BIOL 1111 Lecture Notes - Lecture 2: Allele Frequency, Genotype Frequency, Stabilizing Selection

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The Modes of Natural Selection
Directional: over time, length of the tail is going to shift to one of the two extremes
Stabilizing: the intermediate range phenotype is favoured (less variation occurs)
Disruptive: two extremes favoured, intermediate not favoured for two separate reasons; two distinct
phenotypes (qualitative variation)
Ex. Directional to Stabilizing Selection (larvae)
-both a parasitic wasp and a predatory bird eat larvae of gallmaking fly
-medium sized larvae survive best because they wasps feed on small, birds feed on large
-this causes opposing directional forces to establish stabilizing selection
Ex. Disruptive Selection (birds and food)
-birds with long beaks can open cactus to eat insides, and birds with short beaks can strip trees of bark
to eat insects underneath
-favoured in cases of drought with limited resources; birds with intermediate beaks may not survive
Population Genetic
-study of how certain factors may influence genetic variation within a population
How to study? -> set up structure, determine what will influence the structure, set up hypothesis of a
mathematical model and then test
Gene pool- sum of all alleles at all loci of all individuals within a population
Genotype frequency- percentage of individuals possessing a certain genotype; pick one gene to study at
a time and study it in a population over time, it is difficult to study all genes at once in a population
Allele frequency- p= frequency of one allele at a locus
- q= frequency of other allele at same locus
Locus- site occupied by a gene on a chromosome; alleles on homologous chromosomes have same loci
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Document Summary

Directional: over time, length of the tail is going to shift to one of the two extremes. Stabilizing: the intermediate range phenotype is favoured (less variation occurs) Disruptive: two extremes favoured, intermediate not favoured for two separate reasons; two distinct phenotypes (qualitative variation) Both a parasitic wasp and a predatory bird eat larvae of gallmaking fly. Medium sized larvae survive best because they wasps feed on small, birds feed on large. This causes opposing directional forces to establish stabilizing selection. Birds with long beaks can open cactus to eat insides, and birds with short beaks can strip trees of bark to eat insects underneath. Favoured in cases of drought with limited resources; birds with intermediate beaks may not survive. Study of how certain factors may influence genetic variation within a population. > set up structure, determine what will influence the structure, set up hypothesis of a mathematical model and then test.

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