Mendel, Darwin and Evolution

Intelligent Design was not the most serious challenge to Darwin's Theory of Evolution in its long history. Scientists in the late nineteenth century couldn't reconcile Darwin's theory with their current understanding of heredity. This flaw was serious enough to cause it to fall out of favour until about 1940 (see Eclipse of Darwinism). Darwin's Theory of Evolution, often a symbol of the clash of religion and science, was revived through the work of Gregor Mendel, a Roman Catholic friar.

Darwin's Theory of Evolution 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 friar 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. ).

Gregor Mendel's arrival at the St.Thomas Abbey was a stroke of luck for its abbot. Cyril Napp had already decided that understanding "what is inherited and how" was key to the study of hybridization [_1_] . Answering this question would require someone with a lot of patience and an unusual attention to detail. That person was Gregor Mendel.

Gregor Mendel took over the monastery's research garden from his mentor, Friar Klacel, in 1846. The research garden is shown below. Klacel had been studying heredity and variation in peas [_2_] . Gregor Mendel 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.

Gregor Mendel encountered problems from the start. If you self-fertilized some tall pea plants they would always produce tall plants even through more than one generation. But if you self-fertilized other tall pea plants they would produce mostly tall plants but some dwarf plants. Although the plants looked similar (same phenotype, tall) they were obviously different genetically (different genotypes). Similar problems occurred with every trait that he was testing. Mendel knew he had to start with a set of plants that when self-crossed would always produce the same phenotype. Developing this set of true-breeding plants took two years [_3_] .

After developing his set of true-breeding plants, Mendel and his assistants spent years making 29000 crosses through multiple generations of plants. This cross-fertilization was tedious work. Pea plants have both male and female organs. To cross-fertilize pea 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 cross-fertilize, Gregor 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 St. Thomas Abbey spent sending Mendel to the University of Vienna paid off in both the design of Mendel's experiments and the analysis of the results. One of his professors was a renowned physicist, Christian Doppler. Mendel would have been taught the design of physical experiments. Doppler's math textbooks contained sections on combinatorial theory and the use of probability. 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. [_4_]

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 as "Experiments in Plant Hybridization" in 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.

Gregor Mendel's work 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 associated with a trait survived in the population there was some probability that the trait encoded by the particles would remain in the population.

In simple terms, Mendel's theory 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) can dominate the other (the recessive). This means that in a pair of alleles with a dominant and recessive allele, the dominant trait will show. The only way for a recessive trait to show is if both alleles were 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.

Gregor Mendel's research was so time and resource intensive that it could never have been completed without the full commitment of the St. Thomas monastery. It took 8 years, involving several members of the monastery [_5_] , and monopolized the monastery's greenhouse and two hectares of research plots. A junior friar could not command such resources but an abbot could. Mendel's project was answering a question that Abbot Napp thought was key to understanding hybridization. Other friars had been working on heredity and variation before Mendel's project began (e.g. Klacel).

There are lessons to be learned from Mendel's story that might help us better understand science and the history of science. Gregor Mendel is not commonly referred to as a genius. You don't need to be a genius to make important contributions to science. Perserverence, attention to detail, and approaching the problem from a different angle works too. Luck helps. When Abbot Napp sent Gregor Mendel to the University of Vienna, Mendel was exposed to two elements key to the success of his work, combinatorics and the proper design of physical experiments. Teamwork and gifted leadership is important in big projects too. Abbot Napp understood and believed in Mendel's work. He continued allocating resources even though Mendel and other friars worked eight years without producing any published results.

Mendel's story informs the discussion of church and science also. The St. Thomas monastery performed a double role of monastery and research institute. Other monasteries had served this double role for centuries. Many priests, friars and monks made many important contributions to science. These include many in scientific methodology, the biological sciences and physical sciences. Other pages on this site discuss these in more detail.

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