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Variation
By
Jeanne Smith, Contributor
ORGANISMS
WHICH belong to the same species differ
in their characteristics. The occurrence
of varying characteristics allows
organisms to be adapted to various
environmental conditions, and is therefore
important for survival of the species.
Examples of variation include different
types of finger prints, skin colour
and height in humans; and in plants,
leaf size and flower colour.
There
are two types of variation, these
are:
*
Continuous variation: there are no
discrete types, there is wide range
of types between extremes, e.g. height
in humans or finger length.
*
Discontinuous variation: there are
discrete types with no gradations
between them. Examples of this are
tongue rolling, widow's peak, mid-digital
hair, sex and type of finger prints
(e.g. loop, arch, whorl, or double
whorl). The organism fits into definite
categories.
CAUSES
OF VARIATION
Some
variation is inherited, for example
sex, other forms are influenced by
the environment. There is therefore
genetic variation which is inherited,
and environmental variation, which
is influenced by environmental conditions.
Genetic
variation may occur as a result of:
*
Meiosis Meiosis is the process
of cell division by which gametes
are formed. In meiosis, random separation
of chromosomes, as well as crossing
over of chromosomes. Random separation
of chromosomes leads to new combinations
of chromosomes in the gametes. Crossing
over results in new combinations of
alleles on the chromosomes.
*
Random fertilisation Any two
gametes, one male, the other female
may fuse at fertilisation. The zygote
produced may have any of several possible
chromosome combinations. The offspring
produced by organisms which have several
genes, are therefore unlikely to be
identical.
*
Mutation A mutation is a change
in the amount, arrangement or structure
of an organism's DNA. Mutations may
produce new characteristics, which
may or may not persist. Mutations
may produce changes in the appearance
of a particular characteristic of
the organism.
ENVIRONMENTAL
VARIATION
The
genotype (gene combination of organism)
determines its characteristics. These
may be influenced by environmental
conditions. Genes determine for example
the height of an individual. However,
even with genes which would allow
the individual to be tall, growth
may be stunted without proper nutrition.
*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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