Download Natural Selection on Polygenic Traits

16-2 Evolution as Genetic Change
Natural Selection on
Polygenic Traits
Natural Selection on Polygenic Traits
How does natural selection affect
polygenic traits?
?
Slide
1 of 40
Copyright Pearson Prentice Hall
End Show
16-2 Evolution as Genetic Change
Natural Selection on
Polygenic Traits
Natural selection can affect the
distributions of phenotypes in any of
three ways:
• directional selection
• stabilizing selection
• disruptive selection
Slide
2 of 40
Copyright Pearson Prentice Hall
End Show
16-2 Evolution as Genetic Change
Natural Selection on
Polygenic Traits
Directional Selection
When individuals
Wh
i di id l att one end
d off th
the curve h
have
higher fitness than individuals in the middle or at
the other end,
end directional selection takes place.
place
The range of phenotypes shifts as some
individuals survive and reproduce while others do
not.
Slide
3 of 40
Copyright Pearson Prentice Hall
End Show
16-2 Evolution as Genetic Change
Natural Selection on
Polygenic Traits
In this case, birds with larger beaks have higher
fitness. Therefore,, the average
g beak size increases.
Slide
4 of 40
Copyright Pearson Prentice Hall
End Show
16-2 Evolution as Genetic Change
Natural Selection on
Polygenic Traits
Stabilizing Selection
When individuals
Wh
i di id l near th
the center
t off th
the curve
have higher fitness than individuals at either end
of the curve,
curve stabilizing selection takes place.
place
This keeps the center of the curve at its current
position but it narrows the overall graph
position,
graph.
Slide
5 of 40
Copyright Pearson Prentice Hall
End Show
16-2 Evolution as Genetic Change
Natural Selection on
Polygenic Traits
Human babies born at an average mass are more
likely
y to survive than babies born either much smaller
or much larger than average.
Slide
6 of 40
Copyright Pearson Prentice Hall
End Show
16-2 Evolution as Genetic Change
Natural Selection on
Polygenic Traits
Disruptive Selection
When individuals
Wh
i di id l att th
the upper and
d llower ends
d off
the curve have higher fitness than individuals near
the middle,
middle disruptive selection takes place.
place
If the pressure of natural selection is strong
enough and long enough
enough, the curve will split
split,
creating two distinct phenotypes.
Slide
7 of 40
Copyright Pearson Prentice Hall
End Show
16-2 Evolution as Genetic Change
Natural Selection on
Polygenic Traits
If average-sized seeds become scarce, a bird
population
p
p
will split
p into two g
groups:
p one that eats
small seeds and one that eats large seeds.
Slide
8 of 40
Copyright Pearson Prentice Hall
End Show