From Individuals to Populations: Epigenetics and the Modern Synthesis

The journal article, "The historical transformation of individual concepts into populational ones: an explanatory shift in the gestation of the modern synthesis," charts a crucial intellectual journey in the history of biology. It argues that the foundational shift in evolutionary thought, which led to the Modern Synthesis, was not just about merging genetics with natural selection, but about a conceptual reorientation: moving away from viewing variation as a property of the individual organism and embracing it as a property of the population. This move from an "individual" to a "populational" perspective was a necessary precursor to understanding evolution on a grander, statistical scale, and it is against this backdrop that the contemporary challenge of epigenetics must be understood.

The Modern Synthesis, as articulated by figures like Dobzhansky, Mayr, Simpson, and Stebbins, synthesized Darwinian natural selection with Mendelian genetics. 

A key tenet of this synthesis was the "gene-centric" view of evolution, where genetic mutations were the sole source of heritable variation and natural selection acted upon these variations to drive adaptation. 

This framework, however, largely neglected non-genetic sources of inheritance. The article highlights that the architects of the synthesis had to first grapple with the idea that variation was not a flaw or a deviation from a "type" or ideal, but rather the very raw material of evolution itself. This required a statistical approach, where the frequency of alleles within a population, rather than the traits of any single organism, became the central unit of study. 

The article's central thesis is that this populational thinking was the "explanatory shift" that made the Modern Synthesis possible.

The rise of epigenetics, however, introduces a new layer of complexity that challenges this gene-centric, population-based model in a profound way. 

Epigenetics refers to heritable changes in gene expression that do not involve changes to the underlying DNA sequence. These changes, such as DNA methylation or histone modification, can be influenced by environmental factors like diet, stress, or exposure to toxins. 

The key implication is that these "epigenetic marks" can be passed down to subsequent generations, sometimes for multiple generations. This phenomenon introduces a new source of heritable variation that is not solely based on random genetic mutations.

The involvement of epigenetics in this historical and theoretical narrative is twofold. First, it reintroduces the individual as a significant locus of evolutionary change in a way that the Modern Synthesis had purposefully de-emphasized. While the synthesis focused on populations to explain large-scale evolutionary trends, epigenetics highlights how an individual's experiences and environment can create heritable changes that can then become subject to selection. A classic example is the Agouti mouse study, where maternal diet altered the coat color and disease susceptibility of offspring, effects that were passed down to subsequent generations. 

This demonstrates a direct link between an individual's environment and a heritable, phenotypic change, bypassing the traditional route of random genetic mutation.

Second, epigenetics challenges the strict separation between heredity and environment that was foundational to the Modern Synthesis. The synthesis, in its attempt to provide a clean, parsimonious model, treated genetic inheritance as the primary source of variation and the environment as the selective agent. Epigenetics blurs this line, showing that the environment can directly influence heritable traits. This challenge to the Modern Synthesis necessitates an expansion if not a replacement of its framework. The "explanatory shift" from individual to population was crucial, but epigenetics suggests a new explanatory shift might be necessary—one that incorporates the dynamic interplay between the individual's environment and its heritable potential.

In conclusion, the historical account of the Modern Synthesis's gestation provided a lens through which to understand the current debates surrounding epigenetics. The article shows that the move to a populational perspective was a necessary intellectual leap to escape typological thinking and establish a framework for evolutionary biology. Epigenetics now challenges this very framework by reintroducing the individual's environmental experiences as a source of heritable variation, and by demonstrating that inheritance is not solely a matter of DNA sequence. It suggests that the "gene" is not the only unit of inheritance and that the explanatory framework of evolution must become more inclusive. As such, the study of epigenetics demands a new synthesis, one that can account for the intricate dance between genes, environment, and heritable epigenetic marks, and thereby create an even more nuanced and comprehensive understanding of the process of life's evolution.