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Microevolution What is Microevolution? Change in a populations alleles over a period of time Calculate allele frequencies Allele Two different forms of a gene (portion of DNA that codes for a trait) Three different combinations of Alleles Homozygous Dominant: DD Heterozygous: Dd Homozygous Recessive: dd Allele Frequencies A measure of how often (frequency) an allele is apparent in a population. What is Natural Selection? “Survival of the Fittest” The best adapted organism will survive and pass traits onto offspring Observation #1 Every population has the ability to grow Observation #2 Every population has limited resources Conclusion #1 Competition for resources Hyena Observation #3 Every individual in a population is different in structure and behavior. Ex-pattern, speed, agility, etc Conclusion #2 Natural Selection-Survival of the Fittest: Best adapted organism will survive Observation #4 Survivors will pass their traits on to their offspring. Conclusion #3 Evolution has occurred. There could be a change to the population. Example #1 Peppered Moth http://www.youtube.com/watch?v=LyRA807djLc Directional Selection • One extreme is selected for causing the other extremes to be selected against. Example Directional Selection: Peppered Moths Black were selected against leaving peppered around When env conditions changed, selection shifted against white Example #2: Pesticide Resistance • When insecticides are first used, vulnerable insects die. • Few individuals that are “immune” to the pesticide, survive and live to pass on that trait. Example #3: Antibiotic Resistance • Antibiotics first came into use in the 1940s. Overuse has led to increase in resistant forms of bacteria. • Over-prescribed antibiotics include penicillin and streptomycin. • Overuse of antibiotics has selected for resistant strains, which now threaten to become more predominant than the susceptible ones. Stabilizing Selection • Heterozygote advantage • Intermediate forms are favored while the extremes are selected against. Stabilizing Selection Example #1: Human Babies • Humans who weigh about seven pounds at birth have the greatest survival chances. Extreme weights are lower survival rate. Example #2 • Siberian Husky is a medium dog weighing between 35-60lbs. • These dogs have strong pectoral and leg muscles allowing it to work through the snow. • How is this an example of stabilizing selection? Disruptive Selection • The intermediate (middle) is selected against while both extremes are selected for. Disruptional Selection Example Butterflies selected for.. African butterfly ranges in color from orange to blue. Orange + Blue mimic other butterflies that when eaten, taste bitter. Birds eat intermediates when feeding to avoid bitter taste. Selected against…….