11.2 Natural Selection in Populations Sponge Please copy down these terms Frequency: refers to the number of times a specific thing occurs Mean: is the average, and the point where the frequency is the highest Range: is the difference of interval between the smallest and largest values in a frequency distribution. Median: Is the middle value in a distribution, with and equal number of values above and below. ex: Mode: is the value occurring most frequently ex: = 2

11.2 Natural Selection in Populations Objectives 8a: Students know how natural selection determines the differential survival of groups of organisms 7.3a: Students know both genetic variation and environmental factors are causes of evolution and diversity of organisms.

11.2 Natural Selection in Populations KEY CONCEPT Populations, not individuals, evolve.

11.2 Natural Selection in Populations Natural selection acts on distributions of traits. A normal distribution graphs as a bell-shaped curve. Traits not undergoing natural selection have a normal distribution. – highest frequency near mean value – frequencies decrease toward each extreme value Extremes

11.2 Natural Selection in Populations Phenotypic distribution: A graph that shows the frequency of each phenotype for a trait in a population You can see how common each phenotype is in a population AND whether or not the population is undergoing natural selection for that trait

11.2 Natural Selection in Populations Natural selection can change the distribution of a trait in one of three ways. Microevolution is evolution within a population. –observable change in the allele frequencies –can result from natural selection

11.2 Natural Selection in Populations Natural selection can take one of three paths. –Directional selection favors phenotypes at one extreme. –Ex: antibiotics have led to this type of selection

11.2 Natural Selection in Populations –Stabilizing selection favors the intermediate (mean/middle) phenotype. Ex: gall fly

11.2 Natural Selection in Populations –Disruptive selection favors both extreme phenotypes, while individuals with intermediate phenotypes are selected against by something in nature. Ex: Lazuli buntings