Evolution of Populations Evolution as Genetic Change

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 Traits 1)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 Traits 2)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 Traits 3)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

Ex: Bottleneck

Genetic Equilibrium=No Evolution/Change Hardy-Weinberg Principle: allele frequencies remain the same if… 1.Random Mating Occurs 2.Large Population 3.No immigration or emigration 4.No mutations 5.No natural selection