History of evolutionary thought


 * This article is about the history of evolutionary thought in biology. For history of evolutionary thought in social sciences, see social evolutionism. For history of evolutionary thought generally, see evolutionism.

The history of evolutionary thought is very long, since the idea of biological evolution has existed since ancient times, but the modern theory wasn't established until the 18th and 19th centuries, with scientists such as Jean-Baptiste Lamarck and Charles Darwin. Darwin emphasized the difference between his two main points: establishing the fact of evolution, and proposing the theory of natural selection to explain the mechanism of evolution.

From ancient times to 1850s
The idea of biological evolution was supported in ancient times, notably among Hellenists such as Democritus and his student Epicurus. As early as 400 BC the Greek atomists taught that the sun, earth, life, humans, civilization, and society emerged over aeons without divine intervention. Around 60 BC the Roman atomist Lucretius wrote the poem On the Nature of Things describing the development of the living earth in stages from atoms colliding in the void as swirls of dust, then early plants and animals springing from the early earth's substance, to a succession of animals including a series of progressively less brutish humans.

Whereas evolutionary ideas more or less died out in Europe after the fall of the Roman Empire, they continued to be propounded in the Islamic world. For example Al-Jahiz considered the effects of the environment on the the likelihood of an animal to survive, and Ibn al-Haitham went even further, writing a book in which he argued explicitly for evolutionism (although not, of course, natural selection), and numerous other Islamic scholars and scientists discussed these ideas. Translated into Latin, these works begain to appear in the West after the Renaissance and probably had a large (though subterranean) impact on Western science.

In 17th century English the word evolution (from the Latin word "evolutio", meaning "unroll like a scroll") began to be used to refer to an orderly sequence of events, particularly one in which the outcome was somehow contained within it from the start. In 1677 Sir Matthew Hale used the term evolution in attacking the atheistic atomism of Democritus and Epicurus. Hale set out the atomist idea that vibrations and collisions of atoms in the void without divine intervention had formed "Primordial Seeds" (semina) which were the "immediate, primitive, productive Principles of Men, Animals, Birds and Fishes." and called this mechanism an "absurdity" because "it must have potentially at least the whole Systeme of Humane Nature, or at least that Ideal Principle or Configuration thereof, in the evolution whereof the complement and formation of the Humane Nature must consist. . . and all this drawn from a fortuitous coalition of senseless and dead Atoms." Thus in evolutionism theories from 1700 to 1850 the earth, life, and universe developed without divine intervention.

Typically of these theories, Gottfried Leibniz in 1714 postulated "monads" inside objects causing motion by internal forces, and maintained that "the 'germs' of all things have always existed . . . [and] contain within themselves an internal principle of development which drives them on through a vast series of metamorphoses" to become the geological formations, lifeforms, psychologies, and civilizations of the present.

In his "Venus Physique" of 1745, Pierre Louis Maupertuis wrote of "Chance" producing "an innumerable multitude of individuals" a small number of which had "fitness" to satisfy their needs, while "another infinitely greater number... perished... The species we see today are but the smallest part of what blind destiny has produced...", anticipating in general terms the idea of natural selection.

Vague and general ideas of evolution continued to proliferate among the mid-eighteenth century philosophers of the Enlightenment. G. L. L. Buffon, in a cautious, tentative fashion, suggested rather than stated the mutability of species and the influence of the forces of nature in moulding organisms. Immanuel Kant, in his Theory of the Heavens (1755), foreshadowed a theory of the development of unformed matter into the highest types of animals and plants, and suggested that the gradations of structure revealed by comparative anatomy pointed to the existence of blood relationship of all organisms, due to derivation from a common ancestor. He appeared to believe, however, that the successive variations and modifications had arisen in response to mechanical laws of the organisms themselves rather than to the influence of their surroundings. J. G. von Herder suggested that increase by multiplication with the consequent struggle for existence had played a large part in the organic world, but his theme remained vague and undeveloped.

Charles Bonnet applied "Evolutionism" to biology in 1762, asserting that each feature of the embryo was performed in the parts; some of which came from the egg and some from the sperm. The performed parts expanded and rearranged themselves to grow into the adult, and so Bonnet was called a "preformationist."

In geology, James Hutton's uniformitarianism of 1785 said that the Earth must be much older than previously supposed, to allow time for mountains to be eroded and for sediment to form new rocks at the bottom of the sea, which in turn rose to become dry land.

Acquired characteristics, Lamarckism, and natural selection
Between 1794 and 1796 Erasmus Darwin wrote Zoönomia suggesting "that all warm-blooded animals have arisen from one living filament ... with the power of acquiring new parts" in response to stimuli, with each round of "improvements" being inherited by successive generations. In his "Temple of Nature" of 1802 he described recurring new earths appearing in a cycle of catastrophes, then life springing forth spontaneously to populate the new earth, with animals competing with each other, driven by sex, hunger, "the strongest and most active ... [surviving to] propagate the species, which should thence become improved." Erasmus Darwin cited the work of James Burnett, Lord Monboddo in his Temple of Nature in 1802.

Jean-Baptiste Lamarck proposed in his Philosophie Zoologique of 1809 a theory, later known as Lamarckism, by which characteristics that were "needed" were acquired (or diminished) during the lifetime of an organism then passed on to the offspring. He saw this resulting in the development of species in a progressive chain of development towards higher forms. Lamarck avoided the word "evolution" as he was describing acquired traits, while popular notions of "evolution" at the time asserted Bonnet's "preformationist" idea that the parent passed on the "germs" given by the grandparents unaltered by the parent's "learning."

William Smith began the process of ordering rock strata by examining fossils in the layers. Independently, Georges Cuvier and Alexandre Brongniart published their explanation of the antiquity of the Earth in 1811 on the principle of stratigraphic succession of the layers of the earth, but rejected evolution in favour of catastrophism.

In 1813, William Charles Wells produced essays assuming that there had been evolution of humans, and recognised the principle of natural selection. Charles Darwin and Alfred Russel Wallace were unaware of this work when they jointly published the theory in 1858, but Darwin later acknowledged that Wells had recognised the principle before them. Augustin de Candolle's natural system of classification laid emphasis on the "war" between competing species.

A radical British school of comparative anatomy which included the surgeon Robert Knox and the anatomist Robert Edmund Grant was closely in touch with Lamarck's school of French Transformationism, which included Étienne Geoffroy Saint-Hilaire. Grant developed Lamack's and Erasmus Darwin's ideas of transmutation and evolutionism, investigating homology to prove common descent. As a young student Charles Darwin joined Grant in investigations of the life cycle of marine animals. He also studied geology under professor Robert Jameson who wrote an anonymous paper in 1826 praising "Mr. Lamarck" for explaining how the higher animals had "evolved" from the "simplest worms" – this was the first use of the word "evolved" in a modern sense. Jameson's course closed with lectures on the "Origin of the Species of Animals".

Patrick Matthew wrote in Naval Timber & Arboriculture published in 1831 of "continual balancing of life to circumstance. ... [The] progeny of the same parents, under great differences of circumstance, might, in several generations, even become distinct species, incapable of co-reproduction." Charles Darwin found out about it after publication of the Origin, and wrote that it "briefly but completely anticipates the theory of Nat. Selection ... a complete but not developed anticipation!"

By 1833 the geologist Charles Lyell in the second volume of his Principles of Geology had set out a gradualist variation of creation beliefs in which each species had its "centre of creation" and was designed for the habitat, but would go extinct when the habitat changed. John Herschel supported this gradualist view and wrote to Lyell urging a search for natural laws underlying the "mystery of mysteries" of how species formed.

The computing pioneer Charles Babbage published his unofficial Ninth Bridgewater Treatise in 1837, putting forward the thesis that God had the omnipotence and foresight to create as a divine legislator, making laws (or programs) which then produced species at the appropriate times, rather than continually interfering with ad hoc miracles each time a new species was required.

By 1836 the anatomist Richard Owen had theories influenced by Johannes Peter Müller that living matter had an "organising energy", a life-force that directed the growth of tissues and also determined the lifespan of the individual and of the species. In the 1850s Owen developed ideas of "archetypes" in the Divine mind producing a sequence of species in "ordained continuous becoming" in which new species appeared at birth, not through natural selection.

In 1837 Charles Darwin started the first of a series of secret notebooks on transmutation. In 1838 he read the new 6th edition of Malthus's Essay on the Principle of Population and compared this with breeders selecting traits. This led to the inception of Darwin's theory, and the publication of Darwin's theory jointly with Wallace in 1858. Wallace considered Malthus' essay the most important book he read, and considered it "the most interesting coincidence" that both he and Darwin were independently led to the theory of evolution through reading Malthus. Charles Darwin was influenced by earlier writers including his grandfather Erasmus Darwin.

Another important work was the Vestiges of the Natural History of Creation, published anonymously in England in 1844. The actual author was Robert Chambers. It proposed a theory of evolution, modeled somewhat after that of Jean-Baptiste Lamarck, and created considerable political controversy in Victorian society for its radicalism and unorthodoxy. The work put forward a theory of evolution which encompassed everything from animals, human psychology, economics, and the solar system. Darwin himself later remarked that it was Vestiges which had prepared the world for his own theory.

Later discrediting of Lamarckism and Orthogenesis
Lamarckism became discredited as experiments simply did not support the concept that purely "acquired traits" were inherited. The mechanisms of inheritance and the notion of the Weismann barrier were not elucidated until later in the 19th century, after Lamarck's death. Lamarckism in toto has largely been discredited as a mechanism in evolution, although some scientists such as Eva Jablonka and colleagues have claimed that certain forms of epigenetic inheritance such as chromatin marking and DNA methylation may involve a form of "soft" Larmarckian evolution, although these claims are controversial. Some have also proposed that Lamarckian evolution may be accurately applied to cultural evolution.

Orthogenesis or orthogenetic evolution was the hypothesis that life has an innate tendency to move, in a unilinear fashion, to ever greater perfection. This hypothesis had a significant following in the 19th century and Jean-Baptiste Lamarck himself accepted the idea, where it had a central role in his theory of inheritance of acquired characteristics. Other proponents included Leo Berg, Henri Bergson and, for a time, the paleontologist Henry Fairfield Osborn. Orthogenesis was particularly accepted by paleontologists who believed that fossils indicated a gradual and constant unidirectional change. Those who accepted orthogenesis in this way, however, did not necessarily accept that the mechanism that drove orthogenesis was teleological (goal-directed).

The orthogenesis hypothesis began to collapse when it became clear that it could not explain the patterns found by paleontologists in the fossil record, which was non-linear with many complications. A few hung on to the orthogenesis hypothesis as late as the 1950s by claiming that the processes of macroevolution, the long term trends in evolution, were distinct from the processes of microevolution.

Anticipation of Darwinian thought
Although the pre-Darwinian writers amongst them invoked nearly every principle that Darwin or his successors have suggested, they failed to carry conviction with regard to evolution, and they neither propounded a coherent philosophy of variation nor suggested a mechanism by which variations that appeared might give rise to new species. The anticipations of Darwin were little more than formal and verbal. As T. H. Huxley pointed out in his essay on the reception of the Origin of Species in the second volume of Darwins Life and Letters:

''The suggestion that new species may result from the selective action of external conditions upon the variations from their specific type which individuals present and which we call spontaneous because we are ignorant of their causation is as wholly unknown to the historian of scientific ideas as it was to biological specialists before 1858. But that suggestion is the central idea of the Origin of Species, and contains the quintessence of Darwinism.''

1850s - early 20th century: Darwin's theory
While transmutation of species was accepted by a sizeable number of scientists before 1859, it was the publication of Charles Darwin's The Origin of Species which provided the first cogent mechanism by which evolutionary change could persist: his theory of natural selection. Darwin was motivated to publish his work on evolution after receiving a letter from Alfred Russel Wallace, in which Wallace revealed his own discovery of natural selection. As such, Wallace is sometimes given shared credit for the theory of evolution. However, Wallace himself backed away from claiming too much credit, admitting that Darwin's formulation of the theory and his work on evolution went far beyond Wallace's conjectures in scope and explanatory power (he would later, to Darwin's great disappointment, back away completely from the idea that humans were evolved by natural means as he began to turn towards spiritualism).



Wilkins identifies Darwin's into seven categories:


 * 1) Transmutationism
 * 2) Common descent
 * 3) Struggle for existence
 * 4) Natural selection
 * 5) Sexual selection
 * 6) Biogeographic distribution
 * 7) Heredity

Darwin's theory, though it succeeded in profoundly shaking scientific opinion regarding the development of life (and indeed resulted in a small social revolution), could not explain several critical components of the evolutionary process. Namely, he was unable to explain the source of variation in traits within a species, and he could not provide a mechanism whereby traits were passed faithfully from one generation to the next. Darwin's theory of pangenesis, while relying in part on the inheritance of acquired characteristics, proved to be useful for statistical models of evolution developed by his cousin Francis Galton and the "biometric" school of evolutionary thought. It was, however, found to be of little use to biologists.

Expanding Darwin's theory: the Mendelian-biometrician debates
While the scientific community generally accepted that evolution had occurred, many disagreed that it had happened under the conditions or mechanisms provided by Darwin. In the years immediately following Darwin's death, evolutionary thought fractured into a number of interpretations, include neo-Darwinism, neo-Larmarckism, orthogenesis, Mendelism, the biometric approach, and mutation theory. Eventually this boiled down to a debate between two camps. The Mendelians, advocating discrete variation, were led by William Bateson (coiner of the word genetics) and Hugo de Vries (coiner of the word mutation). Their opponents were the biometricians, advocating continuous variation; their leaders Karl Pearson and Walter Frank Raphael Weldon, following in the tradition of Francis Galton.

An important issue in the debate between the Mendelians and the biometricians was the nature of variation in species. Darwin and Wallace believed that small variations were more important than large ones, since small variations hewed closely to an already-proven model. The biometricians agreed with this position, while the Mendelians insisted that discontinuous species were unlikely to arise from a continuous process of change. While the immediate issue of speciation was resolved in large part by the clear definition of a species as a reproductively isolated population, the rate of evolution would arise again as a point of contention in the late 20th century with the proposal of punctuated equilibrium. Most other questions resolving variation were resolved with the recognition that the size of a genotypic change did not always correspond with the size of the resulting phenotypic change.

Another source of clashes between Mendelians and biometricians was the debate over the origins of variation. Mendelians argued for intrinsic variations originating from genetic transmission; biometricians, observing primarily the phenotype of the organism, were not yet prepared to abandon Lamarckian views on the heritability of acquired characteristics. August Weismann was among those who demonstrated that acquired characteristics were not always inherited, pointing out the existence of worker ants and worker bees, and the importance of 'germ plasm' or gametes in the biology of reproduction. The recognition of means of postnatal adaptation as inherited traits did much to explain acquired characteristics.

Today, it is recognized that most acquired features of an organism change only the somatic cells of an organism and play no part in heredity or evolution. Acquired and inheritable, or epigenetic, traits do exist -- for example prions and DNA methylation. However, the vast majority of the features of an organism are considered to be either inherited through the DNA of the parent(s) or induced after birth. Elucidating which observed phenotypes are genetically inherited and which are environmentally induced remains an important and ongoing part of the study of genetics, developmental biology, and medicine.

The ideas of both the Mendelian and biometric campus were assembled into the modern evolutionary synthesis by the 1930s. Both camps had valid points: modern theory describes how Mendelian characteristics stem from monogenetic variation (mutations in one gene), while biometric characteristics stem from polygenetic variation (mutations in many genes).

1920s-1940s: the modern evolutionary synthesis
These questions of interpretation were not settled until the early 20th century, beginning with the work of an Austrian monk named Gregor Mendel in the late 19th century, who outlined, through a series of ingeniously devised experiments, a model for inheritance of traits based on the fundamental unit of the gene. Mendel's work was unappreciated at the time and largely ignored by the biological community. When it was "rediscovered" in 1900, it led to a storm of conflict between Mendelians (Charles Benedict Davenport) and biometricians (Walter Frank Raphael Weldon and Karl Pearson), who insisted that the great majority of traits important to evolution must show continuous variation that was not explainable by Mendelian analysis.

Eventually, the two models were reconciled and merged, primarily through the work of the biologist and statistician R.A. Fisher. This combined approach, applying a rigorous statistical model to Mendel's theories of inheritance via genes, became known in the 1930s and 1940s as the modern synthesis of Darwin's theory.

1940s-1960s: developments following molecular biology
In the 1940s, following up on Griffith's experiment, Avery, McCleod and McCarty definitively identified deoxyribonucleic acid (DNA) as the "transforming principle" responsible for transmitting genetic information. In 1953, Francis Crick and James Watson published their famous paper on the structure of DNA, based on the research of Rosalind Franklin and Maurice Wilkins. These developments ignited the era of molecular biology and transformed the understanding of evolution into a molecular process: the mutation of segments of DNA.

During this era of molecular biology, it also became clear that a major mechanism for variation within a population is the mutagenesis of DNA. In the mid-1970s, Motoo Kimura formulated the neutral theory of molecular evolution, firmly establishing the importance of genetic drift as a major mechanism of evolution. The theory sparked the "neutralist-selectionist" debate, partially solved by the development of Tomoko Ohta's nearly neutral theory of evolution.

1960s-1980s: Gene centered view of evolution, punctuated equilibrium


In the mid-1960s, George C. Williams strongly critiqued verbal explanations of adaptations couched in terms of "survival of the species" (essentially group selection arguments). Such explanations were largely replaced by a gene-centered view of evolution, epitomised by the kin selection arguments of W. D. Hamilton, George Price and John Maynard Smith. Models of the period showed that group selection was severely limited in its strength, although these models have since been shown to be too limited and newer models do admit the possibility of significant multi-level selection.

One of the most prominent debates arising during this time period was over the theory of punctuated equilibrium, a theory propounded by Niles Eldredge and Stephen Jay Gould to describe and account for the pattern of fossil species persisting phenotypically unchanged for long periods (what they termed stasis), with relatively brief periods of phenotypic change during speciation.

1970s-2000s: evolutionary biology as a discipline
Evolutionary biology as an academic discipline in its own right emerged as a result of the modern evolutionary synthesis in the 1930s and 1940s. It was not until the 1970s and 1980s, however, that a significant number of universities had departments that specifically included the term evolutionary biology in their titles. In the United States, as a result of the rapid growth of molecular and cell biology, many universities have split (or aggregated) their biology departments into molecular and cell biology-style departments and ecology and evolutionary biology-style departments (which often have subsumed older departments in paleontology, zoology and the like).

Microbiology has recently developed into an evolutionary discipline. It was originally ignored due to the paucity of morphological traits and the lack of a species concept in microbiology. Now, evolutionary researchers are taking advantage our extensive understanding of microbial physiology, the ease of microbial genomics, and the quick generation time of some microbes to answer evolutionary questions. Similar features have led to progress in viral evolution, particularly for bacteriophage.

Recent developments in evolutionary theory
Daniel Dennett (1995) argues in Darwin's Dangerous Idea that natural selection is an algorithmic process applicable to many circumstances besides biological evolution. This conception of evolutionary has been dubbed "universal Darwinism".

Symbiogenesis
Another extension to the standard modern synthesis, advocated by Lynn Margulis, is symbiogenesis. Symbiogenesis argues that acquisition and accumulation of random mutations or genetic drift are not sufficient to explain how new inherited variations occur in evolution. This theory states that species arise from the merger of independent organisms through symbiosis. Symbiogenesis emphasizes the impact of co-operation rather than Darwinian competition. This commonly occurs in multigenomic organisms throughout nature.

Evo-devo
Molecular data regarding the mechanisms underlying development started to accrue quickly during the 1980's and '90's. As scientists began to compare the developmental mechanisms in different organisms, they realized that these mechanisms are conserved through deep evolutionary time. By combining the disciplines of phylogenetics, paleontology and comparative developmental biology, scientists try to infer the way in which early organisms developed, thus spawning the new discipline of "evo-devo."

Neo-structuralist themes in evolutionary theory
In the 1980s and 1990s there was a renewal of structuralist themes in evolutionary biology by biologists such as Brian Goodwin, that incorporates ideas from cybernetics and systems theory, and that emphasizes the role of self-organized processes as being at least as important as the role of natural selection.

It is hypothesized, for example, that the rapid emergence of basic metazoan body plans in the Cambrian Explosion was due in part to changes in the environment acting on inherent properties of cell aggregates, such as differential cell adhesion. Such material-based plasticity led to the spontaneous emergence of metazoan body forms. The resulting forms were later “locked in” by means of stabilizing natural selection. Developmental biologists Stuart Newman and Gerd B. Müller have presented recent work relating to this view in the edited volume, Origination of Organismal Form.

Some extreme variants consider natural selection as the result of biological evolution and not its cause, though most neo-structuralist biologists would not go this far.

Altruism
Altruism has been one of the last (and most deeply embedded) thorns in the side of evolutionary theory, but recent developments in game theory have suggested explanations with an evolutionary context. If humans evolved, then so did human minds, and if minds evolved, then so does behaviour - including, according to these models, altruistic tendencies.

Theories of eusociality and the undoubted advantages of kin selection have made good progress in this direction, but they are far from unproblematic. Some writers have pointed out that the conscience is just another aspect of our mental behaviour, and propose an evolutionary explanation for the existence of conscience and therefore altruism. One recent suggestion, expressed most eloquently by the philosopher Daniel Dennett, was initially developed when considering the problem of so-called 'free riders' in the tragedy of the commons, a larger-scale version of the Prisoner's Dilemma.

Horizontal gene transfer
"While horizontal gene transfer is well-known among bacteria, it is only within the past 10 years that its occurrence has become recognized among higher plants and animals. The scope for horizontal gene transfer is essentially the entire biosphere, with bacteria and viruses serving both as intermediaries for gene trafficking and as reservoirs for gene multiplication and recombination (the process of making new combinations of genetic material)." .

Teilhard de Chardin's and Huxley's theories
Pierre Teilhard de Chardin and Julian Huxley formulated theories describing the gradual development of the Universe from subatomic particles to human society, considered by Teilhard as the last stage. (see Gaia theory). These are not generally recognized as scientifically rigorous.

Nine levels of development are described in their scheme. Stages one through five are grouped into the Lithosphere, also called Geosphere or Physiosphere, where the evolution of the structure of organisms is ruled by mechanical laws and coincidence. Levels six, seven, and eight are the classical biological stages. Stages six through eight are collectively called the Biosphere, where the progress of the structure of the organisms is ruled by genetic mechanisms. The actual stage, stage 9, is called the Noosphere, where the structure of human society is ruled by psychological, informational and communicative processes.