Let’s say there’s no such thing as intrinsic value. (I won’t present an argument to this effect here, although many others have done so.)
As a result, let’s say instead of there being intrinsic values, we project values upon the world. (Again, I won’t present an argument for this here, but let’s run with it for now.)
These values we project on the world can come from any number of sources – such as emotion or reason – but there is no ultimate arbiter of what values are true or best. Call this a kind of value nihilism, if you will.
However, there is a way the world is, and evolution is an important aspect of that world.
As such, certain values will ‘survive’ better than others in certain environments, either by spreading culturally or by lending a selective advantage to their practitioners (or by not lending a selective disadvantage to their practitioners).
One can choose for themselves whatever values they wish and justify them however they wish – basing them on emotion, reason, God, whatever. However, as there is no ultimate arbiter of these values, this constitutes a kind of value relativism.
However, depending on the values adopted, and how they affect behaviour, these values may increase or decrease the fitness of the practitioner. I hasten to add, in light of the above, there is nothing intrinsically good or bad about this fact. It just is.
In fact, the world being what it is, values that survive – or that lend a selective advantage to their practitioners – will tend to out compete values that lend a selective disadvantage to their practitioners. As such, these values will tend to survive and propagate.
If one desires their values to persist (i.e. they value their values), then it would be prudent for them to choose values that will improve fitness and not degrade it, so that their values might propagate. Again, nothing good or bad about this.
If the above gives you the concern that suddenly survival or fitness sneak in to become foundational values, or that self-interest becomes an overriding value, then take heart that, as a matter of empirical fact – being the social creatures we are living on the fragile interconnected planet we do – if you chose values that will survive, you’d do well to choose cooperation, environmental sustainability, peace etc as well as highly conditional ruthlessness when it comes to matters of survival.
There’s nothing intrinsically good or bad about choosing values like this, but there’s nothing intrinsically good or bad about choosing, say, hedonist or Kantian deontology values, except that by choosing the latter two values you might place yourself at a considerable selective disadvantage and, as such, those values might not be likely to last in the long term.
There you have a naturalistic theory of value that is compatible with evolution, that isn’t committed to any metaphysically suspect properties such as intrinsic value, that doesn’t commit the naturalistic fallacy and that promotes things that we intuitively want to promote, like cooperation and environmental sustainability.