I couldn't quite figure out where to jump in on this thread, so I figured I'd start back at the beginning. There was another thread arguing about the relevance of the terms macro- and micro-evolution, so I won't rehash those arguments. I do think a review of some of the basic misconceptions evidenced here might be useful, however.
Apollyon states that s/he understands macroevolution to mean "any evolutionary change at or above the level of species". Although fairly close to what paleontologists mean when they discuss the concept, I still find the term grossly misleading when taken out of the technical context it was intended to describe - the branch nodes in lineages that ultimately give rise to distinct higher taxa. For various reasons, in most cases we can only infer changes in major taxonomic groups while looking at the past history of life. IOW, fossils generally only record gross morphological change in lineages over vast stretches of time. There are, of course, exceptions: trilobites in the fossil record show enough divergence that systematists have classified them into four distinct orders - but they remain trilobites.The trilobite example is a nice lead in to a discussion of just what is meant by "change above the level of species" (a paleontological concept) as it relates to evolution (a biological concept).
I'll start with a "working definition" of evolution - the change in allele frequency in a population of organisms over time. Variation (through mutation and drift) are constantly arising in any population from generation to generation. Here's the catch: evolution doesn’t proceed by sudden leaps — saltation — in major morphology. The basic reason is that the larger the effect of a given mutation, the more likely it will be deleterious — and kill the mutant. That’s not to say small mutations can’t have a deleterious effect on the organism, just that large ones would be almost invariably fatal. What normally happens is that a small mutation occurs in a population, splitting it into two different varieties. Now if these two varieties lived close by one another, the odds are that they would interbreed, and the mutation would be suppressed or eliminated. Allele frequency tends to be pretty hard to budge once fixed. However, what would happen if a portion of the population carrying the mutation were to become geographically or behaviorally isolated from the parent population? The mutation, if it provides a net survival advantage in the new area, will rapidly become fixed in the new population. In addition, both populations continue to change due to environmental conditions, or even chance (genetic recombination, additional mutations, etc leading to changes in allelic frequency in both populations). If the two populations are reunited eventually, and they don’t interbreed, then we can say they are in fact two distinct species. The longer they are apart, the more differences we would expect. This is more or less the commonly accepted (even by most creationists) definition of microevolution (or if you're a creationist, "change within created kinds).
I assume everyone knows scientists classify life based on species, genus, etc. I also assume that everyone is aware that these classifications are simply how systematists show relatedness. What isn't immediately obvious (although it would be if you think about it), is that all these different hierarchies are simply larger groups of species. A genus is a bunch of species that are really closely related - sharing some traits, being different in others. Familes are a group of genera, etc. All these categories are simply names given to ever-larger groupings of related species - nothing more. IOW, any mechanisms (say, natural selection) working at the population or species level will axiomatically operate in identical fashion at the level of a class or even phylum, etc. Why? Because a phylum is simply a very large grouping of species that share some common trait (such as a spinal cord). So when scientists talk about transitions between, say, orders, especially within the fossil record, they're saying they've found a species that shares traits across order boundaries (which really get blurry anyway). In short, they're merely describing the relative closeness of members of two species. There isn't some mystical barrier based on taxonomic nomenclature. It's just two different species - more or less related - and ultimately identical to comparisons between two living species of hare (say, between
Lepus arctus and
Lepus townseii). So although the trick (and lots of glorious arguments among paleontologists) is to determine, based on morphology and other evidence, just how related two temporally separated fossils are, if they share enough traits we can be fairly confident that they are related.
The reality is there is no fundamental difference between macro- and microevolution. The former describes
trends or
patterns in the fossil record, and the latter describes changes within species or populations (assuming Apollyon grants microevolution=speciation). Populations are the only "real" category in nature, and species are the only "real" taxonomic grouping; anything above that (genus, family, order, etc.) are not found in nature, but are convenient organizational categories invented by humans. Once speciation occurs, and if the two species remain separated from each other, then they will evolve differently from each other. As time goes by, they will become increasingly differentiated. In terms of the time scale of the earth, humans have only been around long enough to have observed the tiniest slice of time. If we happen to observe two species who have recently diverged, we see how similar they are to each other and categorize them by placing them into the same genus. If, however, we observe two species that diverged long ago in the distant past, we see that while they might retain some very basic similarities to each other, they are much too different to belong in the same genera and so we place each into a higher category: family, or order, perhaps, depending on our criteria of "difference" for that organism.
I'd like to close with my favorite relevant quote (from Carroll, R, 1997, "Patterns and Processes of Vertebrate Evolution", pg 392, Cambridge Uni Press):
quote:
Nearly all the factors that have been used to distinguish the origin of higher categories can be attributed to the same processes of speciation, behavioral adaptation and the gradual accumulation of morphological differences that characterize evolution at the levels of populations, species, and genera. There are no fundamental differences between the early stages in the radiation of placental mammals in the earliest Cenozoic and what is known to have occurred in the origin of the species flocks in the East African Great Lakes...Although formulation of a distinct theory of macroevolution does not appear to be justified, it may be convenient to retain the terms microevolution and macroevolution to describe the different patterns of evolution that are observed at the level of populations and species versus higher taxonomic levels and time spans exceeding 5-10 million years. (emphasis in original)
Hope this answers both your question on macroevolution, and explains why we don't expect to see it in the lab.
. Bottom line: trying to restrict speciation to macroevolution simply confuses the issue, because whenever the subject comes up in arguments with creationists they constantly and repetitively argue that "they're still trilobites". Getting them to agree that speciation can occur (under their acceptable "microevolution") is the first step in getting them to admit there isn't an difference between population dynamics and the pattern observed over long timescales in the fossil record.
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