Modern evolutionary synthesis

The modern evolutionary synthesis (often referred to simply as the modern synthesis or the evolutionary synthesis), neo-Darwinian synthesis or neo-Darwinism, generally denotes the combination of Charles Darwin's theory of the evolution of species by natural selection, Gregor Mendel's theory of genetics as the basis for biological inheritance, and mathematical population genetics. 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, Julian Huxley, Ernst Mayr, George Gaylord Simpson, and G. Ledyard Stebbins. Essentially, the modern synthesis introduced the connection between two important discoveries; the units of evolution (genes) with the mechanism of evolution (selection). It also represents a unification of several branches of biology that previously had little in common, particularly genetics, cytology, systematics, botany, and paleontology.

In a Nutshell
The major tenets of the evolutionary synthesis, then, were that populations contain genetic variation that arises by random (ie. not adaptively directed) mutation and recombination; that populations evolve by changes in gene frequency brought about by random genetic drift, gene flow, and especially natural selection; that most adaptive genetic variants have individually slight phenotypic effects so that phenotypic changes are gradual (although some alleles with discrete effects may be advantageous, as in certain color polymorphisms); that diversification comes about by speciation, which normally entails the gradual evolution of reproductive isolation among populations; and that these processes, continued for sufficiently long, give rise to changes of such great magnitude as to warrant the designation of higher taxonomic levels (genera, families, and so forth)
 * &mdash; Futuyma, D.J. in Evolutionary Biology, Sinauer Associates, 1986; p.12

History
George John Romanes coined the term neo-Darwinism to refer to the theory of evolution preferred by Alfred Russel Wallace et al. Wallace rejected the Lamarckian idea of inheritance of acquired characteristics, something that Darwin, Huxley et al wouldn't rule out. The most prominent "neo-Darwinian" of the time after Darwin was August Weismann, who argued that hereditary material, which he called the germ plasm, was kept utterly separate from the development of the organism. 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.

In 1900, Mendelian inheritance was "rediscovered", and 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. Mendelism was taken up by many biologists, 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 T. H. 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 the subsequent years, he and his colleagues developed the Mendelian-Chromosome theory of inheritance and Morgan and his colleagues published The Mechanism of Mendelan 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 entititled The Correlation Between Relatives on the Supposition of Mendelian Inheritance, which showed using a model how continuous variation could be the result of the action of many discrete loci. This 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, and works by Ernst Mayr (Systematics and the Origin of Species – systematics), G. G. Simpson (Tempo and Mode in Evolution – paleontology), G. Ledyard Stebbins (Variation and Evolution in Plants – botany), 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 evolutionary synthesis and modern synthesis in his semi-popular work Evolution: The Modern Synthesis in 1942.

Tenets of the modern synthesis
According to the modern synthesis as established in the 1930s and 1940s, genetic variation in populations arises by chance through mutation (this is now known to be sometimes caused by mistakes in DNA replication) and recombination (crossing over of homologous chromosomes during meiosis). Evolution consists primarily of changes in the frequencies of alleles between one generation and another as a result of genetic drift, gene flow and natural selection. Speciation occurs gradually when populations are reproductively isolated, e.g. by geographic barriers.

Further advances
The modern evolutionary synthesis continued to be developed and refined after the initial establishment in the 1930s and 1940s. The work of W. D. Hamilton, George C. Williams, John Maynard Smith and others led to the development of a gene-centric view of evolution in the 1960s. The synthesis as it exists now has extended the scope of the Darwinian idea of natural selection, specifically to include subsequent scientific discoveries and concepts unknown to Darwin such as DNA and genetics that allow rigorous, in many cases mathematical, analyses of phenomena such as kin selection, altruism, and speciation.

A particular interpretation of neo-Darwinism most commonly associated with Richard Dawkins asserts that the gene is the only true unit of selection. Dawkins further extended the Darwinian idea to include non-biological systems exhibiting the same type of selective behavior of the 'fittest' such as memes in culture.

"Increasingly, studies of genes and genomes are indicating that considerable horizontal transfer has occurred between prokaryotes." Horizontal Gene Transfer is called by some "A New Paradigm for Biology " and emphasised by others as an important factor in "The Hidden Hazards of Genetic Engineering". "While horizontal gene transfer is well-known among bacteria, it is only within the past 10 years that its occurrence has become recognized among higher plants and animals. The scope for horizontal gene transfer is essentially the entire biosphere, with bacteria and viruses serving both as intermediaries for gene trafficking and as reservoirs for gene multiplication and recombination (the process of making new combinations of genetic material)." .