Lecture 5: Genetic Variation (models and measurement) Flashcards
Who provided the foundations for “Neo-Darwinism” and the “New Synthesis”?
Fisher, Haldane, and Wright.
What is Neo-Darwinism?
An attempt to reconcile mendel’s laws of genetics (which claim organisms do not change) with Darwin’s theory of evolution—which claims they do.
What factors influence patterns of genetic diversity and evolution? Which increase diversity and which decrease?
- mutation (increase)
- recombination (increase)
- migration (increase)
- genetic drift (decrease)
- Natural selection (decrease/increase)
What is the ultimate source of genetic variation?
Mutation which is caused by errors during replication.
Purifying/negative selection
mutations that reduce fitness and are removed by natural selection
Positive selection (adaptation)
mutations that increase fitness will become fixed in a population
Selection favouring diversity
natural selection acting to maintain diversity over long term (heterozygote advantage)
How does migration affect diversity?
Gene flow influences the structuring of diversity over a large spatial scale.
Polymorphism
Proportion (p) of gene loci that are polymorphic. a gene is polymorphic if more than one allele occupies that gene locus within a population. Each diff. gene must occur at rate of at least 1%.
Heterozygosity
Average frequency of heterozygous individuals per gene locus. Heterozygous + the possession of 2 different alleles of a particular gene.
Theories of what maintains genetic variation?
- Mutation-selection balance
- Selection maintaining variation
- Variation selectively neutral
Mutation-selection balance
Less fit types maintained by repeat mutation
Selection maintaining variation
Heterozygote advantage, frequency-dependent selection, fitness varies in space and time.
Variation selectively neutral
Theory developed by Motoo Kimura. Different types do not differ in their fitness hence none eliminated by selection.
Population Genetic Variation: Classical School
- Morgan and Muller
- negative selection
- low heterozygosity
- low polymorphism
- wild type is normal genotype
Population Genetic Variation: Balance School
- Dobzhansky and Ford
- heterozygote advantage
- high heterozygosity
- high polymorphism
- selection favours diversity
What was early quantitative evidence for existence of genetic variation?
Artificial selection experiments done on groups of organisms (done on drosophila and maize). Selection responses demonstrate that abundant genetic variation exists for polygenic traits but didn’t give any info on polymorphism or heterozygosity.
Who was Richard Lewontin and what did he do?
Used electrophoresis and allozymes to figure out which genes are variable (polymorphic). He initiated large scale surveys of electrophoretic variation in enzymes and proteins in diverse organisms.
Define electrophoresis and allozyme.
Electrophor. - technique used to separate charged molecules.
Allozyme - allelic variants of the same protein. Differs by single allele (one mutation) at a single locus. Very useful in identifying differences between species within the same genus.
Advantages of enzyme polymorphism?
- many loci can be examined
- can be used in nearly any organism
- loci co-dominant, heterozygotes can be identified
- variation examined close to DNA level
- Provides genetic marker loci for other studies
Genetic variation in human population (# genes studied, P, and H)?
71 genes studied, P = 0.28, H = 0.07
What is the rate of DNA polymorphism in humans? What about E. coli?
humans - 1 per 1000 base pairs
E. coli - 1 per 20 base pairs
Humans show a loss of what with increasing distance from East Africa?
Genetic variation (reflects founder event as humans migrated)
Which species has more genetic diversity: Maize or Teosinte?
Teosinte, wild ancestral species of Maize displays more diversity.
