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Darwin's Theory of Descent with Modification, Versus the Biblical

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Darwin's Theory of Descent with Modification, Versus the Biblical Historical and Philosophical Perspective Darwin’s Theory of Descent with Modification, versus the Biblical Tree of Life David Penny* Institute for Molecular BioSciences, Massey University, Palmerston North, New Zealand Thereisastrongurbanmyththat to explain events in the past—an ap- Darwin’s Alternative View of Charles Darwin introduced and/or ad- proach that I interpret as being learned ‘‘Descent with Modification’’ vocated a ‘‘Tree of Life’’ for the classi- from his geological background (Figure 1) fication of living organisms. This has [6,7]. Having established that Darwin was not recently been highlighted by Lawton [1], the originator of the term, my second but dates back to at least Doolittle [2]. It How Old Is the Tree of Life main point is that, instead of using the tree is often combined with the idea that we Concept? of life concept, Darwin referred to his require a ‘‘paradigm shift’’ to take into theory as ‘‘descent with modification’’. For account extensive lateral gene transfer, The Tree of Life is a biblical phrase that example, in the first edition of the Origin of especially in prokaryotes. Yes, extensive predates the 19th century by several Species (Darwin 1859, see http://darwin- lateral gene transfer occurs widely in millennia. In all, the phrase occurs 11 online.org.uk/) Darwin uses the phrases prokaryotes (see [3–5]), and this enriches times in Genesis, Proverbs, and the Book ‘‘theory of descent’’ and/or ‘‘descent with our understanding of evolution by em- of Revelations (http://www.biblegateway. modification’’ 21 times (on pages 12, 14, phasizing a more gene-centered view. In com/quicksearch/). In Genesis, it is found 189, 206, 320, 349, 351, 354, 358, 361, contrast, though acknowledging the prior three times (2:9 and 3:22 and 3:24); in the 399, 429, 444, 477, 479, 484, 493, 494, availability of the tree of life simile, middle of the Garden of Eden were both 496, and 497). As such, it includes the idea Darwin continually referred to his ‘‘the- the Tree of Life and the ‘‘Tree of the of an evolutionary tree but also includes ory of descent with modification’’, an knowledge of good and evil’’ (from which hybrids (long a standard technique for expression that encompasses a wide Eve supposedly ate the fruit). Eating of the plant breeders), of which Darwin was fully variety of fundamental processes includ- fruit of the Tree of Life would apparently aware, as well as ideas from genetics that ing both vertical descent and lateral gene have given eternal life, and that eternal life would have been unfamiliar to him. These transfer, thus removing any need for theme appears again in Revelations 2:7, include mechanisms of evolution such as paradigm shifts. and 22:2, 14, and 19. The other references the transfer of genes between bacteria, or For Western Europe, the Tree of Life are in Proverbs (3:18, 11:30, 13:12, 15:4), from bacteria to eukaryotes by endosym- phrase is biblical in origin and, although in where it appears more metaphorical in biosis (by mitochondria and chloroplasts), Darwin’s words, it is ‘‘a useful simile’’, it is intent. and so on. No doubt there are other best not interpreted literally. The basic Thus, my first point is that Darwin mechanisms, but they all fit well with the confusion may have arisen because it is not certainly did not coin the phrase. Indeed, concept of descent with modification. recognised that Darwin was mainly inter- the phrase has widespread use in many Basically, descent with modification allows ested in the mechanisms of evolution. cultures (see Wikipedia) and is clearly an ‘‘cycles’’ in ‘‘graphs’’ (technically, a tree is These include both whether mechanisms ancient idea with strong mystical over- a ‘‘connected graph without cycles’’ [8]). that could be studied in the present could tones. This ancient origin of the term the In real life, cycles could result from explain past events, and establishing the Tree of Life does not preclude its use in hybridisation, endosymbiotic gene trans- continuity between all living creatures. He science, but it certainly means that we fer, lateral gene transfer, recombination, was not primarily interested in the de- need to be careful how we use it—we lineage sorting, the complexities of gene- scription of the patterns of evolution. always use it in lower case letters (tree of alogical relationships, etc. Importantly, We need to reappraise the different life) to help avoid unintended implications. Darwin’s more general phrases, including concepts, what Darwin meant by them, It perhaps should also be pointed out that descent with modification, emphasise the and how he used the terms. The Tree of searching the same biblical Web source continuity between populations, subspe- Life example is not the only case where does not find any occurrences in either cies, sibling species, etc., which was Darwin’s work has potentially been mis- the Old or New Testament of either the perhaps the more fundamental issue in understood (and I will mention some of ‘‘theory of descent’’, nor of ‘‘descent with the mid-19th century (rather than that of these later). In contrast to being interested modification’’! the precise form of relationships). in describing patterns of relationships, I argue that Darwin appears to have been Citation: Penny D (2011) Darwin’s Theory of Descent with Modification, versus the Biblical Tree of Life. PLoS more interested in the extent to which Biol 9(7): e1001096. doi:10.1371/journal.pbio.1001096 mechanisms that can be studied in the Published July 5, 2011 present are both necessary, and sufficient, Copyright: ß 2011 David Penny. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. The Perspective section provides experts with a Funding: No specific funding was received for this work. forum to comment on topical or controversial issues of broad interest. Competing Interests: The author has declared that no competing interests exist. * E-mail: [email protected] PLoS Biology | www.plosbiology.org 1 July 2011 | Volume 9 | Issue 7 | e1001096 Figure 1. In 1830, Charles Lyell (effectively Charles Darwin’s mentor) published his Principles of Geology as an attempt to explain past geological changes by mechanisms that can be studied in the present. Lyell’s frontispiece was the remains of the ancient Greek Temple of Serapus at Puzzuoli, near Naples in Italy. Known forces of land subsidence and earthquakes could account for its unusual appearance today. It had been built on land, and then subsided over a millennium, which allowed marine molluscs to leave the high water mark. The remains of the temple had then been raised up again in an earthquake in September 1438 to its present position. It was an example of how known forces and mechanisms could be used to explain past events. Continuity was essential, but the known forces could vary in their intensity over time. doi:10.1371/journal.pbio.1001096.g001 This modification with descent phrase is ogies. As such, descent with modification, analogy of a branch of the tree overgrow- used in several contexts, including main especially in relation to continuity over ing, and supplanting a ‘‘feebler branch’’ headings in the Origin. It occurs in the short- and long-term time scales, is central (see Darwin [9], p. 148). The full quote is discussion around difficulties for the ‘‘the- to Darwin’s view of evolution. All this is ‘‘buds give rise by growth to fresh buds, and these, ory of continuity’’—including the absence fully acceptable to modern evolutionists, if vigorous, branch out and overtop on all sides or rarity of transitional varieties (leading to even though it is important to acknowl- many a feebler branch, so by generation I believe it a discussion on the relative completeness edge that Mendel’s particulate view of has been with the great Tree of Life, which fills of the fossil record, and why the fossil inheritance was not at all well known to with its dead and broken branches the crust of the record should not be read literally—still Darwin, and (as I note later) authors in the earth, and covers the surface with its ever excellent reading for paleontologists). It is mid 19th century were still puzzled by the branching and beautiful ramifications’’. As such, used as an explanation for the affinities of principles of inheritance. it is not a description of the relationship the extinct forms of life both to each other between taxa, but rather a suggestion that and to living forms. Another usage is that How Darwin Used the Concept a living tree analogy can be applied to species of different classes do not neces- of the Tree of Life lineages of species competing (and sup- sarily change together, or at the same rate, planting) other lineages or groups of or to the same degree; yet, in the long run, In contrast to the 21 occurrences of the species—whether fungi, plants, or animals. they all undergo some modification. It is theory of descent, in the Origin of Species In other words, microevolutionary pro- also an explanation of the naturalness of there is only a single usage of the Tree of cesses are similar in principle to macro- the classification of eukaryotic groups, and Life. However—and this is my third evolutionary processes, a central part of also an explanation for biogeographic point—it is not used as a description of his thinking [7]. He also comments that distributions. Again, the phrase is used relationships, but rather as an analogy for ‘‘the affinities of all the beings of the same class with reference to variability under domes- competition between species (and groups have sometimes been represented by a great tree’’, tication, and as an explanation for homol- of species) during evolution.
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