Fact sheet 8: Epigenetics in evolution

Dario Galanti, Morgane van Antro, Anupoma Troyee

History and current status of Evolutionary theory

Since Charles Darwin and Alfred Russel Wallace came up with the fundamental and enlightening idea of evolution being the result of natural selection [1, 2], several steps have been taken through a more accurate and universal theory of evolution. Subsequent advances in fields like population genetics and more recently evolutionary and developmental biology (Evo-Devo), kept updating evolutionary theories until the present day. During the 20th Century, Darwin’s basic ideas [3] and Gregor Mendel’s studies on genetic inheritance [4] were joint in a unifying theory. This theory was named “Modern Synthesis” by Julian Huxley [5] and although it was implemented with some differences by its founders (Ernst Mayr, G. Ledyard Stebbins and Theodosius Dobzhansky), the basic principle was common: natural selection is acting on heritable genetic variation generated by mutations. Population genetics, focusing on the variation in allele frequencies and distribution of populations, was initially developed by Sewall Wright, J. B. S. Haldane and Ronald Fisher and offered the mathematical framework for the “Modern Synthesis” [6].

At its latest status, the basic principles of the “Modern Synthesis”, i.e. the basic mechanisms driving evolution, can be summarized as follows [7].

• Mutations are the ultimate source generating genetic variation
• Evolutionary mechanisms such as Natural selection, Genetic drift and few others (Genetic hitchhiking, Epistatic effects…) further shape this variation causing changes in allele frequencies.
• Sexual recombination (when present) reshuffles the distribution of alleles between individuals.
• In addition, landscape is playing a crucial role by causing population subdivision and therefore affecting the occurrence of gene flow and migration.

Despite the accuracy of this theory in describing the genetic implications of evolution, further advances were made in the last 20 years, posing the question whether additional drivers should be included in the evolutionary theory [8]. This is where transgenerational epigenetics comes into play, being one of the most important candidates to be included. Answering to this situation, Massimo Pigliucci and Gerd B. Müller renovated previous forms in a new “Extended Evolutionary Synthesis”, which included additional evolutionary drivers such as multilevel selection, transgenerational epigenetics, niche construction, evolvability and others [9, 10]. Nevertheless the question whether these additional drivers are significantly increasing the accuracy and power of the evolutionary theory is still under debate [9, 11‑13]. It is therefore crucial to determine the relative contribution of epigenetics to evolution in order to correctly implement it in an eventual future evolutionary theory.

The role of Epigenetics

In order to determine the role of epigenetics in evolution, it is crucial to understand to which extent different epigenetics marks are stably inherited across generations. While histone modifications seem to revert and not to be transgenerationally inherited by the offspring [14], there is evidence that DNA methylation variation is at least partially heritable [15‑18].

An important complicating factor, hindering advances in determining the importance of epigenetics in evolution, is posed by the tight link between genetics and epigenetics. When a genetic polymorphism is controlling an epigenetic pattern, the latter will seem to be heritable and under selection, while in reality this is only true for the causal genetic polymorphism. Nevertheless different tools were developed to disentangle this aspect and it has been shown that epigenetic variation can, at least in some cases, be independent from DNA sequence variation [15, 18‑20].

Given these basic requirements of being heritable and independent of DNA sequence variation, epigenetic variation could potentially play a role in evolution. This could specifically happen in two ways: 1) Epimutations, arising stochastically, at a higher rate than genetic mutations [21], could be under natural selection; 2) Environmentally induced epigenetic variation could provide a mean of rapid evolution [22, 23].

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