Mendel, Darwin and Evolution

Darwin's Theory of Evolution has faced many challenges. The most serious was from biologists in the late nineteenth century. They realized that it was impossible to reconcile Darwinism with both the fossil record or the current knowledge of heredity. This delayed the acceptance of Darwin's work until about 1940. This pause became known as the "Eclipse of Darwinism". Darwin's Theory of Evolution, often a symbol of the clash of religion and science, would be revived because of heredity experiments conducted in a monastery, by Gregor Mendel, a Roman Catholic monk (see Mendel Timeline).

The Eclipse of Darwinism

By the turn of the twentieth century, Darwin's Theory of Evolution had already fallen out of favour. With good reason. There were two very big holes in the initial theory. One was that the explosion of life forms in the early Cambrian period had not been preceded by transitional forms. The other was 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. If enough of these selections occured on different beneficial traits you could end up with completely new species. But 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. Gregor Mendel had the answer to Darwin's problem. Traits were not blended, but inherited whole. And according to Mendel's laws of inheritance, a trait that might disappear in one generation might reappear in the following generation. Modern Neo-Darwinism combines both Darwin's and Mendel's work.

Mendel's Genetic Experiments

Gregor Mendel was a patient man. Mendel and his assistants had to perform thousands of crosses between different strains of pea plants. But Mendel's decision to study the pea plant was also important. Mendel chose pea plants because they had easily identifiable features such as wrinkled or round peas or yellow or green pea pods, that they can self-fertilize and it is easy to protect them from cross-fertilization. But he had a problem. 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 [_ 1_] .

After developing his set of true-breeding plants, Mendel and his assistants spent years making thousands of 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.

Mendel saw both randomness and reason in his results. In some ways, Mendel's genetics was similar to a dice game. Any roll of two dice could result in any of 11 different results, 2 through 12. But if you roll long enough you will see patterns. For instance, you might notice that 7 will come up 6 times as often as a 2. One of Mendel's innovations was to look at the inheritance of traits as a random event and analyze the results based on probabilities. This may have been one reason why his paper was ignored. Random events, statistics and probabilities were part of the language used by nineteenth century physicists, but not nineteenth century biologists.

We can follow some of 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 from these crosses had yellow seeds. The green trait had completely disappeared in the first generation. But then Mendel took this first generation (F.1) , and self-crossed them. He found that 6022 of the offspring of the second generation (F.2) had yellow seeds and 2001 had green seeds. From this we could guess that the genetic material from the green peas must have been preserved in the first generation. It must have been masked by something more powerful..the genetic material that coded for yellow peas. Yellow peas were dominant and green peas were recessive. The ratio of the results in the second generation is very close to 3:1. This is significant as well. 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.) of the yellow/green trait.

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.

Mendel's Laws of Inheritance

Mendel developed a set of laws from his experiments. Mendel's experiments told him that there were two "components" used to code for each trait and that some traits seem to dominate others. An individual component is now known as an allele. The first law, the Law of Segregation, states that during fertilization each parent passes on one allele for each trait. Which allele the offspring would get from the parents is random.

The second law, the Law of Independent Assortment, states that transmission of one trait does not affect the transmission of other traits. We only described the experiments for one trait, pea color. In fact, Mendel and his fellow monks conducted experiments on six other traits; pea shape (round or wrinkled), seed color (gray or white), stem length, color of unripe pod (green or yellow), position of the flower (terminal or axial) and form of the ripe pod (inflated or constricted). The second law means that the inheritance of a green unripe pod should not influence the inheritance of a terminal flower.

The third law, the The Law of Dominance, states that one type of allele (the dominant) could mask the other (the recessive). This is now considered a general principle and not a law.

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 and the Church

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, not a Club Med. How Gregor Mendel spent his day was determined by the 'rule' of the religious order and the abbot of the monastery. But that is not the main problem with the myth. Mendel's paper represents the results of a very big project. His paper involved the results of 29,000 crosses. We know what was involved with just one cross. It took 8 years, and for at least two of those years there were 3 full-time researchers (including Mendel) [_ 2_] . It required use of the monastery's greenhouse and its two hectares of research plots. The plants in each location had to be tended. An obvious question here is why would the abbot of the monastery commit so many resources to the project. The answer is that Abbot Napp was an advocate for the scientific study of inheritance even before Mendel arrived [_ 3_] .

Napp was not the only monk that would influence Gregor Mendel. Mendel took charge of the monastery's research garden in 1846. His predecessor at the garden, Matthew Klacel, was also studying heredity and evolution. Klacel had a special interest in the study of peas, perhaps influencing Mendel to continue the study. As Mendel was developing his theories he would have been able to bounce his ideas off his friend, Klacel, and other monks that were also interested in botany [_ 4_] .

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 doesn't just derive from Darwin, but also from Gregor Mendel, a Roman Catholic monk. With Gregor Mendel, there didn't seem to be much conflict between science and religion. His was not an individual achievement, as is often suggested. It was a personal achievement that had the full support of the community that he had committed to at a young age. A community that supported his work when few others would.