Neo-Darwinism, also called the modern evolutionary synthesis, generally denotes the integration of Charles Darwin's theory of evolution by natural selection, Gregor Mendel's theory of genetics as the basis for biological inheritance, and mathematical population genetics. Although this was not the historical meaning of the term neo-Darwinism, it has been the popular and scientific use of the expression since the synthesis of the 1930s. (See Origin of the term neo-Darwinism.) Other terminology used synonymously with neo-Darwinism are modern synthesis, evolutionary synthesis, and neo-Darwinian synthesis.
Neo-Darwinism has been one of the most significant, overall developments in evolutionary biology since the time of Darwin. Bowler (1988) stated that there is "a sense in which the emergence of the modern synthetic theory can be seen as the first real triumph of Darwinism."
Essentially, neo-Darwinism introduced the connection between two important discoveries: the units of evolution (genes) with the mechanism of evolution (natural selection). By melding classical Darwinism with the rediscovered Mendelian genetics, Darwin's ideas were recast in terms of changes in allele frequencies. Neo-Darwinism thus fused two very different and formerly divided research traditions, the Darwinian naturalists and the experimental geneticists. This fusion took place roughly between 1936 and 1947.
While the modern synthesis remains the prevailing paradigm of evolutionary biology, in recent years it has both been expanded and challenged as a result of new developments in evolutionary theory. In particular, concepts related to gradualism, speciation, natural selection, and extrapolating macroevolutionary trends from microevolutionary trends have been challenged.
Major figures in the development of the modern synthesis include Thomas Hunt Morgan, Ronald Fisher, Theodosius Dobzhansky, J. B. S. Haldane, Sewall Wright, William D. Hamilton, Cyril Darlington, Sergei Chetverikov, E. B. Ford, Julian Huxley, Ernst Mayr, George Gaylord Simpson, and G. Ledyard Stebbins.
Originally, the term neo-Darwinism signified something quite different than it does today.
From the time of Darwin until the late nineteenth century, the term Darwinism had come to mean any of a wide diversity of views, including various social philosophies. Some of the views enveloped by the term were not centered around natural selection at all.
Near the end of the nineteenth century, one of the strong debates in evolutionary theory was between those who were promoting the inheritance of acquired characteristics (Lamarckism) and those who were promoting the exclusivity of natural selection. Prominent in this later camp were August Weismann and Alfred Russel Wallace. (Darwin himself was more pluralistic, in that he also found a place in his theory for the inheritance of acquired characteristics.)
In 1896, George John Romanes coined the term "neodarwinism" to designate the type of Darwinism being advanced by August Weismann. Weismann argued that hereditary material, which he called the germ plasm, was kept utterly separate from the development of the organism. He maintained that natural selection was the sole creative agent in evolution, and gave no credence to the inheritance of acquired characteristics. This became the meaning of neo-Darwinism, and Weisman was the most prominent "neo-Darwinian." (This was seen by most biologists as an extreme position, however, and variations of neo-Lamarckism, orthogenesis ("progressive" evolution), and saltationism (evolution by "jumps" or mutations) were discussed as alternatives.)
However, the term neo-Darwinism was not terribly popular in the scientific community. It became popular only after the development of the modern evolutionary synthesis in the 1930s, when the term became synonymous with the synthesis. The modern meaning of neo-Darwinism is not "genealogically linked" to the earlier use (Gould 2002).
The modern synthesis integrated diverse scientific disciplines and theories into a common view of the evolutionary process.
Originally, many branches of biology shared little in common. Genetics, cytology, systematics, botany, embryology, biogeography, population genetics, and paleontology, among the chief examples, involved very different research disciplines, working independently.
Mendelian inheritance, after its "rediscovery" in 1900, was initially seen as supporting a form of "jumping" evolution. The biometric school, led by Karl Pearson and Walter Frank Raphael Weldon, argued against it vigorously, stating empirical evidence indicated that variation was continuous in most organisms. The Mendelian school, led by William Bateson, countered that in some cases the Mendelian evidence was indisputable and that future work would reveal its larger truth. Many biologists took up Mendelism, even though it was still extremely crude at this early stage. Its relevance to evolution was still hotly debated.
A critical link between experimental biology and evolution, as well as between Mendelian genetics, natural selection, and the chromosome theory of inheritance, arose from Thomas Hunt Morgan's work with the fruit fly Drosophila melanogaster. In 1910, Morgan discovered a mutant fly with solid white eyes (wild-type Drosophila have red eyes), and found that this condition—though appearing only in males—was inherited precisely as a Mendelian recessive trait. In subsequent years, he and his colleagues developed the Mendelian-Chromosome theory of inheritance and Morgan and his colleagues published The Mechanism of Mendelian Inheritance in 1915. By that time, most biologists accepted that genes situated linearly on chromosomes were the primary mechanism of inheritance, although how this could be compatible with natural selection and gradual evolution remained unclear. Morgan's work was so popular that it is considered a hallmark of classical genetics.
This issue was partially resolved by Ronald Fisher, who in 1918 produced a paper titled The Correlation Between Relatives on the Supposition of Mendelian Inheritance. In this paper Fisher showed, using a model, how continuous variation could be the result of the action of many discrete loci. This work is sometimes regarded as the starting point of the synthesis, as Fisher was able to provide a rigorous statistical model for Mendelian inheritance, satisfying both the needs (and methods) of the biometric and Mendelian schools.
Morgan's student Theodosius Dobzhansky was the first to apply Morgan's chromosome theory and the mathematics of population genetics to natural populations of organisms, in particular Drosophila pseudoobscura. His 1937 work Genetics and the Origin of Species is usually considered the first mature work of neo-Darwinism. Mayr (1982) claimed that this work "heralded the beginning of the synthesis, and in fact was more responsible for it than any other."
Works by Ernst Mayr (Systematics and the Origin of Species–systematics), G. G. Simpson (Tempo and Mode in Evolution–paleontology), and G. Ledyard Stebbins (Variation and Evolution in Plants–botany) soon followed. With Dobzhansky's book, these are considered the four canonical works of the modern synthesis. C. D. Darlington (cytology) and Julian Huxley also wrote on the topic. Huxley coined both the phrases "evolutionary synthesis" and "modern synthesis" in his semi-popular work Evolution: The Modern Synthesis in 1942.
Mayr felt that an international symposium at Princeton, New Jersey, January 2-4, 1947, marked the formal completion of the synthesis (Hull 1988; Mayr 1982). Thus, Mayr places the key dates for the development of the synthesis between 1937, with Dobzhansky's work, and the Princeton symposium of 1947.
At the heart of the modern synthesis is the view that evolution is gradual and can be explained by small genetic changes in populations over time, due to the impact of natural selection on the phenotypic variation among individuals in the populations (Mayr 1982; Futuyama 1986). According to the modern synthesis as originally established, genetic variation in populations arises by chance through mutation (it is now known to be caused sometimes by mistakes in DNA replication and via genetic recombination—the crossing over of homologous chromosomes during meiosis). This genetic variation leads to phenotypic changes among members of a population. Evolution consists primarily of changes in the frequencies of alleles between one generation and another as a result of natural selection. Speciation, the creation of new species, is a gradual process that generally occurs when populations become more and more diversified as a result of having been isolated, such as via geographic barriers, and eventually the populations develop mechanisms of reproductive isolation. Over time, these small changes will lead to major changes in design or the creation of new taxa.
A major conclusion of the modern synthesis is that the concept of populations can explain evolutionary changes in a way that is consistent with the observations of naturalists and the known genetic mechanisms (Mayr 1982).
Though agreement is not universal on the parameters of the modern synthesis, many descriptions hold as basic (1) the primacy of natural selection as the creative agent of evolutionary change; (2) gradualism (accumulation of small genetic changes); and (3) the extrapolation of microevolutionary processes (changes within species) to macroevolutionary trends (changes about the species level, such as the origin of new designs and broad patterns in history). Evolutionary change is a shift of the frequency of genes in a population, and macroevolutionary trends come from gradual accumulation of small genetic changes.
It has been reported that the synthesis during the initial stages was more pluralistic, subsequently hardening into its later canonical formulations (Depew and Weber 1985; Gould 1982).
Since the initial formulation of the synthesis, the scope of the Darwinian idea of natural selection has been extended, specifically to include subsequent scientific discoveries and concepts unknown to Darwin, such as DNA and genetics, which allow rigorous, in many cases mathematical, analyses of phenomena such as kin selection, altruism, and speciation.
The modern synthesis, while remaining the prevailing paradigm of evolutionary biology since the middle of the twentieth century, has been expanded and even challenged by a number of developments in evolutionary theory. In particular, the traditional, neo-Darwinian views of gradualism, speciation, and natural selection have been challenged, and the acceptability of extrapolating macroevolutionary trends from observations on the microevolutionary level has also come under fire.
Among ideas questioning the gradualist assumptions of the modern synthesis are punctuational models of change. The theory of punctuated equilibrium, and models for the origin of major new designs via ‘‘rapid transitions,’’ have caused a re-evaluation of the traditional gradualist position that evolution proceeds by the slow accumulation of small changes over time—with major new designs taking millions of years (See punctuational models.).
The assumption that speciation takes place due to geographic isolation and gradual divergence of populations is being expanded by the concepts of punctuational speciation and speciation models that do not require geographic isolation. In both polyploidy (multiplication of the number of chromosomes beyond the normal diploid number) and chromosomal speciation (large changes in chromosomes due to genetic accidents)—two examples of punctuational speciation—reproductive isolation can arise rapidly, independently of geographic isolation, and without natural selection playing the creative role (although it may assist in subsequent adaptations). Sympatric, clinal, and area-effect speciation are three models of speciation whereby a population can separate into two species without geographic isolation. (See speciation.)
The orthodox view that natural selection is the creative force in evolution and acts on individuals in populations is being challenged by concepts of selection taking place below and above the level of the individual, and by the theory of neutralism (as well as by the punctuational models of speciation noted above). Richard Dawkins proposed that the gene is the only true unit of selection, while some evolutionists posit that natural selection can also act on groups of organisms, such as species selection (Gould 2002). The theory of neutralism holds that most mutant genes are selectively neutral—that is, unaffected by natural selection, since they are functionally equivalent in terms of an individual’s survival and reproduction—and yet they become passively fixed within species. That is, if evolution involves a change in allele frequency, then most evolutionary change and variability within species are not caused by natural selection, but a random drift of mutant genes (Kimura 1979).
Whether it is justified to extrapolate macroevolutionary changes from forces that work on the microevolutionary level is also a point of contention. (See macroevolution.)
The challenges mentioned above are seen by some scientists and evolutionary historians as a severe test of neo-Darwinism, concluding that “there is no longer a universal consensus in favor of the synthetic theory” (Bowler 1988), or that the theory has broken down on its fundamental claims and thus, "if Mayr's characterization of the synthetic theory is accurate, then that theory, as a general proposition, is effectively dead, despite its textbook orthodoxy” (Gould 1980, 1982). However, what some see as threats to the modern synthesis, others see as theories that can be included within the umbrella of a broader, more pluralistic modern synthesis (Gould 2002).
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