In this post, I would like to briefly explore some reasons why I think those committed to the theory of evolution (via the extended, modern, or some variation of the core synthesis) should give very careful consideration to their implicit metaphysical commitments. What often gets lost in popular public bantering between proponents/expositors of evolution and (typically religious) detractors is that the theory makes certain claims about reality. And these claims purport to be true, or at least the evolutionary theorist usually claims they are true in some way. For example, natural selection is something that occurs and acts on genes within species structure, population groups, etc. Another example might be common ancestry and speciation events are actual historical events. These examples might be oversimplifications, but my point is simply that are a lot of claims baked into the theory. One must decide how important for components of the theory are literally true. Without a commitment to some form of realism, the evolutionist loses the grounds by which to make a truth claim about his theory.
To start, let us say that the evolutionist is a nominalist. Where does this leave the evolutionary theory? It seems we would be left with anti-realism. One reason is that “species” (or class, phyla, etc.) would be a mere linguistic convention or some logical or semantic placeholder that does not correspond to anything real. If there is nothing essential to a species, then how can we construe speciation events along realist lines? Without a commitment to real species distinction, it is difficult to see how the theory can make consistent truth claims. For example, that at some point there was a Pan-Homo split.
The evolutionist could perhaps be a conceptualist. On this view, something like homo habilis would be a (presumably reducible) mental concept. You or I could think and speak about the same general idea (maybe), but there would still not be anything extra-mentally real about homo habilis. This also leads to anti-realism about the evolutionary theory. There are numerous other anti-realist views. And each of these would be equally unhelpful to the evolutionist.
If one is going to be realist about evolutionary theory, then some type of metaphysical realism is necessary. A committed naturalist/materialist might balk at this. But they have no other good options. If a class, species, group, etc. are not real things in some way, then what can the theory help us know about the genuine history of biological organisms, their variation, operation, and so on? Can one really execute a proper taxonomy or determine LCAs without upholding some type of realism? Surely, one would not want to argue that a critical speciation event such as CHLCA was not a real occurrence (whether more complex and gradual or sudden). But if species are not real things, then what can we make of CHLCA? The answer is not very much. The evolutionist is aiming at the truth in scientific results, and realism must be part and parcel of that.
Since the evolutionist should be some type of realist, what type of realist should he be? Platonism might be an option. There are, of course, myriad variations, but one might take a basic version and think the particulars of biological study are merely instantiations of universals that exist in another realm. There is a library of Platonic forms by which the evolutionary biologist could classify and organize his work. Besides the standard objections to Platonism, I think there is some extra baggage in applying this theory to evolutionary biology. The Third Trilobite Argument is surely a problem to address. But questions of supervenience, causal relations, and others seem like they would be quite difficult to overcome. Here I think some of the issues raised against Erik Weilenberg’s Atheistic Moral Platonism would apply. Still, I will leave this as a potential, yet unlikely, option for the time being.
I think a more plausible and ultimately helpful account would be something like an Aristotelian-Thomist moderate realism. This is for two reasons. First, the notion of formal causes or substantial forms would present the necessary ‘natural kind’ or ‘species-essence’ by which to conceive of a species or class in a truth-testable way. Speciation events could be true in principle because there could presumably be a way, via genetics, cladistics, etc., to affirm distinct biological substances, A vs. B. Without this type of realism, what exactly would be correspondent to a speciation event? Species pliability is problematic for evolutionary realism, but nominalism (per above) is a bad option. The evolutionist should want to stay away from holding these things as nominal or mere constructs.
Secondly, the notion of Aristotelian final causes would provide additional lift. Intrinsic final causes, the inherent tendency of X to Y, would underpin the understanding of organisms to act in certain ways. The process of selection itself seems dependent upon this notion for coherence. In the absence of final causes, there would be no compelling reason to even rationalize observed regularity in nature at the fundamental level of physics and chemistry. At a higher level, species are intelligible insofar as they tend toward certain operations and actions. Species A tends toward a certain diet, reproductive cycle, migration pattern, and so forth. Too many aberrational observations in A leads us to re-think A, break out sub-variations within A, or draw other conclusions entirely. It is precisely due to final causes that A is analyzable for science.
I have tried to sketch a few reasons why the evolutionary realist must necessarily step into realism to preserve the truth value of his theory. Aristotelian realism presents the most attractive option, and perhaps the shortest move. Without formal and final causes, essences and goal-directedness, evolutionary theory can only turn to Platonism. There does not seem to be a good reason to force Platonism given the advantages of the Aristotelian apparatus. If evolutionary theory is anti-realist, it cannot be taken as a true or likely even a limiting account of natural history.
Following closely on the heels of these conclusions are deeper questions about natures/essences and final causes. Can we just stop there without asking more questions? Questions that perhaps go beyond the bounds of physical sciences? For Aristotle, the answer is that we cannot prevent further inquiry into the science of causes and being qua being, viz. metaphysics. Studying the physical sciences leads us to seek further knowledge. I think this is very exciting and shows the systematic compatibility of science and metaphysics. There are also theistic implications that come into the fold, which I will leave for a future post.