>One cannot argue with the data, so it must have happened. Why it
>happened, seems to require more that a simple example of how one
>randomly selected line can die out. I can generate examples where
>each line should increase without limit, unless some competition
>exists with limited resources ---- then it is easy.<
The limits to total population size provide the limiting resources.
The basic assumption is that each starting individual has a certain
probability of contributing genes to the next generation, which has a
certain size. Even if there is no difference in evolutionary merit
between two alleles in a population, random fluctuations in
reproductive success will produce varying proportions of the alleles
in the next generation. Such random walks in a finite population
will eventually result in the disappearance of alleles. Thus, a
small population loses alleles rapidly whereas large populations
change slowly. Even negative alleles have some chance of becoming
established. The relative benefit conveyed by the allele, the
initial proportions of the alleles, the rate of mutation, and the
population size (including any changes it is undergoing) are among
the important factors.
Graur and Li, Molecular Evolution, Sinauer Press have a good
discussion if you can get ahold of a copy. It is expensive but might
be in university libraries.
The analogy of surnames is good in that it is fairly obvious that
little selective pressure exists, yet changes clearly take place. A
standard example comes from Johann Sebastien Bach. Despite his
having about 20 kids, no one since the mid-1800's has inherited the
Bach name from his descendants (nor his Y chromosome).
Dr. David Campbell
Old Seashells
University of Alabama
Biodiversity & Systematics
Dept. Biological Sciences
Box 870345
Tuscaloosa, AL 35487 USA
bivalve@mail.davidson.alumlink.com
That is Uncle Joe, taken in the masonic regalia of a Grand Exalted
Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at
Droigate Spa
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