Download Selection on phenotype/behavior changes the underlying genetics

Selection on phenotype/behavior changes the
underlying genetics (allele frequencies)
Directional selection on allele frequency
FF has highest fitness
SS has lowest fitness
Selections alters the allele frequency of the pop.
The rate of change in allele frequency is determined
by the strength of selection (i.e., the fitness
differential of the phenotypes)
Intensity determines rate of increase in allele frequency.
Intensity depends on fitness differentials
Selection on Allele Frequency
• Directional selection—graph it
– One allele will be lost, the other will be “fixed”
• Balancing selection
Balancing Selection
• Allele frequency is maintained by natural
selection at some frequency between 0 and
1.0—Graph it
• Can be due to:
– heterozygote advantage,
– frequency dependent selection, or
– multiple niches
Malaria and the S allele
Balancing selection on allele frequency:
Due to heterozygote advantage
(overdominance)
Fitness w/malaria:
AA
0.9
AS
1.0
SS
0.1
AS has the highest fitness when malaria is present, so natural selection will preserve both
alleles
A and S never have equal frequency in a population, though
Balancing selection on A and S alleles
•
•
•
•
Frequency A stays at about 90%
Frequency S stays at about 10%
No fixation/loss of A or S because AS is best genotype
Why is the frequency of A > S?
Because fitness of
AA = 0.9 (pretty high)
SS = 0.1 (very low).
Therefore, AA will contribute more
alleles than SS.
Balancing Selection on Allele Frequency due to
frequency dependent selection
1 gene, two alleles, dominant allele  right mouthed
Balancing Selection on Allele frequency due to
multiple niches
Black-bellied
Seed crackers
Environment offers
large seeds and small
seeds, but not
intermediate seeds
Adaptation
• As a Process: the population as a whole
becomes better suited for its environment
over time as a result of natural selection
– The avg. fitness of the pop. increases
• As a Product: a feature of an organism that
has been molded by natural selection to suit
its current use
– Examples?
• “The sutures in the skulls of young mammals
have been advanced as a beautiful adaptation
for aiding parturition [birth], and no doubt
they facilitate, or may be indispensable for
this act; …
• …but as sutures occur in the skulls of young
birds and reptiles, which have only to escape
from a broken egg, we may infer that this
structure has arisen from the laws of growth,
and has been taken advantage of in the
parturition of the higher animals.”
Harvey and Pagel, 1991
• To be considered an adaptation, a
characteristic must be derived and have
evolved in response to a specific selective
agent
Why does the giraffe have a long neck?
Why are polar bears white?
Why are pronghorn so fast?
Pronghorn running
Why do birds have feathers?
Why is blood red?
“The Adaptationist
Programme”
Be careful about adaptationist
explanations…
1a) The “use” of a feature is not always obvious
(video)
Be careful about adaptationist
explanations…
1b) The “use” of a feature is not always obvious
(video)
Be careful about adaptationist
explanations…
2) The feature may have arisen in a different
environment
Cheetah in North America until 10,000 ya
Be careful about adaptationist
explanations…
3) Some features were originally selected for
another purpose (exaptation)
Feathers arose before
flight;
Heat retention?
Display?
Be careful about adaptationist
explanations…
4) Neutral theory:
some variation in nature [especially genetic] can be
selectively neutral
Be careful about adaptationist
explanations…
5) A feature may be a by-product of selection
for another trait
Why is blood red?
Color is a by-product of high iron content
 probably selectively neutral