The Modern Synthesis describes the fusion (merger) of Mendelian genetics with Darwinian evolution that resulted in a unified theory of evolution. It is sometimes referred to as the Neo-Darwinian theory. The Modern Synthesis was developed by a number of now-legendary evolutionary biologists in the 1930s and 1940s.
The Modern Synthesis introduced several changes in how evolution and evolutionary processes were conceived. It proposed a new definition of evolution as "changes in allele frequencies within populations , " thus emphasizing the genetic basis of evolution. (Alleles are alternate forms of the same gene, characterized by differences in DNA sequence that result in the construction of proteins that differ in amino acid composition.) Four forces of evolution were identified as contributing to changes in allele frequencies. These are random genetic drift, gene flow, mutation pressure, and natural selection . Of these, natural selection—by which the best-adapted organisms have the highest survival rate—is the only evolutionary force that makes organisms better adapted to their environments. Genetic drift describes random changes in allele frequencies in a population. It is particularly powerful in small populations. Gene flow describes allele frequency changes due to the immigration and emigration of individuals from a population. Mutation is a weak evolutionary force but is crucial because all genetic variation arises originally from mutation, alterations in the DNA sequences resulting from errors during replication or other factors. The Modern Synthesis recognized that the majority of mutations are deleterious (have a harmful effect), and that mutations that are advantageous usually have a small phenotypic effect. Advantageous mutations may be incorporated into the population through the process of natural selection. Changes in species therefore occur gradually through the accumulation of small changes. The large differences that are observed between species involve gradual change over extensive time periods. Speciation (the formation of new species) results from the evolution of reproductive isolation, often during a period of allopatry , in which two populations are isolated from one another.
There are several differences between the Modern Synthesis and the older Darwinian conception of evolution. First, mechanisms of evolution other than natural selection are recognized as playing important roles. Second, the Modern Synthesis succeeds in explaining the persistence of genetic variation, a problem that Charles Darwin struggled with. The dominant genetic theory of Darwin's time was blending inheritance, in which offspring were thought to be the genetic intermediates (in-between versions) of their two parents. As Darwin correctly recognized, blending inheritance would result in the rapid end of genetic variation within a population, giving natural selection no material to work with. Incorporating Gregor Mendel's particulate theory of inheritance, in which the alleles of a gene remain separate instead of merging, solves this problem.
There were several key players involved in the Modern Synthesis. The theory relied on the population genetics work of R. A. Fisher and Sewall Wright. Theodosius Dobzhansky made extensive studies of natural populations of the fruitfly Drosophila that supported many aspects of the theory. Ernst Mayr developed the biological species concept and created models concerning how speciation occurs. George Gaylord Simpson helped integrate paleontological observations into the theory behind the Modern Synthesis. G. Ledyard Stebbins contributed tenets (principles) based on his botanical work.
Since the 1990s it has been recognized that the Modern Synthesis omits some biological disciplines that are also relevant to evolution. In particular, much attention has focused on patterns of ontogeny and development.
see also Biological Evolution; Darwin, Charles; Mayr, Ernst; Genes; Genetics; Mendel, Gregor; Simpson, George Gaylord.
Futuyma, Douglas J. Evolutionary Biology, 3rd ed. Sunderland, MA: Sinauer Associates, 1998.
Huxley, Julian. Evolution: The Modern Synthesis. New York: Harper and Brothers, 1942.
Simpson, George Gaylord. Tempo and Mode in Evolution. New York: Columbia University Press, 1944.