Greg argues that since morality is prescriptive, not descriptive, the herd/evolutionary description of morality cannot work because that description does not engage the fundamental element of morality, which is: how we "ought" to behave. What is the herd morality? I've covered it in some detail here. Greg's explanation is obviously the same, but he includes answers to common questions regarding morality "explained" by naturalistic causes:
"My basic point is this: what naturalists explain when they seek to explain morality in naturalistic, evolutionary terms is not morality at all. They are explaining something different. I get to that by asking a series of questions. Instead of looking backward, I look forward, and I ask a question of moral behavior like "Why ought anyone be unselfish in the future?" for example. The question came up yesterday regarding an observation that was done with chimpanzees. There was a group of chimpanzees which had, in a sense, punished one member for being selfish by withholding food from that member and therefore teaching that member moral behavior. Apparently, the moral rule that under-girded the lesson was that the other chimpanzee ought not be selfish. That's a moral statement and the question I'm going to ask is "Why ought the chimp (or human) not be selfish?" I'm looking for a justification there.
The answer is going to be that when we're selfish, it hurts the group. But you see, that answer isn't enough of an answer because that answer itself presumes another moral value that we ought to be concerned about the health of the group. So, I'm going to ask the question, "Why ought we be concerned about the health of the group?" The answer is going to be because if the groups don't survive, then the species doesn't survive. Then you can imagine the next question. "Why ought I care about the health of the species and whether the species survives or not?" You see, the problem with all of these responses that purport to be justifications or explanations for the moral rule, is that all of these things that are meant to explain the moral rule really depend themselves upon a moral rule before they can even be uttered. Therefore, it can't be the explanation of morality. When I ask the question "Why ought I be concerned with the species?", the next answer ends the series. The answer is, "I ought to be concerned with the species because if the species dies out, then I will not survive. If the species is in jeopardy, then my own personal self interests would be in jeopardy."
So, in abbreviated form, the reasoning goes like this: I ought to be unselfish because it is better for the group, which is better for the species, which is better for me. So why ought I be unselfish? Because it is better for me. But looking at what is better for me, is selfishness. So all of this so-called description of where morality comes from, gets reduced to this ludicrous statement: I morally ought to be unselfish so that I can be more thoroughly selfish. That is silly. Because we know that morality can't be reduced to selfishness. Why do we know that? Because our moral rules are against selfishness and for altruism. They are against selfishness and for the opposite. When you think about what it is that morality entails, you don't believe that morality is really about being selfish. Morality is about being unselfish, or at least it entails that. Which makes my point that this description, based on evolution, does not do the job. It doesn't explain what it is supposedly meant to explain. It doesn't explain morality. It is simply reduced to a promotion of selfishness which isn't morality at all"
Morality is selflessness not selfishness, which is something I believe everyone agrees on. Seemingly moral actions can be observed in animals, but ultimately, as Greg has made obvious, the herd morality is for selfish reasons and not for selfless reasons. Further, when an animal makes a seemingly moral action, it's not truly a moral action because animals are not held accountable to a moral lawgiver. Humans are held to a moral standard and that standard is God. When I make a moral action like, getting up out of my seat to let an older person sit down, I have nothing to gain from that action. I do not hope to get anything in return from the older person, nor do I hope to receive an award, it's simply a selfless action I know I ought to perform.
For further study on morality check out these other posts below.
Objective morals and Euthyprho's false dilemma
What is the basis of our values?
Moral argument for God part 3: the conscience