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Natural Selection on Single Gene Traits
• Natural selection never acts on genes, always on the whole organism
• But, if that organism produce a gene change that affects survival, natural selection will act differently on that organism because of that gene change.
• Ex: Lizard color• Brown vs. Black• Change the survival, change the gene pool
Distribution of Phenotypes in Polygenic Traits• When many genes
affect one trait, you get many phenotypes
• The graph to the right shows the number of individuals (area under the curve) for the different phenotypes
Natural Selection on Polygenic Traits1) Directional Selection: when individuals
at one end of the curve have higher fitness than individuals in the middle or at the other end-Thus the curve will shift left or right
Ex:Beak Shape
Natural Selection on Polygenic Traits2) Stabilizing Selection:when individuals
near the center of the curve have higher fitness than the others-Thus the curve will get narrow and tall
-Survival favors the average
Ex: Birth weight in humans
Natural Selection on Polygenic Traits3) Disruptive Selection:when individuals
at opposite ends of the curve have higher fitness-Thus causes the curve to dip in the middle
-Average struggle to
survive
Ex: Bird Beaks
Genetic Drift
• In small populations, individuals that carry a particular allele may leave more descendants than others, just by chance
• Over time, this can cause a change in phenotype from the original population
• Ex: Founders Effect – occurs when a small population settles in a location separate from the rest of the population.
• Ex: Amish