Gregor Mendel (1822-1884)
Mendel was an Augustinian monk born in Austria who experimented with pea plants and discovered the laws of inheritance. Without even knowing the existence of genes, Mendel found that it was possible that some recessive traits could reappear in future generations. We know that he wrote once to Darwin to share his discovery, but Darwin received the letter, put it in a drawer and never read it. That letter was found after Darwin’s death. One cannot avoid wondering what Darwin would have thought about someone experimentally proving what he supposed to happen in theory. Mendel found that traits came in pairs, were inherited from father and mother, and could be either dominant or recessive.
What Mendel discovered is that if we breed together a population of yellow and green peas, where the yellow color is a dominant trait (AA), and the green color is recessive (aa), the offspring in the first generation will only be yellow peas (Aa). The second generation will get all possible combinations of the trait (AA, Aa, aA and aa). The result will be a ratio 1:3. Green peas will reappear. It was until the 20th century when genes were discovered that the genius of Mendel’s work received the credit he deserved.
Ronald Fisher, Sewall Wright, Theodosius Dobzhansky y Ernst Mayr
The variation that Darwin saw in his pigeons and barnacles, whose origin, he had not the faintest idea how to explain, emerge as the DNA sequences are changing. Before 1920, the mechanisms that allow evolution to act were unknown. Genes, DNA, and mutations were unknown. There were no explanations as to why there are differences between two populations of the same species. In the early 1920’s, scientists began to realize that mutations had a great impact on evolution. Ronald Fisher (1890-1962), an English statistician, and Sewall Wright (1889-1988), an American geneticist, integrated natural selection and genetics, positioning Darwin’s theory on more solid foundations. Fisher contributed demonstrating that natural selection progresses by the accumulation of small changes, as opposed to the idea of sudden dramatic variations. A significant step was the publication in 1937 of the book by Theodosius Dobzhansky (1900-1975), a Soviet scientist who emigrated to the US, titled Genetics and the Origin of Species. Working with his naked eye and using a microscope, Dobzhansky was able to identify differences in chromosomes by studying diverse populations of fruit flies (Drosophila).
Ernst Mayr (1904-2005) was inspired by Dobzhansky´s book and understood why the traits of the birds he studied in New Guinea, varied from town to town: there was gene flow. By 1940, the architects of the new modern synthesis had shown that genetics, zoology, and paleontology, all told the same story. Mutations were the foundation of evolutionary change. Those mutations, together with the laws of heredity, gene flow, natural selection and geographic isolation, could create new species and new forms of life. If all these changes worked for millions of years, it was possible that the transformations seen in the fossil record would appear. The success of the modern synthesis, also known as Neo-Darwinism, has been the force that has transformed research in evolutionary biology since 1950.