Microevolution lecture notes.doc

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24 Mar 2012
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Microevolution
Slide 3:
-combination of Darwin and Wallace
-must look at variation within populations rather than individuals
-in population there is a range of variation and in many cases, the variation is
genetic and heritable
-when we want to see natural selection, we need to measure and monitor variation
within the population
Slide 5:
-Mendel gives us an heritable basis for variation
Slide 6:
-Huxley combines genetics with evolution and gives us calculations in population genetics
that become predictable for changes in variation
-we need mechanisms to measure change and to quantify change
-the Synthetic Theory of Evolution (populations genetics + natural selection
based on Mendelian genetics) allows us to measure change
Slide 7:
Microevolution
-changes in alleles within the population
-change in allele frequencies = variation
-microevolution occurs when there is change in allelic frequencies
Slide 8:
allele
-carries traits
-traits present in two forms associated with a homologous chromosome
phenotype
-visual expression of an underlying gene; does not tell allele frequency
-not an expression of genotype
genotype
-underlying frequency of the alleles
homozygous
- identical alleles of the gene are present on both homologous chromosomes
heterozygous
- two different alleles of a gene
dominant and recessive
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Slide 11:
p2 + 2pq + q2 = 1
-if everything is random and pooled together, you can predict what the reassembly should
be
-p and q represent allelic frequencies
Slide 13:
Hardy-Weinberg Principle
-no matter the initial frequency, you can predict the outcome (Hardy-Weinberg
Equilibrium)
-a way to understand population genetics
-tells you if frequencies have changed
-allelic frequency change = microevolution
-makes assumptions:
-no natural selection; no allele combinations are favourable and all combinations
are neutral (doesn't increase fitness)
-no mutation; no allele has been turned into something else
-no genetic drift; the population is sufficiently large that we don't have small size
sampling (needs a large number samples for accuracy)
-no gene flow; nobody is migrating in and out to change allelic frequency
-random mating; there is no preference for mating other than the opposite sex
-tells us whenever there is no microevolution
-if you see allele frequencies changing, microevolution is occurring and we try to figure out
what is changing and why it is changing
Slide 18:
Hardy-Weinberg Principles: Effects of Selection
-fixation
-from selective pressure, even after a thousand generations, there is little change
in allele frequencies
-selective pressure has no impact (all alleles remain)
-we need a severe selective pressure in order to change allele frequency
-under severe selection (i.e.: 1-2% selection) the allele that is being selected for
disappears out of the population
-the population becomes fixed on the other allele
-under severe selection (a situation where there are a minimal number of survivors
and a maximal number of organisms that don't survive) we lose genetic variability
within a population; if we are selecting an agricultural crop and breeding for
certain traits, we may also losing other variability through selection (i.e.:
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resistance; selecting for survivors of the treatment and knocking out huge
numbers of organisms and their genetic variability or breeding and inbreeding
agricultural plants)
-hard selection makes for uniform / homogenous genetics but removes variability
-hard selection fixes the genetic compliment and we lose variability
-too much selection causes fixation of the other allele
Slide 20:
-selection for recessive allele
-the allele will never disappear
-unlike heavy selection, natural selection that works against allelic frequencies
will never eliminate a deleterious allele
Slide 21:
-selection for heterozygote
-even if we take out the heterozygous, it will always remain in the gene pool
-heterozygotes have advantages (i.e.: incomplete expression)
-heterozygote advantage
-we maintain alleles within population
Slide 22:
-heterozygote advantage
-sickle cell anemia
-homozygous = bad
-heterozygous for sickle cell = advantage
-malaria parasite invades sickle cells and healthy RBCs but the liver
constantly destroys sickle cells and the malaria parasite with it
-heterzygotes can fight the malaria parasite which is advantageous in
malaria regions
Slide 23:
Multiple Loci Traits
-most traits are not on a single allele
-most traits are controlled by multiple alleles (i.e.: height)
-cannot measure HWE with multiple loci
-need to compare shape of normal distribution
-when we can't measure allelic frequencies, we look at the shape of the
distribution curve
Slide 24:
Directional Selection
-look at the width of the curve and the location of the median of the curve (the mean and
the standard deviation)
-changing the mean in one direction, but the shape of the curve doesn't change
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