The epigenetic system, evo-devo, and the Extended Evolutionary Synthesis.

This blog covers the core concepts and arguments typically involved.

Integrating Development and Inheritance: The Epigenetic System, Evo-Devo, and the Case for an Extended Evolutionary Synthesis

The theory of evolution, as solidified in the 20th century's Modern Synthesis (MS), provided a framework centered on genes as the primary unit of inheritance and variation. Natural selection, genetic drift, mutation, and gene flow acting on randomly generated genetic variation were seen as the principal engines of evolutionary change.

However, accumulating evidence from diverse fields, particularly developmental biology and epigenetics, has prompted a call by researchers for an 'Extended Evolutionary Synthesis' (EES). This proposed extension seeks to integrate phenomena inadequately addressed or conceptualized by the MS, arguing that factors like developmental processes and non-genetic inheritance play crucial causal roles in evolution.

The interconnectedness of the epigenetic system and insights from Evolutionary Developmental Biology (Evo-Devo) provides compelling support for this broader perspective.

The Modern Synthesis largely treated development as a 'black box' – the process translating genotype into phenotype was acknowledged but not central to the evolutionary mechanism itself. Variation arose randomly through mutation, and selection sorted these variations based on fitness outcomes.

The organism was often seen as a passive vehicle for its genes. Evo-Devo fundamentally challenges this perspective by placing development at the heart of evolutionary change. It investigates how alterations in developmental processes – the timing, location, and level of gene expression, orchestrated by complex gene regulatory networks – generate phenotypic variation. Evo-Devo highlights concepts like developmental bias, where the pathways of development constrain the types of variation possible, meaning variation is not entirely random in its phenotypic effects. 

It also emphasizes phenotypic plasticity, the ability of a single genotype to produce different phenotypes in response to environmental cues.

These developmental mechanisms demonstrate that the organism is not merely a passive recipient of genetic instruction but actively participates in generating the variation upon which selection can act.

Parallel to the rise of Evo-Devo, the field of epigenetics has illuminated a layer of regulation 'above' the DNA sequence itself. The epigenetic system encompasses molecular mechanisms like DNA methylation, histone modifications, and non-coding RNAs that alter gene expression patterns without changing the underlying DNA code. These epigenetic marks are crucial for normal development, cellular differentiation, and responses to environmental stimuli. Critically, they possess characteristics that challenge the MS's exclusive focus on DNA sequence for inheritance. 

Firstly, epigenetic marks can be stably inherited across cell divisions within an organism, ensuring tissue identity. Secondly, and more controversially from an evolutionary standpoint, some epigenetic modifications can be transmitted across generations – transgenerational epigenetic inheritance (TEI).

The implications of TEI are profound. It suggests a mechanism for the inheritance of acquired characteristics, a Lamarckian concept largely dismissed by the MS. If environmental factors (like diet, stress, or toxins) can induce epigenetic changes that are subsequently passed down to offspring, influencing their phenotypes, then the environment has a more direct role in shaping heritable variation than previously thought. This provides a molecular basis for some forms of phenotypic plasticity to become heritable, at least for a few generations, potentially facilitating faster adaptation to changing environments or influencing the trajectory of genetic evolution by stabilizing adaptive phenotypes before genetic mutations catch up.

The confluence of epigenetics and evo-devo provides strong arguments for the EES framework. Epigenetic mechanisms are integral to the developmental processes studied by evo-devo; they are the molecular machinery that often mediates gene regulation, developmental plasticity, and cellular memory. For instance, environmental cues triggering a plastic developmental response often do so via epigenetic modifications that alter gene expression. If these epigenetic states have a degree of heritability (TEI), then plasticity itself can become evolutionarily significant in a way not fully captured by the MS, which primarily views plasticity as an outcome of past genetic evolution rather than a potential driver of future change.

Furthermore, epigenetic processes contribute to the concept of developmental bias. Epigenetic landscapes can channel development along certain pathways, making some phenotypic outcomes more likely than others, irrespective of the underlying genetic variation. 

This suggests that the direction of evolution is not solely determined by selection pressures acting on random variation but is also shaped by the inherent properties and constraints of the developmental system, including its epigenetic layer.

The EES, therefore, proposes a shift in perspective. It aims to pass the core tenets of the MS. It argues that developmental processes (informed by evo-devo) and non-genetic inheritance systems (like epigenetics) are not just consequences of evolution but active participants and causes in the evolutionary process. Other concepts central to the EES, such as niche construction (where organisms modify their environments, thereby altering selection pressures), also fit within this framework of reciprocal causation between organism, development, environment, and inheritance.

Bevers changing their environment changing others environment 

In conclusion, the study of the epigenetic system provides tangible mechanisms underlying key evo-devo concepts like developmental plasticity and bias. The potential for transgenerational epigenetic inheritance offers a non-genetic route for transmitting adaptive responses to the environment, challenging the gene-centric view of heredity. Together, epigenetics and evo-devo paint a picture of evolution where development is a creative force, inheritance extends beyond DNA, and the organism plays an active role in its own evolution. These insights form a significant part of the empirical and conceptual foundation upon which the arguments for an Extended Evolutionary Synthesis are built, promising a richer and more comprehensive understanding of the complexities of life's history.