Text b Development of Evolutionary Theory II Population Genetics and the Modern Synthesis

As the science of genetics advanced during the 1920s and 1930s, several key scientists forged a link between Mendel's laws of inheritance and the theory of natural selection proposed by Darwin and Wallace. British mathematician Sir Ronald Fisher, British geneticist J.B.S. Haldane, and American geneticist Sewall Wright pioneered the field of population genetics. By mathematically analyzing the genetic variation in entire populations, these scientists demonstrated that natural selection, and not just mutation, could result in evolutionary change.

Further investigation into population genetics and such fields as paleontology, taxonomy, biogeography, and the biochemistry of genes eventually led to what is called the modern synthesis. This modern view of evolution integrated discoveries and ideas from many different disciplines. In so doing, this view reconciled the many disparate ideas about evolution into the all-encompassing evolutionary science studied today. The modern synthesis was advanced in such books as "Genetics and the Origin of Species", published in 1937 by the Russian-born American geneticist Theodosius Dobzhansky; "Evolution: The Modern Synthesis' (1942) by British biologist Sir Julian Huxley; and "Systematics and the Origin of Species" (1942) by German-born American evolutionary biologist Ernst Mayr. In 1942, American paleontologist George Gaylord Simpson demonstrated from the fossil record that rates and modes of evolution are correlated. New kinds of organisms arise when their ancestors invade a new niche, and evolve rapidly to best exploit the conditions in the new environment. In the late 1940s American botanist G. Ledyard Stebbins showed that plants display evolutionary patterns similar to those of animals, and especially that plant evolution has demonstrated diverse adaptive responses to environmental pressures and opportunities.

In addition, biologists reviewed a broad range of genetic, ecological, and anatomical evidence to show that observation and experimental evidence strongly supported the modern synthesis. The theory has formed the basis of evolutionary science since the 1950s. It has also led to an effort to classify organisms according to their evolutionary history, as well as their physical similarities. Modern scientists use the principles of genetics and molecular biology to study relationships first proposed by Carolus Linnaeus more than 200 years ago.

New Techniques in Molecular Biology

In 1953, American biochemist James Watson and British biophysicist Francis Crick described the three-dimensional shape of DNA, the molecule that contains hereditary information in nearly all living organisms. In the following decade, geneticists developed techniques to rapidly compare DNA and proteins from different organisms. In one such procedure, electrophoresis, geneticists evaluate different specimens of DNA or proteins by observing how they behave in the presence of a slight electric charge. Such techniques opened up entirely new ways to study evolution. For the first time geneticists could quantitatively determine, for example, the genetic change that occurs during the formation of new species.

Electrophoresis and other biochemical techniques also demonstrated to geneticists that populations varied extensively at the molecular level. They learned that much of population variation at the molecular or biochemical level has no apparent benefit. In 1968 Japanese geneticist Motoo Kimura proposed that much of the variation at the molecular level results not from the forces of natural selection, but from chance mutations that do not affect an organism's fitness. Not all scientists agree with the neutral gene theory.