Adaptation & Descent with
Modification as consequences of Natural
Selection
Consider a species
of garden snails that
displays a range of variation in shell colour, from very light to very
dark. As in
other snail species, shell phenotypes
are largely due to genotype (heritability). In two different populations,
natural selection proceeds in
two ways.
Under stabilizing selection, both extreme phenotypes (the lightest
and
darkest snails) are less successful in the struggle for existence than
the median type
[perhaps they are seen more easily by predators]. Selection against
these individuals leaves a decreased
range of variation (variance) relative to the parental population.
Under directional
selection, one extreme
phenotype is less successful and the other more succesful than average
[darker forms may be especially cryptic to predators, on a darker
surface]. Selection then results in a shifted
average (mean) relative to the parental population
The surviving individuals reproduce, in such a way
that population size remains
constant. Each population is
now better adapted
to its local circumstances, in
that
the character distribution of its members is such that they are are on
average more likely to survive and reproduce
than those in the preceding generation.
Since offspring resemble their parents, the next generation will have a
different character distribution than the preceding. Descent with
modification has
occured, such that each population has greater fitness. Further,
the populations have diverged from
each other: if continued indefinitely, the populations may over time
evolve into distinct species.
Figure modified from the original; text
material
© 2004 by Steven M. Carr