Concept


Key to the theory of evolution by natural selection are new trait variations that arise spontaneously and make an organism more competitive in the struggle for survival. Following publication of Darwin's On the Origin of Species, in 1859, field stations were established where scientists could study the unique characteristics of organisms that had evolved to inhabit different environments. However, field observation could not explain the origin of variation or how new traits are inherited. The new sub-discipline of experimental evolution emerged at the turn of the 20th century with the goal to recreate evolution in controlled experiments with agricultural plants and animals. It soon became clear that mutations in genes are the source of variation and that Mendelian genetics offered a statistical method for analyzing the inheritance of new mutations. By the early 1920's, experimental evolutionists had quietly become the first generation of geneticist.

Animation


Hello, I'm George Shull. I was very interested in Darwin's theories, especially in how variations arise and accumulate in a species. In 1905, I began experimenting with corn hybrids to see how variations are inherited. In an open field, corn plants are normally cross-pollinated by wind. These plants vary a lot in size, vigor, and yield. I started my experiments by self-pollinating corn plants. Look what happened when I self-fertilized these plants for seven generations. I realized that these self-pollinated lines were pure-bred lines of corn, similar to Mendel's pure-bred pea plants. I made several pure-bred lines this way, each carrying a different variation. These pure-bred lines all grew very poorly and had a lower yield compared to corn openly pollinated in the field. When I cross fertilized two pure-bred lines, I noticed an increase in productivity in the hybrids. They grew better than either of the parental strains or those openly pollinated in the field. I counted the kernel rows on each ear as a way to measure yield. Compared to the parental inbred strains, the hybrid offspring had more than double the yield. Compared to corn openly pollinated in the field, the hybrid progeny had a 10-20% higher yield. I saw immediately that "hybrid vigor" was a significant factor in improving corn production. My experiment, which started as a way to study species variation, became a Mendelian analysis of gene inheritance. Most corn grown today is derived from my experimental design and hybrids.

Gallery


1849 portrait of Darwin, drawn about the time of Beagle voyage.
Charles Darwin, around 1859.
Caricature of Darwin after the publication of Origin of Species.
Interior of Darwin's study, towards the windows.
Interior of Darwin's study, showing mantlepiece, corner and Darwin's chair on wheels.
Darwin's chair with the first edition of Origin of Species

Audio/Video


Audio Glossary

Mutation

Video Interviews

Garland Allen

Garland Allen is a Professor in the Evolutionary and Population Biology Program at Washington University in St. Louis. He authored Thomas Hunt Morgan: The Man & His Science, and several texts, including Matter, Energy and Life and The Study of Biology.

Clip 1 (0:45)
What is experimental evolution and who were the experimental evolutionists?

Clip 2 (1:00)
The experimental evolutionists' efforts to prove/disprove Darwin's theory.

Biography


 

Charles Darwin changed the world with his theory of evolution. This theory also provided the impetus for scientists to reexamine the question of heredity and inheritance, leading to the rediscovery of Mendel's laws and the evolution of the field of genetics. George Shull worked at the Station of Experimental Evolution. He used genetics to improve agricultural yields in corn and other crops.

CHARLES ROBERT DARWIN (1809-1892)

Charles Darwin was born in Shrewsbury, England. Charles Darwin's grandfather was the naturalist, philosopher and doctor Erasmus Darwin, who had published a four volume treatise containing his own views on the development of the species. Charles Darwin's father was also a physician, and wanted his son to carry on the family tradition. At 16, Darwin was sent to Edinburgh University to study medicine. Darwin was more interested in his zoology and geology classes. Eventually, his father withdrew him from Edinburgh and sent him to Cambridge to study divinity.

In 1831, Darwin was invited by Captain Fitz-Roy to be the science officer on the H.M.S. Beagle on an exploratory survey. During the next five years, the Beagle toured the South American coasts, the Galapagos Islands, New Zealand, Australia and South Africa. The voyage was an eye-opening experience for Darwin. He collected, sketched and made notes of fossils, and a wide assortment of living organisms. Based on what he saw, Darwin believed that life evolved, and could be traced through fossils and living examples. In this he was influenced by what he knew of geology and the stratification of rock layers. Darwin concluded that the adaptations and changes in many of the species he saw, especially the finches and tortoises on the Galapagos Islands, came about through a process of natural selection.

On his return to England in 1836, Darwin did not immediately publish his theory on evolution. He spent time sorting and cataloguing his collection of material, and publishing popular accounts of his voyage on the Beagle. In 1857, a young biologist, Alfred Russel Wallace, sent Darwin a letter outlining the "theory of evolution through natural selection." Wallace wanted Darwin's opinion and possible endorsement for presentation of the theory to the Linnean Society of naturalists. Darwin recognized the similarity of his work to Wallace's, and actually offered to burn his own work to avoid being thought of as an idea stealer. In the end both Darwin and Wallace presented the theory to the Linnean Society. It was Darwin's book however, On the Origin of Species, published in 1859, which garnered all the attention and the controversy. The book had a profound impact on the social, religious, political and scientific thinking of the general public and the scientific community. Darwin himself was uncomfortable with all the controversy and avoided it whenever possible.

Darwin wrote and published books and papers on other subjects relating to natural history, The Variation of Animals and Plants under Domestication (1868), The Descent of Man (1871). Darwin never forgot Wallace's contributions to the natural selection theory. Darwin died in 1882 and is buried in Westminster Abbey.

GEORGE HARRISON SHULL (1874-1954)

George Shull was born on a farm in Clark County, Ohio, and his knowledge of plants and hybrids came at an early age. Shull's father was a farmer and his mother was an amateur horticulturist. Being needed on the farm, Shull's early formal education was sporadic. However, it was supplemented by intense home study such that by 1892, he was teaching public school, and attending Antioch College.

In 1901, he graduated from Antioch, and went to the University of Chicago to do graduate work. With his knowledge of botany and experience with farming practices, Shull passed a Civil Service exam, and was appointed as Botanical Assistant at the U. S. National Herbarium. He also worked at the U. S. Bureau of Plant Industry as a Botanical Expert examining the flora and fauna of Chesapeake Bay and Currituck Sound. The data he collected during this time was to comprise his Ph.D. thesis. However, Shull had become interested in the statistical analysis of variation in plants. He contacted Charles Davenport at the Station of Experimental Evolution, and in 1904 was appointed to be in charge of plant work at the Station. Shull published his uncompleted Ph.D. thesis as a paper, and went to Cold Spring Harbor.

At Cold Spring Harbor, Shull studied and bred a large variety of plants, for example, the evening primrose (Hugo de Vries' favorite species), shepherd's purse, corn and peas. He published many papers on his observations of plant traits and inheritance. In 1905, he began work on corn, maize, with the intent of examining the quantitative inheritance of corn traits. Following Mendel's example, Shull obtained pure-bred lines of corn through self-pollination. The pure-bred lines were less vigorous and productive, but when he crossed the pure-bred lines, the hybrid yields were better than any of the parents or those pollinated in the open fields. He immediately recognized the potential for using this strategy to improve crop yields. By the 1930's and 40's, most farms were growing the hybrid stocks, and improved yields contributed greatly to the war effort and the rehabilitation of post-war Europe.

In 1915, Shull accepted a professorship at Princeton University. At his instigation, Princeton University Press began the publication of a new journal, Genetics. Shull was the managing editor for ten years. Genetics is still one of the top international science journals.

Shull retired in 1942. He and his wife spent most of their twilight years with their children and grandchildren.

Factoid

Links


 

Links

The Darwin-Wallace 1858 Evolution Paper

This site has the text of the Darwin-Wallace 1858 Evolution Paper. The site also talks about the interaction between Charles Darwin and Alfred Wallace.

Works of Charles Darwin

This site has the text of the three books by Darwin: Voyage of the Beagle, On the Origin of Species and The Descent of Man.

Terra-Quest

This site has a virtual Galapagos Island tour (click on "QT VR Gallery"). It also has a history section on Darwin and his voyage. You can read the page on Darwin by clicking "History" from the top navigation bar and clicking on "Darwin's Visit" on the left hand side.

BBC Online: Evolution Website

This excellent website contains a range of materials directly related to the various television programs shown on BBC TWO over a special weekend devoted to evolution, in addition to a large amount of original online content. Be sure to check out the article by philosopher Steven Rose on Darwin, Genes, and Determinism, as well as an audio interview with Randal Keynes, the great great grandson of Charles Darwin.

Mendel Web

This is a site where you can get a lot of information about Gregor Mendel, his life and scientific achievements. This site has an essay, Mendel in America: Theory and Practice, 1900-1919. There is a section in this essay about The Invention of Hybrid Corn.

Bibliography

  • Mangelsdorf, Paul C., 1955, George Harrison Shull: a Biography, Genetics, 40: frontpiece, Genetics Society of America.

  • Moore, John A., 1985, Science as a Way of Knowing, American Society of Zoologists, Thousand Oaks.

  • Rushton, Alan R., 1994, Genetics and Medicine in the United States 1800 to 1922, Johns Hopkins University Press, Baltimore.

  • Stubbe, Hans, 1972 (English Translation), History of Genetics, The Massachusetts Institute of Technology, Cambridge, Massachusetts.

Glossary


Mutation - A permanent structural alteration in DNA. In most cases, such DNA changes either have no effect or cause harm, but occasionally a mutation can improve an organism's chance of surviving and passing the beneficial change on to its descendants.

Children resemble their parents.
Genes come in pairs.
Genes don't blend.
Some genes are dominant.
Genetic inheritance follows rules.
Genes are real things.
All cells arise from pre-existing cells.
Sex cells have one set of chromosomes; body cells have two.
Specialized chromosomes determine gender.
Chromosomes carry genes.
Genes get shuffled when chromosomes exchange pieces.
Mendelian laws apply to human beings.
Mendelian genetics cannot fully explain human health and behavior.
DNA and proteins are the molecules of the cell nucleus.
One gene makes one protein.
A gene is made of DNA.
Bacteria and viruses have DNA too.
The DNA molecule is shaped like a twisted ladder.
A half DNA ladder is a template for copying the whole.
RNA is an intermediary between DNA and protein.
DNA words are three letters long.
A gene is a discrete sequence of DNA nucleotides.
The RNA message is sometimes edited.
Some viruses store genetic information in RNA.
RNA was the first genetic molecule.
Mutations are changes in genetic information.
Some types of mutations are automatically repaired.
A chromosome is a package for DNA.
Higher cells incorporate an ancient chromosome.
Some DNA does not encode protein.
Some DNA can jump.
Genes can be turned on and off.
Genes can be moved between species.
DNA responds to signals from outside the cell.
Different genes are active in different kinds of cells.
Master genes control basic body plans.
Development balances cell growth and death.
A genome is an entire set of genes.
Living things share common genes.
DNA is only the starting point for understanding human biology.
adi_at_dnaftb