3.3 History of Evolutionary Thought
Evolutionary thought, the concept that species change over time, has its roots in antiquity, in the ideas of the Greeks, Romans, Chinese and Arabs. However, until the 18th century, Western biological thinking was dominated by the belief that every species has essential characteristics that are unalterable. This began to change when, 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; the emergence of palaeontology with the concept of extinction further undermined the static view of nature. In the early 19th century, Jean-Baptiste Lamarck proposed his theory of the transmutation of species
The debate over Darwin’s work led to the rapid acceptance of the general concept of evolution, but the specific mechanism he proposed, natural selection, was not widely accepted until revived in developments in the 1930s which were cemented in the 1940s. Most biologists argued that other factors were responsible for 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 biological fields, resulting in a widely applicable theory of evolution that encompassed much of biology —the modern evolutionary synthesis.
The Tree of Life as depicted by Ernst Haeckel in “The Evolution of Man” (1879) illustrates the 19th-century view that evolution was a progressive process leading towards man.
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. Palaeontology 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 protein sequences and immunological tests, and later incorporating RNA and DNA studies. The gene-centred view of evolution rose to prominence in the 1960s, followed by the neutral theory of molecular evolution, sparking debates over adaptationism, the units of selection, and the relative importance of genetic drift versus natural selection. In the late 20th century, DNA sequencing led to molecular phylogenetics and the reorganization of the tree of life into the three-domain system. In addition, the newly recognized factors of symbiogenesis and horizontal gene transfer introduced yet more complexity into evolutionary history.
Greek philosophers discussed ideas that involved forms of organic evolution. Anaximander (c. 610–546 BC) proposed that life had originally developed in the sea and only later moved onto land, and Empedocles (c. 490–430 BC) wrote of a non-supernatural origin for living things. Empedocles suggested that adaptation did not require an organizer or final cause.
Plato (c. 428–348 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 the Demiurge had created the cosmos and everything in it because, being good, and hence, “… free from jealousy, He desired that all things should be as like Himself as they could be”. The creator created all conceivable forms of life, since “… without them the universe will be incomplete, for it will not contain every kind of animal which it ought to contain, if it is to be perfect”. This idea, that all potential forms of life are essential to a perfect creation, is called the plenitude principle, and greatly influenced Christian thought.
Aristotle (384–322 BC) writings on biology were the result of his research into natural history on and around the isle of Lesbos. His four books, De anima (on the essence of life), Historia animalium (inquiries about animals), De generatione animalium (reproduction), and De partibus animalium (anatomy) summarize his thoughts. Aristotle’s works contain some remarkably astute observations and interpretations, along with sundry myths and mistakes. However, for Charles Singer, “Nothing is more remarkable than Aristotle’s efforts to exhibit the relationships of living things as a scala naturæ.” This scala naturæ, described in Historia animalium, classified organisms in relation to a hierarchical “Ladder of Life” or “Chain of Being”, placing them according to their complexity of structure and function, relative to organisms that showed greater vitality and ability to move described as “higher organisms”.
Ideas on evolution were expressed by ancient Chinese thinkers such as Zhuangzi (Chuang Tzu), a Taoist philosopher who lived around the 4th century BC. According to Joseph Needham, Taoism explicitly denies the fixity of biological species and Taoist philosophers speculated that species had developed differing attributes in response to differing environments. Humans, nature and the heavens are seen as existing in a state of “constant transformation” known as the Tao, in contrast with the more static view of nature typical of Western thought.
Titus Lucretius Carus (d. 50 BC), the Roman philosopher and atomist, wrote the poem On the “Nature of Things” (“De rerum natura”), which provides the best surviving explanation of the ideas of the Greek Epicurean philosophers. It describes the development of the cosmos, the Earth, living things, and human society through purely naturalistic mechanisms, without any reference to supernatural involvement.
Augustine of Hippo
In line with earlier Greek thought, the 4th century bishop and theologian, St. Augustine of Hippo, wrote that the creation story in Genesis should not be read too literally. In his book “De Genesi ad literam” (“On the Literal Interpretation of Genesis”), he stated that in some cases new creatures had come about through the “decomposition” of earlier forms of life. For Augustine, “plant, fowl and animal life are not perfect… but created in a state of potentiality”, unlike what he considered the theologically perfect forms of angels, the firmament and the human soul. Augustine’s idea ‘that forms of life had been transformed “slowly over time”‘ prompted Father Giuseppe Tanzella-Nitti, Professor of Theology at the Pontifical Santa Croce University in Rome, to claim that Augustine had suggested a form of evolution.
3.3.2 Middle Ages
Islamic philosophy and the struggle for existence
Whereas Greek and Roman evolutionary ideas died out in Europe after the fall of the Roman Empire, they were not lost to Islamic scientists and philosophers. In the Islamic Golden Age, early ideas on evolution were taught in Islamic schools. John William Draper, the 19th-century scientist, philosopher and historian, discussed the 12th-century writings of al-Khazini as part of what he called the “Mohammedan theory of evolution”.
The first Muslim biologist and philosopher to speculate about evolution was the Afro-Arab writer al-Jahiz in the 9th century. He considered the effects of the environment on an animal’s chances for survival, and described the struggle for existence. Ibn Miskawayh‘s al-Fawz al-Asghar and the “Brethren of Purity‘s Encyclopaedia of the Brethren of Purity” (The Epistles of Ikhwan al-Safa) set forth ideas on how species developed: from matter into vapour 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 Europe after the Renaissance and may have had an impact on Western science.
Christian philosophy and the great chain of being
During the Early Middle 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 to Islamic thought. Christian thinkers of the scholastic school, in particular Abelard and Aquinas, combined Aristotelian 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 specie. Therefore no species could ever move from one position to another. Thus, in this Christianized version of Plato’s perfect universe, species could never change, but remained forever fixed, in accordance with the text of Genesis. For humans to forget their position was 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 the saying: natura non facit saltum (“nature does not make leaps”).
3.3.3 Renaissance and Enlightenment
Pierre Belon compared the skeletons of birds and humans in his Book of Birds (1555).
Some evolutionist theories explored between 1650 and 1800 postulated that the universe, including life on Earth, had developed mechanically, entirely without divine guidance. Around this time, the mechanical philosophy of René Descartes began to encourage the metaphor of the universe as a machine that would come to characterise the scientific revolution. 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, the French natural philosopher 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 further speculated that the 200 or so species of mammals then known might have descended from as few as 38 original forms. Buffon believed each of the original forms had arisen through spontaneous generation and that each was 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 man had descended from primates, but also that, in response to the environment, creatures had found methods of transforming their characteristics over long time intervals. Charles Darwin’s grandfather, Erasmus Darwin, published Zoönomia in 1796, 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 all of its modern diversity.
Early 19th century
Diagram of the geologic timescale from an 1861 book by Richard Owen showing the appearance of major animal types
3.3.4 Palaeontology 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, based on the stratigraphic succession of rock layers. These works helped establish the antiquity of the Earth.
Knowledge of the fossil record continued to advance rapidly in 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 the predominant fauna of each: 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, like Cuvier, they attributed the progression to repeated catastrophic episodes of extinction followed by new episodes of creation.
From 1830 to 1833, Charles Lyell published his multi-volume work “Principles of Geology”, which, building on Hutton’s ideas, advocated an 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, 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.
3.3.5 Transmutation of Species
In 1844, the Scottish publisher Robert Chambers anonymously published an extremely controversial but widely read book 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 leads 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 only the last step in the ascent of animal life incensed many conservative thinkers.
Idealists such as Louis Agassiz and Richard Owen believed that each species was fixed and unchangeable because it represented an idea in the mind of the creator. They believed that relationships between species could be discerned from developmental patterns in embryology, as well as in the fossil record, but that these relationships represented an underlying pattern of divine thought, with progressive creation leading to increasing complexity and culminating in humanity. Owen developed the idea of “archetypes” in the Divine mind that would produce a sequence of species related by anatomical homologies, such as vertebrate limbs. Owen led a public campaign that successfully marginalized Robert Grant in the scientific community. Darwin would make good use of the homologies analyzed by Owen in his own theory, but the harsh treatment of Grant, and the controversy surrounding Vestiges, would contribute to his decision to delay publishing his ideas.
3.3.6 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 Darwin named those he knew about in an introductory appendix, “An Historical Sketch of the Recent Progress of Opinion on the Origin of Species”, which he expanded 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 Russell 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. 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” (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 edition 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 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, as historian of science Peter J. Bowler says, “Through a combination of bold theorizing and comprehensive evaluation, Darwin came up with a concept of evolution that was unique for the time.”
T. H. Huxley said 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 are ignorant of their causation is as wholly unknown to the historian of scientific ideas as it was to biological specialists before 1858.”
Darwin‘s first sketch of an evolutionary tree from his First Notebook on Transmutation of Species (1837)
3.3.7 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 led 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. Darwin did not publish his ideas on evolution for 20 years but he shared them with other naturalists and friends, such as Joseph Hooker, with whom he discussed his unpublished 1844 essay on natural selection.
Unlike Darwin, Alfred Russell Wallace, influenced by the book “Vestiges of the Natural History of Creation”, already suspected 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 stating that every species originated in close proximity to an already existing closely allied species. In February 1858 Wallace, unaware of Darwin’s unpublished ideas, composed his thoughts into an essay and mailed them to Darwin, asking for his opinion. The result was the joint publication in July of an extract from Darwin’s 1844 essay along with Wallace’s letter. Darwin also began work in earnest on “The Origin of Species”, which he would publish in 1859.
3.3.8 1859–1930s: Darwin and his Legacy
By the 1850s whether or not species evolved was a subject of intense debate, with prominent scientists arguing both sides of the issue. However, it was the publication of Charles Darwin‘s “On the Origin of Species” (1859) that fundamentally transformed the discussion 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.
The British anatomist Thomas Henry Huxley was one of the first to be convinced. Huxley recognized that unlike the earlier transmutational ideas of Lamarck and Vestiges, Darwin’s theory provided a mechanism for evolution without supernatural involvement, even if he was not completely convinced that natural selection was the key evolutionary mechanism. By the early 1870s, in English-speaking countries evolution had become the mainstream scientific explanation for the origin of species. In his campaign for public and scientific acceptance of Darwin’s theory, Huxley made extensive use of new evidence for evolution from palaeontology. This included evidence that birds had evolved from reptiles. Another important line of evidence was the finding of fossils that helped trace the evolution of the horse from its small five-toed ancestors. However, acceptance of evolution among scientists in non-English speaking nations such as France, and the countries of southern Europe and Latin America were slower. An exception to this was Germany, where both August Weismann and Ernst Haeckel championed this idea: Haeckel used 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 altering scientific opinion regarding the development of life. However, this theory could not explain several critical components of the evolutionary process. Specifically, Darwin was unable to explain the source of variation in traits within a species, and could not 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.
3.3.9 Application to Humans
This illustration was the frontispiece of Thomas Henry Huxley‘s book “Evidence as to Man’s Place in Nature” (1863).
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”. In the first half of the 19th century, the scientific community believed that 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 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 view of human history was more compatible with an evolutionary origin for humanity than was the older view. On the other hand, at that time there was no fossil evidence to demonstrate human evolution. 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” centred on the similarities and differences between humans and modern apes. Carolus Linnaeus had been criticised in the 18th century for grouping humans and apes together as primates in his ground breaking classification system. Richard Owen vigorously defended the classification suggested by Cuvier and Johann Friedrich Blumenbach that placed humans in a separate order from any of the other mammals. On the other hand, Thomas Henry Huxley sought to demonstrate a close anatomical relationship between humans and apes. 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 1863 book “Evidence as to Man’s Place in Nature”. Another viewpoint was advocated by Charles Lyell and Alfred Russell Wallace. They agreed that humans shared a common ancestor with apes, but questioned whether any purely materialistic mechanism could account for all the differences between humans and apes, especially some aspects of the human mind.
In 1871, Darwin published “The Descent of Man” and “Selection in Relation to Sex”, which contained his views on human evolution. Darwin argued that the differences between the human mind and the minds of the higher animals were a matter of degree rather than of kind. He argued that all the differences between humans and apes were explained by a combination of the selective pressures that came from our ancestors moving from the trees to the plains, and sexual selection.
3.3.10 Alternatives to Natural Selection
Evolution was widely accepted in scientific circles within a few years of the publication of “Origin”, but the acceptance of natural selection as its driving mechanism was much less widespread. The four major alternatives to natural selection in the late 19th century were theistic evolution, neo-Lamarckism, orthogenesis, and saltationism.
i- Theistic evolution was the idea that God intervened in the process of evolution to guide it in such a way that the living world could still be considered to be designed. However, this idea gradually fell out of favour among scientists, as they became more and more committed to the idea of methodological naturalism and came to believe that direct appeals to supernatural involvement were scientifically unproductive. By 1900, theistic evolution had largely disappeared from professional scientific discussions, although it retained a strong popular following.
ii- ii- 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. Critics of neo-Lamarckism, such as the German biologist August Weismann and Alfred Russell Wallace pointed out that no one had ever produced solid evidence for the inheritance of acquired characteristics. Despite these criticisms, neo-Lamarckism remained the most popular alternative to natural selection at the end of the 19th century, and would remain the position of some naturalists well into the 20th century.
iii- iii- Orthogenesis was the hypothesis that life has an innate tendency to change, in a unilinear fashion, towards ever-greater perfection. Orthogenesis was popular among some palaetologists, who believed that the fossil record showed a gradual and constant unidirectional change.
iv- iv- Saltationism was the idea that new species arise as a result of large mutations. It was seen as a much faster alternative to the Darwinian concept of a gradual process of small random variations being acted on by natural selection. It became the basis of the mutation theory of evolution.
3.3.11 Mendelian Genetics, Biometrics and Mutation
The so-called rediscovery of Gregor Mendel‘s laws of inheritance in 1900 ignited a fierce debate between two camps of biologists. In one camp were the Mendelians, who were focused on discrete variations and the laws of inheritance. Their opponents were the biometricians, who were interested in the continuous variation of characteristics within populations. The biometricians rejected Mendelian genetics on the basis that discrete units of heredity, such as genes, could not explain the continuous range of variation seen in real populations.
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 laboratory by mutation alone. Instead, his work between 1910 and 1915 reconfirmed Mendelian genetics and provided solid experimental evidence linking it to chromosomal inheritance. His work also demonstrated that most mutations had relatively small effects, such as a change in eye colour, and that rather than creating a new species in a single step, mutations served to increase variation within the existing population.
3.3.12 Population Genetics
The Mendelian and biometrician models were eventually reconciled with the development of population genetics. A key step was the work of the British biologist and statistician R.A. Fisher. In a series of papers starting in 1918 and culminating in his 1930 book “The Genetical Theory of Natural Selection”, Fisher showed that the continuous variation measured by the biometricians could be produced by the combined action of many discrete genes, and that natural selection could change gene frequencies in a population, resulting in evolution. Another British geneticist, J.B.S. Haldane, applied statistical analysis to real-world examples of natural selection, such as the evolution of industrial melanism in peppered moths, and showed that natural selection worked at an even faster rate than Fisher assumed.
The American biologist Sewall Wright, who had a background in animal breeding experiments, focused on combinations of interacting genes, and the effects of inbreeding on small, relatively isolated populations that exhibited genetic drift. In 1932, Wright introduced the concept of an adaptive landscape and argued that genetic drift and inbreeding could drive a small, isolated sub-population away from an adaptive peak, allowing natural selection to drive it towards different adaptive peaks.
3.3.13 Modern Evolutionary Synthesis
In the first few 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 helped to bridge the divide between the foundations of microevolution developed by the population geneticists and the patterns of macroevolution observed by 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. In Great Britain E.B. Ford, the pioneer of ecological genetics, continued throughout the 1930s and 1940s to demonstrate the power of selection due to ecological factors.
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 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 “Tempo and Mode 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 palaeontologists had claimed supported orthogenesis and neo-Lamarckism did not hold up to closer examination.
The evolutionary synthesis provided a conceptual core that tied together many, but not all, biological disciplines. It helped establish the legitimacy of evolutionary biology. The synthesis also resulted in a considerable narrowing of the range of mainstream evolutionary thought: by the 1950s, natural selection acting on genetic variation was virtually the only acceptable mechanism of evolutionary change (panselectionism), and macroevolution was simply considered the result of extensive microevolution.
3.3.14 1940s–1960s: Molecular Biology and Evolution
The middle decades of the 20th century saw the rise of molecular biology, and with it an understanding of the chemical nature of genes as sequences of DNA and their relationship, through the genetic code, to protein sequences. At the same time, increasingly powerful techniques for analyzing proteins, such as protein electrophoresis and sequencing, brought biochemical phenomena into realm of the synthetic theory of evolution. In the early 1960s, biochemists Linus Pauling and Emile Zuckerkandl proposed the molecular clock hypothesis: that sequence differences between homologous proteins could be used to calculate the time since two species diverged. By 1969, Motoo Kimura and others provided a theoretical basis for the molecular clock, arguing that most genetic mutations are neither harmful nor helpful, and that genetic drift, rather than natural selection, is responsible for a large portion of genetic change.
From the early 1960s, molecular biology was increasingly seen as a threat to the traditional core of evolutionary biology. Established evolutionary biologists were extremely sceptical of molecular approaches, especially when it came to the connection (or lack thereof) to natural selection. The molecular clock hypothesis and the neutral theory were particularly controversial, spawning the neutralist-selectionist debate over the relative importance of drift and selection, which continued into the 1980s without a clear resolution.
3.3.15 Late 20th Century
In the mid-1960s, George C. Williams strongly critiqued explanations of adaptations worded in terms of “survival of the species” (group selection arguments). Such explanations were largely replaced by a gene-centred view of evolution. 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; though 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 with which it was co-evolving. Hamilton, Williams and others suggested that this idea might explain the evolution of sexual reproduction: the increased genetic diversity caused by sexual reproduction would help maintain resistance against rapidly evolving parasites, thus making sexual reproduction 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.
W. D. Hamilton’s work on kin selection contributed to the emergence of the discipline of sociobiology. The existence of altruistic behaviours has been a difficult problem for evolutionary theorists from the beginning. Significant progress was made in 1964 when Hamilton formulated the inequality in kin selection known as Hamilton’s rule. Other theories followed, some derived from game theory, such as reciprocal altruism. In 1975, E.O. Wilson published the influential and highly controversial book “Sociobiology: The New Synthesis” which claimed evolutionary theory could help explain many aspects of animal, including human, behaviour. Critics of sociobiology, including Stephen Jay Gould and Richard Lewontin, claimed that sociobiology greatly overstated the degree to which complex human behaviours could be determined by genetic factors.
A phylogenetic tree showing the three-domain system. Eukaryotes are collared red, Archaea green, and Bacteria blue.
3.3.17 Evolutionary Paths and Processes
One of the most prominent debates arising during the 1970s was over the theory of punctuated equilibrium. Niles Eldredge and Stephen Jay Gould proposed that there was a pattern of fossil species that remained largely unchanged for long periods (what they termed stasis), interspersed with relatively brief periods of rapid change during speciation. Improvements in sequencing methods resulted in a large increase of sequenced genomes, allowing the testing and refining of evolutionary theories using this huge amount of genome data. Comparisons between these genomes provide insights into the molecular mechanisms of speciation and adaptation. These genomic analyses have produced fundamental changes in the understanding of the evolutionary history of life, such as the proposal of the three-domain system by Carl Woese. Advances in computational hardware and software allow the testing and extrapolation of increasingly advanced evolutionary models and the development of the field of systems biology. Discoveries in biotechnology now allow the modification of entire genomes, advancing evolutionary studies to the level where future experiments may involve the creation of entirely synthetic organisms.
3.3.18 Microbiology and Horizontal Gene Transfer
Microbiology was largely ignored by early evolutionary theory. This was due to the paucity of morphological traits and the lack of a species concept in microbiology. Now, evolutionary researchers are taking advantage of 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.
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 was first recognized since it played a major role in the spread 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. These high levels of horizontal gene transfer have led to suggestions that the family tree of today’s organisms, the so-called “tree of life”, is more similar to an interconnected web or net.
3.3.19 Evolutionary Developmental Biology
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. With the evolutionary biologist Richard Lewontin, they 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. It became clear that the diversity of animal morphology was not 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“.
More recent work in this field has emphasized phenotypic and developmental plasticity. It has been suggested, for example, that the rapid emergence of basic animal 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 stabilized by natural selection.
3.3.20 Unconventional Evolutionary Theory
Pierre Teilhard de Chardin‘s non-scientific Omega point theory describes the gradual development of the universe from subatomic particles to human society, which he viewed as its final stage and goal.
James Lovelock proposed that the living and nonliving parts of Earth can be viewed as a complex interacting system with similarities to a single organism. The Gaia hypothesis has also been viewed by Lynn Margulis and others as an extension of endosymbiosis and exosymbiosis. This modified hypothesis postulates that all living things have a regulatory effect on the Earth’s environment that promotes life overall.
Futurists have often viewed scientific and technological progress as a continuation of biological evolution. Among these, transhumanists often view such technological evolution itself as a goal in their philosophy, possibly in the form of a technological singularity.