Random Mutations and Evolutionary Change:
Ronald Fisher, JBS Haldane, & Sewall Wright (2 of 2)

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 Map of malaria distribution
Map of sickle-cell anemia distribution

Natural Selection in the Wild
Population genetics became one of the key elements of what would be called the Modern Synthesis. It showed that natural selection could produce evolutionary change without the help of imaginary Lamarckian forces. Scientists have used the mathematical tools developed by Fisher, Wright, and Haldane to measure evolutionary change in the wild with exquisite precision. Their insights have even allowed medical researchers to decipher the puzzle of some hereditary diseases. Sickle-cell anemia, for example, is caused when children inherit two defective copies of a gene involved in making hemoglobin. But a single copy of this allele can give some protection against malaria (see figures, right). Natural selection finds a balance between the reproductive disadvantage of being born with two copies of the allele and the advantage of having one. Genetic disorders such as sickle-cell anemia are actually the agonizing byproduct of natural selection acting on our ancestors.

Left top: Distribution of malaria in southern Europe, southwest Asia, and Africa around 1920 (green), prior to mosquito eradication programs. Left bottom: Distribution of the sickle-cell allele within the same area. The darker the blue,the greater the percentage of people carrying the allele. Note the correlation between these maps.

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