Well first off, morality is very evolutionarily beneficial.Le'ts go back in time 50 thousand years or so and find out why:
We're in an ice-age. Ugg, Logg, and Skugg just killed a sabertooth tiger for their clan. Each of them want to bring more than a third back to their families. Logg, seeing an opportunity, and stabs Ugg with his spear. He turns to Skugg and in cave-man says "Aight brah, now we each got half a kit-cat to bring home to our bitches and kids. Later dude". Skugg, thinking that while that's cool, his family would much rather have the whole saber-tooth tiger, promptly stabs Logg in the face and brings the whole carcass home to his family. A week later, out hunting alone, Skugg is killed by a giant ground-sloth, and the entire clan shortly starves to death.
In another clan, a very similar scenario happens, but everyone resists the urge to murder everyone to have more food. Someone probably goes a little hungry, but everyone lives, and the genetic traits leading to this morality live on through humanity.
If undivinely guided evolution was true, then logically we would be more than capable of understanding that our morality is arbitary
If this was true then everyone would have realized there was no real reason not to stab each other in the face, they would have done so, see first scenario for likely results.
There are plenty of great books on Evolutionary Morality out there. I'd recommend picking one up if you want something that sounds more eloquent than a 3rd grader's book report.
I think Kenny is missing the important element of your question, though I think he has answered it indirectly, which is if understand, "If nature, which simply compiles coincidence into some living, sentient being, has no guiding hand, and operates according to laws that were not put into place by a higher power, then is it that any morality developed through a natural, unguided process is inherently flawed and inherently transient because natural circumstances are forever changing?" Even if this is not what you are getting at, it is a good question.
The laws of nature are the only things that actually are constant in the current state of the universe. The state of the universe before the Big Bang, is perhaps another story. Regardless, those laws are subject to no interpretation and cannot be changed in the current universe. They are by definition then, objective. To propose a hypothetical like " if those laws were differnet, our morality would be different" implies that those laws can be changed. They simply cannot, so any situation where those laws are changed is a logical impossibility.
We must "accept" these laws because they an inescapable fact
That however does not dismiss the possibility that all the coincidences that formed life on Earth could have gone a different way and humans would be slightly different or non-existant. In that case, natural selection would have done exactly as it did to us and develop a morality that works best in those animals' situation. Does this fact mean that the moral instincts, the dispositions, that nature has produced in us are flawed? Absolutely not. Just because other functional moralities could exist for another hypothetical species does not impugn the functionality of our own. We can see that our morality is functional simply by acknowledging that we would like to act on baser instinct if the threat of dire retribution were not forever present.
Kenny answered the rest of the question above rather aptly, so I won't take up extra space answering the last bit.
First of all, a "yardstick" is an arbitrarily long measuring tool! Though the fact that you attempted to compare God to a yardstick does provide an excellent jumping off point for discovering that God is as arbitrary as the length of a yardstick. You have assumed that there is an objective, perfect morality against which actions can be weighed. Any morality that you, Vertigo_One, or anyone else, point to as the example of a perfect morality is inherently arbitrary because YOU chose it. Simply by picking a definition of God, and therefore the perfectly moral being, you have arbitrarily chosen what is, to you, the perfect morality.
To add to what Kenny said, God is limited fundamentally by what humans can conceive. In order to attribute any of the qualities that make God a god, you must be assigning only terms that humans have conceived.
God, AS ANY HUMAN CAN UNDERSTAND HIM (that's the important part), is a being limited to qualities that we can give him.
God is conceived of differently by each person. The God that one uses as a "yardstick" is a personal creation that takes on whatever qualities you assign him. God is no less an arbitrary measuring tool than a yardstick.
To tie this back to my original point, examining our nature is a better source for understanding what will or will not cause problems for society than examining our personal conception of God.
I can practically hear your response in my head...
I think I'll just put together my rebuttal to the logical counterargument to my observation now. The argument:
"Just because we can't know which conception of God is correct doesn't mean that there isn't a right answer."
That is a completely logical, true argument. However, because we cannot prove that any conception of God is correct, any guess at which moral code we should follow is as good as the next. We therefore have no logical reason to choose one answer to which morality is best over the other.
To give this some semblance to relevance, any religion-based morality is based on an arbitrary definition of God. They can't all be right, so some must be at least partly wrong. So, even if you do assume that God exists, religions do not provide the best basis for morality.
Any morality that you, Vertigo_One, or anyone else, point to as the example of a perfect morality is inherently arbitrary because YOU chose it. Simply by picking a definition of God, and therefore the perfectly moral being, you have arbitrarily chosen what is, to you, the perfect morality.
No, you choose it based on rationality. If we assume a divine universe then we also assume we have an objective as opposed to arbitary rationality, therefore we use that rationality to examine the claims and otherwise of each individual religion etc.
If that were true (and there are many problems with that assumption), and we all have an objective rationality, how could we each come to different conclusions about which religions are true?
The only possible answer, given all the evidence, is that as children we are diposed to take what our parents tell us as truth. We are indoctrinated in their way of thinking, and we assume that the moral code taught to us by our parents is a good one.
How else could the fact that people almost always hold the same religious beliefs as their parents, regardless of the religion, until they self-examine and realize that they have been indoctrinated?
Also, to connect this to the point of discussion, religions are a cop-out for true philosophical exercise. Religions, unless you invent one yourself, inhibit critical thinking about your values and stunt cultural advancement. Each individual ought to discover for themselves what they believe concerning spirituality and morality after, not before, having been educated in science, history, and critical thinking. In this way, religions are a detriment to mental progress and are a frictional force to personal growth.
My point still stands. If nature was different, morality would be differnent.
Now, please explain to me why that's wrong.