Download Natural Selection results in increase in one (or more) genotypes

Natural Selection results in increase in one (or more) genotypes relative to
other genotypes.
Fitness
- The fitness of a genotype is the average per capita lifetime contribution of
individuals of that genotype to the population after one or more generations,
measured at the same stage in the life history.
Example: parthenogenic weevils
Portion of eggs of genotype A that survive to reproductive age is 0.05. Each
adult lays an average of 60 eggs.
Fitness is (0.05) (60) = 3.
This is the per capita replacement rate (R), also called the Absolute Fitness.
For Genotype A, R = 3
For Genotype B, R = 4
Calculate Relative Fitness (w)
WA = 3/4 = 0.75
WB = 4/4 = 1.00
Absolute fitness of the
genotype divided by the
absolute fitness of the
most fit genotype.
Can also calculate average fitness of the population, w.
For the parthenogenic weevils,
frequency of A = p
frequency of B = q
w = (p) (wA) + (q) (wB)
Rates of change due to selection depend on w, not R. R can describe the rate of
increase in population size, not fitness.
Another result of selection is that the average fitness, w, increases.
Coefficient of Selection (s)
Measures the intensity of selection against a genotype.
sA = 1 - wi
s for the most fit genotype = 0
Modes of Natural Selection
• Directional
Selection for one extreme
•Stabilizing
Selection for the “average”
•Disruptive
Selection for both extremes
Directional Selection
One Gene models
• Selection for the dominant phenotype
• Selection for one homozygote with incomplete dominance
• Selection for the recessive phenotype
Selection for one homozygote with incomplete dominance
Genotype
Frequency
Fitness
AA
p2
1
AA’
2pq
1-(s/2)
A’A’
q2
1-s
p = (1/2)spq
(1-sq)
Selection will continue as long as the allele A’ is present.
Selection for the dominant phenotype
Genotype AA
Frequency p2
Fitness
1
AA’
2pq
1
p = spq2
1-sq2
p increases relative to q2 rather than q.
A’A’
q2
1-s
Natural Selection predicts a low level of genetic variability in populations.
However, populations tend to have relatively high levels of genetic variablity.
Polymorphisms and Variation
Why might we observe polymorphisms?
Transitional polymorphisms - one allele is replacing
another
Balanced polymorphisms - stable frequency with both
alleles present.
Balancing selection – heterozygote advantage
Selection in a variable environment
A fluctuating environment may favor different genotypes at different times.
Temporal fluctuations in the environment may slow down fixation due to
selection, but it will generally not preserve both alleles.
Spatial variations, with a mosaic of resources is more likely to maintain
polymorphisms (multiple-niche polymorphism).
Protein Polymorphism
Protein polymorphism in Drosophila natural populations is about 42%.
Extend this to all genes, then there are about 3000 polymorphic loci. This amount can
not be maintained by balanced polymorphisms.
Segregational Load
Most individuals would be less than optimally fit at many loci, greatly
reducing the average fitness of the population. Unrealistic that polymorphisms are
maintained by balance.
Two possible explanations
Unrealistic for population to be at maximum
possible fitness.
Different alleles are selectively neutral.
(Neutral Theory)
Solutions:
Combination of factors
- neutral
- selection in a heterogeneous environment
- hitchhiking
- selection for heterozygote
Hard Selection vs. soft selection
Hard Selection – basically, the organism is either equipped to survive in a particular
environment, or it is not.
Soft Selection – organisms of different genotypes can survive, but they will compete
with each other.
Hard Selection can promote speciation.
Soft Selection can promote multiple-niche polymorphisms.
Frequency Dependent Selection
Fitness depends on the frequency of a particular phenotype (genotype) in the
population.
Inverse frequency-dependent selection –
The rarer a genotype is in the population, the greater its fitness. This process can
easily maintain polymorphisms.
Positive frequency-dependent selection –
The fitness of a genotype increases as its frequency in the population increases.
Natural Selection with multiple loci
Each locus affects different characters. These loci should evolve independently of
each other.
However, they may be linked ort here may be a functional relationship.
The “Adaptive Landscape”
Sexual Selection
Success in mating (and producing offspring) is based on features that are not
necessarily associated with other aspects of fitness.
Different strategies for males and females
• Females produce a limited number of gametes and therefore there is a high
cost to inappropriate matings.
• Males produce a large number of gametes and there is little cost to
inappropriate matings.
Intrasexual selection
Intersexual Selection
•Mate choice, usually female choice.
Benefits of sexual selection