Download Types of Selection!

 Polygenic
traits
 traits controlled by more than one gene
that show continuous variation
 are susceptible to environmental
influences, as a shift in the environment
can lead to a corresponding shift in the
phenotypes of organisms.
 Natural
selection can affect these
phenotypes in one of three ways…
 Let’s
imagine Darwin’s finches on the Galapagos
Islands as an example.
 There is slight variation in beak length among the
finches, ranging from…
-small beaks (for obtaining small seeds)
-medium beaks (for obtaining medium seeds)
-large beaks (for harder, thick-shelled seeds)
 Imagine
that the environment changes and the
supply of small and medium-sized seeds runs low.
Which beak size will be favored?
 Those birds with the larger beak will be able to
survive on the large seeds!
 In “directional selection,” natural selection favors
a characteristic on the extreme of the
phenotypic variation.
 How does this affect variation?
 Variation of beak size shifts in ONE direction.
Original
distribution of
beak sizes
How
distribution of
beak sizes
change due to
a changing
environment
 Imagine
that the environment changes and the supply
of small and large-sized seeds runs low. Which beak
size will be favored now?
 Those birds with the medium beak will be able to
survive on the medium seeds!
 In “stabilizing selection,” natural selection favors
characteristics near the middle of the phenotypic
distribution.
 How does this affect variation?
 Variation of beak size shifts in towards the middle,
decreasing genetic variation at the extremes.
 Imagine
that the environment changes and only the
supply of medium-sized seeds runs low. Which beak
size will be favored now?
 Those birds with BOTH the smaller and larger
beak will be able to survive!
 In “disruptive selection,” natural selection favors
characteristics at BOTH extremes of the
phenotypic variation.
 How does this affect variation?
 Variation shifts to favor two subgroups of birds
specialized in eating different sized seeds.
Original
distribution of
beak sizes
How
distribution of
beak sizes
change due to
a changing
environment
In
a population of plants,
flowers with the brightest color
might be selected for in order to
attract the most pollinators
whereas plants of moderate
color or dull color are selected
against.
 There
are two types of seeds to eat
for a population of birds. Either of
two different beak shapes (sharp or
blunt) might be selected for, but a
beak that's the average of the two
shapes might not be particularly
good at eating either seed, so it
would be selected against.
Bright
coloration of many
species of birds evolved
over time to help males
find a mate
Human
height varies,
however most humans tend
to be of average height.
Abnormally tall or short
humans are rare
 The
African butterfly, Pseudacraea
eurytus, has colorations ranging from a
reddish yellow to blue. Different ends of
the color spectrum look like (mimic) other
species of butterflies that are not
normally the prey of other local
predators. Those butterflies that are
moderate in coloration are eaten in far
greater numbers than those at the
extreme ends of the color spectrum
Antlers
on deer
evolved to help males
acquire a mate