So attacking the fickle-ness of the changes in the fossil record wouldn't necessarily be the huge debilitating attack .... in times of environmental stability we would see populations in stasis and the fossil record would reflect this....in times of environmental change, we would see population change.
that's why punctuated equilibrium and gradualism may both be happening. To argue for the P.E. side, most species stay the same if exposed to stable environmental conditions. The deep-sea tends to have numerous species of ancient relic species. Some scientists argue that this is due to environmental stability although it could also be due to less competition for resources from more "advanced" forms. But changes in the environment, like changes in ocean circulation, temperature, volcanism, vicariant events, can all trigger "explosive" evolution.
Take the marine faunas of the Eastern Pacific and Western Atlantic for example. They were once homogeneous before the uplift of the Panamanian Isthmus. When the barrier raised, the Eastern Pacific cooled dramatically and experienced much higher upwelling. The shelf also narrowed. The West Atlantic experienced a dramatic decrease in both depth and upwelling and also warmed considerably. This led to a mass extinction of marine life in almost every major family on both sides but especially in the Atlantic. To fill vacated niches, there was a intense radiation of speciation to recover these lost niches. Examples are gobies, chaenopsid blennies, parrotfish, grunts, Caribbean cones, Panamic columbellids, Atlantic murex, Paicifc cancellarids. The faunas are still very closely related but they still have disparities in certain families. Molecular techniques have dated these radiations close the the time of the Panamanian event
This same thing has happened with Brazil and the Amazonian Plume (related to Caribbean forms), in the Indo-Pacific on the numerous island groups, and especially with Conus in the Cape Verde and cichlids in the rift valley of Africa. It seems like environmental change is the main driving force for the punctuated equilibrium model.
Thanx for the comment, Zoey - I agree completely and Darwin would have agreed also, as shown by his quote I posted yesterday. There are some species alive pretty much unchanged for the last 300 million years. With no substantial change in their environment and no catastrophes that affected them, there was no selection pressure for them to change.
For gradualism, there seems to be continuous change going on even in forms that seem to be "unchanged". The coelocanth Latimeria looks like ancient coelocanths, but it is not only a different genus and species, but a different family. The stalked crinoids of the deep ocean are different genera and species from ancient ancestors. Every "relict" species is different at at least the species level. If you look at fossil beds in the SE, you will find fossils of gastropods that are closely related to living species (Oliva, Scaphella, Murex, etc.) but they are different. This indicates that gradual evolution, as proposed by Darwin, is also occurring.
I would like to add that "punctuated evolution" can also be the result of the incomplete fossil record.
Take for example amphibian evolution. The original forms probably evolved in a localized ecosystem and then began to migrate out.
By the time they appear in the fossil record, it would look like a sudden appearance of new forms.
True... this is apparent in the Indo-Pacific "center of diversity" for numerous marine forms. Reef fish are notoriously rare in the fossil record so their origins are best traced by molecular studies.
