[Trends in Evolutionary Biology 2009; 1:e1] [page 1] Charles Darwin as a theoretical biologist in the mechanistic tradition David Penny Allan Wilson Center for Molecular Ecology and Evolution, Massey University, Palmerston North, New Zealand Abstract Charles Darwin has had more impact on biological sciences, and society generally, than any other 19 th century biologist. Yet his modus operandi as a scientist is poorly known by evo- lutionists, and often seriously misinterpreted. Two important aspects of his reasoning dis- cussed here are his hypothetico-deductive approach and his search for mechanisms to explain past events. A wide range of state- ments from his autobiography and letters show that he worked explicitly in the hypothetico- deductive model. The extracts include strong statements that theories were essential even to know what data to collect; to hold theories only as hypotheses; the necessity to search for data that contradict a cherished theory. He also built on the very mechanistic geological tradi- tion of James Hutton and Charles Lyell, and thus brought into historical biology the search for mechanisms that could be studied in the present to explain events in the past. Taken together, the statements show an excellent sci- entist working effectively on conceptual issues, whilst searching for mechanisms that could be studied in the present and that would have operated in the past. In retrospect, our understanding has been hampered by forcing overly-simplistic binary choices, such as uni- formitarianism and catastrophism. It is impor- tant, especially in teaching and interactions with the public, that Darwin’s mode of working is better known, and we need to be more proac- tive in getting across the message that evolu- tion is good testable science. As an evolutionary biologist, I think the work of Charles Darwin holds a strong mes- sage for all those interested in how good sci- ence should be done and in demonstrating the testability of evolutionary theory. Innovative science and testability of hypotheses are both scientifically important questions but are also helpful for both education and in involving the public in discussions about evolution. In many areas of science, major developments have been followed by many subsidiary develop- ments and improvements in understanding mechanisms, but in evolution there was little major development in theory from 1859 until the new synthesis 1 in the 1930s onwards. 2 I illustrate this by two extracts, the first from 1921 and the second from 1929. “For the moment, at all events, the Darwinian period has passed. … All again is in the melting pot. By now, in fact, a new gen- eration has grown up that knows not Darwin”. 3 “Whilst the fact of evolution is accepted by every biologist the mode in which it has occurred and the mechanism by which it has been brought about are still disputable. … and Darwin, rest on a most insecure basis; the validity of the assumptions on which they rest has seldom been examined and they do not interest most of the younger zoologists” 4 (emphases added). All this has now changed and Darwin’s rep- utation about mechanisms of evolution is re- established among evolutionists. Many areas of the study of evolution are now quantitative and go well beyond what was possible in the mid-nineteenth century. In the 1930s, popula- tion genetics, from the work of the triumvirate of Fisher, Haldane and Wright, became the first area to become quantitative 1 but other areas have followed. Our own area of interest, the study of evolutionary trees, 5 is a more recent example. Consequently it is natural to ask, was there something about Charles Darwin’s thinking/reasoning/knowledge that led him in several areas to be so innovative in his think- ing about the mechanisms of evolutionary biol- ogy? Can we use such information to be more proactive in teaching about evolution? Perhaps the complexity of Darwin’s overall theory 6 was one reason why, qualitatively, his thinking was not surpassed until the 1930s. We have found it useful for analyzing the many components of Darwin’s theory to simplify them to three major aspects: 7 - the microevolutionary processes that can be studied in the present (Figure 1A, summa- rized as ‘natural selection’); - macroevolution in referring to Darwin’s the- ory of descent with modification (Figure 1B); - the Darwinian hypothesis (Figure 1C) that the processes of microevolution that can be studied in the present are sufficient to fully account for macroevolution in Part B. This latter part still needs to be fully addressed by evolutionists. The approach of explaining past events in terms of mechanisms that can be studied in the present is actualism 8 and it makes no assumptions about constant rates, in the way that uniformitarianism is often (unfortunate- ly) interpreted (see below). There are other ways to analyze Darwin’s theory; Mayr 9 consid- ered it as five theories. What is important here is that the overall theory, together with its con- sequences, is relatively complex, even though the individual ideas are both relatively simple and testable. 6 Right back to at least Ghiselin, 10 historians of biology appear to have had a better under- standing of Darwin’s approach to science than do many ordinary biologists. Nevertheless, their work appears (unfortunately) to have had insufficient impact on biologists, and histori- ans do not necessarily emphasize all the issues important to researchers (such as Darwin’s emphasis on mechanisms, discussed later). We need to present the conclusions about Darwinian reasoning in a way relevant to evolutionists, and it will help their interpret- ing evolution to others. The two main themes considered here are Darwin’s explicit use of hypotheses for testing (conjectures and falsifi- cation, or hypothetico-deductive reasoning) and his geological background which led him to search for present-day mechanisms that could explain events in the past. This latter approach comes quite directly from the similar mechanistic reasoning of Charles Lyell in the 1830s, and at the time it was recognized that Lyell had further developed the ideas of James Hutton of the 1790s. Studying the modus operandi of innovative and successful scien- tists should help all scientists. In addition, I emphasize that it is misleading to try to force diverse ideas into overly-simplistic binary choices, such as uniformitarianism versus cat- astrophism. Taken together, it is hoped that the analysis presented here will help evolu- tionists in their own work and in presenting evolution to others. Perhaps it is time that evo- lutionists took more initiative in asserting that evolution is central to all of biology and to our understanding of ourselves and our societies. Evolutionists could be more proactive, rather than waiting to respond to challenges from others. The increase in all types of biological data, including genomics, gives a new begin- ning to many evolutionary studies. Trends in Evolutionary Biology 2009; volume 1:e1 Correspondence: David Penny, Allan Wilson Center for Molecular Ecology and Evolution, Massey University, Palmerston North, New Zealand. E-mail: d.penny@massey.ac.nz Key words: mechanisms of evolution, conjectures and falsification, geology, philosophy of science, theoretical biology, role of hypotheses. Received for publication: 11 July 2009. Revision received: 26 August 2009. Accepted for publication: 26 August 2009. This work is licensed under a Creative Commons Attribution 3.0 License (by-nc 3.0). ©Copyright D. Penny 2009 Licensee PAGEPress, Italy Trends in Evolutionary Biology 2009; 1:e1 doi:10.4081/eb.2009.e1 Non-commercial use only