What is the mechanism that prevents microevolution to become macroevolution?

Just saw mjfloresta's thread Hypermacroevolution? Hypermicroevolution, and I have a question for all those who make the micro/macro distinction:What mechanism prevents microevolution from becoming macroevolution?I mean, as far as I've seen, proponents of evolution argue macroevolution is simply an accumulation of the changes described by mjfloresta in 'microevolution'. If mjfloresta is right, then there must be some reason that this accumulation does not occur. What is it?Not sure if this belongs in the other thread. If not, Theological Creationism forum please.Edited by Ben, : No reason given.

A question to Quetzal, crash, Faith, anybody who knows enough to answer: If an allele exists in a population, isn't it expressed in some individuals? Would it even be possible for a split like this to occur and have different phenotypes expressed? Since the pool of genotypes is exactly the same.The only thing I could think of is that if the environments were different, different chemical reactions might be altered (for example, if I remember correctly, some fur will only become white in cold environments, and is brown if the animal with the fur develops in a warmer environment) and the same pool of alleles might produce different phenotypes. Do we have any examples of such environment / genotype interaction being so severe that organisms with identical genotypes would be considered different species?I'm at the edge of my understanding, but I hope this makes sense. I know that, given mutations exist, such an experiment might not be possible.I'm just surprised nobody addressed this point, as it was beyond anything I've read. As far as I knew, given similar developmental environments, the changes in the phenotypes between the two separated groups could not be due to different frequencies of alleles, since those very alleles must be present (and expressed) in the original population.Thanks!Edited by Ben, : Changed subtitle and message icon

I think Faith is proposing that speciation that we see is part of microevolution and is explained by allele frequency changes of isolated populations. She's denying that mutation necessarily have a role in this "microevolution", i.e. adaptation to new environments and species-specific changes due to isolation.By doing so, she's trying to cut the bridge between such adaptation and large-scale "macroevolution". The bridge in evolutionary theory is mutation; Faith is saying microevolution won't accumulate and lead to macroevolution because microevolution can be explained by allele frequency changes ONLY and thus cuts out the bridge to macroevolution--mutation.Well, whether I can explain it or not, I think the discussion is on-topic.