Annoyingly, I think about this a lot, and it keeps me up at night. I've also been led down hazy deistic paths before. To attempt to sketch out my current thoughts, I'd begin with the acknowledgement that a theory of nature should have empirical observations. And so in looking at morality from a naturalistic perspective, I observe its rudiments in other species, e.g., our ape cousins displaying culture and an understanding of fairness, and elephants displaying empathy and even what looks like mourning, to give two examples.
This suggests to me that morality is an evolved process that enhances group survival and facilitates cooperation, which then promotes gene propagation. A moral faculty is part of our genetic endowment and it develops much in the same way that other aspects of our physiology develop, meaning that it is mediated heavily by environment. It is therefore not merely learned behavior. Infants are endowed with an intuitive sense of right and wrong, and many variables will affect how this faculty develops - perhaps similar to the way in which nutrition affects one's pre-determined height.
As to whether an evolutionary account of morality supports or undermines its objectivity and one can say that moral judgements are true, it's complicated. One can take the view that if other species and human cultures have different moral adaptations, then relativism wins and we cannot say what is right.
On this point, I don't think that it's correct to say that different species have different moral adaptations. Rather, moral thinking presents itself once a species reaches a certain level of intelligence and social complexity. The more advanced the brain, the more sophisticated the moral reasoning, so it is not that, say, insects have evolved a different adaptation; rather, they simply haven't evolved enough to have any moral faculty at all, and in fact elementary morality looks rather uniform across all intelligent species.
I don't accept the idea that morality is merely preference. It's not only a diminution of our most deeply held convictions, but I also recognize that my moral conclusions do not always align with my preferences or emotions. Personally, I rarely have much empathy for other men. I'd quite like to torture and kill many of them. However, I realize that this is wrong, and I would not want to live in a society where this is viewed as acceptable behavior.
There are clearly differences in our moral sensitivity due to different neurological tuning, but I hold that we can arrive at moral truth through our capacity to reason and our rational reflection on morally relevant traits. And on this point I also reject that idea that moral consideration should only be given to other rational agents: hence my annoying commitment to vegetarianism.
I'm not yet sure if all of this is consistent, and I may well change my mind, but one day when I've saved enough money and I can retire from my current job, I'd love to formally study ethics and develop my thoughts further - especially in regard to our treatment of other species.
