by Greg Mayer
Our second installment of Teaching Evolution is a paper by A.W.F. Edwards on the history and logical justification of methods of phylogenetic inference. In teaching evolution, the idea of the history of life is very important. Most students intuitively see the closer genealogical relationship between, say, a man and an ape than a dog, or among any of those as compared to a salmon. But the precise logic of doing so, especially when the degree of genealogical propinquity is less evident, is not easy to convey. I now teach this subject using a likelihood-based logic of justification, and Edwards was a pioneer in this area. Although we are accustomed now to think and speak of the phylogenetic tree as a “tree of life”, Darwin at first referred to it in his notebook as “the coral of life”, which is a more apt analogy, in that only the tips are alive, while the bases of the branches are dead.
For the first installment of what Jerry has called our “mini-MOOC” on evolution– an extract from the Origin by Darwin– I left out the title I gave to that week’s topic in my course: “Unity of type and adaptation”. I’ve now revised the title of that installment to include this in its title. Unity of type and adaptation were the two great classes of organic phenomena that Darwin sought to explain with his theory of descent with modification; with the chief means of modification– natural selection— accounting for the fit of organic beings to their conditions of existence, i.e. their adaptations. Thus Darwin proposed to solve these two great unsolved problems of biology in the first half of the 19th century with a single, unified explanatory theory.
Anthony William Fairbank Edwards (b. 1935) is a British statistician, geneticist, and evolutionary biologist. He is a Life Fellow of Gonville and Caius College and Emeritus Professor of Biometry at the University of Cambridge. An undergraduate student of R. A. Fisher, he has written several books and numerous scientific papers. He is best known for his pioneering work, with L. L. Cavalli-Sforza, on quantitative methods of phylogenetic analysis, and for strongly advocating Fisher’s concept of likelihood as the proper basis for statistical and scientific inference. He has also written extensively on the history of genetics and statistics, including an analysis of whether Mendel’s results were “too good” (they were). His most influential book is Likelihood (expanded edition, 1992), in which he argues for the centrality and sufficiency of likelihood as an inferential principle, often using genetic examples to illustrate his argument.
Study Questions:
1. What was Edwards’ purpose in writing this paper?
2. What is Ockham’s razor? What is the “Darwin principle”? What is the relationship between them?
3. What are some of the various ways in which a method of minimum evolution may be used to estimate phylogeny? What, according to Edwards, is the justification for any of these methods?
[For further discussion of the history of phylogenetic methods, see chapter 10, “A digression on history and philosophy”, in Joe Felsenstein‘s Inferring Phylogenies (Sinauer, 2004).]
