Natural Selection

Thu Jun 10 10:20:00 EST 1993


      Fisher described natural selection in terms of fitness (in mathematical
terms): The greater the genetic variability upon which selection for fitness
may act, the greater the expected improvement in fitness, i.e. the fitness of
a population should increase at a rate that is proportional to the genetic
variability or genetic differences in fitness in the population. Thus if a
population in a particular environment were completely homozygous for all
genes, selection for fitness in a changed environment would have little effect
and produce no genetic improvement. Since populations do not always face
constant environmental conditions, selection is not always for only one
optimal genotype and consequently genetic variability must be maintained for
the population to survive.

      Nature forms new species over a very long length of time (biological
time anyway) in combination with selection, mutation and recombination. The
interbreeding within a population enables that population to share a common
gene pool. At the same time a species may consist of numerous individuals with
various degrees of interbreeding between them. Populations that are widely
separated will have less opportunity to share their genepools than those that
live in closer proximity. The structure of a species is therefore broken into
various geographical subunits. Since the forces acting upon these subunits
will vary in different geographical locations one can observe differences
between populations. For example industrial melanism in Biston betularia,
Drosphilia pseudoobscura (Dobzhansky showed that frequencies of the third
chromosome arrangements are different in a range of environments across the
southwest USA.

      Thus race formation could be accelerated by barriers that reduce gene
exchange between populations. Such barriers can be geographic (eg deserts,
oceans, mountain ranges etc) and occur when populations separate from one
another and occupy different areas. The potential for gene exchange, however,
enables all these different populations to be considered as a single species.
Only when populations have achieved sufficient differences to inhibit gene
exchange (i.e. isolating mechanisms, Meyer separated into pre and postmating)
at all between them that they may be considered to have diverged sufficiently
to have reached the level of species.

      A number of books that might be useful (and I apologise if I have miss-
spelt any authors names): Strickberger-Genetics, books by Maynard-Smith,
Kimura and more recently Steve Jones has coauthored a book about evolution
(published I think by Cambridge University). 

Julian A Hiscox
I.A.H Compton

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