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Pre 1800 1800s 1900 - present History of Evolutionary Thought
The History of Evolutionary Thought   

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

In 1900 several scientists across Europe came to the same realization about heredity that Mendel had some 40 years before. But they arrived at the discovery from a very different direction.

Cell mitosis
Chromosomes, the dark structures in this image, are copied and distributed to the daughter cells as this plant cell reproduces.
Chromosomes contain genetic material
Nineteenth century cell biologists discovered that animal and plant cells had a central compartment known as the nucleus. Each nucleus contained a set of rod-shaped structures, and when a typical cell divided, a new nucleus complete with a new set of rods was created. These rods were named chromosomes for the way they absorbed colored stains. But sperm and eggs contained only half the normal set of chromosomes. When a sperm fertilized an egg, the chromosomes combined to create a full complement.

Scientists realized that the chromosomes stored the information necessary for building an individual, and heredity consisted of the transfer of that information from generation to generation. Each chromosome contained information for many different traits, and scientists dubbed each chromosomal chunk that was responsible for a particular trait a "gene."

Rediscovering Mendel
Dutch botanist Hugo DeVries and several other scientists carried out breeding experiments in the late 1890s and rediscovered Mendel’s three-to-one ratio. But this new generation could offer a clearer interpretation of what was happening in their experiments. We each carry two copies of the same gene, one from each parent, but in many cases only one copy produces a trait while the action of the other is masked. Here was the secret behind Mendel's three-to-one ratio of smooth and wrinkled peas.

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.

Fossil Hominids, Human Evolution: Thomas Huxley & Eugene Dubois

Random Mutations and Evolutionary Change: Ronald Fisher, JBS Haldane, & Sewall Wright

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