I might add, though, that evolution seems to proceed in terms of punctuated quasi-equilibria.
There are gradual changes over long periods interlaced by bursts of evolutionary activity -
a kind of biological revolutions.
I think that's true, but I think it's important to emphasize that even in those instances where evolution "surges" forward, it is still a process that takes amounts of time that are nigh-incomprehensible to our human minds. Even the Cambrian Explosion and other massive speciation and cladogenesis events took tens of millions of years. So even if these are "bursts" of speed relative to the equilibrium rate of stable population evolution, they are still incredibly slow. I don't think it's proper to confuse the situation by underemphasizing the core principle of very slow change over very long periods of time.
Personally, I doubt that the rates of generational variation - the amount by which offspring differ from their parents - is any different in populations undergoing the "punctuated" phenomenon as compared to those in "equilibrium." It's unlikely that mutation rates alter much. Instead, I think segregation causes allele frequencies to change because groups are separated or the "middle ground" (the most common phenotype, assuming bell-curve distribution of traits) suddenly becomes less advantageous than the extremes. So it's not that evolution is moving any "faster" or that genetic changes occur more frequently, but rather that populations show a fast divergence of various allele frequencies, caused by sudden changes in environment that either separate groups that have different alleles by chance or wipe out a large swath of the population relatively quickly, leaving a different concentration of alleles in its wake.
I would argue that punctuated equilibrium illustrates not differing rates of evolution, but rather differently aggressive selection pressures on populations. High disruption of a population and/or its ecosystem causes a situation where alleles are more aggressively selected, which can rapidly deviate the allele concentration from its stable equilibrium, leading to an illusion of more rapidly-changing species. Large, evolutionary events like the Cambrian Explosion are caused not by all the surviving genetic lines suddenly gaining the ability to more rapidly mutate and change only to give up the ability when "stability" returns, but by harsh reductions of existing populations to fractions of their original size and components, drastically changing the most common traits in the remaining populations. These changes across the ecosystem or planet have the effect of shuffling the niches different organisms occupy, which changes the selection pressures, leading to new evolutionary developments. Millions of years later, the Earth can obviously look very different.
Iowa Secularists
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