To: All Nov0993 08:55AM Subject: Punctuated Equilibrium I've been noticing a number of pos

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From: Kevin W. Williams To: All Nov-09-93 08:55AM Subject: Punctuated Equilibrium Organization: This variable implies that we ARE organised From: (Kevin W. Williams) Message-ID: <2boi1d$> Newsgroups: I've been noticing a number of posts on punctuated equilibria recently, but haven't seen a clear concise explanation on how PE works without having to invoke the Magic Mutation Rate Fairy. This post will try to summarise my understanding of the theory for those who don't understand it. Corrections to my understanding are greatly appreciated. Punctuated Equilibrium basically states that there are periods of effective statis in the gene pool, briefly punctuated with periods of rapid shift. While there are many mechanisms that move a population to the point of rapid change, they center around one phenomenon: it is very improbable that a single mutation of single allele will convey a strong survival advantage to the host. Multiple mutations are normally necessary to achieve a significant survival advantage. For illustration, let's assume a hypothetical wolf population. This population of wolves has hit a stable point: it is large enough to kill prey efficiently, and has sufficient endurance that it wins enough chases when a long-term chase is necessary to bring down prey. One wolf in the population mutates such that its size is larger. Now, it is better equipped to kill prey on the first try. But ... it requires more food to fuel its larger body mass, and tires faster on long chases. For the sake of the example, assume that this balances out as either neutral or slightly positive. Another wolf at some distant point mutates so that its muscle tissue is more efficient. At its current size, this is not much of an advantage, because the wolves were sustaining themselves without the greater efficiency. Still, the allele will slowly spread due to the slight survival advantage. These genes will spread slowly through the population. Natural selection for either allele is simply not strong enough to cause the gene pool to shift. Eventually, the genes will meet up: an individual will possess both mutations. Now, you have a larger wolf that doesn't tire quickly, and needs no more food than the rest of the population. Now, you have a strong advantage. This combination of alleles will quickly spread through the population, as the large efficient wolves beat out all other combinations in the quest for survival. Net result: the population sat relatively stable for a very long time, followed by an rapid shift. Many other combinations have similar behavior. Delicate sharp claws dull quickly without a retraction mechanism: dull claws work about the same whether they are retractable or not. Improvements in focusing capability may not be particularly useful without improved light sensitivity on the retina. A better leveraging point for a muscle/bone connection may not be useful without a lengthening of the bone to take advantage of it. In essence, the basis of the shift is not a shift in the mutation rate of the gene pool: it is that the survival advantage conveyed by a group of genes may be much larger than that conveyed by any single member of the group. Random drift will convey the single members: once grouped, natural selection will rapidly shift the population. Small founder populations are frequently invoked for PE. This is a different physical mechanism which produces the same effect. Take our example above, but assume both genes are recessive. In any large population, a recessive which is not being actively selected for will be relatively dormant: very few members would be homozygous for either allele. Now, if one pair of wolves each possessing one copy of one of the recessives gets isolated, there is a subpopulation where each of the good alleles has been artificially boosted to 25% penetration. The chances of an individual appearing which is homozygous for both recessive alleles within a few generations of inbreeding is very good. Once that happens, the same effect will take over. Same genetic mechanics, just a different set of population mechanics to act as a trigger. Kevin Wayne Williams


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