Epigenetics in Evolution

Dario Galanti

History of evolutionary theory

The evolutionary theory as we now know it, originated in the second half of 19th century, when Charles Darwin and Alfred Russel Wallace independently came up with the idea of evolution by natural selection (Darwin 1859; Wallace 1858). This revolutionary theory was opposing the Creationist theory, which considered different species as distinct entities created independently. Darwin and Wallace instead argued that species can produce diverging phenotypes which, given a large enough number of generations, can result in different species. This theory was providing a single explanation for the observation of micro- and macroevolution, justifying the entire diversity of life-forms present on Earth. Through his observations Darwin also understood that the force driving evolution is natural selection, acting on variation arising by chance. In other words, during the lifespan of a generation, slight differences arise stochastically between individuals and are transmitted to the progeny. These slight differences or “variation” make the individuals in the new generation diverge. The new generation is then selected by the environment as only individuals that are more fit for that specific environment will more likely survive and reproduce, propagating further only the “positive variation” arisen by chance. However, how this variation originated and was then passed on to the progeny was an important missing piece. It was not until the beginning of the 20th century that an answer emerged.

When Gregor Mendel published his studies on the heredity of traits in plants (Mendel 1865) he described the heredity of discrete traits and for years his findings seemed to be incompatible with the gradual evolution by natural selection hypothesized by Darwin and Wallace. It was only at the beginning of the 1900s that Darwinian evolution by natural selection and Mendel’s laws of heredity were shown to be compatible and were joined in a common mathematical framework by the work of Ronald Fisher, Sewall Wright and J. B. S. Haldane, giving birth to the field of “Population genetics” (Provine 1978). This unification was initially made possible by showing that continuous traits result from the independent inheritance of several genetic loci with small additive effects (Fischer 1919), solving the apparent contrast between Darwin’s gradual evolution and Mendel’s discrete traits.

This unification, started in the 1920s with the work of population geneticists, continued until the 1940s, further developing the evolutionary theory in what was then called the “Modern synthesis” by Julian Huxley (Huxley 1943). The discovery of DNA in 1953 by James Watson and Francis Crick as the molecule responsible for the inheritance of traits answered the last major question left open. The MS was initially implemented with some numerical differences by its founders (Mayer, Stebbins and Dobzhansky 1950; Smocovitis 1997), but the basic principle was common: natural selection is acting on heritable genetic variation generated by random mutations.