Levi Bryant and I have been going back and forth over at Larval Subjects about the role of formal and final causation in the explanation of living systems. He argues that Darwin forever banished teleology from nature, or at least showed how the apparent purposiveness of organisms is a result of an entirely non-teleological process. I’ll paste my latest response to him below:
You are still construing the argument I referenced at #6 [it is not so much “my” argument as it is Varela and Thompson’s (see “Life After Kant,” 2002, and Mind in Life, 2008)] as though it refers to the purpose or function of distinct traits or variations. That was never my claim. I fully accept that the function of an organ or a trait usually comes after its formation, and that in the course of evolutionary history, the same organ can come to have entirely unforeseen functions. The argument has to do with the immanent teleology of biological individuals, not with the contingent function of their parts. Darwin’s genius was to discover a non-teleological mechanism to account for speciation at the phylogenetic level due to chance variation and inheritance at the ontogenetic level. There is nothing in his theory, or in any additions to his theory in the last 150 years, that explains the existence of biological individuals with immanent purposes. Systems theory has offered descriptions of biological individuals in terms of attractors, but these are descriptions of behavior and not causal explanations. Efficient causality cannot offer a complete explanation for the sentient behavior of living beings. It is of course part of any explanation, but cannot be the whole explanation unless we are willing to ignore the distinct phenomenology of living systems by reducing them to the neutral language of physics (neutral in regard to the taking into account of the perspective of the system one is studying). As Etienne Gilson brilliantly argued (see From Aristotle to Darwin and Back Again: A Journey in Final Causality, 1984), no defender of teleology in nature has ever done so in order to deny the role of mechanism (efficient causation); it is only the mechanists who deny teleology. From Gilson’s perspective, while mechanistic biology can perhaps explain the specifics of the functioning of individual organisms (which is what you have been arguing), they cannot explain the existence of such individuals as such. To account for the existence of biological individuals requires a principle of immanent teleology. You’ve made reference to the reductionistic promissory notes that eventually an explanation in purely efficient terms will be provided for how DNA and RNA replication got started, thereby bypassing Varela/Thompson’s argument about the explanatory priority of autopoiesis; but as I understand the arguments of systems biologists like Stuart Kauffman (see Reinventing the Sacred, 2008), any account of nucleic acid autocatalysis, due the inherently recursive nature of such reactions, will already be in terms of formal and final causes.
Footnotes2Plato 
What do you think?