# Chapter 23

 Microevolution or population genetics Smallest unit of evolution Natural selection acts on physical variation The variation of physical features or behavior is a result of underlying genetic variation Natural selection alters the frequency of alleles in a population Alleles are different forms of a gene Sources of Variation Mutation Changes in DNA nucleotide sequenceCan affect single nucleotide, many nucleotides  Sources of variation  Sexual reproduction Meiosis leads to variation in gametes• Independent assortment, crossing over – Randomness of fertilization• Which sperm fertilizes egg? – Mate selection  Hardy-Weinberg Theorem: Describes why a population isn't evolving. Equilibrium and Conditions A population where allele % do not changefrom one generation to the next – No change in allele %, no microevolution • If equilibrium broken, evolution is occurring– Then can try to explain why  Hardy-Weinberg Theorem: Describes why a population isn't evolving. Equation and Calculations HW equation shows allele frequency p2 +2pq+q2 =1 (p+q=1alsohelpful) • p and q represent the relative frequencies of two possible alleles for a gene (ex. A, a) • p2 and q2 represent the frequencies of the homozygous genotypes (ex. AA, aa) • 2pq represents the frequency of the heterozygous genotype (ex. Aa) Hardy-Weinberg Conditions 1. No Mutations2. Random Mating3. No Natural Selection4. Large Population Size          Genetic drift less significant5. No gene flow           Populations are isolated Genetic Drift: Founder Effect When a few individuals become isolated from a larger population    Typically results in altered allele frequency Small population can have less variationSmall population subject to genetic drift Genetic Drift: Bottleneck Effect Occurs when population size is decimated     Environmental change, natural disaster etc.     Typically results in altered allele frequency New population has less variationSmall population subject to genetic drift  Genetic Drift Can lead to a loss of genetic variationCertain alleles can be lost from populationCertain alleles can be fixed in population Fixed alleles can sometimes be harmful Can explain why some genetic disorders become prevalent in isolated populations  Gene Flow Transfer of alleles in & out of a population– Movement of fertile individuals or gametes– Can alter original allele frequencies Extensive gene flow combines neighboring populations into a single population    1 large population with a common gene pool    Ex. Global human population    Previously separated populations coming together  Modes of Selection Act on phenotypes in a normal distribution– Can change original allele frequency • Directional selection– Favors one extreme trait • Disruptive selection– Favor both extreme trait • Stabilizing selection– Favors intermediate trait Authorkamrunsreno ID171201 Card SetChapter 23 DescriptionExam 1 Updated2012-09-16T17:48:58Z Show Answers