Chapter 23 The Smallest Unit of Evolution Microevolution evolution on its smallest scale change in allele frequencies in a population over generations 3 main mechanisms that cause allele frequency change natural selection genetic drift chance events that alter allele frequency and gene flow transfer of alleles between populations Only natural selection improves organism s match with environment adaptation Genetic Variation Makes Evolution Possible Variation in heritable traits is a prereq for evolution Genetic Variation Genetic Variation differences among individuals in the composition of their genes or other DNA segments important for evolution Phenotype is product of inherited genotype and environmental factors Variation Within a Population Characters that vary may be discrete or quantitative Discrete Characters either or basis mostly determined by single gene locus with different alleles that produce distinct phenotypes Quantitative Characters vary along a continuum within a population usually results from 2 genes on a single phenotypic character Measure variation at whole gene level gene variability at molecular level of DNA nucleotide variability Average Heterozygosity average percentage of loci that are heterozygous ID heterozygous loci by gel electrophoresis to survey protein products of genes Variation Between Populations Geographic Variation differences in the genetic composition of separate population Differences in karyotypes of isolated populations Variation due to drift Cline graded change in a character along a geographic axis Some produced by gradation in an environmental variable Sources of Genetic Variation Originates from mutations gene duplication or other processes producing new alleles genes Sexual reproduction Formation of New Alleles Only mutations in gametes can be passed down Most mutations are slightly harmful Genetic Variation Makes Evolution Possible Sources of Genetic Variation Altering Gene Number or Position Important source of variation begins when genes are duplicated due to errors in meiosis slippage during DNA replication or activities of transposable elements Duplications of large chromosomal segments are usually harmful but duplications of smaller DNA pieces are not always Non harmful leads to a larger genome Rapid Reproduction Mutation rates are low in plants animals 1 100 000 genes per generation RNA has more mutations than DNA Viruses have a short generation time allowing for more mutations Sexual Reproduction Genetic variation results from unique combo of parent gametes Crossing over independent assortment of chromosomes and fertilizations result in shuffling to produce variation The Hardy Weinberg Equation Can be Used to Test Whether a Population is Evolving Presence of genetic variation does not guarantee that a population will evolve Gene Pools and Allele Frequencies Population group of individuals of the same species that live in the same area and interbreed producing fertile offspring Different populations of a species may be isolated geographically Gene Pool consists of all copies of every type of allele at every locus in all members of the population Fixed only one allele in a gene pool p represents frequency of one allele and q for other sum of equals 1 100 The Hardy Weinberg Principle To determine if natural selection is causing a particular locus to evolve we examine the population if it were not evolving at that locus No differences not evolving Hardy Weinberg Equilibrium Hardy Weinberg Principle the frequencies of alleles and genotypes in a population will remain constant from generation to generation provided that only Mendelian segregation and recombination of alleles are at work Possible allele combinations for all of crosses in population Mating occurs at random p 2 2 pq q 2 1 Equilibrium only if genotype frequencies are such that the actual frequency of one homozygote is p 2 the other is q 2 and 2pq for heterozygous The Hardy Weinberg Equation Can be Used to Test Whether a Population is Evolving The Hardy Weinberg Principle cont Conditions for Hardy Weinberg Equilibrium Describes hypothetical population that is not evolving 1 No mutations 2 Random mating 3 No natural selection 4 Extremely large population 5 No gene flow Departure from conditions usually results in evolutionary change Can be in equilibrium for one loci and evolving for another Apply the Hardy Weinberg Principle Initial test of whether evolution is occurring Medical applications to estimate population carrying disease trait Natural Selection Genetic Drift Gene Flow can Alter Allele Frequencies in a Population New mutations can alter allele frequencies Mutations ultimately produce new alleles that strongly influence allele frequency Nonrandom mating affects the frequencies of homo hetero genotypes Natural selection genetic drift and gene flow cause the most evolutionary change Natural Selection Individuals exhibit variation in heritable traits causing those with traits better suited for the environment to produce more offspring Adaptive Evolution consistently favoring some alleles over others better match between organisms and their environment Genetic Drift Genetic Drift allele frequencies to fluctuate unpredictably from one generation to the next especially in small populations Chance events associated with survival and reproduction The Founder Effect Founder Effect few individuals become isolated from a larger population and establish a new population whose gene pool differs from the source Some individuals and alleles are transported to new region but not others The Bottleneck Effect Bottleneck Effect caused by severe drop in population size Certain alleles may be over underrepresented or even absent May have low levels of genetic variation Effects of Genetic Drift 1 Genetic drift is significant in small populations Chance events can cause an uneven allele representation 2 Genetic drift can cause allele frequencies to change at random 3 Genetic drift can lead to a loss of genetic variation within populations Can eliminate alleles from a population 4 Genetic drift can cause harmful alleles to become fixed Natural Selection Genetic Drift Gene Flow can Alter Allele Frequencies in a Population Gene Flow Gene Flow transfer of alleles into or out of a population due to movement of fertile individuals or their gametes Tends to reduce the genetic differences between populations Can result in two populations combining into one Can affect how well populations are