Chromosomes, Mutation, and the Birth of Modern Genetics:
Thomas Hunt Morgan (2 of 2)

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Thomas Hunt Morgan

Mutated Gene = New Species?
Perhaps, scientists speculated, evolution took place as genes were altered. DeVries claimed that if a gene changed—if it “mutated”—it would create a new species in a single jump. But no one could say for sure what mutations did until they could be studied up close. That became possible in the laboratory of a Columbia University biologist, Thomas Hunt Morgan (left).

Red- and white-eyed fruit flies
Morgan’s experiments involved red- and white-eyed fruit flies like these.

Morgan bred fruit flies by the thousands, and his team tried to create mutant flies with x-rays, acids, and other toxic substances. Finally, in one unaltered lineage of flies, the researchers found a surprise. Every single fly in that line had been born with red eyes, until one day a fly emerged from its pupa with white eyes. Something had spontaneously changed in the white-eyed fly.

Mutation Does Not Equal Speciation
Morgan realized that one of its genes had been altered and it had produced a new kind of eye. Morgan bred the white-eyed fly with a red-eyed fly and got a generation of red-eyed hybrids. And when he bred the hybrids together, some of the grandchildren were white-eyed. Their ratio was three red to one white. Here was a mutation, but one that didn’t fit DeVries’s definition. DeVries thought that mutations created new species, but the fly that had acquired the white-eyed mutation remained a member of the same species. It could still mate with other fruit flies, and its gene could be passed down to later generations in proper Mendelian fashion.

Genetics Is Born
The work of scientists such as Morgan established a new science: genetics. It would not be until 1953 that the molecular structure of genes (DNA) would be discovered, and only later did scientists figure out how DNA’s code is used by cells to build proteins. But already by the 1920s, many of the paradoxes about genes that tormented previous biologists dissolved. Genes do not always come in simply two different versions, one dominant and one recessive. Mutations can create many different versions of the same gene (known as alleles). While a single mutation can sometimes create a drastic change to an organism, such as changing red eyes to white, most mutations cannot. That’s because most traits are based on many different genes working together. Mutating any one of those genes often only produces a subtle change, or none at all.

• Morgan image (PA96M3 #44) courtesy of University of Kentucky Special Collections and Archive.
• Flies image courtesy of University of Massachusetts Amherst Biology Department.

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