Download Ch. 13 Population Genetics

Variation, Change, Mutation &
Allele frequencies
Variation
Remember: population =
group of organisms from
the same species
Within a population many different
forms of the same species exist – this
is referred to as variation.
 Variations can be classified as:

Structural
 Behavioural
 Biochemical
 Developmental
 Physiological
 Geographic

Using pages 465-470 of
your text book, find at least
one example for each type
of variation
Monomorphic Vs. Polymorphic

A population that shows no variation
for a particular trait is said to be
monomorphic What do you think the term ‘polymorphic
population’ would mean?

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Continuous Vs. Discontinuous

Continuous variation is the type of
variation where members of a
population vary in a particular trait
across a continuum What do you think the term
‘discontinuous variation’ would mean?

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Preferred Writing Hand
Does this trait
demonstrate
continuous or
discontinuous
variation?
70
Frequency (percentage)
60
50
40
How many
genes are likely
to control this
trait?
30
20
10
0
Left
Right
Trait
Ambidextrous
Frequency (percentage)
Adult Male Human Height
16
14
12
10
8
6
4
2
0
Does this trait
demonstrate
continuous or
discontinuou
s variation?
How many
genes are likely
to control this
trait?
160
170
180
190
Height (centimetres)
200
210
Causes of Variation
– Nature Vs. Nurture

Causes of variation may be:

Environmental (nurture) – these are
caused by variations in environmental
conditions such as:
 pH,
water availability, light intensity in plants
 Nutrition, childhood illnesses in humans

Genetically inherited (nature) – these are
caused purely by genetics irrespective of
environmental factors eg.
 Genetic
disorders
Causes of Variation
– Nature Vs. Nurture

Imagine a population of imaginary
creatures
A
Individual A (red feet)
– has been living in
temperatures below
5ºC
B
Individual B – has at
least 1 parent with
vertical stripes on its
legs
C
Individual C – is not
fully mature yet whilst
individuals A and B
are adults
Causes of Variation
– Nature Vs. Nurture

Which variations are probably caused
by environmental factors?
A
Individual A– has
been living in
temperatures below
5ºC
B
Individual B – has at
least 1 parent with
vertical stripes on its
legs
C
Individual C – is not
fully mature yet whilst
individuals A and B
are adults
Causes of Variation
– Nature Vs. Nurture

Which variations are caused by
inherited factors?
A
Individual A – has
been living in
temperatures below
5ºC
B
Individual B – has at
least 1 parent with
vertical stripes on its
legs
C
Individual C – is not
fully mature yet whilst
individuals A and B
are adults
Causes of Variation
– Nature Vs. Nurture

What traits are monomorphic in this
population? What traits are polymorphic?
A
Individual A – has
been living in
temperatures below
5ºC
B
Individual B – has at
least 1 parent with
vertical stripes on its
legs
C
Individual C – is not
fully mature yet whilst
individuals A and B
are adults
Causes of Variation
– Nature Vs. Nurture

Identical (monozygotic) twins – clones
fertilisation

Genotypes are identical

Variations still exist between twins

Are these variations caused by
environmental or genetic factors?
What do you think monozygotic means?
zygote
Inherited Variations

Can be:
Monogenic – controlled by alleles of only
one gene
 Polygenic – controlled by more than one
gene


Which of these forms of inherited
variations are likely to result in
continuous variation?
Mutations

Mutations that occur in germ-line cells
are a source of new genetic variations

Produces new alleles

Can be spontaneous or induced

What are induced mutations?
Spontaneous?
The Gene Pool

Total genetic information that exists
within a population

This is usually described in terms of
allele frequencies per trait valued
between 0 and 1
Allele frequencies in the gene pool
ww
Ww
W
W
Ww
W
W
ww
W
W
W
W
W
W





ww
Total number of alleles = 12 x 2 = 24
Number of w alleles = 10
Number of W alleles = __
Frequency of w alleles = 10/24 = 0.4
Frequency of W alleles = __/24 = __
W
W
ww
Can be written as freq (w)
or freq (W)
p is usually used to
symbolise the frequency of
the dominant allele and q,
the frequency of the
recessive allele
Hardy-Weinberg Principle

Allele frequencies tend to stay constant
from generation to generation as long
as:
Population is large
 Random mating occurs
 All matings are equally fertile
 Population is closed (no migration)


Until an agent of change is introduced
Agents of Change

These can be:

Selection – competition causes the
individuals best suited to their chosen
habitat/environment to survive

Migration – this can add or remove alleles
from the gene pool

Chance – unpredictable events