1. Population Genetics
Packet #5
Chapter #23
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2. Population Genetics
The study of genetic variability within the population and of the forces that act on it.
Gene Pool
Consists of all alleles, of all genes, present within a freely interbreeding population.
Can be calculated, mathematically, using various frequencies
Genotype frequency
Phenotype frequency
Allele frequency
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3. Important Vocabulary Allele Genotype frequency Phenotype frequency
Different form of a particular gene
Genotype frequency
The proportion of a particular genotype in the population.
Phenotype frequency
The proportion of a particular phenotype in a population.
Allele frequency
The proportion of a specific allele in a population.
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4. Hardy-Weinberg Godfrey Hardy Wilhelm Weinberg
British mathematician
Wilhelm Weinberg
German physician
In 1908, both individuals independently reported a mathematical modelthat describes allele frequencies in a population at any given time.
The Hardy-Weinberg Equation
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5. The Hardy-Weinberg Equation
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6. Variables Letters are used to represent specific variables. p q
Represents the frequency of the dominant allele
Ranges from 0 to 1 q
Represents the frequency of the recessive allele
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7. Hardy-Weinberg Equation
Binomial Equation
(p + q)2
p2 + 2pq + q2
p2 = frequency of homozygous dominant individuals
q2 = frequency of homozygous recessive individuals
2pq = frequency of hetereozygous individuals
For a population segregating two alleles at a particular time, in which p represents the dominant allele and q represents the recessive allele, the total frequency of all alleles will always equal to 1.
p + q = 1.0
Any sexually reproducing population in which the allele frequencies conform to this equation is at genetic equilibrium.
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8. Examples HW Please view virtual lab link provided on course website
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9. Assumptions for the Hardy-Weinberg Equation
Random mating of all genotypes
No net mutations
Large population size due to statistical constraints
No migration (gene flow)
No exchange of alleles with other populations
No natural selection
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10. Microevolution
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11. Microevolution
Change in a population’s allele, or genotype, frequencies over successive generations
Occurs when a population does not meet all of the assumptions of the Hardy-Weinberg principle
The small changes are referred to as microevolution.
There are five micro-evolutionary forces
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12. Microevolution
Non-Random Mating
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13. Non-random Mating Assortative Mating
Individuals select mates on the basis of phenotype—indirectly selecting a corresponding genotype.
This may lead to interbreeding
Leads to an increased homozygous allele composition
May lead to interbreeding depression and lowered “fitness” in the population
Commonly seen in plants
Fitness
Ability to pass on genes to the next generation
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14. Microevolution
Mutations
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15. Mutations
Unpredictable changes in DNA resulting in the production of new alleles
Introduces variation
Cause small deviations from Hardy-Weinberg equilibrium
Mutations in somatic cells are not heritable
Mutations in alleles found in sex cells allow those changes to be passed to the offspring
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16. Microevolution
Genetic Drift
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17. Genetic Drift
Random events that change allele frequencies in small populations
Small populations are more prone to lose alleles present in low frequencies
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18. Genetic Drift
Bottleneck Effect
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19. Bottleneck Effect
Rapid and severe declines in population size due to an adverse environmental factor
Results in an increase in different allele frequencies
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20. Bottleneck Effect Example
Examples
Northern elephant seals
Cheetah
American bison
Wollemi Pine
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21. Genetic Drift
Founder Effect
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22. Founder Effect Occurs when a small population colonizes a new area.
Common in island populations
Finnish population exhibits much less allelic variation than the general European population
Amish population in Pennsylvania has a significant number of individuals with the allele composition for a form of dwarfism.
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23. Microevolution
Gene Flow
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24. Gene Flow
The movement of alleles caused by a migration of individuals between populations
Migration of breeding individuals introduces new allelic frequencies to a population
Tends to counteract natural selection and genetic drift
Causes populations to become more genetically similar
Humans have experienced an increase in gene flow in the last few hundred years.
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25. Microevolution
Natural Selection
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26. Natural Selection
Results in changes in allele composition that lead to adaptation
Based on differential reproduction
Natural selection does not act directly on an organism’s genotypebut acts on the phenotype.
The phenotype represents an interaction between the environment and all the alleles in the organism’s genotype.
Natural selection weeds out those individuals whose phenotypes are less adapted to environmental changes
Allowing the better adapted organisms to survive and pass their alleles to future generations
Natural selection acts indirectly on the genotype.
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27. Natural Selection Stabilizing Selection
Favors intermediate phenotypes
Results in reduced variation in a population
Classic example is human birth weight
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28. Natural Selection Directional Selection
Favors one phenotype over another
Favors one extreme of the normal distribution over the other phenotypes.
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29. Natural Selection Disruptive Selection
Favors phenotypic extremes
Selects for two or more different phenotypes
May result in splitting of a population into 2 or more separate species.
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30. Necessities for Natural Selection
Populations contain genetic variation, a necessity for natural selection, introduced because of
Mutations
Crossing Over
Independent Assortment
Random Fertilization
In order to investigate genetic variation scientists examine genetic polymorphism
The presence of two or more alleles at a given locus.
Locus is the location of a gene on the chromosome.
Scientists compares different forms of a protein using gel electrophoresis
Resulting from slight changes in the gene’s DNA nucleotide sequence
Protein gel electrophoresis allows scientists to work backwards to determine DNA nucleotide sequences.
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31. Genetic Variation
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32. Balanced Polymorphism
Special type of polymorphism in which two or more alleles persist in a population over many generations as a result of natural selection. Due to
Hetereozygote Advantage
Frequency Dependent Selection.
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33. Hetereozygote Advantage
Hetereozygote exhibits greater “fitness” than either homozygotes.
Example
Hetereozygote carriers of the sickle cell allele.
Provides individuals resistance to malaria.
Allows individuals living in areas where malaria is high to survive.
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34. Frequency Dependent Selection
The “fitness” of a particular phenotype depends on how often it appears within a population.
Example
Scale-eating cichids
Two forms
Right pointing mouths & left pointing mouths
Right pointing allele is dominant
Get food by biting off the scales of other fish
Those with right pointed mouths attack prey’s left flanks
If the prey is constantly being attacked from a particular side, then they become more wary to such attacks
When the prey become wary, the advantage then swings to those predators that attack from the opposite side
The reason for the frequency of both forms of cichids have osculating over the last 20 – 30 years.
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35. Neutral Variation
Observed in a population where variation does not alter the ability of an individual to survive and reproduce.
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36. Geographic Variation
Genetic variation among individuals of differentpopulations of the same species.
Cline
Type of geographic variation
Gradual change in a species’ phenotype and genotype frequencies through a series of geographically separate populations due to an environmental gradient.
Example
Common Yarrow (Achileamillefolium)
Wild-flower that grows in various North American habitats.
Exhibits clinal variation in height
Due to different climates at different elevations/altitudes.
Could clinal variation result in a new species?
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