strubenuff

Personally, it seems I've heard more about this than not. An aquaintance of mine has friends in the field who say it's an exciting area of research at the moment. Has anyone else heard of this, or how it's comparing to the tradtional theory?

Camaban

punctuated equilibrium, you mean?

strubenuff

Um...not quite sure. Forgive my ignorance...I'm referring to the idea that evolution, rather than being the long, gradual process most percieve it as, is actually more like a species tree...not a lot going on for awhile and then an abrubt change or split, then back to a period of little change, then another split, etc. Essentially that "evolution" happens quickly when it takes place. I know that's badly worded, but that's the idea.

Gregg

There's nothing really new or radical about punctuated equilibrium. In fact, Darwin suggested it long before Gould and Eldredge. All "punk eek" suggests is that there are long period of stasis where not much evolutionary change occurs. Then something happens to upset the environmental equilibrium (like an asteroid collision, or the beginning or end of an ice age), and a great deal of evolutionary change occurs relatively "rapidly" (but still over thousands or tens of thousands or hundreds of thousands of years) as niches fill and things settle back into a routine.

Punk eek was proposed to help explain why there are "gaps" in the fossil record, rather than smooth, even transitions. The explanation is that because a lot of evolutionary change occurs "rapidly" (in response to environmental changes) and usually within small populations, transitional fossils can be vanishingly rare. Until a transitional form becomes successful and well-established over a long period of time, the probability of a sample specimen becoming fossilized is low.

Nevertheless, many transitions are well documented in the fossil record--the development of the inner ear, the development of the modern horse, and the transition from land mammal to early whale, for example.

butswana

Is ther any reason why gradualism and punk eek cannot coexist? It seems to me that both processes working together are better than one over the other, so what's all of the fuss?

Jobar

p.e. and gradualism *do* coexist. The problem (as best I understand) is that some scientists, for quite a while, denied catastrophism- claiming that there had been no ancient catastrophes large enough to actually wipe out common species abruptly. Their arguments are pretty much put paid by the evidence for the K/T asteroid impact, and by increasing evidence for other earlier impacts. Gradualism takes place in 'mature' ecosystems, where the environmental changes are less than catastrophic.

pz

PE isn't about catastrophism, and it can take place in mature, stable ecosystems. Environments after a catastrophe like the K/T impact are really special cases where the rules of neither gradualism nor punctuated equilibrium properly apply.

The idea behind PE is really about modes of replacement. Gradualism says that change sort of simmers throughout a population, and the whole mass steadily shifts in response to the environment. PE argues that natural selection and the sheer inertia of a large population tends to clamp the bulk of a population into a steady state, with little change possible; meanwhile, small, isolated subpopulations on the marginal edges of the environment are in a state of rapid ferment. Since they are in smaller groups, novel mutations are more likely to be fixed, and possibly because they are in a fringe environment, variability may be more of a reward. If some adaptation arises that gives that group a key advantage, it will expand and relatively abruptly replace the larger main population. No external event is required.

You'll find in the literature that guys like Ernst Mayr say this is all old hat, that Darwin first mentioned it (true), that Simpson discussed it (also true), and that it is just peripatric speciation renamed and repackaged. Gould and Eldredge made grander claims for it, of course, and I think there's some truth to that, as well -- if nothing else, they provided a testable focus for a lot of research and made sure that emphasis was placed on empirical, quantitative aspects of speciation.

Jobar

My own (admittedly nonspecialist) understanding of PE:

Over geological time, the biota in a fairly stable environment evolves to a state wherein all ecological niches are filled by plants or animals which are optimized for that niche in that environment. This is not a steady state; rather, the biota evolves to a metastable state. Small changes, jockeying for tiny advantages, is ongoing; however, it is fair to call this an equilibrium state. Such states may last for appreciable geological time, and for thousands upon thousands of generations of plants and animals, with only minor change occurring.

Then something happens. It may be a 'comma'- say some plant species evolves a toxin which makes it completely inedible to the animals which had consumed it, and kept it in check. The plant then multiplies rapidly, upsetting food chains left and right; until some animal (or plant disease) evolves a way of digesting (or infecting) it. Eventually another equilibrium is reached.

Or it might be a 'period'- some disease germ evolves into such virulence that, say, entire species of large carnivores are wiped out. Herbivores become more abundant, food plants are overgrazed, inedible plants become more common due to lack of competition- again, a whole line of overturned dominoes, until a new metastable state is reached.

Or it might be that a large 'exclamation mark' occurs. A (geologically) rapid climate change, massive volcanism, or the aforementioned asteroid impact changes the environment abruptly, and entire ecologies are wiped out or required to make drastic changes.

I might mention that this interpretation of 'punctuation' is my own, so if you think it's too cutesy I take the blame. Perhaps it is stretching the definition of 'catastrophe' to name all these examples so; still, I don't feel it is too great a stretch.

Am I making some false assumption, or missing some fine but vital point here? I had always understood PE to *require* catastrophes of one degree or another, to provide the 'punctuation'.

pz

I think the only small problem is that your definition of 'catastrophe' is far too loose. I agree that getting replaced by a competitor is catastrophic for the species concerned, but if we use that definition, then most speciation events will be catastrophic.

Your "comma" example is closer to what I consider the most common kind of replacement event considered in punctuated equilibrium. I'm certain that Gould and Eldredge would not regard catastrophe a required component of PE.

Albion

I think the early resistance to catastrophism had something to do with biologists being concerned not to appear to be supporting saltation. Once it was clear that alternatives to gradualism didn't involve a dinosaur giving birth to a sparrow, people became mroe relaxed about it. of course, creationists still portray PE as a dinosaur giving borth to a sparrow, but that's only to be expected.