Do qualia have an evolutionary function, and if so what is it?
Could qualia help us solve problems that Turing machines can't solve?
Could qualia help us solve problems faster than normal computers?
Our brain often works like a general problem solving machine. Could qualia have evolved to optimize this machine? Could evolution limit it to solving only the problems that our genes are "interested" in? Could qualia have other purposes?
Qualia are supposed to be (by definition) not the causes of any physical events, hence there is no way that they could enter into evolutionary or any other kinds of scientific explanation of a phenomenon. For instance, imagine there were ghosts which simply couldn't cause any events in the physical world--couldn't rattle any chains, nor even appear to anybody to scare them. Such ghosts could not even in principle enter in to any scientific account of a phenomenon.
Edit I've received several down votes on this answer, which surprises me, since my answer is clearly correct.
Let me quote some evidence. I'll start with an excellent introduction to the philosophy of mind.
"If qualia cannot be physically described or explained, then they are not part of the network of physical causal relations that are responsible for human behavior. If qualia are not part of that causal network, however, then they make no causal contribution to human behavior." Jaworski, "Philosophy of Mind: An Introduction" Blackwell 2011, p. 213
Somebody in the comment thread below says that Jaworski, a professional philosopher of mind who published a peer-reviewed book on the subject with a reputable publisher is just wrong about the definition of "qualia."
Ok, so let's look and see what other scholars in standard, peer-reviewed reference works thing.
"... this response does not apply to those philosophers who take the view that qualia are irreducible, non-physical entities [e.g. Chalmers, Frank Jackson, etc.]. However, these philosophers have other severe problems of their own. In particular, they face the problem of phenomenal causation. Given the causal closure of the physical, how can qualia make any difference? For more here, see Tye 1995, Chalmers 1996).'' Michael Tye, in the SEP article on Qualia.
The implied answer of the question is: "They can't." Nobody who believes in qualia should count as a physicalist, but rejecting physicalism is not the same thing as rejecting the causal closure of the physical world. Rejecting closure isn't just rejecting physicalism, it's opening the door back up to straightforward substance dualism where my mental properties cause physical events by pushing my pineal gland around or something.
But, hey, maybe the entire scholarly community has gotten Chalmers and company wrong. Maybe Chalmers has simply been slighted by careless, slapdash slanders by wild eyed physicalists who cannot stand to hear their theories contradicted. So, let's look and see what Chalmers himself says about whether qualia have causal powers.
"A problem with the view that I have advocated is that if consciousness is merely naturally supervenient upon the physical, then it seems to lack causal efficacy. The physical world is more or less causally closed, in that for any given physical event, it seems that there is a physical explanation of (modulo a small amount of quantum indeterminacy). This implies that there is no room for a nonphysical consciousness to do any independent causal work. It seems to be a mere epiphenomenon, hanging off the engine of physical causation, but making no difference in the physical world." David Chalmers, The Conscious Mind: In Search of a Fundamental Theory, Oxford University Press, 1996, p. 150.
Chalmers does go on to suggest ways in which he will try to avoid, or at least soften the blow, of having to make qualia epiphenomenal, but whether those responses are successful is a matter of further scholarly controversy. For my money, I don't think any of his proposed fixes, like endorsing causal overdetermination, or extreme humeanism about causation look even remotely plausible.
I would love for people who are still down voting to explain in the comments exactly what kind of evidence they think would be necessary to prove the claim at issue here. Remember, the issue at stake is not the empirical question, "Do psychological states have causal powers?" but rather "Could qualia, those theoretical posits whose properties are stipulated by their use in the philosophy of mind of David Chalmers, Frank Jackson, et al., have causal powers?"
I will assume that we all do have subjective experiences which correspond precisely to the information encoded by specific objective brain states, and that the particular subjective qualities which identify different experiences may safely be referred to as qualia, a set of assumptions which some philosophers doubt.
A common tenet of qualia theory is that our objective brain states are driven by the laws of physics, via objective physical processes, and that our subjective or conscious experience of those brain states merely rides on the back of that.
Some, such as Daniel Dennett, hold that such a concept as qualia is therefore worthless; the brain states are all we need concern ourselves with. Others point to the ''sine qua non'' presence of qualia in the mind of every conscious philosopher and may even suggest that this provides unarguable evidence of a metaphysical aspect to the human mind.
Either way, on this basis the evolution of the brain's rational capabilities is driven by broadly Darwinian evolution and consciousness of such reasoning just comes along for the ride.
Modern information theory is beginning to bring another level of sophistication to the argument. For example Integrated Information Theory explicitly treats the information flows in the brain as the seat of consciousness, with the physiology and activity of the brain serving only to support sufficiently complex information.
This must surely have profound consequences, for example the laws of physics never lie but semantic information often does, but I have not seen this aspect developed anywhere yet.
Qualia are either isomorphic to (if you're a dualist) or identical to (if you're a materialist) computational data structures.
Take the simplest example, the visual field. The visual field is very similar to a PNG file. Any computation that could be done with the visual field could be done just as well with a PNG file.
Most qualia are things like sensory experience, emotions, pleasure and pain. These things are not functional parts of a "general problem solving machine", but rather inputs to a problem solving machine.
