Sunday, October 16, 2016

The Extended Evolutionary Synthesis - a moderate step in the right direction

A discontent grows among the practitioners of evolution theory.

Research results keep outrunning the theory’s capacity to herd them—the results, that is, but the practitioners, too—and corral them inside the Modern Synthesis.

Evidence of discontent keeps piling up. For example, in July 2008 researchers from various fields of biological science convened in Altenberg, Switzerland, to formalize a so-called Extended Synthesis of evolutionary theory. MIT Press published the conference papers as a sourcebook, Evolution - the Extended Synthesis.

The book's contributors reassure readers that the findings presented pose no fundamental threat to the Darwinian model. The collective attitude seems to be that new discoveries in genomics, epigenetics, ecology and related fields merely complicate—without undermining—the natural selection model of evolution. These reassurances reek of cover, of maneuvering to avoid the label, “fringe” or the academic equivalent. Maybe, “Traitor.”

More recently, some of the Altenberg attendees, among others, launched a web site, The Third Way, as an organizing tool for scientists working on a model of evolution that accommodates the research findings that are stretching the Modern Synthesis. More recently the The John Templeton Foundation has awarded a major grant (£5.7m or $8m) to an international team of leading researchers for a three-year research program “to put the predictions of the extended evolutionary synthesis to the test.” The Royal Society in 2015 published an article, The extended evolutionary synthesis: its structure, assumptions and predictions that provides additional background information. And in November 2016 the Society hosted a scientific meeting on the topic, entitled, “New trends in evolutionary biology: biological, philosophical and social science perspectives”.

A discontent among those working in the evolution biz is brewing if not already boiling over. 

 It seems that the evolution theory we learned in school is getting an earnest retooling. The breadth and depth of that retooling divides scientific opinion. It might involve tightening a few knobs, or it might involve a fundamental rebuild—a Kuhnian paradigm shift. Which way things shake out remains to be seen.

Whichever way things do shake out for mainstream evolution theory, the new developments embolden the star larvae hypothesis. The new developments describe a convergence of development and evolution, insofar as the new ideas describe development and evolution as sharing a common set of internal mechanics, of endogenous formative processes. In aggregate, the new discoveries make development and evolution seem to be processes of differentiation, or descent with modification, distinguished primarily by their spatial and temporal scales, not by their causal mechanisms. The following table summarizes the congruence of processes.

DiscoveryEvolution of species in ecologies Development of cell types in organisms
Conservation of DNA Much less genomic variability across species than phenotypic variability suggests Much less genotypic variability across cell types than phenotypic variability suggests
Epigenetic regulatory networks Conserved DNA produces diverse phenotypes via epigenetic regulation of gene expression Conserved DNA produces diverse phenotypes via epigenetic regulation of gene expression
Genomic mosaicism Despite a general conservation of DNA across species, organisms in an ecology exhibit a degree of genetic variance Despite a general conservation of DNA across tissues, cells in a complex organism exhibit a degree of genetic variance.
Pre-Adaptation, or "Anticipatory" Genes Some unexpressed sequences code for tissues needed by descendant species. Unexpressed sequences code for proteins needed by descendant cell types.

As these parallels (and perhaps others, such as might involve “junk” DNA) dovetail into a developmental model of evolution, the star larvae hypothesis stands ready as a paradigm within which the seemingly anomalous data—the coincidences and incommensurabilities—can reside. They can live comfortably as neighbors, having been reconciled. That contention is more fully developed at

A recent paper, cited earlier, elaborates on this contention, incidentally to the intentions of its authors. The paper, The extended evolutionary synthesis: its structure, assumptions and predictions, includes a comparison of the core assumptions held by the Modern Synthesis and by the upstart Extended Synthesis [Table 1]. Each of the six observations to which the authors call attention happens also to make evolution, in its mechanical operations, resemble a process of development. Here are the points to which the authors call attention:

1) Unlike the Modern Synthesis, which assumes the pre-eminence of natural selection, the Extended Synthesis assumes that “Developmental processes, operating through developmental bias and niche construction, share with natural selection some responsibility for the direction and rate of evolution and contribute to organism–environment complementarity.”

That is, natural selection gets marginalized as endogenous processes and niche construction in the Extended Synthesis get a (perhaps equal?) slice of the credit for originating species. Niche construction has to do with organisms altering their environments in ways that have evolutionary consequences. An ontogenetic parallel occurs when cells in an embryo release morphogens, which influence the forms and functions of descendant cells as those cells differentiate, a case of ontogenetic niche construction.

2) Unlike the Modern Synthesis, which assumes that inheritance is genetic only, the Extended Synthesis assumes that “Inheritance extends beyond genes to encompass (transgenerational) epigenetic inheritance, physiological inheritance, ecological inheritance, social (behavioural) transmission and cultural inheritance.”

It’s not clear how literally “cultural inheritance” applies to evolution broadly, but the other considerations apply as much to cells differentiating in a body as they do to species differentiating in an ecology (at least if chemical communication among cells can be considered a social behavior). Inclusiveness in the context of cellular differentiation includes inheritance of cytoplasm and organelles, in addition to genes and epigenetic markers.

3) Unlike the Modern Synthesis, which assumes that genetic variation is random, the Extended Synthesis assumes that variation is non-random, “which means that some phenotypic variants are more likely than others.”

This also is the case with cell phenotypes as cells differentiate in a body.

4) Unlike the Modern Synthesis, which assumes that evolutionary change is gradual, the Extended Synthesis assumes that rates of change vary, depending on actions of regulatory systems or when “coordinated suites of traits” are managed by developmental processes.

This also is the case when cells in a body differentiate; e.g., some stem cells remain stem cells and some mature into their fully specialized (adult?) form. Rates of change can be very variable.

5) Unlike the Modern Synthesis, which assumes a gene-centric perspective; i.e., evolution is about changes in gene frequencies, the Extended Synthesis assumes an organism-centered perspective; i.e., evolution is about changes in phenotypic trait frequencies—whatever is going on with the genes.

This is the perspective assumed with regard to cellular differentiation in a developing organism. (That said, some degree of change in gene frequencies characterizes both the descent of species and that of cell types in a body, the latter, somewhat recent, discovery taking the name, “genomic mosaicism.”)

6) Unlike the Modern Synthesis, which assumes that macroevolution reduces to microevolution; i.e., selection, drift, mutation, and gene flow, the Extended Synthesis assumes that “Additional evolutionary processes, including developmental bias and ecological inheritance, help explain macro-evolutionary patterns and contribute to evolvability.”

Cellular differentiation during development also incorporates these circumstances / mechanisms.

What but spatial and temporal scale is left to distinguish evolution from development—the origin of species in an ecology from the origin of cell types in an organism?


The paper’s authors also compare the ways in which the Modern Synthesis and Extended Synthesis interpret various factors that impinge on the evolutionary process [Table 2.]

These factors are developmental bias, phenotypic plasticity, inclusive inheritance, and niche construction. The Modern Synthesis downplays the significance of these factors in shaping evolutionary outcomes. By contrast, by assigning a considerable significance to these factors as shapers of evolutionary outcomes, the Extended Synthesis funnels the dynamics of the evolutionary process into channels already recognized as facilitators and directors of cellular differentiation during development. Developmental bias, phenotypic plasticity, inclusive inheritance, and niche construction all factor into cellular differentiation.

This analysis of this particular paper might be applied to any number of publications that explicate the Extended Synthesis. The new data always point in the same direction: Evolution is a developmental process. That observation invites teleological assessments, because development is teleological—the outcomes are inherent in the process. If the process is spared meddling from disruptive outside influences and endogenous pathologies, then it can complete its life cycle, the life cycle of an organism, in the case of evolution that would be the stellar organism. But that recognition should cause no discontent.

No comments:

Post a Comment