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Natural Selection Genetic Variation is the Key to Natural Selection • Variation is common among populations. • Only genetic variation has evolutionary consequences. Mutation and Recombination • Generate variation. • Mutations are usually harmless or bad. • Sometimes they can be good. • Recombination (like crossing over) also plays a large role in genetic variation. Variation is Preserved in a Population. • Diploidy – the presence of more than one allele is often hidden by a dominant allele. • Occurs even if the gene that is being hidden is lethal. • Ex. most genetic disorders are preserved because of diploidy. Variation Within a Population • Polymorphism – 2 or more contrasting forms within a population Variation Between Populations • Geographic Variation – between different populations in different areas. • Cline – type of geographic variation that occurs over a gradient. The percentage of the B allele in ABO blood group. Genes that are bad now… …may be good in the future Balanced Polymorphism – when genetic diversity is selected for. • Heterozygote advantage – heterozygotes show a greater survivorship than homozygotes • Example: sickle cell anemia. Balanced Polymorphism – when genetic diversity is selected for. • Hybrid vigor – when 2 inbred varieties are crossed, the hybrids show a greater survivorship and growth pattern. • Recessive alleles are hidden. Balanced Polymorphism – when genetic diversity is selected for. • Frequency dependent selection – reproductive success of one morph declines if that phenotype becomes too common. • Example: one insect mimics a poisonous one. If the nonpoisonous one is too common, predator is less likely to be fooled. Neutral Variation • Has no selective pressure. • Does not influence reproductive success. • Example: fingerprints Types of natural selection: • Directional selection - the population consistently moves in one direction. – Changing environmental conditions or new environment. • Stabilizing selection – acts against extreme phenotypes (favors average) – Most common in stable habitats • Diversifying Selection – Favors extremes (results in balance polymorphism) • Natural selection has no goal, there is no perfect organism. • Natural selection only works with what we have. • Organisms do not develop traits in order to fit the environment. Traits that happen to fit the environment best allow for more reproduction. Mechanical Protection Mechanical Protection Warning Patterns Protective Pattern Cryptic coloration Deceptive Coloration Batesian Mimicry Mullerian Mimicry • Several unpalatable species share warning colors or patterns to evade predation. Predator mimicry Distraction/Confusion Behavior Warning Sound Altruism Physiological Adaptations Commensalism Parasitism Parasitism