Lecture 18. Fitness and Selection Flashcards
What is fitness ?
The ability to survive and reproduce
What must successful phenotypes be ?
Viable and fertile
How is fitness measured ?
As the total number of offspring or alleles an individual contributes to the next generation
What is the absolute fitness ?
A measure of the total number of offspring or alleles that individuals contribute to the generation
How do we measure relative fitness ?
Compare each individuals absolute fitness to a benchmark value
How do we measure genotype fitness ?
Calculate the average number of offspring left by individuals with each genotype
What is the relative contribution of a genotype a product of ?
Its frequency and relative fitness
What are the effects of selection on phenotype ?
- Phenotype distribution
- Genotype frequencies at a single loci
- New alleles
What are the three types of selection ?
- Stabilising selection
- Directional selection
- Disruptive selection
In directional selection (Dominant >= heterozygote >= recessive), what do two homozygotes have ?
Different fitnesses
In directional selection (Dominant >= heterozygote >= recessive), what does the heterozygote have ?
Either an intermediate fitness or a fitness equal to one of the homozygotes
In directional selection (Dominant >= heterozygote >= recessive), what does it result in ?
An increase in frequency of one allele at the expense of the other
What is the advantageous allele ?
An increase in frequency of one allele at the expense of the other
In directional selection (Dominant >= heterozygote >= recessive), what can it be a mechanism of ?
Directional or stabilising phenotypic selection, depending on whether the advantageous allele is a new mutation
In directional selection (Dominant >= heterozygote >= recessive), when does the rate of allele frequency change ?
Depending on starting frequencies and the fitness of the heterozygote
In directional selection (AA = Aa > aa), what does the heterozygote genotype have ?
The same fitness as the fitter homozygote
In directional selection (AA = Aa > aa), what happens when the advantageous allele is at low frequencies ?
It occurs mostly in heterozygotes where it can be selected for. This results in a rapid increase in frequency
In directional selection (AA = Aa > aa), where do high frequencies in the deleterious allele occur ?
Mostly in heterozygotes and cannot be selected against which slows down the rate of frequency change for the advantageous allele
In directional selection (AA = Aa < aa), what does the heterozygote genotype have ?
The same fitness as the less fit homozygote
In directional selection (AA = Aa < aa), when the advantageous allele is at low frequencies where does it occur ?
Mostly in heterozygotes where it cannot be selected for resulting in a slow increase in frequency
In directional selection (AA = Aa < aa), what happens to high frequency homozygote recessive genotypes ?
They become more frequent and can be selected for which speeds up the rate of change
AA > Aa > aa
Advantageous allele is co-dominant or incompletely dominant
AA > Aa < aa
Underdominance
AA > Aa < aa, what type of fitness does the heterozygote have ?
The lowest fitness
In directional selection (AA = Aa > aa), what does this result in ?
Unstable equilibrium
In directional selection (AA = Aa > aa), what type of mechanism is this ?
Disruptive selection
AA > Aa < aa
Underdominance
AA < Aa > aa what type of fitness is this ?
The heterozygote has the highest fitness and results in maintenance of both alleles
In directional selection (AA = Aa > aa), what type of mechanism is this ?
Stabilising selection
AA < Aa > aa
Heterozygote advantage or overdominance
AA < Aa > aa, what is an example ?
Sickle cell anaemia
AA > Aa > aa
Negative frequency dependent selection
AA > Aa > aa what happens to the fitness ?
It decreases as it becomes more common
What is a neutral allele ?
Have no effect on the organisms fitness and are invisible for selection and is determined by drift
What is purifying or negative selection ?
If new alleles at a locus are deleterious, selection will act to remoe them
What is positive selection or adaptive evolution ?
If a new allele at a locus is advantageous, it will act to increase its frequency
What are two major drivers of positive selection ?
- Diet and nutrients
2. Infectious disease ad immunity
