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Making Evolution Chancy

By: , Posted on: January 14, 2022

Figure 1: Charles Darwin, as photographed in 1869 by famed photographer Julia Margaret Cameron. Public domain.

When we look at Darwin’s Origin of Species, we find, of course, many remarkable features. We also find much that is familiar. Many of Darwin’s arguments read as though they could be drawn from any work of contemporary life science – as he canvasses his understanding (limited though it may have been in 1859) of the fossil record, biogeography, species distribution, and so forth, the picture of evolution that we find is less foreign than we might expect for a work now more than one hundred and fifty years old.

This makes all the more surprising those instances where Darwin’s work strikes us as particularly unusual. Most famous among these is doubtless the fact that Darwin wrote entirely in ignorance of the underlying mechanisms of heredity and genetics, not to mention future developments in molecular biology and the structure of DNA. But equally striking is the fact that there is not a single mathematical equation or graph to be found in the Origin of Species. While Darwin often discussed a rather vague concept of subjective probability – the idea that we must reason probabilistically as a way to quantify our ignorance of a complicated system – and had some notion of the law of large numbers, arguing as he did that larger populations, for instance, would be more likely to play host to rare adaptations, his theory of evolution was developed in the absence of a robust notion of chance or probability and the tools of population-level statistics that we consider to be indispensable today.

Over the century from Darwin’s first theoretical notebooks to the early works of the Modern Synthesis (such as R. A. Fisher’s Genetical Theory of Natural Selection in 1930, which featured a thoroughly modern form of population genetics), evolutionary theory radically transformed. A theory about the appearance of adaptations in individual organisms, worked out without the aid of mathematics and an unclear view of the probabilistic nature of natural selection, became a statistical theory of natural selection’s action on populations, which in turn needed to be harmonized with Mendelian genetics (famously “rediscovered” in 1900).

Figure 2: W. F. R. Weldon’s graph of an asymmetric morphological character in specimens of Carcinus maenas, 1893. Public domain.

The Rise of Chance in Evolutionary Theory tells the story of this remarkable transformation. Historians of biology have long recognized a number of key moments in this history. Of course, the rediscovery of Mendel looms large – not only because it would be so important to the future development of genetics, but also because it caused, it is claimed, a huge waste of effort, as biologists of the day set aside natural selection as an explanation for adaptation and turned instead to mutation-driven theories of large-scale, rapid evolutionary change. This was pitted against what is often presented as the largely hopeless “biometrical school,” whose leading figures included Francis Galton, Karl Pearson, and W. F. R. Weldon. As the classic story goes, while the biometrical school got a few things right – their commitments to statistics and the gradual action of natural selection foremost among them – their failure to see the utility of Mendelism locked the field of evolutionary in a fruitless battle that spanned some forty years.

Something of this story is certainly correct. The personal feuds between Pearson, Weldon, and early geneticists like William Bateson (who coined the term “genetics”) and R. C. Punnett are the stuff of legend – Weldon once called their disputes “paltry and dirty beyond measure,” and a critique that he once received, that mathematical approaches to biology are nothing more than a way to say, “with a pompous parade of arithmetic,” something that we all already knew, is the source of the book’s subtitle.

But this classic history obscures more than it illuminates. On the one hand, the work of these biologists was much more complicated than “Mendelian vs. biometrician” would lead us to believe. The intellectual story of the developments in conceptual frameworks, methodological tools, and even underlying philosophical commitments proves to be one of the most complex and exciting periods of innovation in the history of evolutionary theory. And on the other hand, when we analyze that story in detail, we find that a tale of continuity is better supported than one of revolution. Efforts to develop a statistical, populational theory of natural selection were present for decades, even if various technical and conceptual hurdles meant that they would not really come to fruition until the first major works of the Modern Synthesis in the 1930s and 1940s.

Figure 3: A photograph of peas, included in a paper by Weldon from 1902, showing a smooth gradation between characters, like “green,” “yellow,” “smooth,” and “wrinkled” that had appeared as distinct in Mendel’s work. Public domain.

The book thus unearths the theoretical and conceptual developments from Darwin to Fisher that, in the end, paved the way for evolutionary theory as we know it today. The main characters of the story are Charles Darwin, Francis Galton, W. F. R. Weldon, Karl Pearson, and R. A. Fisher. But a careful retelling of their impact involves much more – from the history of statistics, present (though presented in a non-technical manner) in figures like Adophe Quetelet or G. Udny Yule, to a whole tradition of textbook authors and so-called “minor figures” over the first decade of the twentieth century, who inspired the search for a way to integrate statistical and genetic studies of evolution.

In that sense, I have hoped to tell a story compelling for practicing life scientists, philosophers and historians of science, and anyone interested in how evolutionary theory came to be so fully integrated with notions of chance, probability, and statistics. These questions not only illuminate the past and present character of evolutionary theory itself, but even our understanding of our own place in an evolved and evolving world.


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Evolutionary biology