1. Genetic Variation
PACKET #60
CHAPTER #23
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2. Introduction Genetic variation is necessary for natural selection
There are multiple reasons for genetic variability in populations
Mutations
Crossing over
Independent assortment
Random fertilization of eggs by sperm
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3. Examination of Genetic Variation
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4. Examination I
One way of evaluating genetic variation is to examine genetic polymorphism.
The presence of two or more alleles for a given locus
To examine genetic polymorphism, biologists compare different forms of a particular protein.
Each form consists of a slightly different amino acid sequence that is coded for by a different allele.
Gel electrophoresis
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5. Balanced Polymorphsim
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6. Balanced Polymorphism
Maintenance in a population of two or more alleles in equilibrium at frequencies too high to be explained, particularly for the rarer of them, by mutation
Balanced polymorphism, a special type of genetic polymorphism, may be a result of natural selection.
The mechanisms that allow balanced polymorphism to exist while allow natural selection to occur is: -
Hetereozygote Advantage
Frequency Dependent Selection.
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7. 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.
Those that are homozygous recessive are likely to die at an early age.
Those are homozygous dominant, having regular red blood cells, are likely to die if infected with malaria.
What type of mutation caused the sickle shaped cell to occur (i.e. the recessive allele)?
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8. Frequency Dependent Selection
The “fitness” of a particular phenotype depends on how often it appears within a population.
The ability of a phenotype to be “fit” depends on the how often it appears within a population.
Often a phenotype has a greater selective value when rare then when common in a population.
If a phenotype falls within this category, they lose their selective advantage as they become more common.
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9. Frequency Dependent Selection
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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10. Neutral Variation
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11. Neutral Variation
Observed in a population where variation does not alter the ability of an individual to survive and reproduce.
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12. TYPE OF GENETIC VARIATION
Geographic Variation
TYPE OF GENETIC VARIATION
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13. Geographic Variation
Genetic variation among individuals of different populations 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 (Achilea millefolium)
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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14. Review Genetic Variation Balanced Polymorphism Heterozygote Advantage
Frequency Dependent Selection
Neutral Variation
Geographic variation
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