Since the 1940s, evolutionary scientists have rejected the concept of race based on physical characteristics — or phenotype — in determining the variety of races. Evolutionary and social scientists observed that established racial categories and definitions lacked taxonomic validity. They argued that definitions of race were imprecise, arbitrary, derived from customs, had many exceptions and gradations and that the number of races observed varied according to the culture examined. For example, categorizing human populations by skin color was no more useful than using hair color or hair texture, or eye color, or nose size, lip size, or height. Instead, they theorized that analyzing human genotypic and phenotypic variation was better interpreted in terms of populations and clines
One of the leading scientists in advancing modern evolutionary theory was Sewall Wright. His work on inbreeding, mating systems and genetic drift made him, along with R. A. Fisher and J.B.S. Haldane, the founders of population genetics theory. Their work was the basis of modern evolutionary synthesis also referred to as neo-Darwinian synthesis.
The inbreeding coefficient and F-statistics, which are standard tools in population genetics, were developed by Wright. His contribution to the mathematical theory of genetic drift led to its designation as the Sewall Wright effect; this theory represents cumulative changes in gene frequencies that arise from random births, deaths, and Mendelian segregations in reproduction. Wright was involved in a longstanding argument with Fisher, who was convinced that most populations in nature were too large for the effects of genetic drift to be important.
Another important scientist involved in reconceptualizing genotypic and phenotypic variation was anthropologist C. Loring Brace. Brace was responsible for the observation that these variations, insofar as they were affected by natural selection, migration or genetic drift, were distributed along geographic gradations called “clines.” This conclusion drew attention to the fact that phenotypic-based descriptions of races ignore numerous other similarities and differences, such as blood type, which do not correlate highly with race. This led anthropologist Frank Livingstone’s to conclude that "there are no races, only clines."
Theodosius Dobzhansky, an American geneticist born in Russia, is known for his basic work in genetics and conducted much of his research with fruit flies. His writings include Genetics and the Origin of Species (1937), a summary of contemporary knowledge of genetics; Evolution, Genetics, and Man (1955); and Mankind Evolving: The Evolution of the Human Species (1962), which explores cultural and biological evolution.
In 1939, race scientist Carleton Coon published The Races of Europe. By 1950, Coon had co-authored Races: A Study of the Problem of Race Formation in Man with colleagues Stanley Garn and Joseph Birdsell. In Race Formation, a place-based concept of race—a multi-regional theory—emerged that countered the neo-Darwinian interpretation of biological variability, which emphasized natural selection. With multi-regional theory, races were conceptualized as geographical races, whose defining characteristics were seen as the byproduct of adaptation through natural selection based on environmental factors. His hierarchal ranking resembled the scientific racism of the early twentieth century.
American immunochemist William C. Boyd co-authored a publication entitled Races and People with Isaac Asimov in 1958. A worldwide survey of the distribution of blood types made by Boyd and his wife Lyle in the 1930s showed that blood groups are inherited and not influenced by environment. Genetic analysis of blood groups led him to hypothesize that the population differences between human races are found in alleles. This hypothesis prompted him to divide the world population into 13 geographically distinct races with different blood group gene profiles.
Harvard professor Richard Lewontin helped establish the field of molecular evolution in a pair of papers that he co-authored with J.L. Hubby in the journal Genetics in 1966. Lewontin , an evolutionary biologist, geneticist and social commentator, helped establish the mathematics of population genetics and evolutionary theory. He found that the proportion of human variation that could be statistically explained by race was insignificant in a 1972 article. If it could be found that the relative degree of variation among races was significant compared to the variation within a single race, then race could be a statistically valid concept; however, if the relative degree of variation among races was not significant compared to the variation within a race, then race would have to be seen as a less statistically valid measure of biological differences.
A series of papers using larger data sets have replicated Lewontin’s results, demonstrating that statistically “race” does not explain a great deal about human variation. Neo-Darwinian theories which explain animal behavior and social structures in terms of evolutionary strategy — which has been controversially applied to humans, and seen as genetic determinism — as espoused by sociobiologists and evolutionary psychologists such as Edward O. Wilson and Richard Dawkins, has drawn criticism from Lewontin. His opinion is that a more careful understanding of the context of the whole organism as well as its environment is required for a more complete understanding of evolution.