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Genetic Variation. PACKET #60 CHAPTER #23. 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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Genetic Variation PACKET #60 CHAPTER #23
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
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
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.
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.
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.
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.
Neutral Variation • Observed in a population where variation does not alter the ability of an individual to survive and reproduce.
Geographic Variation TYPE OF GENETIC VARIATION
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?
