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GCSE level exam revision notes on Evolution:
5.
Continuous variation and discontinuous variation
in animal characteristics and statistics
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Sub-index of biology notes on all aspects of EVOLUTION
(5) Continuous and discontinuous variation
in animal characteristics and statistical representations
Reminders:
Genetic variation
only results from changes
in the DNA - mutations.
Phenotypic variation results from both
genetic and environmental factors.
Variation is
the differences between individuals of the same species.
Variation can be continuous or discontinuous - illustrated by the two bar charts below.
Continuous variation - where the
characteristic of an individual varies over a wide range of
possibilities between two extremes, with no distinct categories - no gaps between
the bars, and each bar represents a narrow range e.g. 1.70 to 1.75 m
in height.
What we have is
a wide range of
phenotypes for a particular characteristic.
We see this clearly in human statistics of
height, weight, skin colour, intelligence where you observe a
continuous wide range of values.
The height of trees or their number of
leaves also show continuous variation.
A graph joining all the maximum values is called a
distribution curve.
Characteristics that are controlled by
several genes or are influenced by both genetic and environment
factors, often show continuous variation.
e.g. the height of a human or a tree
will be determined by both genetic inheritance AND how
'nutritious' the diet the organism receives.
Discontinuous variation - is where
there are two or more distinct categories. The value/type of
the characteristic must be in one of the categories, there is no
range of intermediate values/types.
This means have a limited number of phenotypes for a
particular characteristic.
The four human blood groups of A,
B, AB and O form a good example of a discontinuous variation of
a type of characteristic - there is no continuous range
of blood type, just 4 distinct options.
For example, in the UK, The percentage of
the population with the blood groups is A 42%, B 10%, AB 4% and
O 44%. Four distinct groups, with gaps between them, only 4
possibilities.
However, things like your white or red blood
cell count, may well show a continuous variation - especially as
they depend on your lifestyle e.g. the state of your health and
quality of your diet e.g. iron and vitamin intake.
Another human discontinuous example is the ability to
roll your tongue.
Characteristics determined by one gene and
not influenced by the lifestyle are likely to show a
discontinuous variation.
The colour of flowers or vegetables often shows discontinuous
variation e.g. just a few characteristic colours.
Eye colour is another example of discontinuous variation e.g.
irises
are classified as being one of six colours: amber, blue, brown,
grey, green, hazel, or red.
It is not influenced by lifestyle
conditions, but the colour is controlled by several genes.
Hand
dominance, right-handed or left-handed, is another example
of discontinuous variation.
Key points
Based on
the syllabus-specifications for students taking the AQA, Edexcel and OCR
GCSE level biology examinations (~US grades 9-10).
Key
ideas and examples on continuous variation and discontinuous variation
Revision notes
on continuous and discontinuous variation in animal
characteristics, along with their statistical importance in
understanding human biology.
Variation in
Animal Characteristics
Variation refers to
differences between individuals within a species. It can be classified
into continuous variation and discontinuous
variation based on how the traits are expressed.
Continuous
Variation
-
In continuous
variation, characteristics show a range of
values without clear, distinct categories.
-
It is controlled by
multiple genes (polygenic inheritance) and can be
influenced by environmental factors.
-
Examples include
height, weight, skin color, and foot size in
humans.
-
These traits are
quantitative, meaning they can be measured
numerically.
-
Continuous variation
is represented graphically by a bell-shaped (normal
distribution) curve, with most individuals falling in the
middle range and fewer at the extremes.
-
Examples in
animals:
-
Tail length in
mice
-
Fur thickness in
dogs
-
Body mass in
elephants
Discontinuous
Variation
-
In
discontinuous variation, characteristics fit into
distinct, separate categories with no intermediates.
-
Usually controlled by
a single gene (monogenic inheritance) with
little or no environmental influence.
-
Examples include
blood group, eye color, ability to roll the tongue, and
presence of a widow’s peak.
-
These traits are
qualitative, meaning they can’t be measured
numerically.
-
Discontinuous
variation is represented graphically by bar charts,
showing the number of individuals in each category.
-
Examples in
animals:
-
Fur color in
rabbits (e.g., black or white)
-
Presence or
absence of horns in cattle
-
Attached or
detached earlobes in humans
Statistical
Analysis and Its Importance in Human Biology
Understanding variation is
essential in human biology, genetics, medicine, and evolutionary
studies.
1.
Normal
Distribution and Continuous Variation
-
Continuous traits
often follow a normal distribution when measured
across a population.
-
Researchers can
calculate mean, median, mode, and standard deviation
to study how traits are spread.
-
Example:
Tracking average height changes in populations
helps in understanding nutritional health and genetic
influences.
2.
Chi-Square
Test in Discontinuous Variation
3.
Genetic and
Environmental Contributions
4.
Medical
Applications
-
Understanding
variation helps in diagnosing genetic disorders
(e.g., Down syndrome, sickle cell anemia).
-
Discontinuous traits
like blood type compatibility are crucial for
safe blood transfusions.
Variation plays a
fundamental role in natural selection, evolution, and
medical research. Continuous and discontinuous traits shape
the diversity seen in populations, influencing everything from
survival advantages to genetic inheritance patterns.
Summary of learning objectives and key words or phrases
-
Be able to describe and explain the differences between
continuous variation and discontinuous variation and their role in the
evolution of animals.
-
Be able to interpret and discuss statistical
representations of continuous variation and discontinuous variation.
-
 Be able to demonstrate an understanding of the
causes of variation, including:
-
You should know and understand that
particular genes or accidental changes in the genes of plants or animals may
give them characteristics which enable them to survive better.
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