BIO120 Lecture 5: Genetic Variation - Models and Measurement (What…
BIO120 Lecture 5: Genetic Variation - Models and Measurement
Why study genetic variation?
Theoretical population genetics initiated by Fisher, Haldane, Wright; showed continuous variation and Darwinian natural selection were consistent with Mendel's laws; led to the three questions below
How much genetic variation exists?
Classical (Morgan, Muller)
Wild type is normal genotype
Balanced (Dobzhansky, Ford)
Selection favours diversity
What processes influence patterns of genetic diversity and evolution?
Increases / decreases diversity (natural selection)
Purifying (negative), where mutations that remove fitness are removed
Positive (adaptation), where mutations that increase fitness become fixed
Selection favoring diversity, where natural selection acts to maintain diversity
Mutation-selection balance: less fit are maintained by repeated mutation
Selection maintaining variation: heterozygote advantage, frequency-dependent selection, fitness varies
Variation selectively neutral: different types do not differ in fitness --> none eliminated
Random genetic drift: change in phenotype frequency over time; becomes more important as population size decreases
Ex. maize has lost genetic diversity compared to teosinte
Ex. humans experience lower levels of diversity away from east Africa --> reflecting founder event b/c humans migrated from this source population
Mutation caused by replication errors
Recombination introduces new combinations
Migration (gene flow), influencing structure of diversity over large space
How can we obtain empirical estimates of variation amounts?
Pre-1966: morphology or cytology (chromosome inversions)
Early evidence for genetic variation involved artificial selection
Abundant variation exists for polygenic traits
Electrophoresis revolution (use of molecular technology) provided answer to - what proportion of genes are variable/polymorphic?
Advantages of enzyme polymorphism studies
Many loci examined
Used in any organism
Heterozygotes can be identified
Variation examined close to DNA level
Provides genetic marker loci for other studies