Chapter 23 The Evolution of Populations The smallest unit of evolution Chapter 23 The Evolution of Populations 1 o o o Natural selection acts on individuals each organism s traits affect its survival and reproductive success compared with other individuals Since beak depth is an inherited trait in finches the average beak depth in the next generation was greater than it had been in the pre drought population The finch population had evolved by natural selection however the individual finches did not evolve The proportion of large beaks in the population increased from generation to generation the population evolved not its individual members Focusing on evolutionary change in populations we can define evolution on its smallest scale called microevolution as change in allele frequencies in a population over generations There are three main mechanisms that can cause allele frequency change natural selection genetic drift and gene flow adaptation o Only natural selection consistently improves the match between organisms and their environment Concept 23 1 Genetic variation makes evolution possible In the origin of species Darwin provided abundant evidence that life on earth has evolved over time and he proposed natural selection as the primary mechanism for that change Darwin realized that variation in heritable traits was a prerequisite for evolution but he did not know precisely how organisms pass heritable traits to their offspring Individual variations often reflect genetic variation differences among individuals in the composition of their Only the genetically determined part of phenotypic variation can have evolutionary consequences As such genetic variation provides the raw material for evolutionary change without genetic variation Genetic Variation genes or other DNA segments evolution cannot occur Variation within a population Discrete characters such as the purple or white flower colors of Mendel s pea plants can be Many discrete characters are determined by a single gene locus with different alleles that classified on an either or basis produce distinct phenotypes Quantitative characters vary along a continuum within a population It is the most heritable variation usually results form the influence of two or more genes on a single phenotypic character We can measure genetic variation at the whole gene level gene variability and at the molecular Gene variability can be quantified as the average heterozygosity the average percentage of loci level of DNA nucleotide variability that are heterozygous Chapter 23 The Evolution of Populations 2 o To measure nucleotide variability biologists compare the DNA sequence of two individuals in a population and then average the data from many such comparisons Gene variability tends to exceed nucleotide variability A gene can consist of thousands of nucleotides A difference at only one of these nucleotides can be sufficient to make two alleles of that gene different increasing gene variability Variation between populations o Geographic variation differences in the genetic composition of separate populations The patterns of fused chromosomes differ from one population of house mice to another Because these chromosome level changes leave genes intact their phenotypic effects on the mice seem to be neutral The variation between these populations appears to have resulted from chance events Drift rather than natural selection Cline a graded change in character along a geographic axis Some clines are produced by a gradation in an environmental variable Sources of Genetic Variation The genetic variation on which evolution depends originates when mutation gene duplication or other processes produce new alleles and new genes Many new genetic variants can be produced in short periods of time in organisms that reproduce rapidly bacteria Formation of new alleles Sexual reproduction can also result in genetic variation as existing genes are arranged in new ways New alleles can arise by mutations a change in the nucleotide sequence of an organism s DNA In multicellular organisms only mutations in cell lines that produce gametes can be passed to offspring changed A change of as little as one base in a gene called a point mutation can have a significant impact on phenotype as in sickle cell disease Because of the redundancy in the genetic code even a point mutation in a gene that encodes a protein will have no effect on the protein s function if the amino acid composition is not A mutant allele may on rare occasions actually make its bearer better suited to the environment enhancing reproductive success Altering Gene Number or Position Chromosomal changes that delete disrupt or rearrange many loci at once are usually harmful In rare cases chromosomal rearrangements may even beneficial The translocation of part of one chromosome to a different chromosome could link DNA segments in a way that results in a positive effect An important source of variation begins when genes are duplicated due to errors in meiosis such as unequal crossing over slippage during DNA replication or the activities of transposable elements o o o o o o o o o o Duplication of large segments are often harmful but smaller pieces of DNA may not be Gene duplications that do not have severe effects can persist over generations allowing Chapter 23 The Evolution of Populations 3 mutations to accumulate Rapid Reproduction variation in populations of these organisms Sexual Reproduction Prokaryotes typically have short generation spans so mutations can quickly generate genetic In organisms that reproduce sexually most of the genetic variation in a population results from the unique combination of alleles that each individual receives from its parents It is the mechanism of sexual reproduction that shuffles existing alleles and deals them at random to produce individual genotypes chromosomes and fertilization Three mechanisms contribute to this shuffling crossing over independent assortment of The combined effects of these three mechanisms ensure that sexual reproduction rearranges existing alleles into fresh combinations each generation providing much of the Concept 23 1 Summary Genetic variation makes evolution possible genetic variation that makes evolution possible Genetic variation refers to genetic to genetic differences among individuals within a population The nucleotide differences that provide the basis of genetic variation arise by mutation and other processes that produce new