History Of Evolution Research Paper

1. Models Of Evolution

The term ‘evolution’ is derived from the Latin evolution, denoting the unrolling of a scroll. In the eighteenth century it was applied to the growth of the embryo, then often described as the mere expansion of a preformed miniature. This application was retained up to the nineteenth century, by which time it was clear that the development of the embryo consisted of the progressive appearance of more complex structures. The philosopher Herbert Spencer generalized the term by applying it to any process of natural progressive development, thereby creating the still-common belief that all evolution must be progressive. In particular, Spencer applied the term to the development of societies and of life on earth.

Charles Darwin did not often use the term when describing his theory of the origin of species, but by the end of the nineteenth century this had become the most common version of ‘evolution.’ But Darwin’s theory did not imply an inevitable progression toward complexity, and much controversy has surrounded the association between evolution and progress. Darwin’s theory of natural selection now dominates biology and is being extended to many other domains, but it is but one of a number of mechanisms which have been suggested to explain evolution, each of which has its own associated implications.

1.1 The Ladder Of Progress

Perhaps the most popular model of evolution, and one which reveals the link with embryology, is of a linear hierarchy of stages of development from inception to maturity. Such a model simply assumes that all change is progressive and that there is only one direction of development. When applied to societies or species, the model can be adapted by supposing that many lines of development each ascend the same hierarchy but at different rates, so that the slow developers illustrate the ancestral stages already passed through by those at the top of the scale.

1.2 Evolutionary Trees

Although the ladder model is still popularly applied to the evolution of life on earth, and was once accepted in the social sciences, biologists and sociologists now realize that there is no single goal toward which evolution is developing—neither the human race nor Western civilization can be treated as the most mature phase of development. Evolution is better represented as a tree or bush with many branches, each of which is constantly subdividing. The end product of one branch cannot be seen as the goal toward which all the others are ascending, and it is impossible for the end product of one branch to illustrate exactly the ancestral form of another branch. The living apes are not immature humans, because even if the common ancestor from which humans and apes have evolved would be classified as an ape, it would no longer be identical to any of the living ape species. If there is progress on the branching-tree model, it has to be defined in more sophisticated terms so that there are many different ways to become more complex.

Darwin argued that branching evolution explains how we are able to classify species into groups within groups (species, genera, families, etc.). Two species descended from a common ancestor will still share many characters derived from that ancestor, and will share some characters with even more distant relatives. Darwin and his modern followers assume that each new character is developed only once, so all species exhibiting the character are descended from a common ancestor which also had the character. This assumption will be invalidated if there are cases where two lines of evolution evolve the same character independently (convergence or homoplasy). It will also be invalidated if there are even small-scale examples of ladder-like evolution where several branches are constrained to evolve along the same pattern of development (parallelism).

1.3 Adaptation

The branching model had already been applied to the evolution of languages before Darwin and his followers used it to transform biology. Darwin’s key insight was that there was no natural mechanism that could force evolution along a single predetermined path. Evolution is driven not by progress but by the necessity for each species to adapt to its environment in an ever-changing world. Branching occurs because a single population can sometimes become divided by a geographical barrier, after which each isolated population will develop independently in response to the different adaptive pressures it encounters. Progress to a more complex structure is at best a by-product of adaptation and is never inevitable in any one branch; indeed, many species are the products of degenerative evolution. In the animal kingdom, two branches of evolution can seldom if ever rejoin via hybridization (although this does happen in plants). Here, there is a clear contrast with social and linguistic evolution, where mergings and borrowings are frequent.

1.4 Darwinism And Lamarckism

Two mechanisms of adaptive evolution have been suggested: natural selection (Darwinism) and the inheritance of acquired characters (Lamarckism). Darwin assumed that the species consists of a population of organisms which exhibit some degree of individual variation and that these variations are inherited—we now explain them as genetic differences produced by mutations. He argued that if the population is exposed to a new environment, some variants will by chance be able to cope better with the new conditions; they will survive and reproduce, transmitting their favorable characters to the next generation. Any that are less fitted to the environment will be unlikely to breed and may even die, so their characters are eliminated from the population. Over many generations this process of natural selection (Spencer called it the ‘survival of the fittest’) will change the species to adapt it to the environment.

Sometime before Darwin, J. B. Lamarck proposed that the inheritance of characters acquired during an organism’s lifetime could accumulate to give adaptive transmutation. An acquired character is produced by the organism’s behavior, which in the wild is usually a response to the environment. Thus, the ancestral giraffes, striving to reach the leaves of trees as an alternative source of food, were able to lengthen their necks, just as a weightlifter can acquire better developed muscles. If such characters are inherited, the next generation of giraffes would be born with longer necks and would then stretch them further. This mechanism was once popular in biology, and corresponds quite closely to how humans transmit new ideas to their descendants. But where teaching and learning provides a natural mechanism for ideas to be transmitted, modern genetics has shown that there is no way in which acquired biological characters can be inherited. Lamarckism has been banished from biology: The giraffe got its long neck because those who by chance inherited the genes producing that character reproduced faster than those who carried the genes for shorter necks. The genetic differences are the product of mutations which are random as far as the requirement of the species are concerned.

1.5 Nonadaptive Evolution

There are also several mechanism of biological evolution which suppose that not all developments are adaptive. The possibility that saltations (evolutionary jumps or macromutations) might found new populations was once taken seriously even by geneticists. The theory of orthogenesis supposed that the species was somehow programmed to vary continuously in a particular direction, forcing groups of species to evolve in parallel. Such ideas are now largely discounted in biology, although the possibility that there might be constraints on the range of possible variation is taken more seriously. In the social sciences, the question of whether all cultural and social differences serve a practical purpose has been much debated.

2. Pre-Darwinian Evolutionism

Darwin’s was not the first theory of evolution, although the publication of his Origin of Species in 1859 triggered the conversion of most scientists to evolutionism. Several earlier versions of the idea were proposed, and although rejected by conservative thinkers and most scientists, they did much to prepare the way for the reception of Darwinism.

2.1 Creation And Design

The early Church Fathers did not take the Genesis story of creation in the Bible literally, and it was the Protestant theologians of the seventeenth century who established the world view which survives in modern ‘young earth’ creationism. Archbishop James Ussher published his much-ridiculed calculation that the earth was created in 4004 BC. In such a climate of opinion a form of natural theology flourished in which it could be argued that God had designed and created all of the modern species just as we see them today, each with the structures and instincts appropriate for its intended habitat. John Ray and many other naturalists stressed the adaptation of species to their environment, and argued that similarities could be used to classify species because they expressed the rational order of God’s plan of creation—a view adopted by Carolus Linnaeus, who founded the modern system of biological classification (Greene 1959). Others exploited the ancient concept of the ‘chain of being,’ in which the plan of creation was a linear hierarchy with the human species at the top of the animal series. This was promoted without any sense of a progression through time, although the influence of the chain was retained in some of the early theories of progressive evolution.

2.2 Evolution In The Enlightenment

The more radical thinkers of the eighteenth-century Enlightenment recognized that the Earth was much older than Ussher’s estimate, and that the planet had changed considerably in the course of its history. Some explored the possibility that life had been produced by spontaneous generation from nonliving matter, and began to doubt that species remained unchanged through time. The French naturalist Georges Leclerc, Comte de Buffon, was especially influential in this respect, as were the materialist philosophers Denis Diderot and the Baron d’Holbach (Roger 1998).

Toward the end of the eighteenth century, Erasmus Darwin (grandfather of Charles) published a theory of transmutation in his Zoonomia, arguing that all living things were derived from a simple origin and had become more complex through their striving to adapt to the environment. These views were echoed in a more sophisticated form in J. B. Lamarck’s Zoological Philosophy of 1809. Lamarck accepted that there was a basic progressive force driving living things to ascend the chain of being over many generations, but he accepted that no linear pattern is visible in the present animal and plant kingdoms, explaining the divergence as a consequence of life’s constant need to adapt to an ever-changing environment. He invoked the inheritance of acquired characters (described above) to explain this process, and his name has been associated with that mechanism ever since (Corsi 1988).

2.3 The Fossil Record

Historians used to think that Lamarck was marginalized by his great rival, Georges Cuvier, who pioneered the study of fossil bones to reconstruct the history of life. Cuvier showed that there was a succession of populations in the course of the earth’s history, but insisted that species disappear abruptly from the record through the action of geological catastrophes. He rejected transmutation, but did not openly endorse the idea of divine creation—although the theory of a succession of creations was popularized by Cuvier’s British followers, including William Buckland. Although they had extended the Earth’s history, Cuvier and Buckland were convinced that the human race was a recent creation. Their work thus endorsed a view of human history in which there was no time for social evolution, many scholars still believing that language and the arts of civilization were taught to the earliest humans by their Creator.

Later historical work has shown that although the theory of successive creations was endorsed by the scientific establishment, more radical thinkers were attracted to Lamarck’s theory (Desmond 1989). Darwin, who developed his theory in the 1830s, concealed it for fear of being associated with this radical movement. The situation changed significantly following the anonymous publication of Robert Chambers’ Vestiges of the Natural History of Creation in 1844. Chambers argued that an ideology of progress was just what the middle class needed to support its demands for reform, and provided a scientific foundation for this ideology based on the idea of progressive evolution. Although he presented evolution as the unfolding of a divine plan, his book’s open recognition that humans are transformed animals, and that the mind is a product of the expansion of the brain caused an outcry from conservative thinkers. Over the following decade, however, Vestiges popularized the idea of progressive evolution, creating a space in which Darwin’s more scientific theory could have an impact.

3. The Development Of Darwin’s Theory

Charles Darwin was converted to evolutionism by his discoveries on the voyage of HMS Beagle (1831–6) and returned to England determined to understand how the process worked. He soon discovered the mechanism of natural selection, and continued to work on the various aspects of his theory in relative secrecy. Much controversy has surrounded Darwin’s discovery, because in addition to the many scientific factors influencing his thinking, he acknowledged the impact of Thomas Malthus’ principle of population, a product of the ideology of laissez-faire economics.

3.1 Scientific Influences

While studying at Cambridge, Darwin was attracted to William Paley’s Natural Theology of 1802, which restated the argument that species are adapted to their environment through the wisdom and benevolence of the Creator. On the voyage of the Beagle, his observations of South American geology converted him to Charles Lyell’s uniformitarian position, which accounted for all geological change in terms of the action of observable forces over vast periods of time. Biogeographical studies, especially those of the birds of the Galapagos Islands, forced him to rethink the idea of Divine Creation, because he saw that individual islands had each acquired a distinct species of immigrant families, such as the finches. On his return to England he soon accepted a complete evolutionary philosophy and began to search for a mechanism (Browne 1995).

Darwin saw that evolution must be a branching process and that the ultimate cause was the adaptation of isolated populations to different environments. He studied animal breeding and realized that breeders produce new varieties by selecting out individual animals with the required character and breeding only from these. Artificial selection draws on the natural and largely random variability of the population and allows only individuals with a selected character to breed, thereby enhancing that character in future generations. Darwin was soon looking for a natural equivalent of this process.

3.2 Malthus And Population

A vital step in Darwin’s thinking came when he read Malthus’ Essay on Population, which argued that populations breed so rapidly that they outstrip their food supply. Drawing on hints in Malthus, Darwin realized that population pressure must generate a struggle for existence in which many individuals die. He was now able to argue that any better-adapted variant would be more successful in the struggle and would be more likely to survive and reproduce, while poorly adapted variants would die, thus establishing the theory of natural selection described in Sect. 1.4. This gave Darwin the basis on which to erect a comprehensive account of evolution. The role played by Malthus’ ideologically loaded principle has, however, led to frequent charges that Darwin’s theory is itself a projection of laissez-faire values on to nature, with the consequence that the theory would be used to justify the competitive ethos of Victorian capitalism (Young 1985). Much controversy also surrounds the degree of Darwin’s commitment to the idea of progress. Although the selection theory undermines the ladder model of progress, historians now accept that Darwin’s thinking was influenced by the assumption that competition is the driving force of a more general form of progress. However, it is clear that Darwin’s commitment to branching evolution went far beyond the conventional social values later linked to his theory, while his application of the population principle undermined Malthus’ own ideology, which was an attempt to discredit the optimism of social progressionists.

Darwin let only a few close contacts know of his theory, while continuing to work on exploring its scientific implications. He came to appreciate that, even in a stable environment, specialization would allow natural selection to continue the process of divergence by which species separate from close relatives. Eventually, he began to write up his ideas for publication, but was interrupted in 1858 by the arrival of a paper by Alfred Russel Wallace describing a similar (although not identical) theory. The joint Darwin–Wallace papers were published in that year, while Darwin rushed to complete the Origin of Species.

4. Darwinism In Biology

The Origin of Species sparked an intense debate which soon led to the conversion of most scientists and educated people to evolutionism (Hull 1973). Despite initial opposition by conservative thinkers, the belief that the human race was the outcome of a progressive trend built into nature was accepted. In science, too, creationism was replaced by the expectation that all living species are the products of natural processes, and here, too, there was a widespread assumption that such processes guaranteed progress, even if super-imposed on a branching model of development. The success of Darwinism needs to be assessed with care because the popularity of this developmental model of evolution meant that the more radical implications of Darwin’s thinking were evaded (Bowler 1988). Far from welcoming the theory of natural selection, most nineteenth-century scientists, including some of Darwin’s leading supporters, were lukewarm about it. Opposition grew toward the end of the century, with many alternative mechanisms of evolution being tried out.

4.1 The History Of Life

Much of the first generation of evolutionists’ efforts focused on reconstructing the course of the history of life on earth from anatomical, embryological, biogeographical, and fossil evidence (Bowler 1996). Darwin himself treated this project with caution, but under the influence of the German biologist Ernst Haeckel, a whole generation of evolutionists strove to complete it. They had some successes, but the problems were much greater than they had anticipated and the project was sidelined by new developments in biology at the beginning of the twentieth century. Darwin had stressed the imperfection of the fossil record, but in a few important areas new discoveries helped to show how the major steps in evolution had taken place. Elsewhere, anatomical and embryological evidence was used to reconstruct evolutionary relationships, on the assumption that new characters were developed only once. In embryology, Darwin’s assumption that early stages of development would illustrate affinities was overtaken by Haeckel’s recapitulation theory, according to which earlier stages could actually illustrate ancestral adult forms. This way of thinking was a product of the progressionist way of thought, presupposing a privileged line of development toward maturity. Only in biogeography did it become clear that evolution was a highly irregular process, depending on the hazards of migration in a world where geological forces were constantly remolding the topography.

4.2 Natural Selection And The Alternatives

Historians have focused on the debates over natural selection, but this is more a product of our modern preoccupation with that mechanism than of its significance in late-nineteenth-century biology. One much discussed theme centers on Darwin’s concept of heredity, which was quite unlike the model of unitcharacter inheritance promoted by Mendelian genetics. It has even been argued that, without the concept of nonblending unit characters, natural selection was fatally flawed. It is true that Darwin was much concerned by Fleeming Jenkins’ 1867 review which argued that if male and female parental characters blend in the offspring, the advantages of a well-favored ‘sport’ or mutation would be diluted rapidly. But as A. R. Wallace pointed out, this objection is irrelevant if there is a continuous range of variation in the population, which is the case for most characters (Gayon 1998).

In fact, many of the objections to natural selection were intended only to show that evolution must be a more purposeful process than any mechanism based on random variation would allow. One major alternative which now became popular was the Lamarckian theory of the inheritance of acquired characters, in which the animals’ own behavior directs their variation. Some objections to natural selection were aimed at the whole idea of adaptive evolution and were intended to show that some internally driven force directed the organisms’ variation. The theory of orthogenesis supposed that variation generated linear evolutionary trends, while the theory of saltations assumed that macromutations could somehow found new breeding populations (Bowler 1983).

5. Human Origins And Social Evolution

Many religious thinkers objected to the idea that the human soul was the product of evolution from an animal ancestry. Much of this initial opposition was overcome by stressing the purposeful nature of evolution, allowing the appearance of humanity to be presented as the culmination of a divine plan (Moore 1979). Darwin’s theory of evolution threatened this assumption, and in his Descent of Man of 1871 he argued that humans had evolved from African apes which had stood upright as an adaptation to a new lifestyle on the open plains. Most late-nineteenth century accounts of human evolution, however, stressed the expansion of the brain as the main driving force of the process. At the same time, the idea of progressive evolution was applied to the history of human cultures and societies. Some evolutionists, including Spencer, stressed competition as the driving force of progress, leading to the charge that they were promoting a ‘social Darwinism’ designed to legitimize capitalist values. While Darwin’s theory was certainly associated with this movement, it was by no means the only scientific justification offered.

5.1 Cultural Evolutionism

Coincidentally with the Darwinian revolution, archaeologists undermined the belief that the human race was a recent creation, and exposed a vast period of prehistory in which our ancestors had used only stone tools. It was this initiative rather than any influence from Darwinism which led anthropologists such as Edward B. Tylor to equate modern ‘savages’ (i.e., people with relatively unsophisticated technology) with the ancestral stages through which civilized humans had passed in prehistoric times on their march toward a more mature culture. Cultural evolutionism was a product of the nineteenth-century developmental viewpoint, based on the ladder model of progress (Bowler 1989a). Tylor himself did not accept that humans had evolved from apes, but other cultural evolutionists, especially John Lubbock, linked the two modes of evolution and argued that ‘savages’ were biologically as well as culturally primitive—living examples of the earlier stages in human biological and mental evolution. Herbert Spencer’s model of social evolution, while stressing ostensibly the divergent nature of evolution, still presented ‘lower’ races as surviving primitives incapable of matching the mental powers of the Anglo-Saxons.

5.2 Social Darwinism

Spencer is most often remembered, however, as the social philosopher who linked the idea of progress to the ideology of free-enterprise individualism. As an exponent of laissez-faire, and the coiner of the term ‘survival of the fittest,’ it has been easy for critics to portray Spencer’s enthusiasm for struggle as the motor of progress as a direct application of the Darwinian selection theory to society, hence the image of him as the founder of ‘social Darwinism.’ Spencer worried about the elimination of the unfit, but in biology he defended Lamarckism, and his enthusiasm for struggle came more from a conviction that competition would stimulate individual self-improvement (which in a Lamarckian scheme could be passed on to future generations). Darwin’s theory is thus only one of the scientific factors which stimulated the ideology of ‘progress through struggle’—remembered preferentially because natural selection has now become the only mechanism by which that metaphor can be put into practice in biology (Bannister 1979). In the later nineteenth century, there was more emphasis on struggle between nations and races, which bypasses the central feature of natural selection within populations.

6. Modern Darwinism

By the end of the nineteenth century, the attempt to reconstruct the course of biological and social evolution had run out of steam. The social sciences abandoned the evolutionary model and any suggestion that human behavior is predetermined by biological factors. Biologists began to study the actual processes of heredity and variation, leading to the emergence of Mendelian genetics. The geneticists destroyed Lamarckism, and although at first hostile to the selection theory, they were eventually convinced that genetic mutation was the source of the random variation that is the raw material of natural selection. By the mid-twentieth century, the ‘Modern Synthesis’ of Darwinism and genetics had come to dominate biology but remained controversial elsewhere.

6.1 The Modern Darwinian Synthesis

In the last decade of the nineteenth century, the statistician Karl Pearson developed a mathematical model of natural selection acting on the variations in a wild population (Gayon 1998). But many biologists were now exploring the idea that new species were produced by saltations or macromutations, and it was partly from this tradition that the ‘rediscovery’ of Gregor Mendel’s laws of particulate inheritance emerged. Ignored since the 1860s, these laws postulated the existence of unit characters transmitted unchanged from one generation to the next. In 1900, Mendel’s laws were proclaimed as the basis for a new science of heredity, which William Bateson soon called ‘genetics.’ But Pearson was already hostile to Bateson’s saltationism, while Bateson rejected Pearson’s commitment to continuity and adaptation. Genetics was thus perceived as a new alternative to Darwinism, although it destroyed the credibility of Lamarckism by suggesting that there is no plausible way in which acquired characters can affect the gene (Bowler 1989b).

T. H. Morgan’s studies of genetic mutations showed that most macromutations are fatal, while small mutations simply add to the genetic variation in the population. Soon, Morgan conceded that natural selection would affect the reproductive success of the various genes. In the 1920s and 1930s, R. A. Fisher, J. B. vs. Haldane, and Sewall Wright brought together the statistical study of populations with the new genetics to create a science of population genetics based on the genetical theory of natural selection (Provine 1971, Mayr 1982). Naturalists such as Julian Huxley and Ernst Mayr now recognized that the adaptation of local populations to their environment was the most likely explanation of how species originate. The new selection theory was integrated with these wider developments to create what Huxley called the ‘Modern Synthesis.’

6.2 Later Developments

The Darwinian synthesis has dominated biology since the mid-twentieth century, although there have been numerous developments within it and some efforts to limit its applicability. Some modern thinkers now argue that the concept of natural selection can be applied to almost all areas where change is observed, sweeping away the whole idea of a universe which is the product of divine purpose. Most controversial are efforts to apply selection to human beings, especially in the area of social behavior. The science of sociobiology has been successful in explaining the social behavior of animals in terms of genetics and reproductive success, via the concept of the ‘selfish gene.’ But E. O. Wilson’s efforts to apply sociobiology to human behavior by suggesting that we are driven by instincts implanted by natural selection have been greeted with much suspicion by social scientists, and have also been dismissed as a revived ‘social Darwinism’ by the political left (Segerstrale 2000).

Bibliography:

1. Bannister R C 1979 Social Darwinism: Science and Myth in Anglo-American Social Thought. Temple University Press, Philadelphia, PA
2. Bowler P J 1983 The Eclipse of Darwinism: Anti-Darwinian Evolution Theories in the Decades around 1900. Johns Hopkins University Press, Baltimore, MD
3. Bowler P J 1988 The Non-Darwinian Revolution: Reinterpreting a Historical Myth. Johns Hopkins University Press, Baltimore, MD
4. Bowler P J 1989a The Invention of Progress: The Victorian and the Past. Basil Blackwell, Oxford, UK
5. Bowler P J 1989b The Mendelian Revolution: The Emergence of Hereditarian Concepts in Modern Science and Society. Athlone, London
6. Bowler P J 1989c Evolution: The History of an Idea. University of California Press, Berkeley, CA
7. Bowler P J 1996 Life’s Splendid Drama: Evolutionary Biology and the Reconstruction of Life’s Ancestry, 1860–1940. University of Chicago Press, Chicago
8. Browne J 1995 Charles Darwin: Voyaging. Jonathan Cape, London
9. Corsi P 1988 The Age of Lamarck: Evolutionary Theory in France, 1790–1830. University of California Press, Berkeley, CA
10. Desmond A 1989 The Politics of Evolution: Morphology, Medicine and Reform in Radical London. University of Chicago Press, Chicago
11. Gayon J 1998 Darwinism’s Struggle for Survival; Heredity and the Hypothesis of Natural Selection. Cambridge University Press, Cambridge, UK
12. Greene J C 1959 The Death of Adam: Evolution and its Impact on Western Thought. Iowa State University Press, Ames, IA
13. Hull D L 1973 Darwin and his Critics: The Reception of Darwin’s Theory by the Scientific Community. Harvard University Press, Cambridge, MA
14. Mayr E 1982 The Growth of Biological Thought. Harvard University Press, Cambridge, MA
15. Moore J R 1979 The Post-Darwinian Controversies: A Study of the Protestant Struggle to Come to terms with Darwin in Britain and America, 1870–1900. Cambridge University Press, Cambridge, UK
16. Provine W B 1971 The Origin of Theoretical Population Genetics. University of Chicago Press, Chicago
17. Roger J 1998 The Life Sciences in Eighteenth-century French Thought. Stanford University Press, Stanford, CA
18. Ruse M 1979 The Darwinian Revolution. University of Chicago Press, Chicago
19. Segerstrale U 2000 Defenders of the Truth: The Battle for Science in the Sociobiology Debate and Beyond. Oxford University Press, Oxford, UK
20. Young R M 1985 Darwin’s Metaphor: Nature’s Place in Victorian Culture. Cambridge University Press, Cambridge, UK