Posted: Jun 19, 2010 7:04 pm
Darwinsbulldog wrote:I realize that mass extinctions, by definition, global. However, it seems that some palaeontologists in the past have talked about a "reef gap", where in fact there existed extensive reefs in some areas, and not in others(1). For sure, the diversity of surviving taxa is measure of recovery, but if and only if the population numbers are sustainable in each species. As I said before, some groups made it though a bad extinction, and petered out soon after. A thousand Koalas without Eucalyptus leaves are dead Koalas. A thousand Koalas of post-reproductive age means the species is doomed to extinction. Exaggerated small numbers in my example, but you get my point.
a) I did read the Erwin paper and it´s certainly interesting. But it doesn´t alter the definition of a mass extinction. There are questions concerning mass extinctions effects on the other metrics introduced (He somewhat skimps the ecospace part, which is odd considering he gave it quite a bit of room in his review paper on disparity, Erwin 2007).
b) I still don´t see how this connects to PE. PE is about the distribution of rates of morphological change, not extinction rates or the rapicity of mass extinctions.
c) Geological time. When we talk about the Permo-triassic extinction event, we do know it happened in less than 200ka. A generation more or less doesn´t even make a difference there. The simple explanation for clades going extinct shortly (that´s still geological time spans measured in millions of years) after mass extinctions is that they survive with only a few species and they go extinct as part of the background. Ammonids made it past KT with 2 species and went extinct in the danian. Did these 2 Ammonid species go extinct because of the ecological effects of KT? Probably not - there are millions of years in between. But of couse a clade containing 2 species is more likely to go extinct than a clade with 100 species or 1000 species.
Darwinsbulldog wrote:And if they are in low numbers, then the fossil record might not record them. So, the record may look like a PE event, in that they might look extinct locally, or another population may re-invade the extinct populations area from some refuge.
There are no "PE events". PE is a hypothesis about the distribution of rates of morphological change.
Darwinsbulldog wrote:A citation please? I did not see much in Gould's "Structure of Evolutionary Theory"
Citation for what? The MBL papers are
Raup et al. 1973 "Stochastic models of phylogeny and the evolution of diversity",The Journal of Geology, 81, 525-542.
Raup & Gould, 1974 "Stochastic Simulation and Evolution of Morphology-Towards a Nomothetic Paleontology" Systematic Zoology, 23, 305-322.
Schopf et al, 1975 "Genomic Versus Morphologic Rates of Evolution: Influence of Morphologic Complexity", Paleobiology, 1, 63-70.
Gould et al., 1975 "The Shape of Evolution: A Comparison of Real and Random Clades", Paleobiology, 3, 23-40.
If you want page numbers for the Structure, you´ll have to wait (don´t have it here, will check on Monday).
Darwinsbulldog wrote:Perhaps you have not seen papers like the one by Hinman & Davidson (2007)? (2). They compare the Genes Regulatory Networks [GRN's] of Sea Stars and SeaUrchins, who shared a common ancestor some 500 myo in the Cambrian. Thus the LUCA of these clades must have had the same GRN as well, where the CORE is conserved and different organisms innovate and co-opt less conserved parts to satisfy their own evolutionary challenges. All methodologies have their problems of course, but palaeontologists can usable only resolve down to families or perhaps genus level, and can only work on morphologies, which is a rather crude and unreliable indicator of diversity.
It´s a cool paper, but it doesn´t seem to have any connection to punctuated equilibrium at all. And of course we can resolve species, we just tend to use genus level data to generate diversity curves, mainly because it is standard practice to exclude singletons (taxa only present in one particular time interval) - simply because they obscure the diversity signal as singletons tend to stem from extraordinary preservation, if there are 56 taxa in the danian of shoreline Y, while there are only 5 for the uppermost Maastrichtian there, but the danian has soft body preservation, while the Maastrichtian doesn´t, this does not indicate diversity went up across the KT boundary. And if you exclude singletons for species, you throw out pretty much everything. So for diversity curves genera are used, but we can resolve to species for other purposes. Only working on morphologies is not a downside if you´re looking at the distribution of rates of morphological change (though we can also work with behavioural data for instance).
Darwinsbulldog wrote:In any case, real recovery seems to depend on ecological balance after the perturbations have ceased, or at least, the ecosystem can buffer those disturbances. A crude bunch of refugee species with large populations is still not out of the woods until autotrophic production is back ot capacity and there is depth to the trophic levels. Just counting mere species, or mere population numbers, is not in itself a robust measure of recovery. It depends very much who those species are, although there are sometimes gaps in niches after a recovery is said to be complete and at carrying capacity.
Not a fan of whatever niche definition you are using there. And again, this doesn´t seem to have anything to do with PE.