Overblog Suivre ce blog
Editer l'article Administration Créer mon blog
28 octobre 2014 2 28 /10 /octobre /2014 08:05

He developed Lamarck's and Erasmus Darwin's ideas of transmutation and evolutionism, and investigated homology, even proposing that plants and animals had a common evolutionary starting point. 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, Sony VAIO PCG-5T3L battery

praised Lamarck for explaining how higher animals had "evolved" from the simplest worms; this was the first use of the word "evolved" in a modern sense.[54][55]

In 1844 the Scottish publisher Robert Chambers anonymously published an extremely controversial but widely read book entitledVestiges of the Natural History of CreationSony VAIO PCG-5T4L battery

This book proposed an evolutionary scenario for the origins of the Solar System and of life on Earth. It claimed that the fossil record showed a progressive ascent of animals, with current animals branching 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. Sony VAIO PCG-61111L battery

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. 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.[56][57] Sony VAIO PCG-61112L battery

Ideas about the transmutation of species were associated with the radical materialism of the Enlightenment and were attacked by more conservative thinkers. Georges 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, Sony VAIO PCG-61411L battery

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 scientific reputation helped keep transmutational ideas out of the mainstream for decades. Sony VAIO PCG-71111L battery

In Britain the philosophy of natural theology remained influential. William Paley's 1802 book Natural Theology with its famouswatchmaker analogy had been written at least in part as a response to the transmutational ideas of Erasmus Darwin.[59] 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.Sony VAIO PCG-7141L battery

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.[51] 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. Sony VAIO PCG-7142L battery

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, Sony VAIO PCG-81113L battery

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, showed him the need to ensure that his own ideas were scientifically sound. Sony VAIO PCG-81114L battery

It is possible to look through the history of biology from the ancient Greeks onwards and discover anticipations of almost all of Charles Darwin's key ideas. For example, Loren Eiseley has found isolated passages written by Buffon suggesting he was almost ready to piece together a theory of natural selection, but such anticipations should not be taken out of the full context of the writings or of cultural values of the time which could make Darwinian ideas of evolution unthinkable.[64] Sony VAIO PCG-81115L battery

When Darwin was developing his theory, he investigated selective breeding and was impressed by Sebright's observation that "A severe winter, or a scarcity of food, by destroying the weak and the unhealthy, has all the good effects of the most skilful selection" so that "the weak and the unhealthy do not live to propagate their infirmities".[65] Sony VAIO PCG-81214L battery

Darwin was influenced by Charles Lyell's ideas of environmental change causing ecological shifts, leading to what Augustin de Candolle had called a war between competing plant species, competition well described by the botanist W. Herbert. Darwin was struck by Malthus's phrase "struggle for existence" used of warring human tribes.[66][67] Sony VAIO PCG-81312L battery

Several writers anticipated evolutionary 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.[68]

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. Sony VAIO PCG-8131L battery

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; Sony VAIO PCG-8141L battery

but he applies it only to the races of man, and to certain characters alone."[69]

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."[70] Sony VAIO PCG-8152L battery

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."[71] Sony VAIO PCG-8161L battery

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." Bowler goes on to say that simple priority alone is not enough to secure a place in the history of science; someone has to develop an idea and convince others of its importance to have a real impact.[72Sony VAIO PCG-9131L battery

] 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. But that suggestion is the central idea of the Origin of Species, and contains the quintessence ofDarwinism. Sony VAIO PCG-3E1M battery

The biogeographical patterns Charles Darwin observed in places such as the Galapagos islands during the voyage of the Beaglecaused him to doubt the fixity of species, and in 1837 Darwin started the first of a series of secret notebooks ontransmutation. 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. Sony VAIO PCG-9Z1L battery

In 1838 he read the new 6th edition of An Essay on the Principle of Population, written in the late 18th century 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. Sony VAIO PCG-7148L battery

However he did share them with certain other naturalists and friends, starting with Joseph Hooker, with whom he discussed his unpublished 1844 essay on natural selection. During this period he used the time he could spare from his other scientific work to slowly refine his ideas and, aware of the intense controversy around transmutation, amass evidence to support them. Sony VAIO PCG-7151L battery

In September 1854 he began full-time work on writing his book on natural selection.[63][74][75]

Unlike Darwin, Alfred Russel 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. Sony VAIO PCG-7152L battery

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. Like Darwin, it was Wallace's consideration of how the ideas of Malthus might apply to animal populations that led him to conclusions very similar to those reached by Darwin about the role of natural selection. Sony VAIO PCG-7153L battery

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 on a short abstract summarising his theory, which he would publish in 1859 as On the Origin of SpeciesSony VAIO PCG-7154L battery

By the 1850s, whether or not species evolved was a subject of intense debate, with prominent scientists arguing both sides of the issue.[77] The publication of Charles Darwin's On the Origin of Species (1859) fundamentally transformed the discussion over biological origins.[78] Darwin argued that his branching version of evolution explained a wealth of facts in biogeography, anatomy, embryology, and other fields of biology. Sony VAIO PCG-7161L battery

He also provided the first cogent mechanism by which evolutionary change could persist: his theory ofnatural selection.One of the first and most important naturalists to be convinced by Origin of the reality of evolution was the British anatomist Thomas Henry Huxley. Huxley recognized that unlike the earlier transmutational ideas of Lamarck and VestigesSony VAIO PCG-7162L battery

Darwin's theory provided a mechanism for evolution without supernatural involvement, even if Huxley himself was not completely convinced that natural selection was the key evolutionary mechanism. Huxley would make advocacy of evolution a cornerstone of the program of the X Club to reform and professionalise science by displacing natural theology with naturalism and to end the domination of British natural science by the clergy. Sony VAIO PCG-7171L battery

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.[79] In his campaign for public and scientific acceptance of Darwin's theory, Huxley made extensive use of new evidence for evolution from paleontology. Sony VAIO PCG-7172L battery

This included evidence that birds had evolved from reptiles, including the discovery of Archaeopteryx in Europe, and a number of fossils of primitive birds with teeth found in North America. Another important line of evidence was the finding of fossils that helped trace the evolution of the horse from its small five-toed ancestors.[80] However, acceptance of evolution among scientists in non-Sony VAIO PCG-7173L battery

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 Haeckelchampioned 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.[81] Sony VAIO PCG-7174L battery

Haeckel and other German scientists would take the lead in launching an ambitious programme to reconstruct the evolutionary history of life based on morphology and embryology.[82]

Darwin's theory succeeded in profoundly altering scientific opinion regarding the development of life and in producing a small philosophical revolution.[83] Sony VAIO PCG-7181L battery

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, Sony VAIO PCG-7182L battery

proved to be useful for statistical models of evolution that were developed by his cousinFrancis Galton and the "biometric" school of evolutionary thought. However, this idea proved to be of little use to other biologists.

Charles Darwin was aware of the severe reaction in some parts of the scientific community against the suggestion made inVestiges of the Natural History of Creation that humans had arisen from animals by a process of transmutation. Sony VAIO PCG-7183L battery

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. Sony VAIO PCG-7184L battery

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 bookGeological 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. Sony VAIO PCG-7185L battery

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, Sony VAIO PCG-381L battery

especially in the critical characteristic of cranial capacity, to modern humans for them to be convincing intermediates between humans and other primates.[86]

Therefore the debate that immediately followed the publication of The Origin of Species centered on the similarities and differences between humans and modern apes. Sony VAIO PCG-382L battery

Carolus Linnaeus had been criticised in the 18th century for grouping humans and apes together as primates in his ground breaking classification system.[87] 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, which by the early 19th century had become the orthodox view. Sony VAIO PCG-383L battery

On the other hand, Thomas Henry Huxley sought to demonstrate a close anatomical relationship between humans and apes. In one famous incident, which became known as the Great Hippocampus Question, Huxley showed that Owen was mistaken in claiming that the brains of gorillaslacked a structure present in human brains. Huxley summarized his argument in his highly influential 1863 book Evidence as to Man's Place in NatureSony VAIO PCG-384L battery

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 all the differences between humans and apes, especially some aspects of the human mind.[86]

In 1871, Darwin published The Descent of Man, and Selection in Relation to Sex, which contained his views on human evolution.Sony VAIO PCG-391L battery

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. For example, he viewed morality as a natural outgrowth of instincts that were beneficial to animals living in social groups. 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, Sony VAIO PCG-393L battery

and sexual selection. The debate over human origins, and over the degree of human uniqueness continued well into the 20th century.

The concept of 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. Sony VAIO PCG-394L battery

The four major alternatives to natural selection in the late 19th century were theistic evolution, neo-Lamarckism, orthogenesis, and saltationism.

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. The term was promoted by Darwin's greatest American advocate Asa Gray. Sony VAIO PCG-3A1L battery

However, this idea gradually fell out of favor 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. Sony VAIO PCG-3A2L battery

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. Sony VAIO PCG-3A3L battery

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'srecapitulation theory of evolution, which held that the embryological development of an organism repeats its evolutionary history.[88][89] Sony VAIO PCG-3A4L battery

Critics of neo-Lamarckism, such as the German biologist August Weismann and Alfred Russel 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.[88][89] Sony VAIO PCG-41112L battery

Orthogenesis was the hypothesis that life has an innate tendency to change, in a unilinear fashion, towards ever-greater perfection. It had a significant following in the 19th century, and its proponents included the Russian biologist Leo S. Berg and the American paleontologist Henry Fairfield Osborn. Orthogenesis was popular among some paleontologists, who believed that the fossil record showed a gradual and constant unidirectional change. Sony VAIO PCG-51211L battery

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, and was popular with early geneticists such as Hugo de Vries, William Bateson, and early in his career, T. H. Morgan. It became the basis of the mutation theory of evolution. Sony VAIO PCG-51311L battery

The 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. They were led by William Bateson (who coined the word genetics) and Hugo de Vries (who coined the word mutation). Sony VAIO PCG-51312L battery

Their opponents were the biometricians, who were interested in the continuous variation of characteristics within populations. Their leaders, Karl Pearson and Walter Frank Raphael Weldon, followed in the tradition of Francis Galton, who had focused on measurement and statistical analysis of variation within a population. The biometricians rejected Mendelian genetics on the basis that discrete units of heredity, Sony VAIO PCG-51411L battery

such as genes, could not explain the continuous range of variation seen in real populations. Weldon's work with crabs and snails provided evidence that selection pressure from the environment could shift the range of variation in wild populations, but the Mendelians maintained that the variations measured by biometricians were too insignificant to account for the evolution of new species.[90][91] Sony VAIO PCG-51412L battery

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. Sony VAIO PCG-51511L battery

His work 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, mutations served to increase variation within the existing population.

The Mendelian and biometrician models were eventually reconciled with the development of population genetics. Sony VAIO PCG-51513L battery

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. Sony VAIO PCG-5N2L battery

In a series of papers beginning in 1924, 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.[92][93] Sony VAIO PCG-5N4L battery

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, Sony VAIO PCG-5P2L battery

isolated sub-population away from an adaptive peak, allowing natural selection to drive it towards different adaptive peaks. The work of Fisher, Haldane and Wright founded the discipline of population genetics. This integrated natural selection with Mendelian genetics, which was the critical first step in developing a unified theory of how evolution worked. Sony VAIO PCG-5P4L battery

In the first few decades of the 20th century, most field naturalists continued to believe that Lamarckian and orthogenetic mechanisms of evolution provided the best explanation for the complexity they observed in the living world. But as the field of genetics continued to develop, those views became less tenable.[94] Theodosius Dobzhansky, Sony VAIO PCG-5R1L battery

a postdoctoral worker in T. H. Morgan's lab, had been influenced by the work on genetic diversity by Russian geneticists such as Sergei Chetverikov. He 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.Sony VAIO PCG-5R2L battery

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 a more accessible form. Sony VAIO PCG-5S1L battery

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 including the ability to maintain genetic diversity through genetic polymorphisms such as human blood types. Ford's work would contribute to a shift in emphasis during the course of the modern synthesis towards natural selection over genetic drift.[ Sony VAIO PCG-5S2L battery

Evolutionary biologist Ernst Mayr was influenced by the work of the German biologist Bernhard Rensch showing the influence of local environmental factors on 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. Sony VAIO PCG-5S3L battery

This form of speciation occurs when the 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.[92][93][97] Sony VAIO PCG-5T2L battery

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 paleontologists had claimed supported orthogenesis and neo-Lamarckism did not hold up to closer examination. Sony VAIO PCG-5T3L battery

In 1950, G. Ledyard Stebbins published Variation and Evolution in Plants, which helped to integrate botany into the synthesis. The emerging cross-disciplinary consensus on the workings of evolution would be known as the modern evolutionary synthesis. It received its name from the 1942 book Evolution: The Modern Synthesis by Julian Huxley.[92][93] Sony VAIO PCG-5T4L battery

The evolutionary synthesis provided a conceptual core—in particular, natural selection and Mendelian population genetics—that tied together many, but not all, biological disciplines. It helped establish the legitimacy of evolutionary biology, a primarily historical science, in a scientific climate that favored experimental methods over historical ones.[98] Sony VAIO PCG-61111L battery

The synthesis also resulted in a considerable narrowing of the range of mainstream evolutionary thought (what Stephen Jay Gould called the "hardening of the synthesis"): 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. Sony VAIO PCG-61112L battery

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 of 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. Sony VAIO PCG-61411L battery

In the early 1960s, biochemists Linus Pauling andEmile 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—at the molecular level at least—Sony VAIO PCG-71111L battery

most genetic mutations are neither harmful nor helpful and that mutation and genetic drift (rather than natural selection) cause a large portion of genetic change: the neutral theory of molecular evolution.[101] Studies of protein differences within species also brought molecular data to bear on population genetics by providing estimates of the level of heterozygosity in natural populations.[102] Sony VAIO PCG-7141L battery

From the early 1960s, molecular biology was increasingly seen as a threat to the traditional core of evolutionary biology. Established evolutionary biologists—particularly Ernst Mayr, Theodosius Dobzhansky and G. G. Simpson, three of the architects of the modern synthesis—were extremely skeptical of molecular approaches, especially when it came to the connection (or lack thereof) to natural selection. Sony VAIO PCG-7142L battery

The molecular-clock hypothesis and the neutral theory were particularly controversial, spawning the neutralist-selectionist debate over the relative importance of mutation, drift and selection, which continued into the 1980s without a clear resolution.

In the mid-1960s, George C. Williams strongly critiqued explanations of adaptations worded in terms of "survival of the species" (group selection arguments). Sony VAIO PCG-81113L battery

Such explanations were largely replaced by a gene-centered view of evolution, epitomized by the kin selection arguments of W. D. Hamilton, George R. Price and John Maynard Smith.[105] This viewpoint would be summarized and popularized in the influential 1976 book The Selfish Gene by Richard Dawkins.[106] Models of the period seemed to show that group selection was severely limited in its strength; though newer models do admit the possibility of significant multi-level selection.[107] Sony VAIO PCG-81114L battery

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: Sony VAIO PCG-81115L battery

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.[108][109]

However, contrary to the expectations of the red queen hypothesis, Hanley et al. found that the prevalence, Sony VAIO PCG-81214L battery

abundance and mean intensity of mites was significantly higher in sexual geckos than in asexuals sharing the same habitat.[110] Furthermore, Parker, after reviewing numerous genetic studies on plant disease resistance, failed to find a single example consistent with the concept that pathogens are the primary selective agent responsible for sexual reproduction in their host.[111Sony VAIO PCG-81312L battery

]At an even more fundamental level, Heng[112] and Gorelick and Heng[113] reviewed evidence that sex, rather than enhancing diversity, acts as a constraint on genetic diversity. They considered that sex acts as a coarse filter, weeding out major genetic changes, such as chromosomal rearrangements, but permitting minor variation, such as changes at the nucleotide or gene level (that are often neutral) to pass through the sexual sieve. Sony VAIO PCG-8131L battery

The adaptive function of sex, today, remains a major unresolved issue in biology. The competing models to explain the adaptive function of sex were reviewed by Birdsell and Wills.[114] A principal alternative view to the red queen hypothesis is that sex arose, and is maintained, as a process for repairing DNA damage, and that genetic variation is produced as a byproduct.[115][116] (Also see Evolution of sexual reproduction.) Sony VAIO PCG-8141L battery

The gene-centric view has also led to an increased interest in Darwin's old idea of sexual selection,[117] and more recently in topics such as sexual conflict andintragenomic conflict.

W. D. Hamilton's work on kin selection contributed to the emergence of the discipline of sociobiology. The existence of altruistic behaviors has been a difficult problem for evolutionary theorists from the beginning.[118] Sony VAIO PCG-8152L battery

Significant progress was made in 1964 when Hamilton formulated the inequality in kin selection known asHamilton's rule, which showed how eusociality in insects (the existence of sterile worker classes) and many other examples of altruistic behavior could have evolved through kin selection. Other theories followed, some derived from game theory, such as reciprocal altruism.[119]Sony VAIO PCG-8161L battery

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, 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. Sony VAIO PCG-9131L battery

They also claimed that the theories of sociobiologists often reflected their own ideological biases. Despite these criticisms, work has continued in sociobiology and the related discipline of evolutionary psychology, including work on other aspects of the altruism problem.

One of the most prominent debates arising during the 1970s was over the theory of punctuated equilibrium. Sony VAIO PCG-3E1M battery

Niles Eldredge andStephen 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.[122][123] (Also seeSpeciation.) Improvements in sequencing methods resulted in a large increase of sequenced genomes, Sony VAIO PCG-9Z1L battery

allowing the testing and refining of evolutionary theories using this huge amount of genome data.[124] Comparisons between these genomes provide insights into the molecular mechanisms of speciation and adaptation.[125][126] 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.[127] SONY VAIO PCG-81111V battery

Advances in computational hardware and software allow the testing and extrapolation of increasingly advanced evolutionary modelsand the development of the field of systems biology.[128] One of the results has been an exchange of ideas between theories of biological evolution and the field of computer science known as evolutionary computation, SONY VAIO PCG-81211V battery

which attempts to mimic biological evolution for the purpose of developing new computer algorithms. 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.

Microbiology was largely ignored by early evolutionary theory. SONY VAIO PCG-51111V battery

This was due to the paucity of morphological traits and the lack of a species concept in microbiology, particularly amongst prokaryotes.[130] 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.[131] SONY VAIO PCG-51111M battery

These studies are revealing unanticipated levels of diversity amongst microbes.[132][133]

One important development in the study of microbial evolution came with the discovery in Japan in 1959 of horizontal gene transfer.[134] This transfer of genetic material between different species of bacteria came to the attention of scientists because it played a major role in the spread of antibiotic resistance.[135SONY VAIO PCG-51112M battery

] 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.[136] 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.[137][138] SONY VAIO PCG-51211M battery

Indeed, the endosymbiotic theory for the origin of organelles sees a form of horizontal gene transfer as a critical step in the evolution of eukaryotes such as fungi, plants, and animals.[139][140] The endosymbiotic theory holds that organelles within the cells of eukorytes such as mitochondria and chloroplasts, had descended from independent bacteria that came to live symbiotically within other cells. SONY VAIO PCG-51212M battery

It had been suggested in the late 19th century when similarities between mitochondria and bacteria were noted, but largely dismissed until it was revived and championed by Lynn Margulis in the 1960s and 70s; Margulis was able to make use of new evidence that such organelles had their own DNA that was inherited independently from that in the cell's nucleus. SONY PCG-7134M battery

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. SONY PCG-7131M battery

The evolutionary biologist Stephen Jay Gould revived earlier ideas ofheterochrony, 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, or even a structural novelty, Sony Vaio VGN-CR13/W Battery

could arise incidentally as an accidental result of selection on another structure, rather than through direct selection for that particular adaptation. They called such incidental structural changes "spandrels" after an architectural feature.[142] Later, Gould and Vrba discussed the acquisition of new functions by novel structures arising in this fashion, calling them "exaptations".[143]Sony Vaio VGN-CR13G Battery

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.[144] Sony Vaio VGN-CR13G/B Battery

These proteins became known as the "developmental toolkit".[145] Such perspectives influenced the disciplines ofphylogenetics, paleontology and comparative developmental biology, and spawned the new discipline of evolutionary developmental biology also known as evo-devo.

One of the tenets of the modern evolutionary synthesis was that macroevolution Sony Vaio VGN-CR13G/L Battery

(the evolution of phylogenic clades at the species level and above) was solely the result of the mechanisms of microevolution (changes in gene frequency within populations) operating over an extended period of time. During the last decades of the 20th century some paleontologists raised questions about whether other factors, Sony VAIO PCG-5N1M battery

such as punctuated equilibrium and group selection operating on the level of entire species and even higher level phylogenic clades, needed to be considered to explain patterns in evolution revealed by statistical analysis of the fossil record. Near the end of the 20th century some researchers in evolutionary developmental biology suggested that interactions between the environment and the developmental process might have been the source of some of the structural innovations seen in macroevolution, Sony VAIO PCG-5S1M battery

but other evo-devo researchers maintained that genetic mechanisms visible at the population level are fully sufficient to explain all macroevolution.Epigenetics is the study of heritable changes in gene expression or cellular phenotype caused by mechanisms other than changes in the underlying DNA sequence. By the first decade of the 21st century it had become accepted that Epigenetic mechanisms were a necessary part of the evolutionary origin of cell differentiation.[150] Sony Vaio VGN-CR13G/W Battery

Although epigenetics in multicellular organisms is generally thought to be a mechanism involved in differentiation, with epigenetic patterns "reset" when organisms reproduce, there have been some observations of transgenerational epigenetic inheritance. This shows that in some cases non genetic changes to an organism can be inherited and it has been suggested that such inheritance can help with adaptation to local conditions and affect evolution.[151][1Sony Vaio VGN-CR13G/P Battery

52] Some have suggested that in certain cases a form of Lamarckian evolution may occur.

Teilhard de Chardin's ideas have been seen by advocates of the Gaia theory proposed by James Lovelock, which holds that the living and nonliving parts of Earth can be viewed as a complex interacting system with similarities to a single organism,[154] as being connected to Lovelock's ideas.[155] Sony Vaio VGN-CR13G/R Battery

The Gaia hypothesis has also been viewed by Lynn Margulis[156] and others as an extension of endosymbiosis and exosymbiosis.[157] This modified hypothesis postulates that all living things have a regulatory effect on the Earth's environment that promotes life overall.Sony VAIO PCG-31111M battery,Sony VAIO PCG-31112M battery,Sony VAIO PCG-31113M battery

Partager cet article

Repost 0
Published by batterys
commenter cet article