Types of Natural Selection
effect of
The natural selection allele
frequencies depends the selection the population
-
on on
pressures acting on .
There 3 types selection disruptive selection
stabilising selection directional
:
and
-
are
Year 2 Bio AQA
.
only
,
& This is in
%
"
stabilising selection
Directional Selection Disruptive selection ¥
""" ÷
÷÷÷ . . ÷÷¥
→
extreme
→ selection for →
Selection for individuals with
Selection against one extreme extreme
phenotypes .
phenotype .
phenotypes rather than those with
Preserves of of of
phenotype changes the
phenotypes phenotypes around the
'
mean
average a
•
a
population by favouring average population by favouring phenotypes
the population .
of selection
individuals . that
vary
in one direction from the
-
The opposite stabilising
mean of the
population .
Example : Fur
length in mammals
( SPEC EXAMPLE : Human birth rates ) Example : Bird Beak Size
Example : Cheetah speeds
Optimum fur
( SPEC Example : Antibiotic
I resistance )
¥- , -
"
^
total
jE Population
,
selection "" -9
-9 o
pressure total
jessore
i o
o
± , , I
I o
population
→
*
.
-
breeding É "" "
"
I ¥ ,
,§
$ ± " " "" "
" " " "" "s
g.
:
'
4 ⑦
, ☐
&
'
'
I →
0.5 1.0 I. 5 2.0 2.5 E
fur length / on
§ £
I >
LL
>
small large
slow fast µ,
i sii:
i. ,
if
É i
n ^
±
.
*
§
I §
µµ
o '
no 5
j
É , H
•
±
I 9
o.s-i.io 1.5 2.0 as
%
fur length /on B
I
§£
→
I §
1-
of fur
Initially wide
range length .
I
small
1 >
fur
1 large
Extreme lengths are maintained beak size
slow fast
by rapid breeding in
years
when
speed
average temperature is warmer or
colder 1
Individuals that have alleles for 1 Birds with and small beak
.
large
2 When temperature is
consistently increased speed are more likely to alleles more likely to survive as
around 10°C individuals with catch than slower individuals environment of
, very prey . consists
only large
long or
very short fur are
2 More likely to survive ,
reproduce and and small seeds .
from alleles More
likely
2 to
eliminated population .
pass on their . survive,
reproduce
3
Frequency of alleles for high speed and
pass
on their alleles .
increases .
3
Frequency of alleles for large and
small beaks increases .
effect of
The natural selection allele
frequencies depends the selection the population
-
on on
pressures acting on .
There 3 types selection disruptive selection
stabilising selection directional
:
and
-
are
Year 2 Bio AQA
.
only
,
& This is in
%
"
stabilising selection
Directional Selection Disruptive selection ¥
""" ÷
÷÷÷ . . ÷÷¥
→
extreme
→ selection for →
Selection for individuals with
Selection against one extreme extreme
phenotypes .
phenotype .
phenotypes rather than those with
Preserves of of of
phenotype changes the
phenotypes phenotypes around the
'
mean
average a
•
a
population by favouring average population by favouring phenotypes
the population .
of selection
individuals . that
vary
in one direction from the
-
The opposite stabilising
mean of the
population .
Example : Fur
length in mammals
( SPEC EXAMPLE : Human birth rates ) Example : Bird Beak Size
Example : Cheetah speeds
Optimum fur
( SPEC Example : Antibiotic
I resistance )
¥- , -
"
^
total
jE Population
,
selection "" -9
-9 o
pressure total
jessore
i o
o
± , , I
I o
population
→
*
.
-
breeding É "" "
"
I ¥ ,
,§
$ ± " " "" "
" " " "" "s
g.
:
'
4 ⑦
, ☐
&
'
'
I →
0.5 1.0 I. 5 2.0 2.5 E
fur length / on
§ £
I >
LL
>
small large
slow fast µ,
i sii:
i. ,
if
É i
n ^
±
.
*
§
I §
µµ
o '
no 5
j
É , H
•
±
I 9
o.s-i.io 1.5 2.0 as
%
fur length /on B
I
§£
→
I §
1-
of fur
Initially wide
range length .
I
small
1 >
fur
1 large
Extreme lengths are maintained beak size
slow fast
by rapid breeding in
years
when
speed
average temperature is warmer or
colder 1
Individuals that have alleles for 1 Birds with and small beak
.
large
2 When temperature is
consistently increased speed are more likely to alleles more likely to survive as
around 10°C individuals with catch than slower individuals environment of
, very prey . consists
only large
long or
very short fur are
2 More likely to survive ,
reproduce and and small seeds .
from alleles More
likely
2 to
eliminated population .
pass on their . survive,
reproduce
3
Frequency of alleles for high speed and
pass
on their alleles .
increases .
3
Frequency of alleles for large and
small beaks increases .
