If we take morality to be "of, pertaining to, or concerned with the
principles or rules of right conduct or the distinction between right
and wrong" and ethics as "the rules of conduct recognized in respect to a
particular class of human actions or a particular group, culture,
etc.," we have the conditions for a morality that is both constant and evolves. Morality
involves rules -- one cannot do X to
anyone one considers to be a "person" -- that delineate moral action.
The existence of rules, though, does not preclude evolution. With the
above stated rule, we have moral action that can evolve as our
definition of "person" evolves. And notice that this fits with the
definition of ethics given, since it is action within a particular
social context. Not murdering your brother would be a moral action that
is also an ethical action in every culture (since one's brother is
always considered to be a person, regardless of culture). Not murdering
your wife would be an ethical action in one
culture, while murdering her would not necessarily be unethical in
another, because of the differing definitions of "person" in each
context. (This does not preclude a metaethical rule by which one can judge the ethics of being able to murder one's wife, though.) The moral rule of "one cannot go X to anyone one considers to
be a person" is thus found in every cultural situation (morals), while
differing in expression from culture to culture (ethics). What is not
universal, and what is in part culturally determined (and can evolve, as
cultures evolve) is who gets defined as a "person" and who does not.
Moral truth, then, is the moral rule, around which the ethical expressions orbit/evolve. Now, one can have a moral rule prohibiting murder, where murder is defined as "one should never purposefully kill, except in self-defense, a person" and another rule "every member of the species Homo sapiens is a human being" that would be an ethical rule, that would result in the moral-ethical rule "one should never purposefully kill, except in self-defense, a member of the species Homo sapiens". This is how one gets constancy with change.
But perhaps we should try to generalize the moral rule. Can we reduce it down to one moral rule?
"You may not intentionally harm any person, except to defend against or punish an equal or greater harm against yourself or another person."Harm would of course include murder, theft, rape, and "bearing false witness," which is lying to harm another. A person is anyone you can empathize with. This is the variable that leads one to ethics.
This definition of morality would also mean that there would be both a moral ethics (such as involving who is or is not defined as a person) and a nonmoral ethics. What would be included in a nonmoral ethics? It would be things that do not fit the moral rule, yet fits the above definition of ethics. Thus, Jewish and Muslim prohibition on the eating of pork would be an issue of ethics, but not of morals. Such a prohibition comes about in order to help people avoid illness, and is affected by the feeling of disgust.
It also occurs to me that there would also be a moral justice and a nonmoral justice. A moral justice would involve the second half of the above statement, involving defense and punishment. However, there is also a nonmoral justice, as demonstrated by the ultimatum game, in which a person is given money and has to split the money with a second, who can choose whether or not to accept the offer -- with the consequence that if the second turns down the offer, neither get money. The ultimatum game shows people will reject low offers, because they think them to be unfair. This is nonmoral because you cannot harm someone by preventing them from getting what the did not already have, but only by taking from then what is already theirs.
Of course, again, this all requires much more to work.