The question of the Fixity of species is one which, as a Theologian interested in issues surrounding the “Intelligent Design and Evolution” controversy, has often struck me as a senseless question. It seems to me that the semantic difference between what defenders of the idea of the fixity of species call “Macro” evolution and “Micro” evolution, is non-existent; it is a distinction without a difference. In other words, it has seemed to me that the difference between micro-evolution and macro-evolution is merely a matter of time.
The whole notion of a species is a curious one to begin with. Biologically speaking the definition of a species is either plastic (in the philosophical sense) or else it is at least arbitrary and not meant to be reflective of anything metaphysical. Consider, for example, the suggestion that two organisms belong to the same species just in case they can successfully procreate, which is a standard definition (though this definition needs some nuance, since as it stands that would imply that all male members of what we normally call an animal species would be in a different species from all other such males, along with infertile females not belonging to the species, etc). This definition obviously doesn’t carry the distinction metaphysically, since the same species, if its members were divided into two groups for long enough, would not, if brought back together, be able to procreate anymore given that evolution has taken its course differently in both communities.
Those who are committed to the idea of the fixity of species generally want to say that something like ‘all birds’ are part of the same species, even when not all birds can successfully inter-procreate. One might have thought at this point that the idea of species must be established by appeal to some philosophical consideration. However, here too we are met with a curious dead end. As Dennis Bonnette explains:
“Differences in material expression of the same substantial form account for accidental distinctions marking diverse biological populations belonging to the same natural species. Every vegetative organism, from microscopic algae to giant sequoia, participates in the same substantial form.”
Dennis Bonnette, Origin of the Human Species, p.134
Therefore, a mouse and an elephant have the same substantial form, and yet their differences are accounted for based on material dispositions. In the philosophical sense, then, ‘animals’ constitute a species. Perhaps, to be more forthcoming, this Thomistic approach would imply that species are nothing other than substantial forms, which exist first as vegetative, then as sensitive, and finally as ‘rational’ substantial forms (or species). However, there seems to be no meaningful way to distinguish fish from crocodiles, or birds from cats, arguing that they belong to different species metaphysically (for such a suggestion one would have to distinguish a qualitative difference in the disposition of the substantial form, apart from material considerations, which accounts for what Thomistic philosophers have called accidental differences between natural species.)
Whence, then, this desire to affirm the fixity of species? As far as I can tell, it comes from a way of reading the Genesis account, according to which God created different ‘kinds’ of living creatures (for instance, some for the air, some on the land and some in the water). Could there be anything more to this suggestion? I have trouble seeing it.
However, this is worth thinking about more carefully. Dr. William Dembski has suggested, as have some others, that the common line of a single tree of life (that all organisms are related to each other and trace their biological lineage to a single organism) has been questioned in the scientific community and (so he argues) deserves to be challenged given the evidence at hand. Moreover, it does seem to me, to be honest, that the distinction between micro-evolution and macro-evolution may just be that micro-evolution concerns the evolution which could plausibly have been observed by us since we began studying biology, and macro-evolution concerns an extrapolation of a theory of evolution to un-observable past evolutionary genealogies. However, even granting that distinction, it isn’t a scientific distinction, but merely a quasi-empirical distinction. There is absolutely no difference qualitatively between the theory of evolution as it exists in instances of micro-evolution, and the theory of evolution as it exists in instances of macro-evolution. At least there is no difference unless the fixity of species is true, which seems unintelligible to me.
When asked if I think that the fixity of species is true, I usually have to say that, as it stands, I haven’t the faintest idea what it would mean for it to be true. At most perhaps I can understand it to be a challenge to the story of common descent, but that doesn’t go very far, and it seems to me it isn’t theologically relevant, since it seems as though all sensitive animals are in-formed by the same substantial form, which is the metaphysical foundation of that whole species. Moreover, I’m not convinced that this reading of Genesis is correct, and I feel no need, and see no reason, to think that Genesis commits Catholics (or any other Christians for that matter) to this idea of the fixity of species.