Barbara McClintock showed that genes could transpose within chromosomes; that they could move around (the so-called "jumping genes"). This was done through the investigation of maize (corn) genetics through careful hybridization. Her work with genetics came only twenty-one years after the rediscovery of Mendel's principles of heredity, at a time when acceptance of those general principles was not wide-spread. Thus her work, which to some now seems to have been ignored when it first appeared, was simply too advanced for many to comprehend at the time.
She also traced the evolutionary history of domesticated maize to determine the genetic ancestor of the grass we now call corn.
She received the Nobel prize for medicine in 1983. Her Nobel lecture was entitled "The Significance of Responses of the Genome to Challenge." In the conclusion to her lecture, referring back to those subjects she had spoken about, she stated
The examples chosen illustrate the importance of stress in instigating genome modification by mobilizing available cell mechanisms that can restructure genomes, and in quite different ways. A few illustrations from nature are included because they support the conclusion that stress, and the genome's reaction to it, may underlie many species formations.
She was speaking on no less than a mechanism whereby evolution may occur.
More information and a photo can be found at the Marine Biology Lab at Woods Hole with which she was associated for a short time.
On her 90th birthday her students and colleagues wrote essays on her and her work which were collected into the book "The Dynamic Genome, Barbara McClintock's Ideas in the Century of Genetics" (Plainview: Cold Spring Harbor Laboratory Press, 1992).
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