1. Genes Within Populations
Chapter 20
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2. Gene Variation is Raw Material of Evolution
Evolution – how an entity changes through time.
Darwin used the phrase “Descent With Modification”
Darwin’s phrase of Biological evolution:
- All species arise from other pre-existing species. However through time, species accumulate differences such that ancestral and descendent species are not identical.
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3. Gene Variation is Raw Material
Natural Selection and Evolutionary Change
Some individuals in a population possess certain inherited characteristics that play a role in producing more surviving offspring than individuals without those characteristics.
As a result, the population gradually includes more individuals with advantageous characteristics.
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4. Gene Variation In Nature
Measuring Levels of Genetic Variation
Blood Groups – 75 genetically variable genes in humans
Enzymes – 5% are heterozygous
Enzyme Polymorphism
A Locus with more variation that can be explained is termed polymorphic.
Natural populations tend to have more polymorphic loci than can be accounted for by mutation.
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5. Hardy-Weinberg Principle
Population Genetics - Study of the properties of genes in populations.
Theory of Blending Inheritance (Phenotypic Inheritance) was widely accepted.
If correct new genetic variants would quickly be diluted to point of disappearance in subsequent generations.
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6. Hardy-Weinberg Principle
Hardy-Weinberg Principle - proposed by G.H. hardy & G. Weinberg to explain why genetic variation persists.
Hardy-Weinberg - Original proportions of genotypes in a population will remain constant from generation to generation as long as 5 assumptions are met.
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7. Hardy-Weinberg Principle
Necessary Assumptions :
Population size is very large.
Random mating is occurring.
No mutation takes place.
No gene input from external sources. (No immigration takes place)
No selection occurs
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8. Hardy-Weinberg Principle
Calculate Genotype Frequencies With a Binomial Expansion
(p+q)2 = p2 + 2pq + q2
p = Individuals homozygous for dominant allele.
2pq = Individuals heterozygous for alleles.
q = Individuals homozygous for recessive allele.
Because there is only two alleles, p plus q must always equal 1.
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9. Example:
Consider a population of 100 cats with 84 black and 16 white cats. Assume white is homozygous recessive, calculate allelic frequencies of the two alleles in the population and genotype frequencies.
If bb = .16, then q2 = .16 and q = .4
If p + q = 1, then p = .6
Therefore,
p2 = (.6)2 = and 2pq = 2(.6)(.4) = .48
B = Dominant allele
b = Recessive allele
p2 + 2pq + q2
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10. Hardy-Weinberg Equilibrium
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11. Example:
Given the allele A is present in a large random-mating population at a frequency of 54 per 100 individuals, what is the proportion of individuals in that population expected to be
p2 + 2pq + q2
Heterozygous for the allele?
Homozygous dominant?
Homozygous recessive?
p = .54 q = .46
= 5/10
= 3/10
= 2/10
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12. Establishes a convenient baseline against which to measure changes.
Importance of Hardy-Weinberg Principle:
Establishes a convenient baseline against which to measure changes.
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13. Five Factors that alter proportions of homozygotes & heterozygotes
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14. Five Agents of Evolutionary Change
1. Mutation
Mutation rates are generally so low they have little effect on Hardy-Weinberg proportions of common alleles.
Ultimate source of genetic variation and thus makes evolution possible.
2. Gene Flow
Movement of alleles from one population to another.
Tend to homogenize allele frequencies.
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15. Example: Inbreeding (mating w/relatives)
3. Nonrandom Mating
phenomenon in which individuals with certain genotypes sometimes mate with one another more commonly then would be expected on a random basis
Example: Inbreeding (mating w/relatives)
- cause frequencies of a particular genotype to
differ greatly.
- increases proportions of homoygous individuals
- increases the expression of recessive alleles
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16. Five Agents of Evolutionary Change
4. Genetic Drift
In small populations frequencies of particular alleles may change drastically by chance alone.
Two causes of decreases in population size:
Founder Effect - Few individuals become founders of a new populations (Small allelic pool).
Bottleneck Effect - Drastic reduction in population, and gene pool size.
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17. Raven - Johnson - Biology: 6th Ed
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18. Five Agents of Evolutionary Change
5. Selection
Artificial - Breeders exert selection.
Natural - Nature exerts selection.
- Variation must exist among individuals.
Variation must result in differences in numbers of viable offspring produced.
Variation must be genetically inherited.
Natural Selection is a process, and Evolution is the historical record of change through time; it’s an outcome.
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19. Five Agents of Evolutionary Change
Selection Pressures:
Avoiding Predators
Matching Climatic Condition
Pesticide Resistance
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20. Evolutionary Forces Maintaining Polymorphism
Adaptive Selection Theory
Heterogeneous environments produce many alleles.
Neutral Theory
Balance between mutation and genetic drift.
Gene Flow versus Natural Selection
Gene flow may decrease frequency of an allele favored by selection.
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21. Heterozygote Advantage
Heterozygote Advantage will favor heterozygotes, and maintain both alleles instead of removing less successful alleles from a population.
Example: Sickle Cell Anemia
Homozygotes exhibit severe anemia, have abnormal blood cells, and usually die before reproductive age.
Heterozygotes are less susceptible to malaria.
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22. Raven - Johnson - Biology: 6th Ed
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23. 3 kinds of Natural Selection: Disruptive Selection
Forms of Selection
3 kinds of Natural Selection:
Disruptive Selection
Selection eliminates intermediate types.
Directional Selection
Selection eliminates one extreme from a phenotypic array.
Stabilizing Selection
Selection acts to eliminate both extremes.
Fitness - Number of surviving offspring passed to the next generation.
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24. Types of Natural Selection
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25. Evolution Requires Genetic Variation
Limits to Selection
Evolution Requires Genetic Variation
1. Intense selection may remove variation from a population at a rate greater than mutation can replenish.
Thoroughbred Horses
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26. 2. Selection Against Rare Alleles
Limits to Selection
2. Selection Against Rare Alleles
As selection only operates on phenotypes, rare recessives cannot be effectively selected against except in homozygote form.
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27. Hardy Weinberg Principle Five Agents of Evolutionary Change
Review
Gene Variation
Hardy Weinberg Principle
Five Agents of Evolutionary Change
Forces Maintaining Polymorphism
Heterozygote Advantage
Forms of Selection
Limits to Selection
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28. The End
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