Punc Eq Happens
A controversial study in Science found evidence for punctuated equilibria.1
A long-standing debate in evolutionary biology concerns whether species diverge gradually through time or by punctuational episodes at the time of speciation. We found that approximately 22% of substitutional changes at the DNA level can be attributed to punctuational evolution, and the remainder accumulates from background gradual divergence. Punctuational effects occur at more than twice the rate in plants and fungi than in animals, but the proportion of total divergence attributable to punctuational change does not vary among these groups. Punctuational changes cause departures from a clock-like tempo of evolution, suggesting that they should be accounted for in deriving dates from phylogenies. Punctuational episodes of evolution may play a larger role in promoting evolutionary divergence than has previously been appreciated.
Mark Pagel et al. studied 122 lineages for molecular changes and found more than could be explained by natural selection. They believe genetic drift sped up in certain lineages, then settled down after speciation. Some groups, they said, evolve gradually, but others, like the ginger family, went through bursts of accelerated genetic change. Elisabeth Pennisi wrote about this paper in Science Now.
For some biologists, “punctuated equilibrium” is a radical idea. The term was coined in the 1970s to describe an uneven pace of evolution in the fossil record. But because it posits that evolution happens in bursts, punctuated equilibrium goes against the notion that evolution inches forward in tiny steps guided by natural selection. Now evolutionary biologists have shown that evolution in the genome also has fast and slow speeds, and that natural selection isn’t always governing genetic change.
She points out that the paper “is causing a stir,” but that some critics are not sure Pagel et al. accounted for factors that could have skewed their results.
1Mark Pagel, Chris Venditti, Andrew Meade, “Large Punctuational Contribution of Speciation to Evolutionary Divergence at the Molecular Level,” Science, 6 October 2006: Vol. 314. no. 5796, pp. 119-121, DOI: 10.1126/science.1129647.
The analysis by Pagel’s team assumes evolution from start to finish, from initial conditions to conclusions, from variables to constants. So despite the mathematical wizardry, no sound conclusions can be expected from such an incestuous reasoning process. What’s interesting about the paper is that the war started by Gould and Eldredge in the 1970s about the pace of evolution is not over. Both fossils and genes fail to show the gradual change Darwin expected. Evolutionists are still having to scramble to force-fit the observations into their imaginary picture of the world.