Monday Article #75: The extended evolutionary synthesis: A modern interpretation of evolution
Last week, we took a road down memory lane where we discussed how the theory of evolution as we know of has changed since it first became known to the public. Currently, the most dominant form of the theory of evolution is the Modern Synthesis (MS) theory which combines natural selection, Mendelian genetics, and population genetics to explain how evolution occurs. However, following the advent of the MS, the field of evolutionary biology has continued to evolve (no pun intended). Hence, the evolutionary theory today is vastly more sophisticated than the original MS, resulting in some groups of scientists suggesting an alternative theory that unifies multiple disciplines of biology that were previously neglected. Enter—the extended evolutionary synthesis (EES) theory!!
Before diving in, let us clear up one major misconception about the EES. Many may think the EES aims to undermine the MS theory and render it useless, which is simply not true. Instead, the EES is a developing line of contemporary evolutionary thought that exists within the field, not a denial of past discoveries.
With that out of the way, let’s take a look at the perspective of EES on modern evolution.
Evolutionary developmental biology (evo-devo)
First, let’s take a look at evolutionary developmental biology (evo-devo). Evo-devo provides a causal-mechanistic understanding of evolution by using comparative and experimental biology to identify the developmental principles that underlie phenotypic differences (Laland et al., 2015). Most evo-devo research is compatible with standard assumptions in MS, though some findings are conflicting. One of which is the observation that the processes of development can bias phenotypic variation, known as developmental bias (Stephen Jay Gould, 2002). Neo-Darwinian states that the process behind evolutionary change is natural selection acting upon heritable variation caused by genetic mutations (Douglas John Emlen and Zimmer, 2020). However, natural selection acts on phenotypes, not genotypes (ie: natural selection does not care how you evolve wings, as long as you do), and for a mutation to readily alter a phenotype, it has to alter the ontogenetic trajectory, called developmental reprogramming (Arthur, 2000). Some kinds of reprogramming are just inherently more likely to occur due to the nature of the genotype-phenotype map, thus biasing the system to evolve in a particular direction, instead of randomly as would MS predicts (Wagner and Altenberg, 1996). This bias acts in two ways in that it can limit or facilitate evolution, known as developmental constraints (Smith et al., 1985), or developmental drive respectively (Arthur, 2001). An example of this would be a dragon (a giant reptile-like creature with two pairs of limbs and a pair of wings). This creature is ontogenetically impossible as in vertebrates, the fore-limbs and wings are homologous characters (ie: our humerus is developmentally the same as the wings of a bird), thus making them mutually exclusive. Developmental bias may contribute to convergent selection, by allowing evolution to occur on selected pathways only, such as in soil-dwelling centipedes with an enormous variation in the number of pairs of legs, but only having them in odd numbers (Arthur, 2002).
Figure 1: Homologous structures that render the co-development of wings AND anterior limbs as mutually exclusive. Image taken from Mometrix.
While plasticity has been studied as a consequence of evolution, it has rarely been suggested as a cause. The contribution of plasticity to evolution acts through phenotypic and genetic accommodation (Moczek et al., 2011). Phenotypic accommodation refers to the mutual and functional adjustment of parts of an organism during development in the absence of genetic mutation. This accommodation could promote genetic accommodation if the induced phenotypes are subsequently stabilized and fine-tuned across generations by selection of genetic variation (West-Eberhard, 2003). This largely contradicts traditional MS that implies only genetic variation can induce phenotypic variation, not the other way around. An example of this is when spadefoot tadpoles, normally omnivorous, are raised solely on meat, can spontaneously develop more carnivore-like features (Harmon, Evans and Pfennig, 2023). This mechanism of plasticity can provide an explanation for rapid adaptation to novel environments that are subsequently stabilized and maintained over-time.
Traditionally, it was only thought that transmission of genes from parents to offspring was the only way of inheritance. However, it is increasingly recognized that there are multiple mechanisms that contribute to heredity. Components of the egg and post-fertilization resources (e.g. hormones), behavioral interactions, parental modifications of the biotic and abiotic environment, and inheritance of symbionts (e.g. gut microbes) can directly influence the development of the offspring, providing opportunities for some acquired characteristics to be inherited (Badyaev and Uller, 2009). Besides, non-genetic inheritance can bias the expression and retention of environmentally induced phenotypes (Badyaev, 2009). For example, parents who lack the “like-to-exercise” genes, but otherwise have learned to like exercising influence their children to like exercise, who subsequently influence their own children, and so on and so forth leading to the stabilization of the “like-to-exercise” genes in further generations. Of course, there is also increasing evidence that epigenetic inheritance (ie the inheritance of DNA modification) occurs, which demonstrates that inheritance does not necessarily only act through our genes (Heard and Martienssen, 2014).
Figure 2: Inclusive inheritance implies that we inherit more than just genes from our parents. Image taken from Avatars of information: towards an inclusive evolutionary synthesis (Danchin, 2013).
Constructive development and reciprocal causation
Constructive development refers to the ability of an organism to shape its own developmental trajectory (Gerhart and Kirschner, 2007). It emphasizes that gene expression and environment are interdependent, rather than one-sided. Reciprocal causation on the other hand is similar to constructive development and states that process A can cause process B, but process B can also cause process A. Both of these principles illustrate the interdependent nature of evolutionary factors (Laland et al., 2011). As an example, niche construction refers to the process whereby the metabolism, activities, and choices of organisms modify their environments, and thereby affecting the selective pressure acting on themselves or other species. For instance, despite living on land for millions of years, earthworms retained the freshwater physiology of their ancestors. They process the soil in ways that allow them to draw water into their bodies more effectively, thereby constructing a simulated aquatic environment on land, rather than adapting to become more terrestrial-like (J. Scott Turner, 2009).
Figure 3: Constructional development and reciprocal causation imply interdependent effects of our genome and other factors. Image taken from The extended evolutionary synthesis: its structure, assumptions and predictions (Laland et al., 2015)
Essentially, EES takes on a more organismal-level approach to evolution, rather than attributing everything to natural selection and genes. This recognition of a variety of distinct routes to phenotype-environment fit fills in the gaps that traditional perspectives lack. Perhaps it is due time that we no longer dwell on the dogmas of old legends, and embrace this new perspective that enables us to explore the complexities of evolution with greater nuance and depth.
Article prepared by: Jared Ong Kang Jie, R&D Director of MBIOS 2023/2024
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