be the one to do it. The point is that I do understand what he is working towards, and the resulting log-log plots of change against time are elegant.
“The picture that emerges is clear, argues Gingerich: evolution is fast on a generational time scale, but once you start calculating rates over many generations most of this averages out.”
The bulk of the book is three long chapters sprawling over some 150 pages that analyse rates of phenotypic change in experimental studies, field studies, and in the fossil record. Gingerich has taken a large number of datasets from older and newer studies (including some classics) and reanalysed the data to determine rates of change. The presentation here is somewhat repetitive, especially in explanations of conventions used in figures. The upshot is that each of these studies can be consulted individually, out of order. For those who want to inspect the raw data or have a stab at analysing other studies, the author has provided datasets and R scripts online.
This deep immersion pays off big time once you make it to the discussion in the last six chapters. The results are not what you might intuitively expect! Rates of change in experimental and field studies are high, while those in the fossil record are often several orders of magnitude lower. Gingerich argues this is a sampling artefact: this dichotomy between fast and slow rates exists because we sample dichotomously. Fossil studies provide snapshots in time that hide what happened in between on the resolution of single generations, while field and experimental studies rarely continue long enough. By combining the two you can start to fill in the void in between and discern that there is a continuum. Other chapters here deal with a critical rereading of Eldredge and Gould’s ideas, the question of whether relatedness between taxa can throw a spanner in the works of quantitative comparisons, and the applications of Gingerich’s findings to genetic modelling and the idea of adaptive radiations.
“[Evolution] is only goal-directed in the sense of chasing shifting peaks in a fitness landscape […] Is this a case of “the more things change, the more they stay the same”?”
The picture that emerges is clear, argues Gingerich: evolution is fast on a generational time scale, but once you start calculating rates over many generations most of this averages out. One reason he shortly touches on is that of limits. As mentioned in The Equations of Life, “physics is life’s silent commander”. There are limits to e.g. an animal’s size, and evolution acts rapidly enough to thoroughly explore that envelope of possibilities over the course of deep time.
The other reason is one that I feel Gingerich does not really mention, so allow me some speculation of my own here. When you say “rates of evolution”, I say “compared to what?” Surely, the answer must be “compared to rates of environmental change”. We know that evolution as a process has no ultimate goal – it is only goal-directed in the sense of chasing shifting peaks in a fitness landscape. Environments constantly fluctuate and the resulting back-and-forthing as organisms adapt means rapid change in the short term, but, on average, not much movement in any direction on the long term. Is this a case of “the more things change, the more they stay the same”?
Rates of Evolution is a technical book squarely aimed at advanced students and researchers. Though the subject matter is complex in places, I found Gingerich’s writing and argumentation clear and his discussion and interpretation thought-provoking. For any evolutionary biologist or palaeontologist interested in the question of whether evolution is fast or not and why it matters (and, let’s be honest, that should be all of us) this book is obligatory reading. It is easy to see how this monograph will be a benchmark for years to come.
Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.
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