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Microevolution Levels of Evolutionary Study • Microevolution: examines changes to the genes (alleles) within populations – Population Genetics: studies the changes in the numbers & types of alleles in a population • Examines evolution within a species. • Small changes that do not lead to new species, but can lead to new variations. • Macroevolution: examines the evolutionary changes that create new species – Speciation: the formation of new species over time Processes of Microevolution • Microevolution examines the processes by which inherited traits change over time in a population –Natural Selection –Migration –Mate Choice –Mutation –Genetic Drift Processes of Microevolution • Natural Selection: changes in environmental pressures can cause an increase or decrease in certain alleles (traits) in a population – Favorable alleles stay in population (selected for) – Unfavorable alleles are eliminated (selected against) • Migration: as organisms move from one population to another, their genes move with them – Causes the numbers and types of alleles within each population to change. Processes of Microevolution • Mate Choice: if parents are selective (picky) or limited in their choice of mates, only a limited set of traits will be passed on • Mutation: can add a new allele to a population – Mutations are rare, and must occur in egg or sperm to be passed on • Genetic Drift: population sizes affect the change in alleles (traits) in a population – Large populations experience slower changes in alleles – Smaller populations can change more quickly Microevolution: Changes within Populations Populations evolve NOT individuals evolve Natural selection works on the phenotypes within a population Individuals cannot evolve within their lifetime, because they cannot evolve a new phenotype due to a change in the environment Evolution is brought about by breeding that occurs between organisms within a population. Natural selection maintains favorable (useful) genotypes, and eliminates unfavorable (not useful) genotypes. Natural Selection - Explains how organisms change over time 1. Over Production: Most organisms produce more offspring than will survive 2. Variation: Members of the population have variations 3. Selection: Certain variations will increase the likelihood of survival 4. Adaptation: Over-time organisms with that variation make up most of the population and may look different than their ancestors Patterns of Natural Selection Stabilizing selection: Directional selection: Favors average individuals; reduces variation Favors EXTREME variations; evolution is fast Disruptive selection: Favors two extreme variations; no intermediate and eventually two species. Stabilizing Selection • “Average” individuals are better able to survive • Alleles for the “extreme” are eliminated • A narrow range of phenotypes is favored resulting in a narrow bell shaped curve Directional Selection • ONE extreme is favorable, but other is not • Alleles for other extreme and normal phenotypes become less common in the population • Results in FAST evolution! • Bell curve is shifted off of center Disruptive Selection • BOTH extremes are favorable! • Results in increasingly distinct phenotypes, the “normal” phenotype is selected against • Bell curve is ‘disrupted’ and pushed apart into two peaks