Everything about History Of Evolutionary Thought totally explained
» This article is about the history of evolutionary thought in biology. For the history of evolutionary thought in the social sciences, see social evolutionism. For the history of evolutionary thought generally, see evolutionism.
Evolutionary thought, the idea that species change over time, has roots in antiquity, in ideas of the
Greeks,
Romans,
Chinese and
Muslims. However, until the 18th century,
Western biological thinking was dominated by
essentialism, the idea that living forms are unchanging. During the
Enlightenment, evolutionary
cosmology and the
mechanical philosophy spread from the physical sciences to
natural history. Naturalists began to focus on the variability of species and the emergence of
paleontology with the concept of
extinction undermined the static view of nature. In the early 19th century
Jean-Baptiste Lamarck proposed his theory of the
transmutation of species, which was the first fully formed scientific theory of evolution.
The evolutionary theory often referred to as
Darwinism was first put forward by
Charles Darwin and
Alfred Russel Wallace and discussed in detail in Darwin's
On the Origin of Species (1859). Unlike Lamarck's theory, Darwinism proposed
common descent and a branching
tree of life. It was based on the idea of
natural selection, and it synthesized evidence from
animal husbandry,
biogeography,
geology,
morphology, and
embryology.
Darwin's work led to the rapid acceptance of
evolution, but the mechanism he proposed, natural selection, wasn't widely accepted until the 1940s. Most biologists argued that other factors drove evolution, such as inheritance of acquired characteristics (neo-Lamarckism), an innate drive for change (
orthogenesis), or sudden large mutations (
saltationism). The synthesis of natural selection with Mendelian genetics during the 1920s and 1930s, founded the new discipline of
population genetics. Throughout the 1930s and 1940s, population genetics became integrated with other branches of biology, finally resulting in a unified theory of evolution - the
modern evolutionary synthesis.
Following the establishment of
evolutionary biology, studies of
mutation and
variation in natural populations, combined with
biogeography and
systematics, led to sophisticated mathematical and causal models of evolution. Paleontology and
comparative anatomy allowed more detailed reconstructions of the
history of life. After the rise of
molecular genetics in the 1950s, the field of
molecular evolution developed, based on
DNA,
RNA, and
protein sequences. The
gene-centered view of evolution then rose to prominence in the 1960s, followed by the
neutral theory of molecular evolution, sparking debates over
adaptationism, the
units of selection, and the importance of
genetic drift. In the late 20th century, genetic
sequencing led to a reorganization of the tree of life into the
three-domain system, and the newly-recognized factors of
symbiogenesis and
horizontal gene transfer have introduced yet more complexity into evolutionary history.
Antiquity
Greek thought
Some
Greek philosophers discussed ideas that involved forms of organic evolution.
Anaximander claimed that life had originally developed in the sea and only later moved onto land, and
Empedocles also discussed a non-supernatural origin for living things. Empedocles even suggested a form of
natural selection, which Aristotle summarized as, "Wherever then all the parts came about [tobe] just what they'd have been if they'd come to be for an end, such things survived, being organized spontaneously in a fitting way; whereas those which grew otherwise perished and continue to perish..."
Plato (427/8–347/8 BC) was, in the words of biologist and historian
Ernst Mayr, "the great antihero of evolutionism," as he established the philosophy of
essentialism, which he called the
theory of forms. This theory holds that objects observed in the real world are only
reflections of a limited number of essences (
eide). Variation is merely the result of an imperfect reflection of these constant essences. In his
Timaeus, Plato set forth the idea that God had created the
cosmos and everything in it because He is good, and hence, "... free from jealousy, He desired that all things should be as like Himself as they could be." God created all conceivable forms of life, since "... without them the universe will be incomplete, for it won't contain every kind of animal which it ought to contain, if it's to be perfect." This idea, that all potential forms of life are essential to a perfect creation, is called the
plenitude principle, and it greatly influenced Christian thought.
Aristotle, (384–322 BC), one of the most influential of the Greek philosophers, is the earliest natural historian whose work has come down to us in any real detail. His writings on biology were the result of his research into natural history on the isle of
Lesbos, and have survived in the form of four books, usually known by their
Latin names,
De anima (on the essence of life),
Historia animalium (inquiries about animals),
De generatione animalium (reproduction), and
De partibus animalium (anatomy). These works contain some remarkably astute observations and interpretations by Aristotle, along with sundry myths and mistakes — reflecting the uneven state of knowledge during his time. However, for
Charles Singer, "Nothing is more remarkable than [Aristotle's] efforts to [exhibit] the relationships of living things as a
scala naturæ".
Roman thought
Titus
Lucretius Carus (d. 50 BC), the Roman
Epicurean and
atomist, wrote the poem
On the Nature of Things (
De rerum natura), describing the development of the living earth in stages: from atoms colliding in the void as swirls of dust to early plants and animals springing from the early earth's substance, then a succession of animals, including a series of progressively less brutish humans. Lucretius may be seen as the earliest believer in
hard inheritance. He said "For if each organism hadn't its own genetic bodies, how could we with certainty assign each to its mother?". The essence of Lucretius' ideas was naturalism, and the avoidance of supernatural interventions or explanations.
Middle Ages
Christian thought and the great chain of being
During the so-called
Dark Ages, Greek classical learning was all but lost to the West. However, contact with the
Islamic world, where Greek manuscripts were preserved and elaborated on, soon led to a massive spate of
Latin translations in the 12th century. Europeans were thus re-introduced to the works of Plato and Aristotle, as well as
Islamic thought.
Christian thinkers combined Aristotlean classification with Plato's ideas of the goodness of God, and of all potential life forms being present in a perfect creation, to organize all inanimate, animate, and spiritual beings, into a huge interconnected system: the
Scala Naturæ, or
great chain of being.
Within this system, everything that existed could be placed in order, from "lowest" to "highest", with
Hell at the bottom and
God at the top — below God, an angelic hierarchy marked by the orbits of the planets, mankind in an intermediate position, and worms the lowest of the animals. As the universe was ultimately perfect, the Great Chain was also perfect. There were no empty links in the chain, and no link was represented by more than one species. But this also implied that, since every link is occupied, and none can be occupied twice, then no species can ever move from one position to another. To do so would leave one level empty and put two species on another. Thus, in this Christianized version of Plato's perfect universe, species could never change, but must remain forever fixed, in accordance with the text of
Genesis. For humans to forget their position was even seen as sinful, whether they behaved like lower animals or aspired to a higher station than was given them by their Creator.
Creatures on adjacent steps were expected to closely resemble each other, an idea expressed in a saying which
Charles Darwin often quoted:
natura non facit saltum ("nature doesn't make leaps").
The first Muslim biologist and philosopher to put forth detailed speculations about evolution was the
Afro-Arab writer
al-Jahiz in the 9th century. He considered the effects of the environment on the likelihood of an animal to survive and evolve, and first described the
struggle for existence.
Ibn Miskawayh's
al-Fawz al-Asghar and the
Brethren of Purity's
Encyclopedia of the Brethren of Purity (
The Epistles of Ikhwan al-Safa) expressed ideas about how species appeared: from
matter into
vapor and thence to
water, then
minerals into
plants and then
animals, leading to
apes and, finally,
humans. The
polymath Ibn al-Haytham wrote a book in which he argued for
evolutionism (although not natural selection). Numerous other Islamic scholars and scientists, such as
Abū Rayhān al-Bīrūnī,
Nasir al-Din Tusi, and
Ibn Khaldun discussed and developed these ideas. Translated into
Latin, these works began to appear in the West after the
Renaissance and may have had an impact on Western science. However, most contemporary theories of evolution, such of those of
Gottfried Leibniz and
J. G. Herder, held that evolution was a fundamentally
spiritual process. In 1751,
Pierre Louis Maupertuis veered toward more materialist ground. He wrote of natural modifications occurring during reproduction and accumulating over the course of many generations, producing races and even new species, and he anticipated in general terms the idea of natural selection. Later in the 18th century
G. L. L. Buffon suggested that what most people referred to as species were really just well-marked varieties modified from an original form by environmental factors. For example he believed that Lions, Tigers, leopards and house cats might all have a common ancestor. He speculated that the 200 or so species of mammals then known might have descended from as few as 38 original forms. Buffon’s evolutionary ideas were strictly limited. He believed each of the original forms had arisen through spontaneous generation and that they were shaped by “internal moulds” that limited the amount of change. Between 1767 and 1792
James Burnett, Lord Monboddo included in his writings not only the concept that that man had descended from primates, but also that, in response to their environment, creatures had found methods of transforming their characteristics over long time intervals. In 1796 Charles Darwin’s grandfather,
Erasmus Darwin, published
Zoönomia, which suggested "that all warm-blooded animals have arisen from one living filament. In his 1802 poem
Temple of Nature, he described the rise of life from minute organisms living in the mud to its modern diversity.
19th century before On the Origin of Species
Paleontology and geology
In 1796,
Georges Cuvier published his findings on the differences between living
elephants and those found in the
fossil record. His analysis demonstrated that
mammoths and
mastodons were distinct species different from any living animal, effectively ending a long-running debate over the possibility of the extinction of a species.
William Smith began the process of ordering
rock strata by examining fossils in the layers while he worked on his geologic map of
England. Independently, in 1811, Georges Cuvier and
Alexandre Brongniart published an influential study of the geologic history of the region around Paris, which was based on the
stratigraphic succession of layers of rock. These works helped establish the antiquity of the earth. Cuvier advocated
catastrophism to explain the patterns of extinction and
faunal succession revealed by the fossil record.
Knowledge of the fossil record continued to advance rapidly during the first few decades of the 19th century. By the 1840s, the outlines of the
geologic timescale were becoming clear, and in 1841
John Phillips named three major eras, based on their predominant fauna: the
Paleozoic, dominated by marine invertebrates and fish, the
Mesozoic, the age of reptiles, and the current
Cenozoic age of mammals. This progressive picture of the history of life was accepted even by conservative English geologists like
Adam Sedgwick and
William Buckland; however, also like Cuvier, they attributed the progression to repeated catastrophic episodes of extinction followed by new episodes of creation. Unlike Cuvier, Buckland and some other advocates of
natural theology among British geologists made efforts to explicitly link the last catastrophic episode to the
biblical flood.
From 1830 to 1833,
Charles Lyell published his multi-volume work
Principles of Geology, which advocated a
uniformitarian alternative to the catastrophic theory of geology. Lyell claimed that, rather than being the products of cataclysmic (and possibly supernatural) events, the geologic features of the earth are better explained as the result of the same
gradual geologic forces observable in the present day — but acting over immensely long periods of time. Although Lyell opposed evolutionary ideas (even questioning the consensus that the fossil record demonstrates a true progression), his concept that the earth was shaped by forces working gradually over an extended period, and the immense age of the earth assumed by his theories, would strongly influence future evolutionary thinkers such as
Charles Darwin.
Transmutation of species
Jean-Baptiste Lamarck proposed in his
Philosophie Zoologique of 1809 a theory of the transmutation of species. Lamarck didn't believe that all living things shared a common ancestor. Rather he believed that simple forms of life were created continuously by
spontaneous generation. He also believed that an innate life force drove species to become more complex over time, advancing up a linear ladder of complexity that was related to the great chain of being. Lamarck also recognized that species were adapted to their environment. He explained this by saying that the same innate force driving increasing complexity, also caused the organs of an animal (or a plant) to change based on the use or disuse of that organ, just as muscles are affected by exercise. He argued that these changes would be inherited by the next generation and produce slow adaptation to the environment. It was this secondary mechanism of adaptation through the inheritance of acquired characteristics that would become known as
Lamarckism and would influence discussions of evolution into the 20th century. A radical British school of comparative anatomy that included the
anatomist Robert Grant was closely in touch with Lamarck's school of French
Transformationism, which contained scientists such as
Étienne Geoffroy Saint-Hilaire. Grant developed Lamarck'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. In 1826 an anonymous paper, probably written by
Robert Jameson, praised Lammarck for explaining how higher animals had “evolved” from the simplest worms, which was the first use of the word “evolved” in a modern sense.
In 1844 the Scottish publisher
Robert Chambers anonymously published an influential, and extremely controversial book of popular science entitled
Vestiges of the Natural History of Creation. This book proposed an evolutionary scenario for the origins of the solar system and life on earth. It claimed that the fossil record showed a progressive ascent of animals with current animals being branches off a main line that lead progressively to humanity. It implied that the transmutations lead to the unfolding of a preordained plan that had been woven into the laws that governed the universe. In this sense it was less completely materialistic than the ideas of radicals like Robert Grant, but its implication that humans were just the last step in the ascent of animal life incensed many conservative thinkers. The high profile of the public debate over
Vestiges, with its depiction of evolution as a progressive process, would greatly influence the perception of Darwin's theory a decade later.
Ideas about the transmutation of species were associated with the radical materialism of the
enlightenment and were attacked by more conservative thinkers. Cuvier attacked the ideas of Lamarck and
Geoffroy Saint-Hilaire, agreeing with Aristotle that species were immutable. Cuvier believed that the individual parts of an animal were too closely correlated with one another to allow for one part of the anatomy to change in isolation from the others, and argued that the fossil record showed patterns of catastrophic extinctions followed by re-population, rather than gradual change over time. He also noted that drawings of animals and animal mummies from Egypt, which were thousands of years old, showed no signs of change when compared with modern animals. The strength of Cuvier's arguments and his reputation as a leading scientist helped keep transmutational ideas out of the scientific mainstream for decades.
In Britain, where the philosophy of
natural theology remained influential,
William Paley wrote the book
Natural Theology with its famous
watchmaker analogy, at least in part as a response to the transmutational ideas of
Erasmus Darwin. Geologists influenced by natural theology, such as Buckland and Sedgwick, made a regular practice of attacking the evolutionary ideas of Lamarck, Grant, and
The Vestiges of the Natural History of Creation. Although the geologist
Charles Lyell opposed scriptural geology he also believed in the immutability of species, and in his
Principles of Geology (1830–1833), he criticized Lamarck's theories of development.
Anticipations of natural selection
Several writers anticipated aspects of Darwin's theory, and in the third edition of
On the Origin of Species published in 1861 he named those he'd learnt about in an introductory appendix,
An Historical Sketch of the Recent Progress of Opinion on the Origin of Species, which he added to in later editions.
In 1813,
William Charles Wells read before the Royal Society essays assuming that there had been evolution of humans, and recognising 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, writing that the paper "An Account of a White Female, part of whose Skin resembles that of a Negro" was published in 1818, and "he distinctly recognises the principle of natural selection, and this is the first recognition which has been indicated; but he applies it only to the races of man, and to certain characters alone." When Darwin was developing his theory, he was influenced by
Augustin de Candolle's
natural system of classification, which laid emphasis on the war between competing species.
Patrick Matthew wrote in the obscure book
Naval Timber & Arboriculture that was 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 discovered this work after the initial publication of the
Origin. In the brief historical sketch that Darwin included in the 3rd addition he says "Unfortunately the view was given by Mr. Matthew very briefly in an Appendix to a work on a different subject ... He clearly saw, however, the full force of the principle of natural selection."
It is important to understand that it's possible to look through the history of biology from the ancient Greeks onwards and discover anticipations of almost all of Darwin's key ideas. However, there are a couple of major differences between Darwin and his predecessors. Perhaps the most important difference is that the vast majority of Darwin's predecessors seem to have failed to understand the implications of their own ideas. As an example, Matthew chose to relegate his idea on natural selection to the appendix of a work on an unrelated subject, and William Charles Wells seems to have made little or no attempts to publicise his ideas beyond reading them to the Royal Society. Secondly, despite having enunciated the basic idea of natural selection, precursors of Darwin either assumed that it was self-evidently true, or gave merely
logical arguments for its importance and failed to provide any
empirical data. In other words, the anticipations of Darwin were merely formal and verbal.
T. H. Huxley pointed out in his essay on the reception of the
Origin of Species:
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're 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.
Natural selection
The
biogeographical patterns Charles Darwin observed in places such as the
Galapagos islands during
the voyage of the Beagle caused him to doubt the fixity of species, and in 1837 Darwin started the first of a series of secret notebooks on
transmutation. Darwin's observations lead him to view transmutation as a process of divergence and branching, rather than the ladder-like progression envisioned by Lamarck and others. In 1838 he read the new 6th edition of
An Essay on the Principle of Population, written in the late
1700s by
Thomas Malthus. Malthus' idea of population growth leading to a struggle for survival combined with Darwin's knowledge on how breeders selected traits, led to the
inception of Darwin's theory of
natural selection. Concerned by the intense controversy raging over other transmutational ideas, Darwin would develop this idea in private for the next 20 years, sharing it only with a handful of friendly naturalists through correspondence.
Unlike Darwin,
Alfred Russel Wallace, influenced by the book
Vestiges of the Natural History of Creation, already believed that transmutation of species occurred when he began his career as a naturalist. By 1855 his biogeographical observations during his field work in
South America and the
Malay Archipelago made him confident enough in a branching pattern of evolution to publish a paper that stated that every species originated in close proximity to an already existing closely allied species. Once again it was consideration of how the ideas of Malthus might apply to animal populations that lead Wallace to conclusions very similar to the ones reached by Darwin about the role of natural selection. In February 1858 Wallace, unaware of Darwin's unpublished ideas, wrote up his thoughts into an essay and mailed them to Darwin, asking for his opinion. The result was the joint
publication of Darwin's theory of natural selection with Wallace in July. Darwin also began work in earnest on
The Origin of Species, which he'd publish in 1859.
1859–1930s: Darwin and after Darwin
While transmutation of species was accepted by a sizable number of scientists before 1859, it was the publication of Charles Darwin's
On the Origin of Species that fundamentally transformed the debate over biological origins. Darwin argued that his branching version of evolution explained a wealth of facts in biogeography, anatomy, embryology, and other fields of biology. He also provided the first cogent mechanism by which evolutionary change could persist: his theory of natural selection.
One of the first and most important naturalists to be convinced by
Origin was the British anatomist
Thomas Henry Huxley. Huxley recognized that unlike the earlier transmutational ideas of Lamarck and
Vestiges, Darwin's theory provided a mechanism for evolution without supernatural involvement. Huxley would make advocacy of evolution a cornerstone of the program of the
X-club to reform and professionalize science by displacing
natural theology with
methodological naturalism, ending the domination of British natural science by the clergy. By the early 1870s in English-speaking countries, thanks partly to these efforts, evolution had become the mainstream scientific explanation for the origin of species. However, acceptance of evolution among scientists in non-English speaking nations such as France, and the countries of southern Europe and Latin America was slower. An exception to this was Germany, where both
August Weismann and
Ernst Haeckel championed this idea: with Haeckel using evolution to challenge the established tradition of metaphysical idealism in German biology, much as Huxley used it to challenge natural theology in Britain.
Darwin's theory succeeded in profoundly shaking scientific opinion regarding the development of life and resulted in a small social revolution. However, this theory couldn't explain several critical components of the evolutionary process. Namely, Darwin was unable to explain the source of variation in traits within a species, and couldn't identify a mechanism that could pass traits faithfully from one generation to the next. Darwin's hypothesis of
pangenesis, while relying in part on the inheritance of acquired characteristics, proved to be useful for statistical models of evolution that were developed by his cousin
Francis Galton and the "biometric" school of evolutionary thought. This idea was, however, of little use to biologists.
Application of the theory to humans
Charles Darwin was aware of the severe reaction in some parts of the scientific community against the suggestion made in
Vestiges of the Natural History of Creation that humans had arisen from animals by a process of transmutation. Therefore he almost completely ignored the topic of
human evolution in
The Origin of Species. Despite this precaution, the issue featured prominently in the debate that followed the book's publication. For most of the first half of the 19th century the scientific community believed that, although geology had shown that the earth and life were very old, human beings had appeared suddenly just a few thousand years before the present. However, a series of archaeological discoveries in the 1840s and 1850s showed stone tools associated with the remains of extinct animals. By the early 1860s, as summarized in Charles Lyell's 1863 book
Geological Evidences of the Antiquity of Man, it had become widely accepted that humans had existed during a prehistoric period - which stretched many thousands of years before the start of written history. This new view of human history was more compatible than the older one with an evolutionary origin for humanity. On the other hand, there was no fossil evidence of human evolution that was known at the time. The only human fossils found before the discovery of
Java man in the 1890s were either of anatomically modern humans, or of
Neanderthals that were too close, especially in the critical characteristic of cranial capacity, to modern humans for them to be convincing intermediates between humans and other primates.
Therefore the debate that immediately followed the publication of
The Origin of Species centered on the similarities and differences between humans and modern
apes. Richard Owen vigorously defended the traditional classification, suggested by
Carolus Linnaeus and Cuvier, that placed humans in a completely separate order from any of the other mammals. On the other hand, Huxley sought to demonstrate a close anatomical relationship between humans and apes. In one very famous incident, Huxley showed that Owen was mistaken in claiming that the brains of
gorillas lacked a structure present in human brains. Huxley summarized his argument in his highly influential 1863 book
Evidence as to Man's place in Nature. Another viewpoint was advocated by Charles Lyell and Alfred Russel Wallace. They agreed that humans shared a common ancestor with apes, but questioned whether any purely materialistic mechanism could account for some of the differences between humans and apes, especially some aspects of the human mind.
In the late 19th century the term
neo-Lamarckism came to be associated with the position of naturalists who viewed the inheritance of acquired characteristics as the most important evolutionary mechanism. Advocates of this position included the British writer and Darwin critic
Samuel Butler, the German biologist
Ernst Haeckel, and the American paleontologist
Edward Drinker Cope. They considered Lamarckism to be philosophically superior to Darwin's idea of selection acting on random variation. Cope looked for, and thought he found, patterns of linear progression in the fossil record. Inheritance of acquired characteristics was part of Haeckel's
recapitulation theory of evolution, which held that the embryological development of an organism repeats its evolutionary history.
When
T. H. Morgan began experimenting with breeding the fruit fly
Drosophila melanogaster he was a saltationist who hoped to demonstrate that a new species could be created in the lab by mutation alone. Instead, the work at his lab between 1910 and 1915 reconfirmed Mendelian genetics and provided solid experimental evidence linking it to chromosomal inheritance. It also demonstrated that most mutations had relatively small effects (such as a change in eye color), and that rather than creating a new species in a single step, they served to increase variation within the existing population.
Modern evolutionary synthesis
In the first couple of decades of the 20th century most field naturalists continued to believe that Lamarckian and orthogenic mechanisms of evolution provided the best explanation for the complexity they observed in the living world. However, as the field of genetics continued to develop, those views became less tenable.
Theodosius Dobzhansky had been a postdoctoral worker in T. H. Morgan's lab and had been influenced by the work on genetic diversity done by
Russian geneticists such as
Sergei Chetverikov. He would help to bridge the divide between the population geneticists and the field biologists with his 1937 book
Genetics and the origin of species. Dobzhansky examined the genetic diversity of wild populations, and showed that contrary to the assumptions of the population geneticists, these populations had large amounts of genetic diversity with marked differences between sub-populations. The book also took the highly mathematical work of the population geneticists and put it into more accessible form.
Ernst Mayr was influenced by the work of the
German biologist
Bernhard Rensch on how local environmental factors influenced the geographic distribution of sub-species and closely related species. Mayr followed up on Dobzhansky's work with the 1942 book
Systematics and the Origin of Species, which emphasized the importance of
allopatric speciation in the formation of new species. This form of speciation occurs when geographical isolation of a sub-population is followed by the development of mechanisms for
reproductive isolation. Mayr also formulated the
biological species concept that defined a species as a group of interbreeding or potentially interbreeding populations that were reproductively isolated from all other populations. In the 1944 book
Mode and Tempo in Evolution George Gaylord Simpson showed that the fossil record was consistent with the irregular non-directional pattern predicted by the developing evolutionary synthesis, and that the linear trends that earlier paleontologists had claimed supported orthogenesis and neo-Lamarckism didn't hold up to closer examination. In 1950
G. Ledyard Stebbins published
Variation and Evolution in Plants, which helped integrate
botany into the synthesis. The emerging cross-discipline consensus on how evolution worked would be known as the
modern evolutionary synthesis. It received its name from the book
Evolution: the modern synthesis by
Julian Huxley.
1940s–1960s: Molecular biology
In the 1940s, following up on
Griffith's experiment on
bacterial transformation,
Avery,
MacLeod and
McCarty definitively identified
deoxyribonucleic acid (DNA) as the
transforming principle responsible for transmitting genetic information. In 1953,
Francis Crick and
James D. 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
mutations 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.
Since the 1960s:
Gene centered view of evolution
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 R. Price and
John Maynard Smith. This viewpoint would be summarized and popularized in the influential 1976 book
The Selfish Gene by
Richard Dawkins. 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.
In 1973
Leigh Van Valen proposed the term
Red Queen, which he took from
Through the Looking Glass by
Lewis Carroll, to describe a scenario where a species involved in one or more
evolutionary arms races would have to constantly change just to keep pace with the species it was
co-evolving with. Hamilton, Williams and others suggested that this idea might help explain the
evolution of sexual reproduction, because the increased genetic diversity caused by sexual reproduction would help maintain resistance against rapidly evolving parasites. They felt this might explain why sexual reproduction was so common despite the tremendous cost from the gene-centric point of view of a system where only half of an organism's
genome is passed on during reproduction. The gene-centric view has also led to an increased interest in Darwin's old idea of
sexual selection, and more recently in topics such as
sexual conflict and
intragenomic conflict.
Punctuated equilibrium
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 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.
Sociobiology
W. D. Hamilton's work on kin selection also contributed to the emergence of the discipline of
sociobiology.
Altruism has been a difficult problem for evolutionary theorists going all the way back to Darwin. Significant progress was made in 1964 when Hamilton formulated the inequality known as
Hamilton's rule which showed how
eusociality (sterile worker classes) in insects and many other examples of altruistic behavior could have evolved through kin selection. Other theories, some derived from
game theory, such as
reciprocal altruism followed. In 1975
E.O. Wilson published the influential and highly controversial book which claimed evolutionary theory could help explain many aspects of animal, including human, behavior. Critics of sociobiology, including Stephen Jay Gould, and
Richard Lewontin, claimed that sociobiology greatly overstated the degree to which complex human behaviors could be determined by genetic factors. They also claimed that the theories of sociobiologists often reflected their own ideological biases. Despite these criticisms, work in sociobiology and the related discipline of
evolutionary psychology, including work on other aspects of the altruism problem, has continued.
Evolutionary paths and processes
Improvements in
sequencing methods have resulted in a large increase of sequenced
genomes, allowing the testing and refining of the theory of evolution using this huge amount of genome data. This research is providing insights into the molecular mechanisms of speciation and adaptation. Such genetic analysis has produced fundamental changes, such as
Carl Woese's
Three-domain system, in our understanding of the
evolutionary history of life. Advances in computational hardware and software have allowed for the testing and extrapolation of increasingly advanced evolutionary
models and the development of the field of
systems biology. Discoveries in
biotechnology are now producing methods for the synthesis and modification of entire genomes, driving evolutionary studies to the level where future experiments may involve the creation of entirely synthetic organisms.
Microbiology and horizontal gene transfer
Microbiology has just 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, particularly amongst
prokaryotes. Now, evolutionary researchers are taking advantage their improved understanding of microbial physiology and ecology, produced by the comparative ease of microbial
genomics, to explore the taxonomy and evolution of these organisms. These studies are revealing completely unanticipated levels of diversity amongst microbes, demonstrating that these organisms are the dominant form of life on Earth.
One particularly important outcome from studies on microbial evolution was the discovery in Japan of
horizontal gene transfer in 1959. This transfer of genetic material between different species of bacteria has played a major role in the propagation of
antibiotic resistance. More recently, as knowledge of
genomes has continued to expand, it has been suggested that lateral transfer of genetic material has played an important role in the evolution of all organisms. Indeed, as part of the
endosymbiotic theory for the origin of
organelles, horizontal gene transfer has been a critical step in the evolution of eukaryotes such as fungi, plants, and animals.
Evo-devo
In the
1980s and
1990s the tenets of the
modern evolutionary synthesis came under increasing scrutiny. There was a renewal of
structuralist themes in evolutionary biology in the work of biologists such as
Brian Goodwin and
Stuart Kauffman, which incorporated ideas from
cybernetics and
systems theory, and emphasized the
self-organizing processes of development as factors directing the course of evolution. The evolutionary biologist
Stephen Jay Gould revived earlier ideas of
heterochrony, alterations in the relative rates of developmental processes over the course of evolution, to account for the generation of novel forms, and, with the evolutionary biologist
Richard Lewontin, wrote an influential paper in 1979 suggesting that a change in one biological structure could arise incidentally as an accidental result of selection on another structure, rather than through direct selection for that particular adaptation.
Molecular data regarding the mechanisms underlying
development accumulated rapidly during the 1980s and '90s. For example, it became clear that the diversity of animal morphology wasn't the result of different sets of proteins regulating the development of different animals, but from changes in the deployment of a small set of proteins that were common to all animals. These proteins became known as the
"developmental toolkit". These various perspectives came to inform the disciplines of
phylogenetics,
paleontology and comparative developmental biology, spawning the new discipline of "evo-devo."
More recent work in this field has emphasized
phenotypic and developmental plasticity. It has been 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 material properties of cell aggregates, such as differential cell adhesion and biochemical oscillation. The resulting forms were later “locked in” by means of stabilizing natural selection. Experimental and theoretical research on these and related ideas has been presented in the multi-authored volume
Origination of Organismal Form.
Computer sciences
Recent decades have seen a rising interest in evolution within the
computer sciences. Evolutionary computation, specifically
evolutionary algorithms have found many applications in science and engineering as a means to solve complex problems, called
combinatorial optimization problems. These algorithms have underlying mathematical principles based on an analogy with evolution, sharing concepts such as populations, generations, selection and mutation. The performance of these algorithms, also compared to other, more traditional optimization methods, are seen as evidence by some in support of evolutionary biology and its validity.
Unconventional evolutionary thought
Gaia hypothesis
Pierre Teilhard de Chardin 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), but his ideas were not accepted by the scientific community. However, this hypothesis was later developed in a more limited and rigorous form by
James Lovelock, who proposed that the living and nonliving parts of Earth can be viewed as a complex interacting system with similarity to a single organism. This modified hypothesis postulates that all living things have a regulatory effect on the Earth's environment that promotes life overall. Although not fully accepted by the scientific community, this hypothesis has been a useful spur to further research and is a topic of current scientific debate.
Further Information
Get more info on 'History Of Evolutionary Thought'.
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