I don't think "micro" and "macro" is a real distinction at all. That is,
the supposed boundary between "microevolution" and "macroevolution" is
whether there is difference in "kind"... but all the various "kinds" of
animals are defined based on essentially arbitrary criteria, not on some
real boundary we can observe in nature. For example, we might say that
"having feathers" is a unique property of birds. But this does not mean
that having feathers is an intrinsically significant sort of thing, nor
that an evolutionary change producing feathers is any different than one
producing, say, a difference in coloration.
In other words, if you grant that evolutionary change happens, then you
must grant that "macro" changes can happen. This is because the only
difference between a macro and a micro change is whether or not the change
places a creature on the other side of a boundary between definitions.
Simply put: the difference between a micro and a macro change is a purely
semantic one, not an actual difference in the sort of thing that occurs.
So, to answer your question, if it is possible to mutate a fly, then yes it
is possible to mutate a fly past the point of being a fly at all. This
will simply happen when the mutations become significant enough in number
and scope that the resulting beastie no longer meets our definition of
"fly-kind".
Regards,
Mike Hardie
<hardie@globalserve.net>
http://www.globalserve.net/~hardie/dv/