Modes of natural selection.
<P>
   Natural selection favors certain genotypes by acting on the phenotypes
   of individuals.
<P>
   The selection can take place in 3 possible modes.
<P>
<lI> Directional  selection  favors  individuals  at  one  end  of  the
phenotypic range.
<lI> Disruptive  selection  favors  individuals at both extremes of the
phenotypic range.
<lI> Stabilizing selection favors intermediate phenotypes.