Darwin,Malthus, Süssmilch,and Euler: The Ultimate Origin of the Motivation for the Theory of Natural Selection

It is fairly well known that Darwin was inspired to formulate his theory of natural selection by reading Thomas Malthus’s Essay on the Principle of Population. In fact, by reading Darwin’s notebooks, we can even locate one particular sentence which started Darwin thinking about population and selection. What has not been done before is to explain exactly where this sentence – essentially Malthus’s ideas about geometric population growth – came from. In this essay we show that eighteenth century mathematician Leonhard Euler is responsible for this sentence, and in fact forms the beginning of the logical chain which leads to the creation of the theory of natural selection. We shall examine the fascinating path taken by a mathematical calculation, the many different lenses through which it was viewed, and the path through which it eventually influenced Darwin.     

Darwin’s two theories, 1844 and 1859

Darwin’s first two, relatively complete, explicit articulations of his theorizing on evolution were his Essay of 1844 and On the Origin of Species published in 1859. A comparative analysis concludes that they espoused radically different theories despite exhibiting a continuity of strategy, much common structure and the same key idea. Both were theories of evolution by means of natural selection. In 1844, organic adaptation was confined to occasional intervals initiated and controlled by de-stabilization events. The modified descendants rebalanced the particular “plant and animal forms … unsettled by some alteration in their circumstances.” But by 1859, organic adaptation occurred continuously, potentially modifying the descendants of all organisms. Even natural selection, the persistent core of Darwin’s theorizing, does not prove to be a significant basis for theory similarity. Consequently, Darwin’s Origin theory cannot reasonably be considered as a mature version of the Essay. It is not a modification based on adjustments, further justifications and the integration of a Principle of Divergence. The Origin announced a new “scientific paradigm” while the Essay did little more than seemingly misconfigure the operation of a novel mechanism to extend varieties beyond their accepted bounds, and into the realm of possible new species. Two other collections of Darwin’s theorizing are briefly considered: his extensive notes of the late 1830s and his contributions to the famous meeting of 1 July 1858. For very different reasons, neither constitutes a challenge to the basis for this comparative study. It is concluded that, in addition to the much-debated social pressures, an unacknowledged further reason why Darwin did not publish his theorizing until 1859, could have been down to his perceptive technical judgement: wisely, he held back from rushing to publish demonstrably flawed theorizing.     

Sexual selection: Another Darwinian process

Why was sexual selection so important to Darwin? And why was it de-emphasized by almost all of Darwin's followers until the second half of the 20th century? These two questions shed light on the complexity of the scientific tradition named “Darwinism”. Darwin's interest in sexual selection was almost as old as his discovery of the principle of natural selection. From the beginning, sexual selection was just another “natural means of selection”, although different from standard “natural selection” in its mechanism. But it took Darwin 30 years to fully develop his theory, from the early notebooks to the 1871 book The Descent of Man, and Selection in Relation to Sex. Although there is a remarkable continuity in his basic ideas about sexual selection, he emphasized increasingly the idea that sexual selection could oppose the action of natural selection and be non adaptive. In time, he also gave more weight to mate choice (especially female choice), giving explicit arguments in favor of psychological notions such as “choice” and “aesthetic sense”. But he also argued that there was no strict demarcation line between natural and sexual selection, a major difficulty of the theory from the beginning. Female choice was the main reason why Alfred Russel Wallace, the co-discoverer of the principle of natural selection, engaged in a major controversy with Darwin about sexual selection. Wallace was suspicious about sexual selection in general, trying to minimize it by all sorts of arguments. And he denied entirely the existence of female choice, because he thought that it was both unnecessary and an anthropomorphic notion. This had something to do with his spiritualist convictions, but also with his conception of natural selection as a sufficient principle for the evolutionary explanation of all biological phenomena (except for the origin of mind). This is why Wallace proposed to redefine Darwinism in a way that excluded Darwin's principle of sexual selection. The main result of the Darwin–Wallace controversy was that most Darwinian biologists avoided the subject of sexual selection until at least the 1950 s, Ronald Fisher being a major exception. This controversy still deserves attention from modern evolutionary biologists, because the modern approach inherits from both Darwin and Wallace. The modern approach tends to present sexual selection as a special aspect of the theory of natural selection, although it also recognizes the big difficulties resulting from the inevitable interaction between these two natural processes of selection. And contra Wallace, it considers mate choice as a major process that deserves a proper evolutionary treatment. The paper's conclusion explains why sexual selection can be taken as a test case for a proper assessment of “Darwinism” as a scientific tradition. Darwin's and Wallace's attitudes towards sexual selection reveal two different interpretations of the principle of natural selection: Wallace's had an environmentalist conception of natural selection, whereas Darwin was primarily sensitive to the element of competition involved in the intimate mechanism of any natural process of selection. Sexual selection, which can lack adaptive significance, reveals this exemplarily.     

Under the influence of Malthus's law of population growth: Darwin eschews the statistical techniques of Aldolphe Quetelet

Charles Darwin, James Clerk Maxwell, and Francis Galton were all aware, by various means, of Aldolphe Quetelet’s pioneering work in statistics. Darwin, Maxwell, and Galton all had reason to be interested in Quetelet’s work: they were all working on some instance of how large-scale regularities emerge from individual events that vary from one another; all were rejecting the divine interventionistic theories of their contemporaries; and Quetelet’s techniques provided them with a way forward. Maxwell and Galton all explicitly endorse Quetelet’s techniques in their work; Darwin does not incorporate any of the statistical ideas of Quetelet, although natural selection post-twentieth century synthesis has. Why not Darwin? My answer is that by the time Darwin encountered Malthus’s law of excess reproduction he had all he needed to answer about large scale regularities in extinctions, speciation, and adaptation. He didn’t need Quetelet.     

Separated at Birth: The Interlinked Origins of Darwin’s Unconscious Selection Concept and the Application of Sexual Selection to Race

This essay traces the interlinked origins of two concepts found in Charles Darwin’s writings: “unconscious selection,” and sexual selection as applied to humanity’s anatomical race distinctions. Unconscious selection constituted a significant elaboration of Darwin’s artificial selection analogy. As originally conceived in his theoretical notebooks, that analogy had focused exclusively on what Darwin later would call “methodical selection,” the calculated production of desired changes in domestic breeds. By contrast, unconscious selection produced its results unintentionally and at a much slower pace. Inspiration for this concept likely came from Darwin’s early reading of works on both animal breeding and physical ethnology. Texts in these fields described the slow and unplanned divergence of anatomical types, whether animal or human, under the guidance of contrasting ideals of physical perfection. These readings, it is argued, also led Darwin to his theory of sexual selection as applied to race, a theme he discussed mainly in his book The Descent of Man (1871). There Darwin described how the racial version of sexual selection operated on the same principle as unconscious selection. He thereby effectively reunited these kindred concepts.     

Darwin's "beloved barnacles": tough lessons in variation

In 1846, burdened by insecurity and self-doubt, and having been convinced that he needed to study some group of organisms closely, Darwin embarked on an eight-year odyssey in the protean and perplexing world of barnacles. At the time, he was searching for evidence in support of his theory of evolution by natural selection. In the course of his long study of barnacles, however, he was not just validating his preexisting theoretical system, but was also modifying his views on such fundamental aspects as the universality of individual variation, which is the focus of this paper. According to this notion, the members of any population of living things are expected to exhibit sufficient differences from one another for natural selection to operate. By emphasizing the theoretical value of the barnacle project, my analysis contributes to the historiographic tradition which highlights the significance of the period between the first comprehensive formulation of the theory of evolution by natural selection in 1844 and its urgent publication in the late 1850s. In the course of these years, Darwin's theory was not just accumulating empirical laurels, but was also expected to adapt to a changing conceptual landscape.     

Engaging with Lyell: Alfred Russel Wallace’s Sarawak Law and Ternate papers as reactions to Charles Lyell’s Principles of Geology

Alfred Russel Wallace (1823–1913) and Charles Darwin (1809–1882) are honored as the founders of modern evolutionary biology. Accordingly, much attention has focused on their relationship, from their independent development of the principle of natural selection to the receipt by Darwin of Wallace’s essay from Ternate in the spring of 1858, and the subsequent reading of the Wallace and Darwin papers at the Linnean Society on 1 July 1858. In the events of 1858 Wallace and Darwin are typically seen as central players, with Darwin’s friends Charles Lyell (1797–1875) and Joseph Dalton Hooker (1817–1911) playing supporting roles. This narrative has resulted in an under-appreciation of a more central role for Charles Lyell as both Wallace’s inspiration and foil. The extensive anti-transmutation arguments in Lyell’s landmark Principles of Geology were taken as the definitive statement on the subject. Wallace, in his quest to solve the mystery of species origins, engaged with Lyell’s arguments in his private field notebooks in a way that is concordant with his engagement with Lyell in the 1855 and 1858 papers. I show that Lyell was the object of Wallace’s Sarawak Law and Ternate papers through a consideration of the circumstances that led Wallace to send his Ternate paper to Darwin, together with an analysis of the material that Wallace drew upon from the Principles. In this view Darwin was, ironically, intended for a supporting role in mediating Wallace’s attempted dialog with Lyell.     

Sex ratios in geographically structured populations

In his book on sexual selection (1), Darwin documented evidence that the primary sex ratio (the proportion of males at conception) is about 1/2 in a wide variety of species. Otherwise, he explained, a newly conceived member of the rare sex will, on average, have more offspring than one of the common sex, since each offspring has one mother and one father; thus there is frequency-dependent selection in favour of parents producing the rare sex. Darwin formulated this explanation in the first edition (1871) for monogamous species, but he failed to extend it to polygamous species, and in the second edition (1874) he retracted it completely. It was left to Fisher (2) to develop the theory in the more general form that there should be equal parental expenditure on the two sexes, allowing for the possibility that one sex may cost more to produce than the other. Despite the wide applicability of Fisher's principle, recent work on sex ratio evolution has focused on situations where it breaks down (3). Hamilton (4) first pointed out that Fisher's argument assumes population-wide competition for mates, whereas most natural populations have a geographical population structure in which limited dispersal imposes constraints on mating patterns. What are the consequences for the sex ratio?     

Darwin would have loved DNA: celebrating Darwin 200

Analysis of DNA sequences now plays a key role in evolutionary biology research. If Darwin were to come back today, I think he would be absolutely delighted with molecular evolutionary genetics, for three reasons. First, it solved one of the greatest problems for his theory of evolution by natural selection. Second, it gives us a tool that can be used to investigate many of the questions he found the most fascinating. And third, DNA data confirm Darwin''s grand view of evolution.     

Why Darwin rejected intelligent design

As a Cambridge University undergraduate Charles Darwin was fascinated and convinced by the argument for intelligent design, as set forth in William Paley’s 1802 classic, Natural Theology. Subsequently, during his five-year voyage on HMS Beagle (1831–1836), Darwin interpreted his biological findings through a creationist lens, including the thought-provoking evidence he encountered during his historic visit to the Galápagos Islands in September and October 1835. After his return to England in 1836 and his subsequent conversion to the idea of organic evolution in March 1837, Darwin searched for a theory that would explain both the fact of evolution and the widespread appearance of intelligent design. His insight into the process of natural selection, which occurred in September 1838, provided this alternative explanation. Darwin’s Origin of Species (1859) exemplifies his skillful deployment of the hypothetico-deductive method in testing and refuting the arguments for intelligent design that he had once so ardently admired.     

Charles Darwin's debt to malthus and Edward Blyth

Conclusion It is not justifiable to accuse Darwin of conscious or unconscious plagiarism. This charge is contrary to the historical evidence and to the extensive information that we have about his character. When Darwin listed the writers on the origin of species by natural selection before himself, he did not mention Blyth, and this omission did not disturb the cordial relations between Darwin and Blyth. Blyth continued to supply Darwin with information which Darwin used in his later publications with due acknowledgment to Blyth. For example, in The Descent of Man, Darwin cited Blyth: Mr. Blyth, as he informs me, saw Indian crows feeding two or three of their companions which were blind.⁶³ Blyth felt no resentment. If he did, he would have so informed Darwin. Blyth did not regard himself as in any sense a predecessor of Darwin and he certainly did not resent Darwin as a plagiarizer of himself. Moreover, Darwin went to a great deal of trouble to find his own predecessors and to give them proper credit.⁶⁴ After Darwin had completed his work on natural selection, he wrote a letter to the Reverend Baden Powell in which he clearly showed recognition of the contribution of others to his own work:No educated person, not even the most ignorant, could suppose I mean to arrogate to myself the origination of the doctrine that species had not been independently created. The only novelty in my work is the attempt to explain how species became modified, and to a certain extent how the theory of descent explains certain large classes of facts; and in these respects I received no assistance from my predecessors.⁶⁵ *** DIRECT SUPPORT *** A8402011 00002     

Darwin on woman

In his 1871 book The Descent of Man, Darwin exposed the idea of sexual selection as a major principle of human evolution. His main hypothesis, which was already briefly presented in The Origin of Species, is that there exists, besides “natural selection”, another form of selection, milder in its effect, but no less efficient. This selection is operated by females to mate and reproduce with some partners that are gifted with more qualities than others, and more to their taste. At more evolved stages, sexual selection was exerted by men who became able to choose the women most attractive to their taste. However, Darwin insists, sexual selection in the human species is limited by a certain number of cultural practices. If Darwin's demonstration sometimes carried the prejudices of his times regarding gender differences he was the first who took into account the importance of sexual choices in his view on evolution, and who insisted on the evolutionary role of women at the dawn of humanity. Thus, he opened the space for a rich reflection, which after him was widely developed and discussed in anthropological and gender studies.     

Alfred Russel Wallace, past and future

The naturalist Alfred Russel Wallace (1823–1913) has for many years been standing in the shadow of his more famed co‐discoverer of the principle of natural selection, Charles Darwin. Despite outward similarities between the two men's formulation of the principle, Wallace had fit his appreciation of natural selection into views on evolution that were quite different from Darwin's. A closer examination of what Wallace had in mind suggests a model of process in which natural selection per se acts as the negative feedback mechanism (actually, a ‘state‐space’) in the relation between population and environment, and environmental engagement as made possible by the resulting selection of traits acts as the positive feedback part of the cycle. Thus, it may be better to contextualize adaptive structures as entropy‐relaying biogeochemical facilitators that only ‘generate a potential for evolution’ than to portray them as the end results of evolution. This systems point of view better lends itself to appreciations of the biogeographical context of evolution than does the tree‐thinking of a more conventional style of speciation‐focused Darwinism, which sometimes confuses process with result.     

A guide to the genomics of ecological speciation in natural animal populations

Interest in ecological speciation is growing, as evidence accumulates showing that natural selection can lead to rapid divergence between subpopulations. However, whether and how ecological divergence can lead to the buildup of reproductive isolation remains under debate. What is the relative importance of natural selection vs. neutral processes? How does adaptation generate reproductive isolation? Can ecological speciation occur despite homogenizing gene flow? These questions can be addressed using genomic approaches, and with the rapid development of genomic technology, will become more answerable in studies of wild populations than ever before. In this article, we identify open questions in ecological speciation theory and suggest useful genomic methods for addressing these questions in natural animal populations. We aim to provide a practical guide for ecologists interested in incorporating genomic methods into their research programs. An increased integration between ecological research and genomics has the potential to shed novel light on the origin of species.     

Towards a postmodern synthesis of evolutionary biology

In 2009, we are celebrating the 200th anniversary of Charles Darwin and the 150th jubilee of his masterpiece, the Origin of Species. Darwin constructed the first coherent and compelling narrative of biological evolution and thus founded evolutionary biology—and modern biology in general, remembering the famous dictum of Dobzhansky. It is, however, counter-productive, and ultimately, a disservice to Darwin’s legacy, to define modern evolutionary biology as neo-Darwinism. The current picture of evolution, informed by results of comparative genomics and systems biology, is by far more complex than that presented in the Origin of Species, so that Darwinian principles, including natural selection, are incorporated into the evolving new synthesis as important but certainly not all-embracing tenets. This expansion of evolutionary biology does not denigrate Darwin in the least but rather emphasizes the fertility of his ideas.     

Mystery of mysteries: Darwin and the species problem

Darwin offered an intriguing answer to the species problem. He doubted the existence of the species category as a real category in nature, but he did not doubt the existence of those taxa called “species”. And despite his scepticism of the species category, Darwin continued using the word “species”. Many have said that Darwin did not understand the nature of species. Yet his answer to the species problem is both theoretically sound and practical. On the theoretical side, Darwin’s answer is confirmed by contemporary biology, and it offers a more satisfactory answer to the species problem than recent attempts to save the species category. On the practical side, Darwin’s answer frees us from the search for the correct theoretical definition of “species”. But at the same time it does not require that we banish the word “species” from biology as some recent sceptics of the species category advocate. © The Willi Hennig Society 2010.     

Monkeys into Men and Men into Monkeys: Chance and Contingency in the Evolution of Man, Mind and Morals in Charles Kingsley’s Water Babies

The nineteenth century theologian, author and poet Charles Kingsley was a notable populariser of Darwinian evolution. He championed Darwin’s cause and that of honesty in science for more than a decade from 1859 to 1871. Kingsley’s interpretation of evolution shaped his theology, his politics and his views on race. The relationship between men and apes set the context for Kingsley’s consideration of these issues. Having defended Darwin for a decade in 1871 Kingsley was dismayed to read Darwin’s account of the evolution of morals in Descent of Man. He subsequently distanced himself from Darwin’s conclusions even though he remained an ardent evolutionist until his death in 1875.     

The Darwin of pangenesis

The Darwin of pangenesis is very much another Darwin. Pangenesis is Darwin's comprehensive theory of generation, his theory about all sexual and asexual modes of reproduction and growth. He never explicitly integrated pangenesis with his theory of natural selection. He first formulated pangenesis in the 1840s and integrated it with the physiology, including the cytology, of that era. It was, therefore, not consilient with the newer cytology of the 1860s when he published it in 1868. By reflecting on the role of pangenesis in Darwin's life and work, we can learn to take a wider view of his most general theorising about animal and plant life.     

Character displacement and the origins of diversity

In The Origin of Species, Darwin proposed his principle of divergence of character (a process now termed "character displacement") to explain how new species arise and why they differ from each other phenotypically. Darwin maintained that the origin of species and the evolution of differences between them is ultimately caused by divergent selection acting to minimize competitive interactions between initially similar individuals, populations, and species. Here, we examine the empirical support for the various claims that constitute Darwin's principle, specifically that (1) competition promotes divergent trait evolution, (2) the strength of competitively mediated divergent selection increases with increasing phenotypic similarity between competitors, (3) divergence can occur within species, and (4) competitively mediated divergence can trigger speciation. We also explore aspects that Darwin failed to consider. In particular, we describe how (1) divergence can arise from selection acting to lessen reproductive interactions, (2) divergence is fueled by the intersection of character displacement and sexual selection, and (3) phenotypic plasticity may play a key role in promoting character displacement. Generally, character displacement is well supported empirically, and it remains a vital explanation for how new species arise and diversify.