ALLEN MACNEILL: In the previous episode in this series, we learned how variation-- that is, the differences between individuals in populations-- is key to evolution. Without variation, evolution is virtually impossible. With variation, evolution is virtually inevitable. Darwin realized that the key to understanding evolution was variation.
Therefore, he spent the first two chapters of The Origin of Species talking about variation in domesticated animals and plants and their counterparts in the wild. He pointed out that there's much more variation between domesticated animals and plants than there is between wild animals and plants. He went on to explain that this difference is the result of natural selection, which has the effect of limiting variation.
However, Darwin's critics also realized that variation is the key to evolution and realized that Darwin's arguments for variation were not very strong. Indeed, in chapter 5 of The Origin of Species, which is on the subject of variation, Darwin summarized his work by saying, "Our ignorance of the laws of variation is profound." In chapter 5, Darwin also cited Lamarck's principle of use and disuse as a possible source for the variations between individuals in populations.
SPEAKER: One of Darwin's chief critics was also one of his closest friends. Asa Gray was an American botanist and a professor at Harvard University. He was also deeply religious and was troubled by the lack of supernatural design in Darwin's theory.
In an early review of The Origin, Gray pointed out that if a deity intervened in the process of natural selection, the most likely place for such intervention would be in the production of specific variations. In this way, a deity could guide natural selection to particular goals, especially the evolution of humans and human society.
At first, Darwin praised Gray's efforts to reconcile his theory with religion, going so far as to finance the publication of a pamphlet praising Gray's views. However, as many scientists attacked Gray's hypothesis as unnecessary, Darwin changed his mind and sided with the majority of the scientific community. The engines of variation appeared to have no built-in direction but were essentially random.
Fleeming Jenkin, a Scottish engineer who was famous for his work on the first transatlantic telegraph cable, wrote a much more critical review of The Origin. He pointed out an essential flaw in Darwin's argument, a flaw that once again focused on the problem of variation.
Jenkin cited the views of animal and plant breeders who universally agreed that inheritance was blended. That is, when individuals with two different characteristics were mated, those characteristics were blended in their offspring. For example, the offspring of a tall person and a short person were almost always intermediate in height between them. The same was the case for most plants. Tall plants crossed with short plants produced plants of medium height.
Jenkin pointed out that this would mean that eventually, all of the variation in any population would eventually disappear as the offspring of various crosses would all converge on some average set of characteristics. Without variation, natural selection would grind to a halt and so would evolution.
ALLEN MACNEILL: Darwin took Jenkin's criticisms very seriously. He eventually went on to publish a two-volume work on the variations in domesticated animals and plants. In it, he proposed a theory for the origin of variation that was essentially the same as Lamarck's theory of the inheritance of acquired characteristics as the result of use and disuse.
Darwin proposed that all the cells in a living organism produced tiny invisible particles called "pan genes." These pan genes were produced by the things that happened to the cells in the organism during its life. They then migrated through the circulatory system of the organism to the sex cells, where they were then passed on to the offspring. In this way, Darwin could explain how changes in the organism during its life could be passed on to its offspring as the result of mating.
Darwin's hypothesis not only explained the inheritance of characteristics that were acquired during an organism's lifetime. It also provided an explanation for the source of the variation that Darwin said was necessary for evolution by natural selection. This variation was the result of the different experiences that each organism had during its life and which were passed on to the offspring by the pan genes that Darwin proposed existed and were carried through the circulatory system.
Darwin's pan gene hypothesis was also potentially testable. Since the pan genes were supposedly carried through the circulatory system, blood could be transfused from one organism to another to determine if the things that had happened to the organism whose blood was transfused would show up in the characteristics of the organisms that receive the transfusion.
Unfortunately for Darwin's theory, this was not confirmed. Instead, transfusions and other experiments tended to indicate that there were no such thing as pan genes and furthermore, eventually, it was determined that acquired characteristics could not be inherited. As brilliant and as original as Darwin's pan gene hypothesis was, it was simply not supported by the evidence.
And so as is the case in science always, it was rejected. Darwin's theory therefore did not contain a key ingredient. It did not contain any explanation for the origin of the variations that were necessary for natural selection to occur.
SPEAKER: Unknown to Darwin, a solution to both the problem of blending inheritance and the crucial problem of the source of variation had already been solved. Gregor Mendel, an Augustinian monk living in what was then part of Austria, had formulated and tested a revolutionary theory. Mendel was a university-trained physicist. Using Newton's concept of invisible, indivisible particles, Mendel proposed that such particles were contained in all cells and encoded all the characteristics of living organisms.
Unlike Darwin's pan genes, however, Mendel's particles of inheritance worked in the opposite direction. They spread out through the bodies of organisms as they developed and most importantly, they produced characteristics that were not blended when combined.
Mendel crossed tens of thousands of garden peas and showed that the traits that he observed were not blended from generation to generation. Instead, some traits, called "dominant traits," masked the expression of other recessive traits when the two were combined. However, the recessive traits could be combined together to produce unblended recessive offspring.
Mendel had solved the problem of blending inheritance and set the stage for the solution to the deeper and more important problem of variation, as well. At the end of the 19th century, criticism of Darwin's theory of natural selection had increased dramatically as new information about the origin of variations poured in from laboratories and field stations around the world.
In England, William Bateson, now recognized as one of the founders of the science of genetics, published a huge compendium that gathered together this evidence. In it, Bateson showed thousands of examples of variation that appeared suddenly in the offspring of parents who lacked them. Bateson's work seemed to undermine Darwin's theory because implicit in the theory of natural selection was the idea that evolutionary change was gradual, not sudden.
Darwin had asserted that natural selection worked on very slight differences and so required immense lengths of time to produce the diversity we see in the living world. But new discoveries in genetics pointed to an opposite conclusion, that variations appeared abruptly and altered large parts of the anatomy and physiology of animals and plants.
In particular, Hugo de Vries, a Dutch botanist, identified many large-scale changes in the plants he studied. Furthermore, these changes, which he called "mutations," were inherited unblended from parents to offspring. In 1900, de Vries rediscovered the same principles that Mendel had discovered in the 1860s and discovered Mendel's publications, as well. de Vries used Mendel's ideas to explain his own work and proclaimed the birth of the new and revolutionary science of "Mendelian genetics."
ALLEN MACNEILL: The new Mendelian geneticists believed that their theory provided a more testable hypothesis for the origin of new species than Darwin's. Rather than natural selection being the cause of the origin of new species, mutations were the cause. And so as the science of Mendelian genetics developed around the turn of the 20th century, Darwin's theory of evolution by natural selection fell into disfavor and by the year 1900 was declared to be all but dead.
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Published 150 years ago, Charles Darwin's
On the Origin of Species provided the foundation for the modern science of biology. It also set in motion a revolution in the sciences and in our understanding of ourselves and our place in nature.
This CyberTower Study Room is a brief introduction to Darwin's theory and its implications. Beginning with an overview of Darwin's predecessors, we learn how Jean Baptiste Lamarck set the stage for Darwin's monumental achievement with his Philosophie Zoologique, which advanced a theory of evolution by means of the inheritance of acquired characteristics.
Darwin, whose academic training at Cambridge University was in Anglican theology, became an acclaimed naturalist and science writer following the five-year voyage of HMS Beagle. Using the notes and specimens that he had collected during the voyage, Darwin spent twenty years refining his theory, first published in 1859, of evolution by natural selection.
In the last segment of this Study Room, we visit the Museum of the Earth in Ithaca, New York, whose director, Dr. Warren Allman, discusses the importance of such museums to the science of evolutionary biology. We also hear from Cornell professor William Provine, who discusses Darwin's work and its importance to the history and philosophy of biology.
This video is part 5 of 6 in the Darwinian Revolutions series.