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Natural
selection
By
Jeanne Smith, Contributor
CHARLES
DARWIN proposed the theory of Natural
Selection even before much was understood
about heredity. He suggested that
the organisms which passed on their
characteristics to successive generations
were those which were best adapted.
Today, with knowledge of genetics,
we can say that genes which produce
advantageous characteristics are more
likely to be passed on to successive
generations than those which do not,
through natural selection.
Natural
selection ensures that organisms remain
well adapted to their natural habitats.
Such organisms have a greater chance
of survival, and are therefore likely
to reproduce, thus passing on their
advantageous characteristics to their
offspring. Those organisms which do
not possess these favourable characteristics
are less likely to survive and reproduce.
WHICH
ORGANISMS SURVIVE?
Organisms
which can adapt to environmental changes
also have a better chance of survival.
The more successful species are those
which are able to adapt to and live
in a range of habitats. Such organisms
are able to colonise new areas to
ensure survival if conditions become
unfavourable in their natural habitats.
Genetic
variation facilitates natural selection.
Variation ensures that there are various
phenotypes within the population.
This increases the chance that some
individuals will survive if there
is a disastrous event.
A
good example of natural selection
can be seen in the peppered moth,
Biston betularia, found in parts of
England. The moth is nocturnal, and
rests on tree trunk during daylight
hours. Prior to 1850 all peppered
moths collected had speckled wings,
this was a very effective camouflage
on the lichen-covered tree trunks.
However
the speckled variety would have been
quite conspicuous on a tree trunk
blackened by soot. A black (melanic)
form of the moth was first seen in
about 1850. The number of dark moths
increased rapidly in areas that were
highly polluted. The black moth had
an advantage in polluted areas, as
it was effectively camouflaged on
the blackened surfaces, and are less
likely to be predated by birds.
ARTIFICIAL
SELECTION
In
natural selection, nature selects
those characteristics which are passed
on from one generation to the next.
In artificial selection, man selects
these characteristics. Artificial
selection, also called selective breeding,
involves the selection and crossing
of varieties of domestic animals and
cultivated crops which have desirable
characteristics. Man is able to select
and breed these organisms until a
variety which is true breeding for
the desirable trait is obtained. These
traits desirable to man, may be of
little importance to the organism,
but are economically important to
man. Examples of these characteristics
are fruit size, fruit yield, appearance
of fruits, vegetables and animals
(for example dogs, goats or cattle),
milk and meat production in animals,
shorter breeding time and disease
resistance. These organisms are very
different from the wild types, and
may have lost some genes found in
wild varieties.
*Jeanne
Smith teaches Biology at the Queen's
school. Send your questions and comments
to the CXC Study Guide, the Gleaner
Company Ltd., 7 North Street, Kingston;
or email us at jcampbell@gleanerjm.com
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