Cambridge International A Level Biology Revision Guide

Genetic variation, whether caused by the reshuffling
of alleles during meiosis and sexual reproduction or by
the introduction of new alleles by mutation, can be passed
on by parents to their offspring, giving differences in
phenotype. Genetic variation provides the raw material
on which natural selection can act. Variation within a
population means that some individuals have features
that give them an advantage over other members of that
population. Variation in phenotype is also caused by the
environment in which organisms live. For example, some
organisms might be larger than others because they had
access to better quality food while they were growing.
Variation caused by the environment is not passed on by
parents to their offspring.
Number of individuals
Chapter 17: Selection and evolution
In continuous (quantitative) variation:
■■
■■
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b
different alleles at a single gene locus have small effects
on the phenotype
different genes have the same, often additive, effect on
the phenotype
a large number of genes may have a combined effect on
a particular phenotypic trait; these genes are known
as polygenes. Measurement
QUESTION
17.1 Explain why variation caused by the environment
cannot be passed from an organism to its offspring.
Continuous and discontinuous variation
Phenotypic differences between you and your friends
include qualitative differences such as blood groups and
quantitative differences such as height and mass.
Qualitative differences fall into clearly distinguishable
categories, with no intermediates – for example, you have
one of four possible ABO blood groups: A, B, AB or O.
This is discontinuous variation.
In contrast, the quantitative differences between your
individual heights or masses may be small and difficult to
distinguish. When the heights of a large number of people
are measured, there are no distinguishable height classes.
Instead there is a range of heights between two extremes
(Figure 17.2). This is continuous variation.
The genetic basis of continuous and
discontinuous variation
Both qualitative and quantitative differences in phenotype
may be inherited. Both may involve several different genes.
However, there are important differences between them.
In discontinuous (qualitative) variation:
■■
■■
different alleles at a single gene locus have large effects
on the phenotype
different genes have quite different effects on the
phenotype.
Number of individuals
a
Number of individuals
Measurement
Measurement
Figure 17.2 a Distribution curve and b a histogram, showing
continuous b
variation.
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