BIO 115 Lecture Notes - Lecture 20: Balancing Selection, Genome Size, Heterozygote Advantage
Charlie Kent
Bio 115
2017 Fall
Human Biology
● Mutation generates variation
○ Mutation rates for any given gene are low
○ But, considering genome size and population size many new mutations arise each
generation
■ Estimate in humans: 9.8 billion new mutations
○ Source of variation for selection and drift to act
● Mutation—selection balance
○ Equilibrium frequency reached through tug-of-war between negative selection
and new mutation
○ Explains persistence of rare deleterious mutations in populations
■ Takes a long time to get rid of
○ Would selection be more effective at eliminating a rare, recessive deleterious
allele or a rare, dominant, deleterious allele? Can you explain why?
○ Some forms of selection maintain the diversity in populations (balancing
selection)
■ Negative frequency-dependent selection
● Takes place when rare genotypes have higher fitness than common
genotypes. This process can maintain genetic variation in
populations
■ Heterozygote advantage (sickle cell anemia)
● Can also lead to the maintenance of diversity at a locus.
● Selection against that sickle form of the RBC/hemoglobin.
● Where malaria is common, heterozygous have high advantage over
homozygous.
● Balancing selection. Neither allele is eliminated.
● Fitness level of heterozygous means neither allele is eliminated.
● Negative frequency-dependent selection
○ Bumble bees move pollen across the orchids. Part of the orchids fitness-need to
attract bees.
○ Can measure how success these plants are at having bees take away their pollen
and fertilize elsewhere.
○ How good are these plants at having their pollen taken away?
○ High numbers=higher relative fitness.
○ Population frequency of the yellow morph.
○ For the yellow morph plants do better when they’re rare.
■ These plants produce no reward. Unrewarding plants.
■ Rewarding for the bee perspective.
■ Bee is visiting because it wants something, these plants have nothing.
■ As yellow frequency increases, their reproductive success goes down.
○ Frequency of one type can’t get too high—balancing.
● Heterozygous advantage
Document Summary
Mutation rates for any given gene are low. But, considering genome size and population size many new mutations arise each generation. Estimate in humans: 9. 8 billion new mutations and new mutation. Source of variation for selection and drift to act. Equilibrium frequency reached through tug-of-war between negative selection. Explains persistence of rare deleterious mutations in populations. Would selection be more effective at eliminating a rare, recessive deleterious. Takes a long time to get rid of selection) Takes place when rare genotypes have higher fitness than common genotypes. This process can maintain genetic variation in populations. Can also lead to the maintenance of diversity at a locus. Selection against that sickle form of the rbc/hemoglobin. Where malaria is common, heterozygous have high advantage over homozygous. Fitness level of heterozygous means neither allele is eliminated. Negative frequency-dependent selection attract bees. and fertilize elsewhere. Bumble bees move pollen across the orchids.
