Re: Punctuated Equilibrium Theory
Wed, 23 Sep 1998 23:03:09 EDT

In a message dated 98-09-23 17:15:59 EDT, (Robin Hanson) writes:

> On 9/10/98 -0500, Eliezer S. Yudkowsky wrote:

>>Orr challenges the theory that evolution consists of many tiny genetic >>mutations: "The distribution of mutations causing adaptation neatly fits an
>>exponential curve: While few major mutations are needed, the number of more >>minor mutations rises exponentially with their genetic insignificance. Orr's
>>theory is based on mathematical modeling and computer simulations, and assumes

 >that a population is well-positioned to adapt to environmental pressures. He
 >now plans to use a common laboratory technique called quantitative trait
 >locus, or QTL, analysis -- capable of examining how species' genetic
 >compositions differ -- to examine whether his theory holds up."

> I finally obtained this paper and read it last night. It is a theory paper,
>and the key assumption is that "the phenotypic optimum changes suddenly and
> then remains fixed during the bout of adaptation studied." The paper then
> models a hill-climbing search to a peak. In such searches, one makes big
> moves early on, and then smaller and smaller moves as one approaches the
> peak.

> As genes get fixed along the way, the genotype ends up reflecting this
> distribution of moves; some genes embody very large moves and others
> very small moves. The paper notes that you don't get this effect "if the
> phenotypic optimum perpetually drifts, moving away from the population at
> about the same rate that the population evolves to `keep up.'"

> The bottom line is that this paper *assumes* punctuated equilibrium,
> and so is not evidence in favor of it.

Not exactly. The paper provides a theoretical model for adaptation, which predicts punctuated equilibrium when adaptive changes to new situations is involved. The evidence is not from the paper, but from previous results on quantitative genetic traits which indicate the effects of genes causing differences between species follow an exponential distribution. I found it quite impressive that a simple mathematical theory has good agreement with the evidence. A quantitative theory of adaptation has been a holy grail of evolutionary biology for some time, and this may qualify.

If speciation resulted from isolation and drift, then the Orr model wouldn't apply and speciation would not be punctuated.