Re: Natural Selection - Question

Willem de Winter (
24 Jan 1995 07:02:28 GMT (Brad Swanson) writes:

>Boy Richard, thanks for that indepth analysis of the situation. Any
>reason why it wouldn't be considered natural selection? It certainly is
>evolution, since evolution is the change in allele frequencies in a
>population. What extinction
>produces is a loss of all the alleles for that population. So evolution
>is occurring, the forces of evolution are genetic drift, mutation,
>assortative mating, migration, and natural selection. Hmm, which one of
>these fits, not drift, as the extinction certainly wasn't a chance event,
>wasn't a mutation, assortative mating couldn't do it, and the dispersal
>patterns probably wouldn't do it either. Looks like we only have one
>candidate left. That being said, it doesn't mean that we shouldn't worry
>about driving species extinct. We are changing the world at a much more
>rapid rate than new genetic material can be introduced to allow evolution
>via natural selection to act. With mutation rates of around 1e-6 (at
>least for nuclear non-repetative sequence DNA) we are asking the
>populations to respond at a faster rate then they physically can. Is it
>natural selection, I would argue not natural, the things we are doing are
>not natural (pollution, global warming, etc) but maybe artificial.
>However that is just in MHO. I suppose that it is not that different than
>the mass extinctions produced at the K/T boundary and the other really big
>extinctions. Rapid environmental change followed by mass die-offs as
>organisms failed to find the genetic variation quickly enough to handle
>the new situations. Anyways, a quick poll of the grad students here says
>it is natural selection.

I think the question would be answered differently by different
evolutionists. Some hold a broad view of natural selection and would argue
that the extinction of one species due to competion from another
constitutes species selection. Others apply more rigorous definitions
such as the one referred to by Brad, ie. directional change in allele
frequencies. However, I think Brad confuses absolute with *relative*
changes in allele frequencies. Natural selection in the narrow sense
refers to systematic changes in the relative frequencies of alleles at a
single locus. The problem we're talking about here involves a reduction
in population size due to competetion from another species. This does not
necessarily involve changes in the relative frequencies of alleles in the
gene pool of the population. Conceivably, these could remain unchanged
down to the 'last of the Mohicans' (though not very likely...). It is
important to distinguish such changes in the composition of a gene pool
from the elimination of the gene pool as such. Therefore, in the more
rigorous, genic or population genetic sense, I would say that such cases
do not constitute Natural Selection Proper.


Willem de Winter
Dept of Anatomy and Human Biology
University of Western Australia.