Population Genetics
In the decade between 1858 and 1868, two of the greatest of all biological theories were formulated: Darwinian evolution and Mendelian genetics. Each was to have its own profound influence on biological thought. Yet in the early 1900s, the two theories appeared to be on a collision course. One--genetics--was based on the notion that changes occur as the result of combinations of discrete units (genes), and thus take place along a discontinuous path. The other--evolution--analyzed changes in terms of slow, continuous variation from one generation to the next. Many biologists saw no common ground between these two approaches. Bitter arguments developed as to which was the "correct" way to analyze change in living organisms.
One solution to the problem was suggested by the British mathematician George Udney Yule (1871-1951) in 1902. Yule thought that more complex forms of change, such as those studied by evolutionists, might be explainable if one were to assume that a number of factors (genes), rather than a single factor, are involved in such changes. Unfortunately, Yule's views were not widely adopted for a number of years. Over time, however, it became apparent that Yule's hypothesis held the key to resolving the debate between Mendelians and evolutionists. The solution was to find mathematical techniques for studying the complex interaction of multiple genes as they passed from generation to generation. The product of this line of research is the field of science now known as population genetics.
The moving force behind the development of population genetics was a group of biometricians, mathematicians who use mathematical or statistical techniques to study living organisms. One of the most famous of the early biometricians was Ronald Fisher, who later became professor of genetics at Cambridge. In a series of remarkable papers, Fisher used statistics to show how Mendelian principles can be used to understand the process of natural selection on which evolutionary change is based. Others who joined in this effort included John Burdon Haldane, Sewell Wright (1889-1988), Reginald Crundall Punnett, and S. S. Chetverikoff. Haldane analyzed the process of selection by mathematical techniques and, in 1932, achieved the first estimate for the rate of mutation for a single human gene. Wright worked extensively with populations of guinea pigs and found that statistical problems of sampling were important in developing theories of evolution. Punnett developed a tabular system for analyzing genetic changes, a system that is well known to all beginning genetics students today. Chetverikoff was a Russian geneticist whose mathematical discoveries in many cases actually preceded those of his Western colleagues.
Perhaps the single most important accomplishment in the early years of population genetics was the discovery of the Hardy-Weinberg equilibrium. In 1908, English mathematician Godfrey Harold Hardy (1877-1947) and German physician Wilhelm Weinberg (1862-1937) published similar papers that appeared about six months apart. In these papers, Hardy and Weinberg developed a mathematical system that described the stability of gene frequencies in succeeding generations of a population. The law states that a population reaches genetic equilibrium after only a single generation and that the equilibrium can be disturbed by the occurrence of mutations within the population, migration into or out of the population, non-random mating, and other factors. Since these factors almost always occur within a population, the Hardy-Weinberg Equilibrium is important in calculating the effect they have on gene distribution within the population.
Critical to the spread of population genetics were such books as Genetics and the Origin of Species (1937) and Evolution, Genetics, and Man (1955) by Theodosius Dobzhansky, and The Meaning of Evolution (1949) by George Gaylord Simpson (1902-1984). These volumes made the findings of population genetics widely available to specialists and non-specialists alike.
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