TYPES
OF NATURAL SELECTION
The chapter EVOLUTION in NCERT Biology text of
class 12 hints that there three types of natural selection happening in nature:
Stabilizing, Directional and Disruptive . Apart from putting three graphs related to these types of selections,
no examples are given to illustrate the same.
Lets have some simple examples so that we have a
clearer concept of Stabilizing, Directional and Disruptive selection
processes
a) Stabilizing selection:
more individuals acquire mean character
value of a phenotypic range. In other words, individuals with average
phenotypic value of a phenotypic range are selected and those with minimum and
maximum values are not selected by nature. If this selection process continues
in population, more and more individual with average phenotypic values will
remain and others will be perished. The populations “stabilizes” in terms of a particular phenotypic value, we
can say. (Simply think of the
stabilizers we use with televisions and air conditioners which avoids both
highs and lows of the electric current and provides only the steady average
value to the equipment )
Example for Stabilizing selection:
A classic example of this is human birth
weight. Babies of low weight lose heat more quickly and get ill from infectious
diseases more easily, whereas babies of large body weight are more difficult to
deliver through the pelvis. Infants of a more medium weight survive much more
often. For the larger or smaller babies, the baby mortality rate is much higher
B) Directional selection: In this case , more individuals acquire value other than
the mean character value of a phenotypic range . It is the selection in which
individuals with phenotypic values towards one end of a range are selected that is either
those with maximum value or minimum but never those with mean or average value
.
Example for directional selection:
Assume that we do breeding of dogs and while
selecting parents for the next generation , our focuss is on ability to run
very fast, we will selects dogs that can run very fast while those with average
and slow speed will be avoided . ( If we
select slow runners , it will be also an example for directional selection, but those dogs will never be good guards
for us!)
Disruptive selection : more individuals acquire peripheral character value at both
ends of the
distribution curve. In simple terms, nature selects individuals with phenotypic
values from both ends of a range, only those with average values are discarded.
Example for Disruptive selection:
Limpets
Limpets are aquatics snails with three
colors of shells: Dark , Grey and pure white. These colors represent a phenotypic
range . They rest on either of the two different habitats : Dark rocks, or white colored barnacles . So dark shelled
limpets resting on dark rocks and pure white limpets resting on white barnacles
are easily escaped from predators while grey colored limpets gets no such a camouflage
and are easily predated. So nature selects Dark and white limpets which occupy
at two end of a range and grey colored ones in the middle are not selected.
Nice sir
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