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3 Causes of Deviation from Equilibrium 1. Natural Selection ○ Stabilizing selection ○ Disruptive selection ○ Directional selection 2. Genetic Drift ○ Founder Effect ○ Bottleneck Effect 3. Gene flow 1. Natural Selection When natural selection occurs, variation of traits in the populations change over time 3 patterns of change due to natural selection A. Stabilizing selection B. Disruptive selection C. Directional selection Bell Curve All of the patterns are compared to the standard bell curve. Shows that most members of the population share similar values for a certain trait. What do you observe? A. Stabilizing Selection Individuals with average (intermediate) forms of a trait has the highest fitness and the extreme traits have a lower fitness Extremes- selected against Intermediate- selected for Ex: Human birth weight A. Stabilizing Selection What do you observe? What do you observe? B. Disruptive Selection Selection where both extreme traits are considered more “fit” for the environment than the intermediate trait Both Extremes- selected for Intermediate- selected against Ex: African Swallowtail B. Disruptive Selection What do you observe? C. Directional Selection C. Directional Selection Individuals in a population with one extreme trait have higher fitness than the individuals with the average trait One extreme is selected for The average trait and the other extreme trait are selected against Ex: Beak size in finches Guppy experiment Guppy Experiment To Summarize… 2. Genetic Drift Causes deviation from equilibrium Genetic drift- change in gene pool due to chance The smaller the population, the greater the impact it has on the population ○ Ex: Founder effect ○ Ex: Bottleneck effect Founder Effect When alleles occur at a higher frequency in a population isolated from the general population The founding individuals could only contain a fraction of the total genetic diversity of the original gene pool. So the new population has only the alleles that the founders can provide Ex: The Amish Founder Effect Polydactyly in the Amish •The Amish in Lancaster County are an isolated population that was begun by German founders. •Some of the founders carried the recessive allele which causes polydactylism (extra fingers) •Today, as many as 1 in 14 individuals carries that recessive allele. Bottleneck Effect Occurs when a species is subjected to near extinction due to natural disaster, overhunting, disease, or habitat loss Only a few survivors are left from the event Bottleneck effect prevents the majority of the original genotypes from participating in the next generation. Have much less genetic variation than the original population Bottleneck Effect 3. Gene Flow Gene flow- gaining or losing alleles when individuals move into or out of a population Immigration- moving into Emigration- moving out of Could be moving for new breeding ground, place to hibernate, or more food sources Ex: Salmon migration Salmon Migration Concluding Questions What are the three instances where populations will deviate from equilibrium? Natural selection, genetic drift and gene flow What is the name for the changes in gene pool due to chance? Genetic drift What is this picture an example of? Founder Effect What types of selection do each of these curves represent? Directional Disruptive Stabilizing