Complete revision notes for Topic 18 of the CIE IGCSE Biology course: Variation and selection. Explanations with diagrams for every specification point. These notes are written for candidates taking the Extended paper.
Define variation as differences between individuals of the same species
Distinguish between phenotypic variation and genetic variation
Phenotypic variation is the difference in features between individuals of the same species.
Some of these differences are caused by differences in genes, which is genetic variation.
State that phenotypic variation is caused by both genetic and environmental factors
Phenotypic variation can be caused in two main ways:
It can be genetic – controlled entirely by genes
Or it can be environmental – caused entirely by the environment in which the
organism lives
Environmental
Characteristics of all species can be affected by environmental factors such as climate, diet,
accidents, culture and lifestyle.
In this instance ‘environmental’ simply means ‘outside of the organism’ and so can include
factors like climate, diet, culture, lifestyle and accidents during lifetime.
Examples include:
An accident may lead to scarring on the body
Eating too much and not leading an active lifestyle will cause weight gain
Being raised in a certain country will cause you to speak a certain language with a
certain accent
A plant in the shade of a big tree will grow taller to reach more light
Continuous/discontinuous
Discontinuous variation is usually caused by genetic variation alone. Examples:
Blood group
Eye colour
Gender
Ability to roll tongue
Whether ear lobes are free or fixed
Continuous features often vary because of a combination of genetic and environmental
causes, for example:
Tall parents will pass genes to their children for height
Their children have the genetic potential to also be tall
, However, if their diet is poor then they will not grow very well
Therefore, their environment also has an impact on their height
State that continuous variation results in a range of phenotypes between two extremes, e.g.
height in humans
State that discontinuous variation results in a limited number of phenotypes with no
intermediates, e.g. tongue rolling
State that discontinuous variation is mostly caused by genes alone, e.g. A, B, AB and O blood
groups in humans
Phenotypic variation can be divided into two types depending on how you are able to group
the measurements:
Continuous variation is when there are very many small degrees of difference for a
particular characteristic between individuals and they are arranged in order and can
usually be measured on a scale
o Examples include height, mass, finger length etc. where there can be many
‘in-between’ groups
Discontinuous variation is when there are distinct differences for a characteristic
o For example, people are either blood group A, B, AB or O; are either male or
female; can either roll their tongue or not – there are no ‘in-betweens’
Record and present the results of investigations into continuous and discontinuous variation
When graphs of these data are plotted, continuous variation gives smooth bell curves (a
result of all the small degrees of difference), whereas discontinuous gives a ‘step – like’
shape:
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