Evolution

Charles Darwin was the leading transmutationist of the nineteenth century. The grandson of Erasmus Darwin, and the son of a fairly successful country physician, Robert Waring Darwin, Charles was born into an educated and affluent English family that fostered his interest in natural history. His mother, Susanna Wedgwood, was the daughter of the famed industrialist and potter Josiah Wedgwood (1730–1795) and heir to a family fortune. Charles was the youngest of two sons and had three sisters, who doted on him after the premature death of his mother when Charles was only eight. His scholastic achievements were less than stellar, though he early on developed a passion for natural history. He first made an attempt to study medicine at Edinburgh University but gave up after witnessing an operation on a young child without the aid of anesthesia. Although his formal medical studies disappointed him, Darwin did enjoy interactions with local experts in natural history. In particular he fell under the influence of an ardent transmutationist, Robert E. Grant (1793–1874), a keen student of marine invertebrates who encouraged Darwin's interest in natural history and also encouraged him to consider the leading transmutationist theories of the day. Under Grant's tutelage at Edinburgh, Darwin was exposed seriously to the scientific theories of Lamarck and to the insights of his own grandfather Erasmus, which he entertained but did not enthusiastically accept, at least at that time. He also made some of his first scientific observations using a microscope on the mode of fertilization of the marine polyp Flustra.

His second attempt at formal education shortly thereafter in theology at Cambridge University exposed him to the popular views of natural theology, with its focus on adaptation and design, especially the work of William Paley (1735–1805) and his Natural Theology (1802). Darwin's only formal scientific training had been in geology under the tutelage of Adam Sedgwick (1785–1873) when John Stevens Henslow (1796–1861), the professor of botany and his mentor, recommended him to the Admiralty for a geographic expedition to chart the coast of South America. Under the command of Captain Robert FitzRoy (1805–1865), the HMS Beagle set sail in 1831, with Darwin on board serving as gentleman-companion to the captain and increasingly playing the role of ship's naturalist. The five-year voyage, which charted the coastline of South America and then continued to the Galapagos Islands, Tahiti, New Zealand, and Australia, exposed Darwin to the variation and distribution of living organisms in both continental and island environments. Darwin collected extensive specimens of flora and fauna and made notable observations of the geological history of the locales he visited. Frequently referring to Charles Lyell's (1797–1875) recently published Principles of Geology (1830–1833), which was part of the personal library that he had taken with him, Darwin sought to understand the geographic distribution of plants and animals in terms of a uniformitarian geology. He was especially struck by the manner in which related forms appeared to replace each other as one traveled up and down the coast of eastern South America, by the resemblance of extinct fossil forms to extant life, by the similarity between island species in the Galapagos to nearby continental areas like western South America, and by the differences displayed between those island species on the Galapagos. All of these patterns suggested that some natural and gradual process that involved migration and adaptation to local environments had taken effect, rather than some act of unique or special creation.

Darwin increasingly sought a general explanation for his observations of the natural world during the five-year voyage, and after returning to England he devoted himself to this end by revising his journals, reexamining his specimens, consulting with noted experts, and reading extensively in the scientific literature available to him. Between 1837 and 1838—generally regarded as the crucial years in the formulation of his famous theory—Darwin read the Essay on the Principle of Population by Thomas Robert Malthus (1766–1834). More than any other work, the essay provided Darwin with the intellectual backdrop for his theory by suggesting that competition for natural resources was a fact of life and that populations remained stable as a result of processes that included checks and balances. In this competitive world where there was a struggle for existence, those organisms with the most favorable characteristics would be favored to survive and reproduce themselves. Given enough time, those with favorable traits and characters would diverge from the ancestral forms to give rise to new species. The new elements to transmutationism that Darwin introduced thus included the struggle for existence, but also the fact that heritable variation that was favorable and that conferred an advantage would be likely preserved in the process of reproduction.

Darwin recorded the development of his ideas at this time in a series of notebooks that reveal his attempts to understand the branching process for the origin of species, which had "descended" from some common ancestor. Although he had the major features of his theory at this time, Darwin did not make his work public until much later. There is much speculation in the scholarly literature about the delay in publishing his theory, but there is general agreement that Darwin spent the next interval of his life collecting evidence in support of what he knew would be a contentious theory. He wrote to experts collecting information that might be useful in support of his theory, he engaged in detailed taxonomic work on some little-known species of barnacles to familiarize himself with general problems in the taxonomy of a particular group, and most importantly he closely followed the practices of domestic breeders, especially by frequenting exhibitions and shows on popular or fancy breeds of pigeons. In 1842 he wrote a historical sketch outlining his theory and extended it into a longer historical essay in 1844, neither of which was made public, while all the while compiling the data he was amassing from experts all over the world and from his own research and reading. He was finally forced into joint publication of an abbreviated version of his theory in 1858 shortly after the English naturalist Alfred Russel Wallace independently formulated his own parallel theory. Up to that point, Darwin had been laboring for some twenty years to complete a comprehensive work he planned to title "Natural Selection."

Under pressure to complete a manuscript, Darwin took less than a year to outline his theory of species change, which he ultimately called "descent with modification" by means of natural selection. The full title of his famous book was On the Origin of Species by Means of Natural Selection; or, The Preservation of Favoured Races in the Struggle for Life. It appeared in bookstores on 24 November 1859 and sold out on the first day. It went through six editions as Darwin modified his theory in response to his many critics. Historians generally cite the first edition of this book for their scholarly attempts to understand Darwin's theory, since subsequent editions included such prolonged attempts to accommodate his critics that the text and scientific explication is considered unclear, if not actually inaccurate.

In addition to natural selection, Darwin included some four or five other ways that species change could take place, including the inheritance of acquired characters. Though he did not address human evolution in this book (only one sentence alludes to human evolution), Darwin's readers quickly made the connection between humans and primates thanks in part to the efforts of earlier transmutationists like Robert Chambers. Darwin turned to human evolution only later, in 1871, when he wrote the two-volume Descent of Man and Selection in Relation to Sex. In this book Darwin corrected earlier misconceptions of his work and made it clear that humans had not evolved from apes or monkeys but that both had shared a common ancestor. The book also included pronounced reflections on Darwin's views of human societies and the evolution of civilizations, some of which supports the present-day idea that Darwin himself was indeed a "Social Darwinist." The second part of this book, sexual selection, was an explication of Darwin's theory of sexual selection that was first articulated in 1859. Sexual selection was the process by which Darwin thought fanciful characters like male plumage had evolved, largely through the process of female mate choice. It was one point that led to a disagreement with Wallace, who did not support sexual selection because he thought it acted against natural selection.

Darwin's theory did face notable criticism in his day. One problem had to do with the absence of any viable theory of heredity in it. This led to the criticism—most closely ascribed to Fleeming Jenkin (1833–1885) in a famous review of 1867—that new or novel characters would be diluted or "swamped" out with subsequent generations. Darwin was aware of this criticism, and in his 1868 book The Variation of Animals and Plants under Domestication, he attempted to formulate his "provisional hypothesis of pangenesis," a theory postulating that organs of the body generate hereditary information in the form of gemmules which become combined in the gonads during reproduction. The hypothesis remained largely just an unproven hypothesis. This problem was eventually addressed, during the "rediscovery of Mendel" in 1900, by the discovery that heredity is particulate in nature.

Another problem was the age of the Earth, which, according to estimates made by William Thomson, later Lord Kelvin (1824–1907), was about one hundred million years old. This was an insufficient amount of time to account for the slow, gradual process that Darwin envisioned. The problem was solved after the late-nineteenth-century discovery of radioactivity, which, when accounted for in estimates of the age of the earth, increased it to nearly five billion years, an estimate of time long enough to account for evolution. Yet another problem came from objections to the randomness of the process and the apparent lack of rigor in Darwin's methodology. Leading this charge against Darwin's theory, the astronomer J. F. W. Herschel (1792–1871) described Darwinian natural selection as the "law of higgledypiggledy." Perhaps most problematic of all was the fact that Darwin had no direct proof for a process that took place over such a long stretch of time and that had not been easily detected in the fossil record (most of his evidence was indirect, based on evidence from biogeography, from analogies to artificial selection, or from "imaginary illustrations"). Darwin knew this, and he predicted that it would take some fifty years for evidence to support this theory. The first direct proof of evolution by means of natural selection in natural populations was finally provided beginning in the 1920s with the example of industrial melanism in the peppered moth, Biston betularia. Though the peppered moth example was later challenged by some who thought that the shift to the melanic form did not constitute a true speciation event, it remains a famous example of evolution in action since it demonstrates the rapid shift in allele frequencies following strong selection pressure. Since the case of industrial melanism became known, numerous studies in the wild or under laboratory conditions have provided definitive evidence in support of Darwinian natural selection. Some of the best examples are the morphological responses in beak shape and size to drought conditions on some of the Darwin finches in the Galapagos, and the evolution of antibiotic resistance in pathogenic strains of microorganisms that cause diseases like tuberculosis.

More difficult to resolve were the theological and philosophical questions that followed from the mechanistic theory of natural selection. Even though Darwin had only one line in his 1859 book on human evolution, the theory implied that humans were subject to the same mechanistic process as plants and animals. Natural selection challenged the argument for God's existence from design and led to a nonpurposive view of the world. To some, this echoed the fears raised earlier by the poet Alfred, Lord Tennyson (1809–1892), that such a competitive and nonpurposive view of nature implied that it was "red in tooth and claw." Darwin's own views of "nature," as embodied in works such as On the Origin of Species, appeared to be much more subtle. Nature, to Darwin, appeared to be benignly passive or indifferent to the drama playing itself out in the struggle for existence. Darwin's religious views became increasingly secular; there was no "death-bed conversion" to religious belief.

Despite considerable controversy over the mechanism, the fact of evolution was rapidly accepted by scientists and by popular

audiences. Some of the greatest advocates and promoters of Darwin and his theory in fact disagreed with some rather major aspects of the theory. In the United States, the leading advocate of Darwin's theory was the botanist Asa Gray (1810–1888) at Harvard University. Though Gray found Darwin's theory useful to biogeography, he found the mechanistic implications of natural selection distasteful. It was Gray who brought Darwinian evolution to the attention of many American scientists and who defended Darwin against critical assaults by figures like his Harvard colleague in zoology Louis Agassiz (1807–1873). In Germany, Darwin and evolution found especially fertile ground in one of his greatest advocates, Ernst Haeckel. Beginning in 1866 with his Generelle Morphologie der Organismen (General morphology of organisms), Haeckel promoted Darwin and his evolutionary theory because of its materialistic flavor yet either misunderstood or disagreed with Darwinian natural selection. Rather than upholding an intricately branching and nonprogressivist view of evolution as Darwin had described in Origin, Haeckel retained a linear, progressive model only with some lateral branching. Haeckel continued to draw on embryological or developmental models for the evolutionary process and believed that evolution was guided by historical evolutionary forms that could still be seen in the process of individual development. In the process of development, the ontogeny (or developmental pattern of the organism) recapitulated the phylogeny (or the evolutionary history of the organism). Though German embryologists like Karl Ernst von Baer (1792–1876) argued against such a crude linear progressivist model of evolutionary development in favor of more complex branching models, the view that "ontogeny recapitulated phylogeny," also known as "the biogenetic law," continued to have mass appeal especially in Germany. Haeckel was a prolific and popular writer whose numerous attempts to reconstruct "phylogenetic trees" with the main trunk running upward to the human "race" as the pinnacle of development shaped popular understanding of evolution in the late nineteenth century. Haeckel's influence also had the unfortunate effect of linking Darwinian evolution with what he thought were materialist progressivist leanings toward the struggle for national development. His Generelle Morphologie and subsequent writings, which gained currency in the late nineteenth century, eventually provided pseudo-scientific justifications for nationalism and racism.

Darwin's most famous, indeed notorious, advocate was his close friend, the anatomist Thomas Henry Huxley. It was Huxley, along with the Kew Gardens botanist Joseph Dalton Hooker (1817–1879) and Charles Lyell, who formed the "inner circle" of friends and supporters who promoted and defended Darwin's name and his theory. Though Huxley earned himself the title of "Darwin's Bulldog" while supporting Darwin's theory—which stressed slow, gradual evolution—he preferred instead the view that evolution could take place more suddenly and rapidly. Like others of his time, Huxley closely linked views of evolutionary progress with social progress; he used the principles of evolution to support his reformist views of English society.

By the turn of the twentieth century, a staggering number of evolutionary theories—that added to, amended, or outright disagreed with Darwin's selection theory—were being actively entertained. These included a revitalization of the inheritance of acquired characters into movements associated with neo-Lamarckism; directed evolution, aristogenesis, and orthogenesis, all of which upheld the view that evolution was guided by an internal driving force; and "creative evolution," a quasi-mystical evolutionary theory endorsed by the French philosopher Henri Bergson (1859–1941), who postulated that living organisms were guided by an élan vital, or special living force.

One of the most popular alternatives to Darwinian selection theory was the "mutation theory" of the Dutch botanist Hugo de Vries (1848–1935). One of the "rediscoverers of Mendel," de Vries upheld a particulate theory of heredity that stressed the importance of what he termed "mutation pressure" in generating evolutionary novelty. Much of his theory was based on observations he made on the evening primrose plant, Oenothera lamarckiana, which appeared to throw off new varieties or species suddenly. De Vries erroneously interpreted these new forms as being entirely new species that had been generated by strong mutation pressure (these new forms of the primrose were subsequently shown to be regularly occurring varieties that resulted from its genetic structure). Natural selection was not ruled out, but it came into play only in selecting the most advantageous of these forms; it therefore played an eliminative role in evolution, while mutation pressure played the more active or creative role. Because it drew on the newer science of genetics, which appeared to be more rigorous because it was experimental, mutation theory was widely adopted by younger scientists at the turn of the century, who did not favor the natural-history-oriented approach associated with Darwin or his naturalist followers. The interval of time between approximately the rediscovery of Mendel and the late 1920s thus saw a period of dissonance between younger geneticists and older naturalists, all of whom sought a viable evolutionary theory that could be a rigorous experimental science that could also explain patterns of adaptation in natural populations. Others, who were strict followers of Darwinian selectionism and stressed the fact that Darwin endorsed a slow, gradual process that operated at the level of small, individual differences, turned to the newer science of statistics to create a new school known as "biometry." Francis Galton (1822–1911), Darwin's famous first cousin, was an exponent of the biometrical school, which tried to understand evolution in statistical terms. He was also the individual who coined the term eugenics in this attempt to formulate a viable theory of heredity and evolution that could then provide social reformers with the tools to "improve society" through selective breeding.

The turn of the century as a whole saw a series of proposed evolutionary theories that some have claimed were in fact "non-Darwinian" evolutionary theories. This was so much the case that Julian Huxley (1887–1975) the famous grandson of Thomas Henry Huxley, designated this interval of time "the eclipse of Darwin," a period of confusion and disagreement over the mechanism and mode of evolution. Only after the mechanism of heredity was understood and only after the science of genetics was integrated with natural history was the debate over the mechanism of natural selection extinguished. This did not take place until the interval of time between 1920 and 1950 and was part of the event called the "evolutionary synthesis." The synthesis brought together Darwinian selection theory with Mendelian genetics in a populational view of evolution to account for the origin of biological diversity. It first drew primarily on the work of three mathematical population geneticists, R. A. Fisher (1890–1962) and J. B. S. Haldane (1892–1964) in England and Sewall Wright (1889–1988) in the United States, all of whom offered models demonstrating the efficacy of natural selection under a range of different parameters. The theoretical work of these modelers was tested in field conditions with natural populations of organisms in the mid-1930s. This then led to the writing of a series of synthetic accounts that integrated Darwinian selection theory with Mendelian genetics.

The new science called evolutionary genetics was most closely associated with Theodosius Dobzhansky (1900–1975) and his synthetic book Genetics and the Origin of Species (1937). In addition to genetics, the "evolutionary synthesis" drew on systematics, botany, paleontology, cytology, and morphology to create what is now called the "synthetic theory of evolution" or the "neo-Darwinian theory of evolution." In addition to drawing on the work of Dobzhansky, it drew on the work of twentieth-century biologists like Ernst Mayr (b. 1904), George Ledyard Stebbins (1906–2000), George Gaylord Simpson (1902–1984), and Julian Huxley. It endorses the view that natural selection is the dominant mechanism that drives evolutionary change. Within such a modern, populational frame of reference, evolution itself became redefined as "any relative change in gene frequencies." This definition has been debated extensively, especially by naturalists-systematists who prefer a more inclusive consideration of the process of speciation.

Accompanying the emergence of the "modern synthesis" (Julian Huxley's exact phrase for the new science of evolution), the first international Society for the Study of Evolution (SSE) was formed in 1946 and sponsored the journal Evolution, the first international journal for the dissemination of scientific knowledge of evolution. With the "evolutionary synthesis," the varied sciences that informed evolution thus became reorganized into the new science of evolutionary biology. Because it drew on so many scientific disciplines, encompassing the breadth of the biological sciences and many of the social sciences, like psychology and anthropology, evolutionary biology began increasingly to play a central, integrative role in the biological sciences, especially beginning in the late 1950s and early 1960s. In 1975 Dobzhansky stated the important fact that "nothing in biology makes sense except in the light of evolution." In stating this, he was stressing the fact that evolution by means of natural selection serves as the central, unifying principle of the modern science of biology.

Though there have been varied attempts to amend or alter the synthetic theory as it was formulated during the "evolutionary synthesis" in the latter half of the twentieth century, the theory remains fundamentally intact. Among the varied points of agreement are included the primacy of natural selection, the continuation between microevolutionary and macroevolutionary processes, and the fact that evolution takes place at the level of small individual differences—all pretty much formulated in the 1930s–1950s. New techniques and methods from molecular biology have led to a virtual revolution in understanding evolution at the molecular level, while more traditional evolutionary biologists continue to mine the fossil record and to explore developmental biology, areas such as behavior and functional morphology as well as biochemistry to give a more detailed account of the evolutionary history of life on earth.