The Eclipse of Darwinism was a period from the late nineteenth century to about 1940. During this time, Darwin's Theory of Evolution was out of favour with biologists. The skepticism of Darwin's work was justified. When first published, it had some pretty big holes. In a ironic twist, it would be a Roman Catholic monk, Gregor Mendel, who rescued Darwinism by plugging its biggest hole, heredity*.
Darwin proposed, that with the natural variations that occur in populations, any trait that is beneficial would make that individual more likely to survive and pass on the trait to the next generation. This process of natural selection could result in completely new species. Darwin did not have an explanation for how the traits could be preserved over the succeeding generations. At the time, the prevailing theory of inheritance was that the traits of the parents were blended in the offspring. But this would mean that any beneficial trait would be diluted out of the population within a few generations. This is because most of the blending over the next generations would be with individuals that did not have the trait.
A Roman Catholic monk from Moravia, Gregor Mendel, had the answer to Darwin's problem. Traits were not blended, but inherited whole. Modern Neo-Darwinism combines both Darwin's and Mendel's work.
*(Another hole was that the explosion of life forms in the early Cambrian period had not been preceded by transitional forms. ).
Mendel's Laws provided a way for Darwin's beneficial traits to be preserved. Instead of mixtures that were blended, Mendel proposed particles that could be recombined. As long as the particles survived there was some probability that the trait encoded by the particles would remain in the population.
In simple terms, Mendelian genetics says that individual traits are "coded' by pairs of particles. In reproduction, one particle would be contributed by each parent for every trait. This observation is known as the Law of Segregation. The particles are known as "alleles". The traits you see in the child are governed by the relationship of the two alleles (see Third Law). Mendel also noticed that the inheritance of one trait doesn't influence the inheritance of other traits ( the Law of Independent Assortment).
The third law, the The Law of Dominance states that one type of allele (the dominant) could mask the effect of the other (the recessive).
Mendel's Laws of Inheritance helped revive Darwin's theory. They would also prove tremendously important to the future of biology and medicine, affecting the lives of billions of people. A completely new discipline within Biology, Genetics, arose from Mendel's work. New hybrid food strains were developed that were either more productive, more nutritious, more disease resistant or had better taste. The Green Revolution and foods that we take for granted such as canola oil were largely the product of Mendelian genetics.
Cyril Napp, the abbot of St. Thomas Abbey, didn't know how lucky he was when Gregor Mendel arrived at the monastery in 1843. Napp had already decided that understanding "what is inherited and how" was important in discussions of breeding [_1_] . Answering this question would require someone with an unusual amount of patience and an unusual attention to detail. That person was Gregor Mendel.
Mendel took over the monastery's research garden from his mentor, Friar Klacel, in 1846. Klacel had been studying heredity and variation in peas [_2_] . Mendel's work would focus on peas as well, perhaps influenced by his mentor. This choice was very important to his eventual success. Pea plants have easily identifiable features, can self-fertilize and are easily prevented from cross-fertilizing. While the choice of pea plant made success more likely, he and his team still had to overcome many hurdles.
Like many researchers today, Mendel encountered problems from the start. If you self-fertilized some pea plants with green pods they would always produce pea plants with green pods even through more than one generation. But if you self-fertilized other pea plants with green pods they would produce mostly plants with green pods but some with yellow pods. Although the plants looked similar (same phenotype) they were obviously different genetically (different genotypes). Similar problems occurred with every trait that he was testing. Mendel knew he had to start with true-breeding plants which means that he had to produce a set of plants that when self-crossed would always produce the same phenotype. This required two years of tedious work before Gregor Mendel could even start his hybridization experiments [_3_] .
After developing his set of true-breeding plants, Mendel and his assistants spent years making 29000 crosses through multiple generations of plants. This was tedious work. Pea plants have both male and female organs. To cross these plants you have to make certain they don't self-fertilize first. Mendel performed surgery on each target plant by cutting off the male organs (stamens) while the plant was still immature. When the time came to make the cross, Mendel and his assistants used a paintbrush to brush some pollen off the anthers of the donor plant and painted the pollen onto the stigma (part of female reproductive structure) of the target plant. A bag was then wrapped around the flower to prevent other pollen from landing on the stigma.
The money the St. Thomas Abbey spent sending Mendel to study at the University of Vienna paid off when it came time to analyze the results. One of his professors was a renowned physicist, Christian Doppler. One of Mendel's innovations was to look at the inheritance of traits as a random event and analyze the results based on probabilities. Random events, statistics and probabilities were part of the language used by nineteenth century physicists, but not nineteenth century biologists.
We can follow Mendels's logic by following one of his experiments. Mendel took true-breeding pea plants that produced only yellow peas and crossed them with true-breeding pea plants that produced only green peas. All offspring had yellow seeds. The green trait had completely disappeared. Then Mendel took this first generation (F.1) and self-crossed them. The green trait showed up again. 6022 of the offspring of the second generation (F.2) had yellow seeds and 2001 had green seeds. Genetic material from the green-seeded plants must have been preserved in the first generation. It was masked by something more powerful..the genetic material that coded for yellow seeds. Yellow-seeded plants were dominant and green-seeded plants were recessive. The ratio of the results in the second generation is very close to 3:1. This ratio can be explained if the inheritance of traits depended on paired elements that are recombined (not blended as Darwin believed) in the offspring. In this experiment a Yellow-Green pair would show as a yellow pea. But if we crossed many Yellow-Green plants we could get only 4 different permutations; Yellow-Yellow, Yellow-Green, Green-Yellow, and Green-Green. Three of them result in yellow peas, and only one, the Green-Green, results in green peas. The diagram below (taken from a early book by Thomas Hunt Morgan) illustrates Mendelian genetics through two generations (F.1 and F.2) .
Why did Mendel use such large numbers of crosses in his experiments? Mendel needed large samples to produce higher confidence in the 3:1 ratio. If Mendel had used smaller sample sizes his work would have been of little value. Charles Darwin had conducted similar experiments with snapdragons but because of his poor understanding of sampling had only used 125 crosses. His result of 2.4:1 could have been interpreted as a 2:1 ratio or a 3:1 ratio ( Darwin, Mendel and Statistics). Mendelian genetics helped support a trend toward a more mathematical approach in biology.
Gregor Mendel's work on genetics was finally published in the the Proceedings of the Natural History Society of Brünn in 1866. No-one seemed to care. The paper was rarely mentioned over the next 35 years. It would dramatically change the field of biology when it was rediscovered around 1900.
Even though Gregor Mendel was a Catholic monk, the role the church played in his life and research is often dismissed. In The God Delusion, Richard Dawkins reduces the church's role to little more than a passive source of funds:
... Mendel, of course, was a religious man, an Augustinian monk; but that was in the nineteenth century. when becoming a monk was the easiest way for the young Mendel to pursue his science. For him, it was the equivalent of a research grant.
The monks who worked and prayed daily with Gregor Mendel would have disagreed. Two years after the publication of Mendel's work the monks of Brno held an election for a new abbot. The unanimous choice was Gregor Mendel.
One myth about Mendel's work is even less believable than Dawkins' speculations. This is the myth of a shy monk discovering the laws of genetics while puttering around in the monastery garden during his spare time. This ignores the obvious. This was an Augustinian monastery. How Gregor Mendel spent his day was determined by the 'rule' of the religious order and the abbot of the monastery.
Another problem with the myth is simply the scale of the work. It was a very big project! It took 8 years, involved multiple researchers [_4_] , and monopolized the monastery's greenhouse and two hectares of research plots. Why were so many resources assigned to Mendel's project? The answer is that Mendel was answering a question that Abbot Napp thought was important. Other friars had been working on heredity and variation before Mendel arrived (e.g. Klacel).
Was the monastery's support for Mendel's work an exception? Not if you look at the history of agriculture in Europe. Europe during the Middle Ages was one of the few areas in the world with highly educated farmers. These were monks, working both inside and outside of monasteries. They had long recognized the importance of breeding, draining land, and irrigation. They also believed strongly in experiment, measurement and keeping records. The image below is the "August" panel from the Ciclo dei Mesi painted in Trento around 1400. The man in the hut is a monk keeping an account of a harvest.
Evolution is a favourite example for those that believe in the conflict between science and religion. And it is Darwin that is the focus of these discussions. But current evolutionary theory derives from both Darwin and Gregor Mendel, a Roman Catholic monk. With Gregor Mendel and his superiors at the St. Thomas Abbey science and religion seemed to go hand in hand.
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