Reply #16: Both [View All]

[Ediacara]

Proponents of extremes don't listen to the opposite extremes. The pattern seen by punc-eq is real. Morphospecies stay roughly static over time, then rapidly change to something else. This is seen in fossils (which are generally astoundingly complete, contrary to popular opinion), and in lab and field experiments.

The difference of opinion is on the process that leads to that pattern. Punc-eq proponents point to founder events, bottle-necking, and peripatric (groups on the periphery of a large population that has lots of gene flow) speciation as the mechanisms of rapidity, and gene flow as the mechanism leading to stasis.

Gradualists point out that larger populations have a larger rate of mutation, so shouldn't actually appear to be static if gene flow is occuring. They point out that many morphological features are changed at thresholds, so the rate of genetic change is linnear, but morphological change looks jumpy.

So what it looks like is this: "traditional" punc-eq mechanisms seem to be dominant in small populations, like those experiencing founder events, and especially those speciating peripatrically. In large populations, step-wise, punctuated, non-speciation events are dominated by genetic threshold crossing events (which look punctuated, but are essentially gradual in nature).

For two really pretty cool books (although Levinton's is dry as toast), check out:

Macroevolution- Pattern and Process by Steven M. Stanley, and Genetics, Paleontology, and Macroevolution by Jeffrey S. Levinton.