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Biology Unit 2
Chapter 7- Variation…read more

Slide 2

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· Living things differ.
· If one species differs from another it is called
interspecific variation.
Two different species of lizard.
· Members of the same species also differ ­
intraspecific variation.
Variation among the same species
e.g size.…read more

Slide 3

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Making measurements
· Sampling: Making sure the sample is
representative by:
· Not sampling by being bias
· Even if bias is avoided by chance a population may still not be representative
· Best way to eliminate bias to carry out random
· For example, number each sample and use numbers generated from a computer.
· We cannot eliminate chance but minimising is
· Use a large sample size
· Analyse the data collected ­ Statistical tests to show the extent of chance influencing
the data.…read more

Slide 4

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Causes of Variation
· Genetic factors:
· Mutations ­ sudden changes in genes and chromosomes.
· Meiosis ­ this form of nuclear division form the gametes therefore, genetic
material is mixed up.
· Fusion of gametes ­ offspring inherit characteristics of both parents. Asexual
organisms variety are only increased by mutations.
· Environmental factors:
· The genes set limits but it is the environment that determines where, within those
limits, an organism lies.
· Factors include; climatic conditions ( e.g. temperature, rainfall and sunlight) , soil
conditions, pH and food availability.
· It is very hard to distinguish between environmental and genetic factors as in most
cases variation is due to both. Due to this, it is difficult to draw conclusions and
any that are drawn are usually tentative and should be treated with caution.…read more

Slide 5

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Types of variation
· Variation due to genetic factors:
· Organisms fit into a few distinct forms and there are no intermediate types. E.g.
You are either blood group, A, B, AB or O.
· This type of variation is controlled by a single gene and is represented on a bar
chart or pie graph.
· Variation due to environmental influences:
· Characteristics overlap leading to a continuum. E.g. in humans height and mass.
· This type of variation is controlled by many genes (polygenes).
· Environmental factors play a major role ­ you may think due to genes you won't
grow more than your parents however due to environmental factors such as diet,
this is not true.
· When you plot this graph it is bell-shaped ­ normal distribution curve.…read more

Slide 6

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Mean and Standard Deviation
· A normal distribution curve differs in its
maximum height and its width.
· Mean :- it provides an average value and is useful when comparing one sample
with another.
· Standard Deviation :- is the measure of the spread of data (the width). It is the
distance from the mean to the point where the curve changes from being convex
to concave. 68% of measurements fall between this range and 95% for standard
deviation.…read more


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