Is that moral system objectively true or is it a matter of your opinion?
...
Theory of evolution is an idea ...
Given relative morality (That everyone's moral views of the world differ in ways subtle to extreme), I don't think that morality is objectively debatable, since your view of the world is coloured by your system of morality. We can debate whether the theory of evolution is true because we have something other that we can compare against: 'Does this model accurately represent what is happening in the world', etc. Doing that with morality is impossible, it is a concept that is relevant only to humans.
A half-baked example would be: If you kill a billion wildflowers, then that has a measurable effect on the surrounding environment, sure. But at the end of the day there is nothing to say whether the effect that had on the environment is good or bad. It is just an event, it happens, there isn't anything deeper here from an objective point of view. Right and wrong only come into it when you have a preference for a particular outcome. So you view this act as good if you have a preference towards the outcome of a city being built, or you view this as bad if you have a preference towards plant life being left intact. Since we all have different preferences for different outcomes, everybody's systems of morality are inherently different -- even if that difference is not pronounced in most cases.
Wait, so morality for you is a difference of opinions?
Kinda like: 'I like chocolate ice cream, you like vanilla ice cream'.
That is one way you could state it, sure.
Why do you follow that system of morality and not another one?
I follow my system of morality because I have a preference against harming life. What I mean by this (ignoring the recursion, of course), is that I have been exposed to an overwhelming amount of stimuli, situations, and recollections where this opinion is presented as the preferable decision (Say, Star Trek, etc.), compared to the alternatives I have been exposed to.