Download Directional selection

• Natural selection: Differential reproduction
by genetically diverse organisms.
• The driving force in evolution, it leads to
greater adaptation in of organisms to their
environment.
• If sub-populations are found in
substantially different environments then
selection can lead to genetic divergence
and speciation.
• Both genetic drift and natural selection
tend to erode variation.
• However, natural selection is non-random
and creates order by increasing the
adaptedness of the population.
• There are three primary modes of
selection: Directional, stabilizing and
disruptive selection.
• Directional selection: One extreme has
higher fitness than the other and the mean
phenotype shifts after selection
• Example 1: The evolution of cryptic
coloration in Biston betularia (Kettlewell
1959).
• Example 2: A shift in bill size in Darwin’s
medium ground finch Geospiza fortis Boag
and Grant (1981).
• Example 3: The evolution of pesticide resistance in
insects.
• 1) Human introduction of new selective pressures, can
provide dramatic illustration of the power of selection:
antibiotics, pesticides.
•
2) Introduction of DDT brought a rapid evolutionary
response:
•
a) Introduced in WWII by the 1946 there was
resistance in houseflies.
•
b) In the Korean War used against body louse, it
evolved resistance within a year.
•
c) By 1965, 165 species had evolved resistance.
• Stabilizing or normalizing selection.
• a) The extremes of a distribution are selected
against, and the mean is at a selective
advantage.
•
b) The most common form of selection.
• c) There are trade-offs involved in most traits.
The advantage that you get from some extreme
of a characteristic is balanced by some negative
effects. Size is a good example
• Stabilizing selection Example 1: Human
birth weights. Mean birth weights have the
highest survival.
Selection Against Both Homozygotes
• Stabilizing selection Example 2: Selection on gall size in
the goldenrod ball gall-inducer Eurosta solidaginis (Weis
et al. 1992).
•
a) Larvae develop inside the galls and are attacked
by several natural enemies.
• Eurytoma gigantea has a short ovipositor and attacks
small galls exerting selection for larger gall size .
•
Eurytoma obtusiventris attacks medium size galls.
•
Birds attack large galls, selecting for smaller gall
size.
• b) Selection differs at different times and
location depending on the prevalence of
different natural enemies. For example, if
there are trees nearby there is more bird
predation. When parasitoids are more
prevalent they can produce directional
selection.
•
c) The opposing selection forces create
normalizing selection.
• Disruptive selection: There is selection
against the mean, and both extremes have
higher fitness. The mean may not change,
but the variance increases.
• Disruptive selection in African swallowtail
butterfly, Papilio dardanus (Shepard 1961).
•
a) The butterfly mimics several different
inedible models.
•
b) There is a polymorphism in the banding
pattern.
• c) Intermediate phenotypes do not look like any
model, and so they are selected against.
• d) Genes for the polymorphism are close
together on the chromosome forming a tight
linkage group a “supergene”.
• Disruptive selection in African swallowtail
butterfly, Papilio dardanus (Shepard 1961).
•
a) The butterfly mimics several different
inedible models.
•
b) There is a polymorphism in the banding
pattern.
• c) Intermediate phenotypes do not look like any
model, and so they are selected against.
• d) Genes for the polymorphism are close
together on the chromosome forming a tight
linkage group a “supergene”.
In the case of a trait that is determined by a single gene with two alleles,
there are five combinations of genotypes that nature can select:
1.
either homozygote (AA or aa but not both)
2.
both homozygotes (AA and aa)
3.
either homozygote and the heterozygote (AA and Aa or aa and Aa)
4.
the heterozygote (Aa)
5.
all alleles (AA, Aa, and aa)
Selection against one of the
homozygotes (aa)
Expected
offspring genotypes
Possible
parent mating
patterns
AA
AA X AA
4
AA X Aa
2
2
Aa X AA
2
2
Aa X Aa
1
2
1
Total
9
( 56% )
6
( 38% )
1
( 6% )
Aa
aa
.
Selection against both homozygotes (AA and aa)
Expected
offspring genotypes
Possible
parent mating
patterns
AA
Aa
aa
Aa X Aa
1
2
1
Total
0
( 0% )
2
( 100% )
0
( 0% )
."
Selection against the heterozygote and one of
the homozygotes (Aa and aa)
Expected
offspring genotypes
Possible
parent mating
patterns
AA
AA X AA
4
Total
4
( 100% )
Aa
aa
0
( 0% )
0
( 0% )
There are at least 5,000 genetically inherited human abnormalities and diseases.
Apparently, many, if not most, of them are caused by recessive alleles.
Selection against the
heterozygotes (Aa)
Expected
offspring genotypes
Possible
parent mating
patterns
AA
AA X AA
4
Aa
AA X aa
4
aa X AA
4
aa X aa
Total
aa
4
4
( 25% )
8
( 50% )
4
( 25% )
Selection against the heterozygotes (Aa)
Expected
offspring genotypes
Possible
parent mating
patterns
AA
AA X AA
4
Aa
AA X aa
(4)
aa X AA
(4)
aa X aa
Total
aa
4
4
( 50% )
0
4
( 50% )