Rate changes for evolution

Hi Abby,Another factor that needs to be considered is the difference between the underlying mutation rate & the fixation rate.Consider organism 1/ which lives in a stable environment that has existed in it for many, many generations. For our purposes it can be considered optimal. Given that this is true, any mutation can be considered neutral/deleterious. For our purposes this means that it is selectively stable, or in stasis.Organism 2/, on the other hand, finds itself in another environment where various loci are selected for (differently to 1/) that are different to the "wild" type. That is that certain loci are likelier to have different mutational changes fixed than the "optimal" organism.In both cases the underlying rate of mutation per locus is the same, but the rate of "evolution" is different.Mark

Abby,Yup, sorry for not being particularly clear. I was drunk & was having one of those I-want-to-hit-that-key-but-always-hit-the-one-next-to-it moments .Mark

PR,It's probably worth noting that there is little to no evidence of PE as Eldridge & Gould formulated it. There is, however, a wealth of evidence for a "weak" version of PE where evolutionary rate changes anagenetically, rather than be associated purely with speciation.The best evidence I have seen for a strong PE is an analysis of (if I remember correctly) snails, where one species is continuous through the given section, & another slightly different one appears alongside it halfway up. Gould interprets this to be his version of PE, where the new species has evolved so rapidly that the resolution in time of the rock unit doesn't record the evolutionary change, followed by stasis of the species in question. It could, however, be a species moving into that habitat from elsewhere that isn't recorded anywhere else in the fossil record. A little too equivocal for my liking.IMHO, Gould has done an excellent job selling PE that goes beyond what is evidencially warranted.Mark

Hi Denesha,The fossils were from the same rock unit, so that their relative ages were easy to determine.Mark

PaulK, I know, but it doesn't change the fact that there is no good evidence of PE in the fossil record. What you'd need to see is a single species give rise to a daughter species that immediately undergoes rapid evolution followed by stasis (& like you say, it is allegedly allopatric so it would be unlikely anyway). There simply isn't any. The particular snails in question could very well have allopatrically speciated, & started competing with the original population at a later date, but how do we know how fast they evolved? It isn't evidence for rapid evolution at all. They could have evolved via the old Darwinian gradualistic mode. The point was that he claimed it was consistent with PE, which it is, of course, but that it is very, very, very equivocal.Don't get me wrong, I have no problem with PE being indicative of reality, but the actual evidence as it stands gives far greater support to anagenetic rate change (which is seen in the fossil record), whereas rate change (as opposed to something appearing in the same strata as-is) being associated with cladogenesis is utterly absent in the fossil record. I do concede that PE would be difficult to see in the fossil record if it is indeed allopatric, but neither is that the fault of the skeptic.You have to remember that Gould claimed the majority of rapid evolution occurred at cladogenesis, the evidence suggests that it isn't so. I don't think pointing this out is being harsh, Gouldian PE may very well occur, but there's no evidential reason to think it occurs with the frequency that he claims, or even think it occurs at all for that matter.IMHO Eldredge & Gould placed unwarranted emphasis on the cladogenetic aspect of PE. True, there is good reason to suspect that a small population evolves faster than a large one, but it has to stay a small population for a relatively significant amount of time for this to be true. But at the same time it is also potentially true of an anagenetic population that experiences fluctuations in population size. So why hang your hat on cladogenesis in the first place?A good idea in need of more evidence. Weak PE wins hands down at the moment.Mark

Hi Abby,Thanks for the link. I agree that the graph is better evidence of gradualism. The N.nunienus part of the graph covers about a million years & it looks fairly linear to me, rather than a rapid burst followed by stasis. Had it have done so it would have shut a lot of nay sayers up.Worth keeping on file though!Mark

PaulK,Gould means by rapid up to 100ky-ish, according to The Structure of Evolutionary Theory. Enough to be potentially observed.Mark[This message has been edited by mark24, 04-01-2004]

Hi Paul, It's the time for the evolutionary burst, rather than speciation itself. After that comes stasis. If I remember correctly it's only meant to be an upper limit ball park figure.Mark