Lecture 8 - Selection Flashcards Preview

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What are the different types of selection?



What are the prerequisites for selection?

-variation in fitness between genotypes
-variation needs to have a genetic basic (be heritable)


What are the features of selection?

-non-random process
-can lead to adaptation
-simplest form: reduces genetic variation


What are the features of adaptation?

-difficult to show that it is the response to selection (mathematical model)
-often a slow process, longer than a human life span


What examples are there of adaptation and selection?

-the increase of Methacillin-resistant Staphylococcus aureus in samples from hospitals
-the peppered moth Biston bistularia


Outline the MRSA example of adaptation and selection

-MRSA infections now respond only to very advanced antibiotics that were never meant to be a first-line defense
-number of MRSA and MSSA samples from hospitals has dramatically increased
-rate of MRSA infections recorded at U.S. academic hospitals doubled between 2003 and 2008
Peppered moth Biston bistularia


Outline the peppered moth example of adaptation and selection

-Before the industrial revolution in Britain, most peppered moths were of the pale variety
-camouflaged against the pale birch trees mutant black coloured moths were easily spotted and eaten by birds
-white variety had an advantage, and they were more likely to survive to reproduce.
-Airborne pollution in industrial areas blackened the birch tree bark with soot
-the mutant black moths were now camouflaged, while the white variety became more vulnerable to predators
-black variety had an advantage, and they were more likely to survive and reproduce
-Over time, the black peppered moths became far more numerous in urban areas than the pale variety.


What factors does the response to selection depend upon?

Strength of selection (dependent upon fitness differences between genotypes)
Allele frequencies
Dominance of the allele
Population size


What are the two types of fitness?

Absolute fitness
Relative fitness


What is the definition of absolute fitness?

Absolute fitness is the genotype specific rate of increase that predicts the absolute number of individuals


What is the definition of relative fitness?

Relative fitness is the fitness relative to one chosen genotype, normally relative to the fittest genotype


What does the basic selection model assume?

Diploid organisms
One locus with two alleles
Sexual reproduction
Non overlapping generations
Random mating
Viability selection
Population in Hw equilibrium
An ideal population (infinite size, no population structure, no mutation)


What are the symbols for relative fitness and absolute fitness?

AA: w1
Aa: w2
aa: w3

Average fitness of a population is w bar


What do the genotype and allele frequencies after selection depend upon?

The initial allele frequencies
The fitness of each genotype
The average fitness in a population


What are the formula for genotype frequencies after selection?

(P^2 x w1) / w bar (total fitness)


What are two implications on the change of allele frequencies?

No change when p=0 or q=0, ie when there is only one allele in the population
No change when all the genotypes have the same fitness


Give an example of the change in allele frequencies

The scarlet tiger moth callimlrpha Dominula
-the medionigra morph reduces spotting on the forewing
-only found in the cothill population
-the morph declined rapidly in the cothill population


Aside from being expressed in terms of w, how else can relative fitness also be expressed?

In terms of a selection coefficient s
S is the difference between 1 and the relative fitness value of the genotype


What is the genotype specific fitness of AA, Aa, and aa when selection is against a recessive phenotype and when selection is against a dominant phenotype? (Fitness in terms of s)

Selection against a recessive phenotype
AA: 1
Aa: 1
aa: 1 - s
Selection against a dominant phenotype
AA: 1 - s
Aa: 1 - s
aa: 1


How does selection act?

Selection interacts with other processes e.g. Genetic drift
And is more efficient in large populations