Joseph E. Laferriere joseph at IRIS.CEAMISH.UAEM.MX
Tue Jun 17 07:34:23 CDT 1997

   With respect to paraphyly and polychotomies at the species level, one
must examine the situation at the level of population genetics. Most
species undergo stabilizing selection the vast majority of the time. If a
species is reasonably well-adapted to its environment, any mutations will
very likely be disadvantageous and eliminated from the gene pool very
quickly. The only time directional selection will be in effect is either
during periods of climatic change or if the species is invading a new
habitat. The same is even more true of chrosomal mutations and other
barriers to interbreeding.  Anything preventing a plant from mating with
members of its parent population is disadvantageous if the parent
population is already well-adapted to the environment. The only time
hybridization barriers would be advantageous would be if the plant
containing the chromosomal mutation contained some combination of genes
making it better-adapted than the parent population.  Again, this is
likely to occur only during times of climatic change or when a taxon is
invading a new habitat.
   Thus, it appears that polychotomies may be very common. Suppose for
example you have a species common across an entire continent, and
well-adapted to the conditions on that continent. Stabilizing selection
will ensure that the species changes slowly if at all. However, seeds
drifting to off-shore islands may experience different environments and
different selection pressures.  They will thus evolve in different
directions, perhaps rather quickly. There are numerous examples of this
in Hawaii and the Galapagos, complexes of species derived from common
and extant continental ancestors.

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