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Chapter 23 The Evolution of PopulationsStudent Guided NotesOverview: The Smallest Unit of EvolutionNatural selection does act on individuals. Each individual’s traits affect its survival and its reproductive success relative to other individuals in the population.The evolutionary impact of natural selection is apparent only in the changes in a population of organisms over time and so it is the __________________________, not the ____________________________, which evolves.Microevolution is defined as ___________________________________________________________ ___________________________________________________________________________________.Three mechanisms can cause allele frequencies to change: ____________________________________, __________________________________________ (chance events that alter allele frequencies), and __________________________________ (the transfer of alleles between populations).Concept 23.1 Genetic variation makes evolution possibleGenetic variation occurs within a population.Individual variation occurs in all species and often reflects genetic variation, ____________________ __________________________________________________________________________________ ___________________________________________________________________________________. Both quantitative and discrete characters contribute to variation within a population.Discrete characters, such as flower color, are usually determined by a single locus with different alleles that produce distinct phenotypes.Quantitative characters, such as plant height, vary along a continuum within a population.Genetic variation occurs between populations.Species also exhibit geographic variation, differences in the genetic composition of geographically separate populations.Geographic variation in the form of graded change in a trait along a geographic axis is called a cline (example: birds have increasing body fat as latitude increases.)New genes and new alleles originate only by mutation.The genetic variation on which evolution depends originates when _____________________________, ______________________________________, or other processes produce new __________________ and new _______________________________. New alleles can arise by mutation, ______________________________________________________ ___________________________________________________________________________________.Some genetic variation in populations represents neutral variation that does not __________________ ________________________________________________________________ (example: fingerprints.)The tendency for natural selection to reduce variation is countered by mechanisms that _____________ ___________________________________________, including diploidy and balanced polymorphisms.Diploidy, having two copies of each chromosome, in eukaryotes prevents the elimination of recessive alleles via selection because recessive alleles do not affect the phenotype in heterozygotes.Even recessive alleles that are unfavorable can persist in a population by “hiding” in heterozygous individuals.Heterozygote protection maintains a huge pool of alleles that may not be suitable under the present conditions but may become beneficial when the environment changes. (balanced polymorphism)Sexual reproduction also increases variety via: _______________________, ____________________ ____________________________ and __________________________________.Concept 23.2 The Hardy-Weinberg equation can be used to test whether a population ________________For a population to evolve, individuals must differ genetically and one of the factors that causes evolution must be at work.A population’s gene pool is defined by its allele frequencies.A population is _____________________________________________________________________ ___________________________________________________________________________________.The total of all the alleles for all of the loci for all of the individuals in a population is called the population’s ____________________________.Each allele has a frequency or proportion in the population’s gene pool.For example, imagine a population of 500 wildflower plants with two alleles (CR and CW) at a locus that codes for flower pigment.Suppose that in the imaginary population of 500 plants, 20 (4%) are homozygous for the CW allele (CWCW) and have ______________________ flowers.Of the remaining plants, 320 (64%) are homozygous for the CR allele (CRCR) and have __________ flowers.These alleles show incomplete dominance, so 160 (32%) of the plants are heterozygous (CRCW) and produce ___________________ flowers.Because these plants are diploid, the population of 500 plants has 1,000 copies of the gene for flower color.The dominant allele (CR) accounts for 800 copies (320 × 2 for CRCR + 160 × 1 for CRCW) 640 + 160 = 800.The frequency of the CR allele in the gene pool of this population is 800/1,000 = 0.8, or ______%.The CW allele must have a frequency of 1.0 ? 0.8 = 0.2, or ______%.When there are two alleles at a locus, the convention is to use p to represent the frequency of one allele and q to represent the frequency of the other.Thus p, the frequency of the CR allele in this population, is _____.The frequency of the CW allele, represented by q, is ______.Allele and genotype frequencies can be used to test whether ____________ is occurring in a population.The Hardy-Weinberg principle describes a non-evolving population.The Hardy-Weinberg principle states that the frequencies of alleles and genotypes in a population’s gene pool will remain constant over generations unless acted upon by agents other than Mendelian segregation and recombination of alleles.The shuffling of alleles by meiosis and random fertilization has no effect on the overall gene pool of a population.Such a gene pool is said to be in _____________________________________________________.The Hardy-Weinberg principle states that the repeated shuffling of a population’s gene pool over generations does not increase the frequency of one allele over another.Theoretically, the allele frequencies in our flower population should remain 0.8 for CR and 0.2 for CW forever.In a population that has two alleles with frequencies p and q, the combined frequencies must add to 1, or 100%.Therefore p + q = 1.0____= frequency of dominant allele (B)____ = frequency of recessive allele (b) In addition, the genotype frequencies must add to 1.0: _____ + _____ + _____= ______._____ = frequency of homozygous dominant (BB) _____ = frequency of heterozygous dominant (Bb)_____ = frequency of homozygous recessive (bb)This general formula is the Hardy-Weinberg equation.Using this formula, we can calculate the frequencies of alleles in a gene pool if we know the frequencies of genotypes, or we can calculate the frequencies of genotypes if we know the frequencies of alleles.TIP: ALWAYS SOLVE FOR q FIRST!!!Five conditions must be met for a population to remain in Hardy-Weinberg equilibrium.Populations evolve because five conditions for non-evolving populations are rarely met for long in nature. A population must satisfy all five conditions to remain in Hardy-Weinberg equilibrium:_______________________________________________________. The gene pool is modified if mutations alter alleles or if entire genes are deleted or duplicated. _______________________________________________________. If individuals pick mates with certain genotypes, or if inbreeding is common, the mixing of gametes will not be random and genotype frequencies will change._______________________________________________________. Differential survival or reproductive success among genotypes will alter allele frequencies._______________________________________________________. In small populations, chance fluctuations in the gene pool will cause allele frequencies to change over time, a process called genetic drift.____________________________________________. Gene flow, the transfer of alleles due to the migration of individuals or gametes between populations, will change the frequencies of alleles.NOTE: Hardy-Weinberg explains NON-EVOLVING populations! If one or more of these 5 conditions ARE NOT met, then that means the population is EVOLVING. We can apply the Hardy-Weinberg principle to a human population.We can use the Hardy-Weinberg principle to estimate the percent of the human population that carries the allele for the inherited disease ___________________________________ (PKU). (refer to Genetic Disorder Chart CH 14/15)From the epidemiologic data, we know that the frequency of homozygous recessive individuals (q2 in the Hardy-Weinberg principle) is one in _____________________, or 0.0001.The frequency of the ___________________________ allele (q) is the square root of 0.0001 = 0.01.The frequency of the ___________________________ allele (p) is p = 1 ? q, or 1 ? 0.01 = 0.99.The frequency of carriers (heterozygous individuals) is 2pq = 2 × 0.99 × 0.01 = 0.0198, or about __%.Thus, about 2% of the U.S. population carries the PKU allele.Concept 23.3 Natural selection, genetic drift, and gene flow can alter allele frequencies in a populationNatural selection is based on differential survival and reproductive success.Individuals with variations better suited _______________________________________________ tend to produce more ______________________________ than those with variations that are less well suited.As a result of selection, alleles are passed on to the next generation in frequencies different from their relative frequencies in the present population.For example, the fruit fly Drosophila melanogaster has an allele that confers resistance ______________ ____________________________, including ______________. In 20 years after the first use of DDT, the allele frequency shifted from 0% to _____%.DDT was a strong ______________________ force, favoring alleles that conferred resistance.Note: If DDT had eliminated ALL pests in the early application (no survivors) it would have been effective.By _________________________________________________________________________ can cause adaptive evolution (evolution that results in a better match between organisms and their environment).Genetic drift results from chance fluctuations in allele frequencies in small populations.__________________________________ occurs when changes in gene frequencies from one generation to another occur because of ________________ events (sampling errors) that occur in small populations.Genetic drift in _______________________ populations may occur as a result of two situations: the bottleneck effect or the founder effect.The __________________________ effect occurs when a new population is started by a small number of individuals who do not represent the gene pool of the larger source population.The bottleneck effect occurs when the numbers of individuals in a large population are drastically ____________________________ by a disaster.FOUR key points about Genetic Drift: _____________________________________________________________________________________ _____________________________________________________________________________________._____________________________________________________________________________________ _____________________________________________________________________________________. _____________________________________________________________________________________ _____________________________________________________________________________________. _____________________________________________________________________________________ _____________________________________________________________________________________. A population may lose or gain alleles by gene flow.___________________________________ is the transfer of alleles among populations due to the migration of ________________________ individuals or ______________________.Gene flow tends to reduce _____________________________________________________________.Gene flow is an increasingly important agent of evolutionary change in _________________ populations. With increased human mobility, mating is more common between previously isolated populations, leading to an exchange of alleles and reducing _____________________________________ between human populations. Concept 23.4 Natural selection is the only mechanism that consistently causes adaptive evolutionEvolution by natural selection is _________________________________________________________ __________________________________________________________________________________________________________________________________________________________________________. Because of this favoring process, the outcome of natural selection is __________________________. Adaptive advantage can lead to greater relative fitness (aka “Darwinian Fitness”): ________________ __________________________________________________________________________________________________________________________________________________________________________.There are three modes of selection: ___________________________, ____________________________, and ________________________________.Natural selection can alter the frequency distribution of heritable traits in three ways, depending on _____________________________________________________________________________________.Directional selection is most common during periods of environmental change or migration and favors _________________________________________________________________________.Disruptive (aka “diversifying”) selection occurs when environmental conditions favor individuals ____________________________________________________________________ ____________________________________________ over those with intermediate phenotypes.Stabilizing selection favors ________________________________________________________ ____________________________________ (eg. human _______________________________.)Sexual selection may lead to pronounced secondary differences between the sexes.Sexual selection results in sexual dimorphism, _________________________________________________ _____________________________________________________ such as size, coloration, and ornamentation._______________________________ selection is direct competition among individuals of one sex for mates of the opposite sex.Intersexual selection, or _________________________, occurs when members of one sex (usually females) ________________________________________________________________________________________.Balancing selection preserve genetic variation._______________________________ selection occurs when natural selection maintains stable frequencies of two or more phenotypes in a population.One mechanism that produces balanced polymorphism is _________________________________________.In some situations, individuals who are heterozygous at a particular locus have greater fitness than homozygotes. (Ex. Individuals who are heterozygous for ______________________________________ are also resistant to ___________________________)A second mechanism that promotes balanced polymorphism is ____________________________________.Frequency-dependent selection occurs when the fitness of any one morph declines if it becomes too _______________________________ in the population.Frequency-dependent selection has been observed in a number of predator-prey interactions in the wild.Natural selection cannot fashion perfect organisms.There are at least four reasons natural selection cannot produce perfection.___________________________________________________________________________________.___________________________________________________________________________________. ___________________________________________________________________________________.___________________________________________________________________________________.With these constraints, evolution does not tend to craft perfect organisms. ................
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