The Changing Face of Evolutionary Theory?
Is evolution driven mainly by random genetic variation, or are there other factors at play? Michael Burdett reviews the scientific debate.
An interesting development is taking place in the biological and anthropological sciences today that has its roots in a decades-old discussion. Whispers in the halls of scientific faculties and hushed conversations in laboratories have solidified into outright dialogue and debate in top scientific journals. Scientists from across the various evolutionary disciplines have locked horns over the accepted mechanisms of standard evolutionary theory (SET) and the significance that ought to be afforded to each mechanism. Let me be clear upfront, the core of Darwinian evolution itself is NOT being questioned. Indeed, as Dobzhansky, echoing Teilhard de Chardin, asserted decades ago, “nothing in biology makes sense except in the light of evolution.” It is uncontested that variation occurs in populations and is subsequently winnowed by natural selection, generating biological change over time. SET contends biological diversity is mostly explained by natural selection, defined as the confluence of random phenotypic variation, genetic inheritance, and differential reproductive success. However, some scientists (proponents of the “extended evolutionary synthesis,” or EES) are challenging the tenet that phenotypic variation is entirely random and that natural selection is entirely driven by genetic inheritance. The dialogue focuses on the processes within evolution, where to put causal emphases, and sometimes just what to call things. These scientists are working to hone our understanding of evolutionary theory at present—even if they disagree on the severity of this honing process. The controversy helps highlight the sheer breadth and intricacy of modern evolutionary theory, which cuts across many interdisciplinary lines.
The topic of these conversations even made their way into our recent symposium on Christian doctrine and evolutionary theory. It came up when our team was deciding what kind of evolutionary scientists it would be important to invite to the symposium. Should we invite mostly evolutionary biologists? Geneticists? Paleontologists? Even psychologists and culture experts? Evolutionary theory plays a significant role for so many scientists today. A complete picture of evolutionary theory would not be possible without referencing how it functions in each of these areas, yet scientists are divided on how to parse out the significance attributed to each of these areas.
A recent article in Nature elucidates well the growing momentum of this conversation, as seen by its provocative title: “Does Evolutionary Theory Need a Rethink?” In the article, two teams of scientists respond to this question. Kevin Laland and colleagues think evolutionary theory does need to be reassessed, while Gregory A. Wray, Hopi E. Hoekstra, and colleagues agree evolutionary theory is fine as it is.
Laland et al. argue that new scientific developments in genomics, epigenetics, developmental biology, social science, and ecology are altering the prevailing, gene-centric view of evolution. They contend that organisms are not simply genetically programmed from birth to fit into a prior environment but instead can “co-construct and co-evolve with their environments, in the process changing the structure of ecosystems.”1 The major issue Laland et al. have with the way evolutionary theory is represented at present is it localizes the central evolutionary processes at the genetic level thus downplaying the role of other mechanisms. Instead of genes being the focal point of evolution, organisms as a whole ought to take that place. Laland et al. propose that standard evolutionary theory going back to the “modern synthesis” in the 1930s and 1940s ought to be renamed the extended evolutionary synthesis (EES) as a way to recognize these new discoveries and the subsequent alterations they make to our understanding of evolution.
What exactly are these processes which are inciting this potential upheaval? Briefly, the article mentions four items: developmental bias, phenotypic plasticity, niche construction, and extra-genetic inheritance. Each of these will be explained below and their significance unpacked.
Developmental bias refers to “a bias on the production of variant phenotypes or a limitation on phenotypic variability caused by the structure, character, composition, or dynamics of the developmental system.”2 Essentially, something within the development of the species constrains the possible set of expressed features, favoring some over others. For instance Laland et al. cite the phenomenon that nearly 1000 centipede species tend to have an odd number of legs despite the different environmental settings around the world and the unique evolutionary history. This can be explained by the development of the centipede and the way in which the physical development of the segments constrains the possible number of legs—thus leading to an odd number of legs in centipede species that evolved independently of each other. SET says phenotypic variation is a random process in its dependence upon underlying genetic mutations, therefore it must acknowledge the incredible coincidence of this kind of parallel evolution. However, Laland proposes phenotypic variation is not always random but rather mechanisms such as developmental bias help to privilege certain phenotypes. EES has no such issue with parallel evolution because “developmental bias and natural selection work together”3 in EES.
Laland et al. explain that phenotypic plasticity is also changing the gene-centered view of evolution. Phenotypic plasticity refers to the way certain organisms can directly alter their morphology, physiology, and behavior in response to an environmental change. What is interesting about these changes is that they occur within the lifetime of the individual organism itself rather than lagging behind in evolutionary time. While plasticity is most drastic with static organisms such as plants (i.e., they cannot move away from their environment and have evolved to adapt directly to their changing environment), it is also visible with insects and animals. As an example, certain grasshoppers, such as Schistocerca gregaria, change from docile, solitary creatures to the well-known, aggressive locusts when surrounded by many others of the same species. They even change color to denote this change in behavior. Laland et al. suggest these immediate phenotypic changes can help prime the genetic pump by helping to select organisms that have the advantageous phenotypic trait—paving the way for the subsequent underlying genes. As Laland et al. says “often it is the trait that comes first; genes that cement it follow, sometimes several generations later.”
The third mechanism is niche construction. Here the common picture of SET meets the claims and findings of ecology. Niche construction avows that organisms do not simply passively adapt to their surrounding environment through the survival of the fittest but will actively alter that environment so that it is often more hospitable for them and their descendants or other species. Beavers and earthworms are two examples. Beavers will construct dams to create pools and wetlands that help the beaver population thrive, even for subsequent generations. Similarly, earthworms alter the chemistry of the surrounding soil making it more fit for other earthworms and plants. Much like the aforementioned mechanisms, niche construction helps bias the relative fitness of particular species. Laland contends that SET treats the environment as merely a “background condition” rather than a central factor involved in the evolutionary process. EES takes into consideration the entire ecology of the system where the environment and organism live in a mutual relationship and where both are substantial players in the evolutionary process.
Finally, extra-genetic inheritance contributes to this tidal shift in the colloquial understanding of evolution. The most cited of these mechanisms is epigenetic markers, but it can also include the transmission of social behavior (i.e., social learning and cultural evolution) and even ecological inheritance (e.g., a beaver passing down his dam to subsequent generations). Epigenetics is one of the most fascinating areas of extra-genetic inheritance and has received a lot of attention in recent years. Epigenetics is the field that looks at “the heritable changes in gene expression (active versus inactive genes) that does not involve changes to the underlying DNA sequence; a change in phenotype without a change in genotype.”5 Without getting into the scientific detail of epigenetics, the important thing to note here is that extra-genetic factors (DNA methylation, histone modification, and non-coding RNA) influence the underlying DNA’s phenotypic expression. An organism’s DNA does not unilaterally produce the specific organism, but rather these extra-genetic factors can suppress or reveal aspects of the genetic code, sometimes altering features of the organism. What is more, these epigenetic markers can be influenced by environmental and behavioral patterns and can be transmitted to progeny up to two to three generations. This means that our actions today can directly influence the phenotype of our children and grandchildren through these epigenetic markers.
Now, those in the “No” camp (Wray, Hoekstra, and colleagues) agree each of these mechanisms play a role in evolutionary development; however, they contend SET already makes room for these mechanisms, and thus, evolution does not require redefinition. The central point of disagreement, then, is the significance these other mechanisms have to the theory of evolution. Laland et al. clearly want the genetic throne of evolutionary theory shared with other extra-genetic features. However, Wray, Hoekstra, and colleagues are hesitant to allow the genetic core to be dissolved and give equal value to extra-genetic mechanisms. They have two central concerns. First, there has not been enough experimental evidence as of yet to warrant changing SET. To do so would be too hasty. The second concerns the priority of the current genetic basis of evolutionary theory to these other extra-genetic mechanisms. At the heart of the article the naysayers pose a very important and illuminating question: Could these extra-genetic mechanisms “lead” evolution rather than merely fine-tune or hone the existing underlying genetic engine?
It seems to me this is an important question and will largely dictate whether the present theory needs significant overhauling. What would it mean to “lead” evolution? Clearly, Laland et al. would contend that evolution can be “led” by many of these extra-genetic mechanisms. For instance, phenotypic plasticity might help lead evolution by providing an immediate advantageous trait in a given environment, helping to select and funnel the underlying genetic code in a particular direction towards the advantageous trait expressed by phenotypic plasticity. This phenomenon has often been referred to as genetic assimilation, and it has a very under-represented scientific heritage.6 It “leads” evolution because the phylogenetic variation and selection occurs without genetic congruence. These extra-genetic mechanisms lead the evolutionary process and are causally prior to the change in the genome. Might we say other extra-genetic mechanisms can also “lead evolution”?
However, perhaps the whole notion of “leading” is ill-formed here and already biases towards a particular response. The very framing has us rank either the genetic basis of evolution over these extra-genetic mechanisms or vice-versa. We are called to give priority of one over the other when empirically we might not want to claim as much at this stage. Indeed, it seems Laland et al. aren’t making such a strong claim such that that these extra-genetic features take greater precedence over the underlying genetic material. Rather, they are making a more conservative claim: that these extra-genetic mechanisms play a far more significant role than they are currently allowed in SET.7
Whatever significance is finally attributed to these extra-genetic mechanisms, the conversation gives substantial insight into the scientific process for the lay person such as myself. The pursuit of scientific knowledge is a dynamic endeavor in which substantial exchanges take place between continued empirical investigation and conceptual interpretation. The relationship between theory and evidence is reciprocal; theories help us to interpret facts, and facts help us to modify theories. The expectation that science is and should be the unproblematic accumulation of facts, to be consulted at our convenience, is simply false. Similarly, the notion that scientific theories are objects of loyal consensus is also false. Scientific theories are ever-evolving things, sensitive to empirical and conceptual progress. Scientific theories can and should change and scientists can and should disagree with one another over the finer details of theory and the interpretation of facts. Thus, faulting a scientific theory for being a moving target and discounting its claims on the basis that the scientific community might be divided on certain aspects of a theory ignores (1) that this is the way science operates and (2) the productivity that can arise from its functioning this way. Yes, intergroup dynamics, with competing labs holding differing views influenced by adherence to group identity, can play a role in working out the minutia of empirical data and theory-construction. This is why the study of the sociology of scientific knowledge is still a fruitful venture. But those working on evolution are not unique—it is a feature of the scientific endeavor and, indeed, all areas of human inquiry. Articles such as the one authored by Laland et al. should not incite religious people to question the validity of the theory in question but to recognize the wealth and diversity of the scientific task and how this actually leads to a stronger, more robust knowledge base.8
The Nature article also highlights an important warning to the non-expert in evolutionary theory. We need to be very cautious when making claims, religious or otherwise, that are based upon popularizations of evolution. As we have seen, the details of evolution are often much more complex than the simple explanation offered in elementary schools. What is more, our intuitions actually work against us when trying to understand evolution. Many notable cognitive scientists have discovered that we are psychologically predisposed to misinterpret evolution. We intuitively favour teleological and purpose-based explanations for natural phenomena (e.g., ‘‘the sun radiates heat because warmth nurtures life”) when this is scientifically incorrect.9 We are psychologically biased to give an essentialist account of species10 and are predisposed to anthropomorphic explanations.11 Each of these cognitive biases can make it harder to understand evolutionary theory, and the non-specialist is at a serious disadvantage. Therefore, any philosophical or theological reflections on current evolutionary science must be made tenuously and with great care.
Indeed, because such rapid advancements are being made in the evolutionary sciences at present it would be important to be cautious in making grandiose claims based upon recent findings for it is likely these words would have a relatively short expiration date.12 Genuine prudence is called for here. However, these rapid advancements also signal an exciting time for the evolutionary sciences. We are in the middle of a watershed moment that deserves to be celebrated. It is a wonderful time to get into these sciences and much work needs to be done.
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