Transcript Population Genetics

Adaptation and Natural Selection
12.4 Warning coloration in a western coral snake
(Micrurus euryxanthus)
The Outcome of Natural Selection Depends Upon:
(1) Relationship between phenotype and fitness.
(2) Relationship between phenotype and genotype.
Determine the relationship between
fitness and genotype.
Outcome determines if there is evolution
12.2 Growth of 2 genotypes in an asexually reproducing
population w/ nonoverlapping generations
% survival to reproduction:
A = 0.05
B = 0.10
Fecundity (eggs produced):
A = 60
B = 40
Fitness A = 0.05 x 60 = 3
Fitness A = 0.05 x 60 = 4
R = Per Capita Growth Rate
= Represents Absolute Fitness
The rate of genetic change in a populations depends upon
relative fitness:
Relative Fitness of A = Absolute fitness A
Highest Absolute Fitness
WA
=
3/4 = 0.75
Often by convention, fitness is
expressed relative to the genotype
with highest absolute fitness.
Thus,
WB = 4/4 = 1.0
The fitness of a genotype is the average lifetime
contribution of individuals of that genotype to the
population after one or more generations, measured
at the same stage in the life history.
12.3 Components of natural selection that may affect
the fitness of a sexually reproducing organism
12.1(1) Modes of selection on a heritable quantitative
character
12.1(2) Modes of selection on a polymorphism
consisting of two alleles at one locus
Incorporating Selection
Individuals may differ in fitness because
of their underlying genotype
Genotype A1A1 A1A2 A2A2
Frequency p2
2pq q2
Fitness
w11
w12 w22
Average fitness of the whole population:
w = p2w11 + 2pqw12 + q2w22
Given variable fitness, frequencies after selection:
Genotype
A1A1
A1A2
A2A2
Freq
p2 w11
2pq w12
q2 w22
w
w
w
New allele frequencies after mating:
p2 w11 + pq w12
w
New Frequency of A1
pq w12 + q2w22
w
New Frequency of A2
Fitness: Probability that one’s genes will be represented
in future generations.
Hard to measure. Often, fitness is indirectly measured:
(e.g. survival probability given a particular genotype)
WAA
1
Fitness is often stated
in relative terms
WAa
1
Waa
1+s
Selection coefficient
gives the selection differential
Persistent Selection Changes Allele Frequencies
(i.e. Evolution by Natural Selection)
Strength of selection is given by the
magnitude of the selection differential
Selection Experiments Show Changes in Allele Frequencies
HW
Cavener and
Clegg (1981)
Food spiked
with ethanol
Selection can drive genotype frequencies
away from Hardy Weinberg Expectations
High frequency (Europe)
High selection/transmisson (Africa)
High frequency (Europe)
Low selection/transmisson (Europe)
Low frequency (Africa)
High selection/transmisson (Africa)
Predicted
change
in allele
frequencies
at CCR5