Peez

I am sympathetic to your position, but there is a place for the distinction (just as there is a place for the distinction between big and small animals, even though there is no clear line between them and people may disagree on what constitutes "big"). Certainly creationists tend to use the terms to obfuscate, and I agree with Kevbo that the key in forums such as this is to challenge creationists to define "macroevolution." Of course, creationists will generally try to dodge this issue.

Peez

MrDarwin

I've generally found that creationists don't dodge the issue at all. They simply define "macroevolution" to mean large-scale change--which is one way that evolutionary biologists use it--but go on to define macroevolution exclusively as the evolution of one major group ("kind") from another, e.g., birds from reptiles.

Where they run into problems is in scale: they seem to think that "macroevolution" means a bird hatching from a lizard egg, which of course doesn't happen and so seemingly proves their point that macroevolution is simply not possible. They overlook the fact that during the evolution of major groups that we recognize today, the fossils of the earliest members of these groups are barely distinguishable from their sister taxa or immediate ancestors. Thus the earliest birds are amazingly similar to theropod dinosaurs, and the earliest tetrapods are amazingly similar to lobe-finned fish because many of the characteristics we recognize as defining modern groups were acquired gradually and stepwise after the origins of these groups. This is a pattern that creationists try desperately to sweep under the rug.

Peez

Perhaps I was using "dodge" too liberally. I meant that they tend to define it sufficiently vaguely that it is difficult to pin them down. No matter what example of how evolution can produce some change, it can continue to be dismissed as "microevolution" as long as the distinction between the two remains shades of grey. Of course the distinction is shades of grey, but it shouldn't be if the creationists were correct.

Peez

KC

I agree. I still see many of them asking for observation of a 'macroevolutionary event', such as one species becoming a species in another genus. They either fail to realize (or, more typically, ignore) the fact that the reason we can distinguish genera is not because of a single 'event', but due to many extinction events of the intermediate forms between them.

Of course, when I am in a more playful mood, I simply tell them that macroevolutionary events are observed today: every time a systematist places a species in a genus, or creates a new genus.



KC

pz

There are really two different ways the term "macroevolution" is used. Both are valid, both are relatively discrete, and both represent a useful distinction from microevolution.

The first is the more mainstream usage, and the one everyone is using here, the phylogenetic definition of macroevolution. It is any change above the species level. That T.O. definition that makes it synonymous with descent with modification encompasses it, as does any definition that ties it to speciation.

There is another definition that is much more radical, much closer to the original intent of Goldschmidt's coinage, and which no one has brought up yet. It's that macroevolutionary events are any rapid large-scale change in a population. This kind of macroevolution isn't necessarily associated with any phylogenetic event at all. It has also fallen out of favor, for a couple of very good reasons: it's a bit vague (how large does a large-scale change have to be to be called macroevolutionary?), and it got blown away as evolutionary change was more and more tightly coupled conceptually to genetic change, and as the extreme disconnect between the magnitude of genetic and phenotypic change was discovered. If a single base pair change can produce radically different phenotypes, minute, genetically microevolutionary steps were thought sufficient to account for all large-scale morphological change.

There are some interesting ideas lurking in the developmentalist camp that may end up changing that assessment, though. It may be that macroevolutionary changes are qualitatively different from microevolution. That was Goldschmidt's idea, that there is another class of mutation behind macroevolution. He postulated systemic changes in the organization of the genome that didn't change the genes, but did change their arrangement and relationship with one another (the funny thing is that he was right, and way ahead of his time...it's just that those concepts got swallowed up by standard genetics as position effects and epistatic interactions became part of the canon).

The most interesting heresy I've heard about, though, is that macroevolutionary change is not genetic at all. Genes are slow and cumbersome things to modify, and you can get much more rapid change to greater extremes by taking advantage of phenotypic plasticity. Change in the environment drives changes in morphology of the population directly, in the absence of any genetic change. Macroevolution occurs before speciation, and genetic change occurs after the fact to reinforce and stabilize a favorable phenotype.

The distinction between micro- and macro-evolution may seem a little bit like a forgettable bookkeeping detail under the neo-Darwinian synthesis. However, that synthesis is grossly incomplete. If you follow much of the evo-devo, or in particular for this problem, the eco-devo literature, there's a minor revolution looming in the near future. I suspect this will become a much more important distinction.

For a brief, general overview of the importance of phenotypic plasticity, see:

Gilbert, S (2001) Ecological developmental biology: developmental biology meets the real world. Developmental Biology 233: 1-12

For an extremely thorough, in-depth, mindblowing discussion of this whole topic, the best book I've seen so far is:

West-Eberhard, MJ (2003) Developmental Plasticity and Evolution. Oxford University Press. Don't ask for page numbers, read the whole thing.