I recently criticized a paper by Lu and Bourrat on the extended evolutionary synthesis [Debating philosophers: The Lu and Bourrat paper]. Pierrick Bourrat responds in this guest post.
Research Fellow, Department of Philosophy
Both Qiaoying Lu and I are grateful to Professor Moran for the copious attention he has bestowed on our paper. We are early career researchers and didn’t expect our paper to receive so much attention from a senior academic in a public forum. Moran claims that our work is out of touch with science (and more generally works in philosophy of biology), that the paper is weakly argued and that some of what we write is false. But in the end, he puts forward a similar position to ours.
Before delving into the substance, I would like to make some general comments, starting first with the claim that philosophers are out of touch with science. Once upon a time, some areas of philosophy had limited contact with science. But philosophy of science, a special sub-field of philosophy, has always had some engagement. These days, especially in philosophy of biology, the norm is to be very engaged. Both of us (authors of the paper) have biology degrees (evolutionary biology in my case) as well as philosophy ones, and we make a considerable effort to be in touch with issues in evolutionary biology, specifically evolutionary theory and models. Theory and philosophy are not that far apart, when one thinks about it a bit. We think it must be the case that Moran likes only some interpretations of evolutionary biology, and that his objections to us are based on how well we do or don’t fit these favourite interpretations. Moran likes non-adaptive evolutionary theory, and especially Michael Lynch’s promotion of it. We do too. It is great to see such blanket endorsement of non-adaptive evolution, and perhaps another time we’ll find an evolutionary topic that requires us to address it in detail (rather than the small mentions we made of it in the current paper).
In his second post, Moran reproaches us for talking about three extensions of the modern synthesis that aren’t significant in comparison to the modifications of evolutionary theory coming from neutral theory and the increased emphasis on genetic drift. We only took the term “modern synthesis” to be one way to characterise some core principles that people tend to agree with. It would be naïve not to recognize that from one evolutionary biologist or school of thought to another there will be some variation in what they mean by “modern synthesis”. Following Gould, Moran thinks the Modern Synthesis is dead. It is one way to interpret what happened in evolutionary theory over the last sixty years or so. Nevertheless, whether some features of current evolutionary theory should be regarded as extensions of the Modern Synthesis or as parts of a different theoretical apparatus is a mere semantic point. What matters is to be clear about what one means.
But the problem with Moran’s criticism is that neutral theory and the increased emphasis on genetic drift are aspects of evolutionary theory that are quite distinct from the main topic of our paper. The core aim of our paper addresses issues surrounding epigenetic inheritance, rather than which aspect(s) of the modern synthesis have been the most challenged by recent theoretical developments and empirical discoveries.
In his third post, Moran claims that we confuse the terms “allele” and “gene”. I recognize that in some parts of the manuscript we could have been more careful and used “allele” instead of “gene”. That said, Moran doesn’t notice that we define the terms “epigene” and “epiallele” precisely in order to avoid the analogous confusion between “allele” and “gene”. It is also fair to note that the confusion between “gene” and “allele” is encountered in various places, including evolutionary biology textbooks. For instance, Moran uses a slide from Pearson Education in which one can read “[t]hree major factors alter allele frequencies […] gene flows” (my emphasis). Note also that Futuyma, in his textbook extensively quoted by Moran, uses the expression “gene flows”. Following Moran’s own point (with which I largely agree), it would be more appropriate to use “allele flows” instead of “gene flows”. Many other textbooks (e.g., Ridley’s Evolution) do not consistently make the distinction between “gene” and “allele” when referring to changes in frequency. Thus, at best, Moran’s point is minor and could be equally attributed to biologists (including some theorists) in addition to philosophers.
In the fourth post Moran accuses us of equating natural selection with evolutionary change. Moran regards our definition of the evolutionary gene as problematic because it “only recognizes genes that cause a difference in phenotype and that such differences are subject to selection” and that we don’t “seem to be aware of neutral or nearly-neutral alleles whose frequency is strongly influenced by random genetic drift”. It is actually not the case that we regard phenotypic differences as necessarily due to selection. We mention trait variation, which is different from selection. We even wrote elsewhere in our article that “our focus here is the concept of the gene, rather than gene selectionism.” The important point here is that we define the phenotype of a gene as “everything [it] makes a difference to when compared to another evolutionary gene.” A difference in phenotype can be a difference in DNA sequence with no other observable difference. For an example of this use see the chapter 6 of Sean Rice’s 2004 book Evolutionary Theory. Thus, the claim that our definition is incompatible with neutral evolution is incorrect.
The second point Moran makes in this fourth post is basically the same as the one he makes in the previous one, namely that we should have used “allele” instead of “gene”. Yes, perhaps David Haig using the term “strategist gene” should have used the term “strategist allele” and perhaps making the distinction would make clearer the point that the two concepts are different. But this is for the great part a semantic point, not a conceptual one. I also note that David Haig and George Williams, on whom we relied for our definition, are acclaimed evolutionary biologists.
Moran’s fifth post is making yet another semantic point. He claims that our definition of the molecular gene is wrong and should be substituted with his definition. The two definitions are very similar, with one emphasizing what a gene typically is, while the second aims for generality, something Moran does not acknowledge. In fact, we clearly write that our definition (which we owe to Griffiths and Stotz) is a stereotyped one. Like any stereotype, it will have exceptions, and in particular contexts it will not be applicable. Rather Moran concludes: “[t]he Griffiths and Stotz definition is wrong because it excludes all DNA sequences that lack an open reading frame and this means that tRNA genes aren't genes by their definition”. At the same time, like us, he recognizes that there is no consensus on the definition of the molecular gene.
In the sixth post, Moran finally presents our main thesis about epigenetic inheritance and whether it can be accommodated within current evolutionary theory. Moran concludes: “it's safe to say that most experts in evolutionary biology do not see epigenetics as a threat to current evolutionary theory, with its heavy emphasis on population genetics, and they do not see any need to shift paradigms”. Clearly, therefore, his conclusion does not contradict in any way what we argue in our paper. Accommodation is our point.
Overall, we are delighted that Moran agrees with us that contemporary evolutionary theory can accommodate heritable epigenetic change, and grateful he took so very much time to echo the exact point of our paper.