Thread: Punctuated Equilibrium

Punctuated equilibrium (as I understand it) makes more sense to me since evolution should lead to an optimum, then enter an equilibrium, as no further improvement is possible. This equilibrium can be broken by diversification of the species (say a few ground dwelling rats enter the trees, so they need to change to survive) or through natural disaster.

What are you're thoughts on this debate? Which do you agree with, punk eek or gradualism? Some information on where this debate is in academic circles would be appreciated.

i must take issue that evolution leads to an optimum - doubt whether that is truly the case
on the other hand, the typical aspect of punctuated equilibrium, the long periods of stasis inbetween short periods of rapid change are likely to be the result of stabilising selection : animals and plants tend to migrate with their niche rather than change
the periods of change would appear to me the result of local extinction followed by migration of a sister species into the vacated niche

Originally Posted by Golkarian

Punctuated equilibrium (as I understand it) makes more sense to me since evolution should lead to an optimum, then enter an equilibrium, as no further improvement is possible. This equilibrium can be broken by diversification of the species (say a few ground dwelling rats enter the trees, so they need to change to survive) or through natural disaster.

What are you're thoughts on this debate? Which do you agree with, punk eek or gradualism? Some information on where this debate is in academic circles would be appreciated.

I'd say it's probably a mixture of both.

I agree with Biologista, and to be honest I don't think there really is such a debate is still going on. We have evidence from the hominid fossil record alone of what appears to be relative periods of stability interspersed with periods of relatively quick change in certain traits. But when we say "quick" in evolutionary terms, we're still speaking of at least a couple hundred thousand years. The Cambrian Explosion took place over 15 million years. When change occurs it still occurs in a largely gradualistic way, it just appears quick compared to the periods of relative stability that came before.

I will join Marnix, though, in cautioning you about thinking in terms of optimums. Natural selection does lead species to optimize as best they can, but only as far as they need to go. Perhaps they are rather inefficient at gathering a certain food source, but it hardly matters because the source is so abundant and is not eaten by any other species - no competition. However, if competition comes along, another species that wants that food source too, then that inefficiency actually becomes a detriment.

My only tuppence in this one is that, of course, the fossil record can only speak of morphological and not of genetic change: it is theoretically possible that the allele frequency in a population changed at an even rate even while morphology stayed almost identical, or showed huge changes. So it partly depends upon how you measure evolutionary change too.

Originally Posted by sunshinewarrior

My only tuppence in this one is that, of course, the fossil record can only speak of morphological and not of genetic change: it is theoretically possible that the allele frequency in a population changed at an even rate even while morphology stayed almost identical, or showed huge changes. So it partly depends upon how you measure evolutionary change too.

As I understand it, some things are less likely to be simple phenotype variations than others.

My only tuppence in this one is that, of course, the fossil record can only speak of morphological and not of genetic change: it is theoretically possible that the allele frequency in a population changed at an even rate even while morphology stayed almost identical, or showed huge changes. So it partly depends upon how you measure evolutionary change too.

I would suggest that the more likely scenario is this. Allele frequency stays fairly constant. The bulk of the population is one of a handful alleles. New alleles arise at a fairly constant rate, but these novel alleles remain as only a small percentage of the population, so that effectively, the allele frequency has not changed much.

When conditions begin to change, those rarer alleles which had previously been only a small percentage come in to play and suddenly explode onto the scene, hence giving the appearance of sudden arrival.

This nicely explains the fossil record because one would expect that most fossils will represent the predominant phenotype. Rarer phenotypes that correspond to rarer alleles would not likely be preserved until one gets a drastic change in the allele frequency and those alleles are selected for.

Evolution doesn't lead to an optimum. Case in point, the giant panda. Talk about an animal that evolution totally fucked over.

So stasis is caused by lack of competition and gradual change by arms races? Is this a better way of describing it?

Originally Posted by Golkarian

So stasis is caused by lack of competition and gradual change by arms races? Is this a better way of describing it?

More simply, a stable environment. As long as you remember that "environment" encompasses the whole of exogenous factors that affect an individual organism, including other organisms and even members of its own species, not just the physical habitat.

Punctuated equilibrium implies... drumroll.... equilibrium.

And equilibrium does not equal optimum.

So I put my weight of support behind Marnix.

if Peter & Rosemary Grant's work in the Galapagos has taught us anything, it's that evolutionary change can be rapid from one year to another, but also that over decades or centuries these rapid changes more often than not cancel each other out, meaning stasis over geological time

on the other hand, if changes in the environment DO maintain an evolutionary change in a certain direction, then it goes to show that these changes will happen equally fast - the punctuation of your equilibrium

Originally Posted by mormoopid

Evolution doesn't lead to an optimum. Case in point, the giant panda. Talk about an animal that evolution totally fucked over.

We could say the same about some of the traits humans possess. Not at all suited to how quickly our circumstances have changed in such a short time. We have good reason to think that allergy may be partly blamed on the fact that we've more or less eliminated parasites in humans in the space of a couple of hundred years. Evolution just can't react in that time scale, at least not in organisms like us and the pandas. We just don't reproduce fast enough.

Originally Posted by mormoopid

Evolution doesn't lead to an optimum. Case in point, the giant panda. Talk about an animal that evolution totally fucked over.

Fossils of giant pandas dating as far back as the middle Pleistocene era (about 600,000 years ago) have been found in central and southern China. This suggests that the prehistoric range of giant pandas was much greater than at present, which is restricted to portions of north-central Sichuan Province and southern Gansu Province, and the Qinling Mountains of Shaanxi Province of China.

http://science.jrank.org/pages/5005/...ification.html

translation: the giant panda has been around 6x as long as the human species. An evolutionary success story therefore.

Originally Posted by Golkarian

What are you're thoughts on this debate? Which do you agree with, punk eek or gradualism? Some information on where this debate is in academic circles would be appreciated.

I don't know the current status of this debate in academic circles, but I suspect it's generally regarded as a manufactured controversy. The division is only relevent in a certain restriction of temporal context, as even the most punctuated evolution is still going to be gradual. Whether or not evolutionary stasis occurs afterwards is entirely dependent on external factors, and not on a conceptual basis alone.

Evolution is always punctual.

One generation at a time.