The term "microevlution" is unfortunately a rather vague one - a bit like cancer or arthritis!!  This leads to much confusion in the type of general discussions as occur on this list, where terms are seldom operationally or precisely defined.  Therefore such statements as < Doesn't all the micro add up to macro?> mean little due to "fuzzy" definition of terms (and in any case, many evolutionists have rejected gradualistic, "Classically Darwinistic" viewpoints, given the extreme paucity of obvious or convincing "missing links"(intermediate forms halfway between two species)).  The term is most often used to refer to adaptations which occur in local populations (a good example would be the HIV virus, which has now been shown to "evolve" within the host organism, until such time as the survivors of the wars between it an the host's immune system are able to prevail and overpower it, being substantially different from the origional invading organisms - hence the large number of "strains" of the virus!) in response to directional stress (or, if facilitated by humans "selective breeding"!).  The **observable** results, however, are always just variations (breeds, strains, etc.) of the same species - even over long time-frames such as that of various food crops, domestic animals, and pets such as dogs: a poodle and a St. Bernard can be interbred easily (provided the Saint is the mother, ie!!), since despite large superficial differences, they are both still dogs, decended from common ancestors.
Most of the problems seem at first blush to stem from the fragmented and extremely partial nature of the "fossil record" - however,  the fact that species are observed to stay essentially the same in external form for the duration of their existances, is troubling to many who believe speciation (the inferred process by which one species is transformed via accumulated mutations into a different species) should be regarded as a "fact" ( ie, something so strongly supported by cold, hard observations as to be considered essentially "proven" by the majority of the scientific community).  Hence, the recent profusion of such hypotheses as "punctuated equilibrium" and "hopeful monster" (both of which i'll leave to someone else to summarize!) - which postulate that speciation usally occurs in such a short period of time that vanishingly few (i have seen none i would call truly convincing!)  demonstrably intermediate forms between two observed (ie, as fossils) species, are preserved in the fossil record, nor indeed observed at the present time.
        In summary, i think micro-"evolution" is best described as adaptation within a given population of a species, often via preferrential expression of pre-existing alleles (forms of a given gene) already present in the gene pool of  that species, but sometimes though observable mutations as well.  Macro-evolution is best described as the process of speciation - one species giving rise to a separate one.  Even most die-hard scientists will agree that the former has been observed abundantly, while the evidence for the latter is largely "circumstantial" - it has never been observed directly, even though many would say the evidence strongly points towards it!
Seldom concise in Canada (and occasionally even mistaken :-o  !!)
Ross.
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