Population Evolution ch. 23 One misconception is that organisms evolve, in the Darwinian sense, during their lifetimes Natural selection acts on individuals, but only _____________________ ______________________ in populations contribute to evolution _____________________ is a change in allele frequencies in a population over generations populations evolve Genetic variations Microevolution mutation and sexual reproduction Two processes, ____________________________, produce the variation in gene pools that contributes to differences among individuals Variation __________________ in individual genotype leads to variation in individual phenotype Not all _______________ variation is heritable Natural selection can only act on _____________with a genetic component phenotypic variation

Fig. 23-1 Figure 23.1 Is this finch evolving by natural selection?

Fig. 23-2a (a) Figure 23.2 Nonheritable variation

Fig. 23-2b (b) Figure 23.2 Nonheritable variation

Variation Within a Population Both discrete and quantitative characters contribute to variation within a population ___________________ can be classified on an either-or basis ___________________ vary along a continuum within a population Discrete characters Quantitative characters Population geneticists measure polymorphisms in a population by determining the ____________________________ at the gene and molecular levels amount of heterozygosity Variation Between Populations geographic variation Most species exhibit __________________, differences between gene pools of separate populations or population subgroups Cline - __________________________________________________ a graded change in some trait along a geographic axis

Fig. 23-3 Geographic variation in isolated mouse populations on Madeira 1 2.4 3.14 5.18 6 7.15 8.11 9.12 10.16 13.17 19 XX 1 2.19 3.8 4.16 5.14 6.7 Figure 23.3 Geographic variation in isolated mouse populations on Madeira 9.10 11.12 13.17 15.18 XX

Figure 23.8 Clinal variation in a plant variations within a population geographic variation

Ldh-B b allele frequency Fig. 23-4 A cline determined by temperature 1.0 0.8 0.6 Ldh-B b allele frequency 0.4 0.2 Figure 23.4 A cline determined by temperature 46 44 42 40 38 36 34 32 30 Latitude (°N) Maine Cold (6°C) Georgia Warm (21°C)

Mutation _______________are changes in the nucleotide sequence of DNA Mutations cause new ___________________ to arise Only mutations in cells that produce ______________ can be passed to offspring Mutations genes and alleles gametes A ____________________ is a change in one base in a gene point mutation The effects of point mutations can vary: Mutations in noncoding regions of DNA are often ______________ harmless Chromosomal mutations that delete, disrupt, or rearrange many loci are typically ________________ Duplication of large chromosome segments is usually ______________ harmful harmful animals and plants Mutation rates are low in _______________________________ The average is about one mutation in every 100,000 genes per generation Mutations rates are often ______________ in prokaryotes and ____________ in viruses lower higher

Sexual Reproduction _____________________ can shuffle existing alleles into new combinations In organisms that reproduce _____________, ____________________ _________________ is more important than ___________ in producing the genetic differences that make adaptation possible Sexual reproduction sexually recombination of alleles mutation The Hardy-Weinberg equation can be used to test whether a population is evolving A _________________ is a localized group of individuals capable of interbreeding and producing _____________ offspring A _______________ consists of all the alleles for all loci in a population A locus is fixed if all individuals in a population are homozygous for the same allele population fertile gene pool

One species, two populations Fig. 23-5 One species, two populations Porcupine herd MAP AREA CANADA ALASKA Beaufort Sea NORTHWEST TERRITORIES Porcupine herd range Figure 23.5 One species, two populations Fortymile herd range ALASKA YUKON Fortymile herd

p + q = 1 Hardy-Weinberg Principle The frequency of an allele in a population can be calculated For ___________ organisms, the total number of alleles at a locus is the total number of individuals _______ diploid x 2 p and q By convention, if there are 2 alleles at a locus, _______________ are used to represent their _________________ The frequency of all alleles in a population will add up to 1 For example frequencies p + q = 1 The Hardy-Weinberg principle describes a population that is ____ evolving If a population does ____________ the criteria of the Hardy-Weinberg principle, it can be concluded that the population is ____________ NOT not meet evolving

Alleles p + q = 1 p=A, q=a p2 + 2pq + q2 = 1 AA Aa aa no migration Hardy Weinbeg Equation: Requirements for Hardy Weinberg Equilibrium: 1. 2. 3. 4. 5. p + q = 1 p=A, q=a p2 + 2pq + q2 = 1 AA Aa aa no migration no natural selection no net mutations large population random mating

Hardy-Weinberg Equilibrium The Hardy-Weinberg principle states that _______________ of ____________________________ in a population remain constant from _____________________________ In a given population where gametes contribute to the next generation randomly, allele frequencies will ________ change Mendelian inheritance ______________ genetic variation in a population frequencies alleles and genotypes generation to generation NOT preserves Hardy-Weinberg equilibrium describes the constant frequency of alleles in such a gene pool If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then where p2 and q2 represent the frequencies of the _____________ genotypes 2pq represents the frequency of the _________________ genotype p2 + 2pq + q2 = 1 homozygous heterozygous

Conditions for Hardy-Weinberg Equilibrium The Hardy-Weinberg theorem describes a _______________ population In real populations, allele and genotype frequencies ____change over time hypothetical DO The five conditions for nonevolving populations are rarely met in nature: __________________________ No mutations Random mating No natural selection Extremely large population size No gene flow Natural populations can evolve at some loci, while being in Hardy-Weinberg equilibrium at other loci

Natural selection, genetic drift, and gene flow can alter allele frequencies in a population Three major factors alter allele frequencies and bring about most evolutionary change: _____________________ Natural selection Genetic drift (small populations) Gene flow (migration) Natural Selection Differential success in reproduction results in certain alleles being passed to the next generation in ______________________ greater proportions

Genetic Drift The smaller a sample, the greater the chance of deviation from a predicted result __________________ describes how allele frequencies fluctuate unpredictably from one generation to the next Genetic drift tends to __________ genetic variation through _______ of alleles Genetic drift reduce loss The ________________ occurs when a few individuals become isolated from a larger population Allele frequencies in the small founder population can be different from those in the larger parent population founder effect The ________________ is a sudden reduction in population size due to a change in the environment The resulting gene pool may no longer be reflective of the original population’s gene pool If the population remains small, it may be further affected by genetic drift bottleneck effect

- floods - volcanoes - ice ages not in Hardy Weinberg equilibrium Genetic Drift not in Hardy Weinberg equilibrium p and q are not staying constant throughout the generations Bottleneck Effect: the population undergoes a dramatic decrease in size - floods - volcanoes - ice ages

Case Study: Impact of Genetic Drift on the Greater Prairie Chicken Loss of prairie habitat caused a severe reduction in the population of greater prairie chickens in Illinois. The surviving birds had low levels of genetic variation, and only 50% of their eggs hatched Fig. 23-10a Pre-bottleneck (Illinois, 1820) Post-bottleneck (Illinois, 1993) Range of greater prairie chicken Figure 23.10 Bottleneck effect and reduction of genetic variation Researchers used DNA from museum specimens to compare genetic variation in the population before and after the bottleneck The results showed a loss of alleles at several loci Researchers introduced greater prairie chickens from population in other states and were successful in introducing new alleles and increasing the egg hatch rate to 90%

Effects of Genetic Drift: A Summary Genetic drift is significant in _________________________ Genetic drift causes allele frequencies to change at ____________ Genetic drift can lead to a loss of _______________ within populations Genetic drift can cause harmful alleles to become ___________ small populations random genetic variation fixed Gene Flow movement Gene flow consists of the ______________ of alleles among populations Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen) Gene flow tends to __________________ between populations over time Gene flow is more likely than mutation to alter allele frequencies directly reduce differences

Fig. 23-11 Gene Flow Figure 23.11 Gene flow and human evolution

Gene flow can decrease the fitness of a population 70 NON- MINE SOIL MINE SOIL NON- MINE SOIL 60 50 Prevailing wind direction Index of copper tolerance 40 30 20 10 20 20 20 40 60 80 100 120 140 160 Distance from mine edge (meters) Figure 23.12 Gene flow and selection In bent grass, alleles for copper tolerance are beneficial in populations near copper mines, but harmful to populations in other soils Windblown pollen moves these alleles between populations The movement of unfavorable alleles into a population results in a decrease in fit between organism and environment

Only natural selection consistently results in ______________________ Natural selection is the only mechanism that consistently causes adaptive evolution Only natural selection consistently results in ______________________ adaptive evolution Natural selection brings about adaptive evolution by acting on an organism’s ______________ phenotype Relative Fitness The phrases “struggle for existence” and “survival of the fittest” are misleading as they imply direct competition among individuals Reproductive success is generally more subtle and depends on many factors __________________ is the contribution an individual makes to the __________________ of the next generation, relative to the contributions of other individuals Selection favors certain ______________ by acting on the ____________ of certain organisms Relative fitness gene pool genotypes phenotypes

Directional, Disruptive, and Stabilizing Selection Three modes of selection: _____________________ favors individuals at one end of the phenotypic range _____________________ favors individuals at both extremes of the phenotypic range _____________________ favors intermediate variants and acts against extreme phenotypes Directional selection Disruptive selection Stabilizing selection

Frequency of individuals Original population Frequency of individuals Phenotypes (fur color) Original population Evolved population Figure 23.13 Modes of selection (a) Directional selection (b) Disruptive selection (c) Stabilizing selection

The Key Role of Natural Selection in Adaptive Evolution Natural selection increases the frequencies of alleles that enhance _______________________________ Adaptive evolution occurs as the match between an organism and its environment _________________ survival and reproduction increases Because the environment can change, adaptive evolution is a _______________ process Genetic drift and gene flow do not consistently lead ______________ evolution as they can increase or decrease the match between an organism and its environment continuous to adaptive

Movable bones (b) Movable jaw bones in snakes Examples of adaptations Fig. 23-14b Movable bones Examples of adaptations Figure 23.14 Examples of adaptations (b) Movable jaw bones in snakes

(a) Color-changing ability in cuttlefish Fig. 23-14a Figure 23.14 Examples of adaptations (a) Color-changing ability in cuttlefish

Sexual Selection Sexual selection is natural selection for _____________________ It can result in ___________________, marked differences between the sexes in secondary sexual characteristics mating success sexual dimorphism ___________________ is _______________ among individuals of one sex (often males) for mates of the opposite sex _______________________, often called mate choice, occurs when individuals of one sex (usually females) are choosy in selecting their mates Male showiness due to mate choice can increase a male’s chances of attracting a female, while decreasing his chances of survival Intrasexual selection competition Intersexual selection

Fig. 23-15 Figure 23.15 Sexual dimorphism and sexual selection

The distinctive feature of this monkey is their protruding nose The distinctive feature of this monkey is their protruding nose. It’s not defined about the purpose of the large nose, but it has been determined as the result of sexual selection. The female Proboscis Monkey prefers big-nosed male, thus propagating the trait.

► Heterozygote Advantage ► Frequency-Dependent Selection The Preservation of Genetic Variation Various mechanisms help to preserve genetic variation in a population ► Balancing Selection Balancing selection occurs when natural selection maintains _________ frequencies of two or more ________________________ in a population stable phenotypic forms ► Diploidy Diploidy maintains genetic variation in the form of ___________________ alleles hidden recessive ► Heterozygote Advantage Heterozygote advantage occurs when heterozygotes have a ___________ fitness than do both homozygotes Natural selection will tend to maintain two or more alleles at that locus The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance higher ► Frequency-Dependent Selection In frequency-dependent selection, the fitness of a phenotype _____________ if it becomes ________________ in the population Selection can favor whichever phenotype is _________________ in a population declines too common less common

Plasmodium falciparum (a parasitic unicellular eukaryote) 7.5–10.0% Fig. 23-17 Frequencies of the sickle-cell allele 0–2.5% Figure 23.17 Mapping malaria and the sickle-cell allele 2.5–5.0% 5.0–7.5% Distribution of malaria caused by Plasmodium falciparum (a parasitic unicellular eukaryote) 7.5–10.0% 10.0–12.5% >12.5%

“left-mouthed” individuals Fig. 23-18 Frequency-dependent selection in scale-eating fish (Perissodus microlepis) “Right-mouthed” 1.0 “Left-mouthed” “left-mouthed” individuals Frequency of 0.5 Figure 23.18 Frequency-dependent selection in scale-eating fish (Perissodus microlepis) 1981 ’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89 ’90 Sample year