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Lecture 5

BIO120H1 Lecture Notes - Lecture 5: Neo-Darwinism, Population Genetics, Dna Replication

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Jean Jiang Nash

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Lecture 5 - Model systems in ecological genetics
What processes influence the maintenance of genetic variation in populations?!
How do we measure genetic variation and how much exists in populations?
Foundations of population genetics
R.A. Fisher, J.B.S. Haldane, and S. Wright provided the foundations for “Neo-Darwinism” and
the “New Synthesis”
They showed that continuous variation and Darwinian natural selection were entirely
consistent with Mendel’s Laws
They also demonstrated the evolutionary significance of genetic variation leading to several
key questions and development of the field of ecological and evolutionary genetics
Key questions in ecological and evolutionary genetics
What processes influence patterns of genetic diversity in natural populations?
How much and what types of genetic variation occur in populations?
How can we obtain empirical estimate of the amount of variation in populations?
What are the important parameters used to measure patterns of genetic variation?
Polymorphism (P): proportion of gene loci that are polymorphic
Heterozygosity (H): average frequency of heterozygous individuals per gene locus (a site on a
chromosome that is usually a gene)
What processes influence patterns of genetic diversity?
i) Mutation (increases diversity)!
- ultimate source of genetic variation!
- caused by random errors during DNA replication!
ii) Random Genetic Drift (decreases diversity)!
- random sampling effects every generation!
- drift important when populations become small!
iii) Natural Selection (can decrease or increase diversity)!
a) purifying (negative selection): mutations that reduce fitness removed by selection!
b) positive selection (adaptation): mutations that increase fitness become fixed!
c) balancing selection: natural selection maintains diversity; e.g. heterozygote advantage
What maintains genetic variation?
1. Mutation - selection balance!
- Less fit types maintained by repeated mutation
2. Selection maintaining variation!
- heterozygote advantage (if Aa is fitter than AA or aa, will maintain variability), frequency-
dependent selection (where natural selection varies depending on the frequency of the
genotypes in the population), fitness varies in space and time (certain genotypes do better in
certain environments)
Early evidence for the existence of genetic variation
Selection experiments on quantitative traits in different groups of organisms
Involves controlled breeding and selection of individuals for many generations
“Artificial selection”
Example: breeding for bristle number in fruit flies!
- chose ones with highest bristle number, bristle number eventually increased, showed that
bristle number had genetic variability!
Example 2: selecting maize (corn) for oil, once selection is relaxed, random mating will re-
approach a mean
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