Week 9 Flashcards
Describe the expected change to population mean fitness under directional selection.
Mean fitness increases with successive generations of selection (as the allele with lower fitness is purged from the population)
Provide a real-world example of directional selection.
(1) Resistance to rat poison Warfarin in the UK 1950s:
Mutation trade-off where resistance also means rats are less efficient at regenerating vitamin k. So, resistance evolved rapidly after poison exposure but declined afterwards due to the cost of adaptation.
(2) Insecticide resistance
Describe examples of heterozygote advantage and disadvantage
Heterozygote advantage:
When natural selection acts on individual loci independently:
Sickle-cell anemia
S allele encodes sickle cell hemoglobin
A allele encodes normal hemoglobin
SS genotype suffer severe sickle-cell anemia
AA genotypes have normal hemoglobin, but are susceptible to malaria
AS genotypes suffer slight anemia but are resistant to malaria
Heterozygote disadvantage:
Seed cracker birds in South Africa
Antagonistic selection- multiple niche polymorphism example
What are some examples of antagonistic selection and describe how it can give rise to multiple niche polymorphism?
Seed Cracker birds in South Africa
Similar to Darwin’s finches, differ in their mandible morphology
Two different types of seeds these birds feed on based on their beak width
Intermediate beak width at the disadvantage because they lack the dexterity for small seeds but the strength for large seeds.
This results in a bimodal peak or two separate fitness categories or “most fit” individuals
Distinguish between soft and hard selection.
Soft selection:
Occurs when the number of survivors in a patch of a particular microenvironment is determined by competition for a limiting factor. The relatively superior genotype has a higher probability of survival.
Hard selection:
Likelihood of survival of an individual in a microenvironment depends on its absolute fitness, not the density of competitors
Explain the ecological conditions that can produce frequency-dependent selection.
Frequency dependent selection– the rarer the phenotype is in the population, the greater its fitness
Arises in numerous ecological contexts (competition, mate choice, crypsis, mimicry, etc.)
“If the alternative phenotype is abundant, making it more costly, a selective pressure working against the alternative may increase the fitness of an individual’s phenotype, thus making it more common. This outcome is called positive frequency dependence.”
Ex)
Even sex ratios: selection favours individuals who produce a sex ratio that is biased towards the sex that is the minority in the popn
What is antagonistic selection and how does it arise?
A source of selection opposes another source of selection on a trait.
Can be due to:
Temporal fluctuations: fluctuating environments favour different genotypes across generations
Spatial variation: different genotypes are best adapted to different microhabitats or resources
Is soft or hard selection more likely to produce multiple-niche polymorphism? Why?
Soft selection is more likely to produce a multiple-niche polymorphism.
Subtle variation at a locus owing to superior fitness of different genotypes under different conditions of a varying environment.
Multiple-niche polymorphisms also more likely if each individual only experiences a microenvironment.
What is positive frequency dependent selection?
Provide an ecological example of its occurrence.
Positive frequency dependent selection– the fitness of a genotype is greater the more frequent it is in a population
Ex)
Asposematic (warning colouration in South American butterflies…
Müellerian mimicry:
two or more unpalatable species mimic each others’ warning colourations
Explain the function gene flow has in maintaining alleles and helping/hindering local adaptation.
Advantageous alleles should go to fixation, but deleterious alleles can persist in a population due to recurrent mutation (or by gene flow from another population)
Explain the relationship between gene flow and selection.
Gene flow-selection balance
Width of the cline is proportional to V/s
where V is the measure of gene flow (distance genes disperse) and s is the coefficient of selection
What is the relationship between genetic drift, selection, and population size?
Effect of genetic drift is negligible if selection (s) on a locus is strong relative to population size.
If selection is sufficiently weak that allele frequencies change mostly by genetic drift, the alleles are nearly neutral.
Genetic drift and natural selection can work in concert to move a population from one adaptive peak to another (a peak shift)
Fixation Index
Fixation index is a measyre of the variation in allele frequency among different populations
*practice using this formula
End of Species Concepts slides
