“Physicalism (also known as Materialistic Monism . . . is the philosophical position that everything which exists is no more extensive than its physical properties, and that the only existing substance is physical. Therefore, it argues, the mind is a purely physical construct, and will eventually be explained entirely by physical theory, as it continues to evolve.” ~ PhilosophyBasics.Com
1) Presupposing consciousness, evolutionary biology and neurophysiology may account (more or less adequately) for changing states of mind by correlating them with changes in physical structure and processes. Nevertheless, it is difficult to imagine, in principle, how they might explain the advent of consciousness, per se. For if (for billions of years) we imagine electro-chemical processes taking place “in the dark”, there is simply no reason to think that such processes should ever become “illuminated” — no reason to think that, at some point, there should suddenly be “something that it is like” (on the inside) to be one of those processes (cf. “What is it like to be a bat?“, by Thomas Nagel).
2) But even if we assume, per chance, the advent of this or that “emergent state of consciousness”, there is no reason to speak of such a state as being “selected for” (in evolutionary terms) unless it is not ultimately reducible to the material substrate on top of which it appears. Genes are material… Brain-states are material… Natural selection as ordinarily understood operates materially… So unless we are willing to grant that, at some point, conscious minds began to contribute something over and above their material substrate, there is no reason to say that consciousness, qua consciousness, is selected for.
If, however, we want to say that consciousness, as such, does indeed contribute something over and above its material substrate–that the physical structures associated with sentience, perception, or discursive thought are selected for with a view to a qualitative contribution to life that somehow flows through them (and not just for their quantitative features, as such), we are at that point leaving a strict physicalism behind in favor of what I would call an “emergent dualism” or mind/body “interactionism”. If on the other hand, there is no such contribution, then it is simpler to think of some rudimentary “consciousness” (or “interiority”) as being present from the beginning, in any and all material forms (cf. Spinoza or Teilhard de Chardin). But if that is the case, reality is not (and has never been) merely physical.
An alternative would be to argue for the primacy of consciousness (as do, for example, Peter Russell and Bernardo Kastrup), but either way, the mystery of consciousness endures and would seem to remain irreducible….