Biology II Chapter 23

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Biology II Chapter 23
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2011-10-06 18:27:42
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Biology Campbell Evolution Populations
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Chapter 23 of Campbell's Biology Textbook 8th - The Evolution of Populations
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  1. What is microevolution?
    The evolutionary change in populations on its smallest scale as the changes in allele frequencies in a population over generations.

    The three mechanisms that can cause allele frequency change include:

    • 1) Natural selection
    • 2) Genetic drift (Chance events that alter allele frequencies)
    • 3) Gene flow (the transfer of alleles between populations)
  2. What are the three mechanisms that can cause allele frequency change in populations?
    • 1) Natural selection
    • 2) Genetic drift (Chance events that alter allele frequencies)
    • 3) Gene flow (the transfer of alleles between populations)
  3. (T/F) Mutations and sexual reproduction produce the genetic variation that makes evolution possible.
    True.
  4. (T/F) Some phenotypic variation is not heritable.
    True.
  5. Characters that vary within a population may be discrete or quantitative. What are discrete characters and what are quantitative characters?
    Discrete characters, such as the purple or white flowers of Mendel's pea plants can be classified on an either-or basis (each plant has flowers that are either purple or white). Many discrete characters are determined by a single gene locus with different alleles that produce distinct phenotypes.

    Quantitative character: A heritable feature that varies continuously over a range rather than in an either-or fashion. Most quantitative characters usually result from the influence of two or more gene on a single phenotypic character.
  6. Genevariation in a population, at both the whole-gene level (gene variablity) and the molecular level of DNA (nucleotide variability) can be quantified as the _____________, the average percent of loci that are heterozygous.

    A heterozygous individual has two different alleles fora given locus, whereas a homozygous individual has two identical alleles for that locus.
    average heterozygosity

    Average heterozygosity is often estimated by surveying the protein products of genes using gel electrophoresis.
  7. What is geographic variation?
    Differences in the genetic composition of separate populations.
  8. What is a cline?
    A graded change in a character along a geographic axis.

    Some clines are produced by a gradation in an environmental variable, as illustrated by the impact of temperature on the frequency of a cold-adaptive alleles in mummichog fish. Such clines are probably result from natural selection or mutation.
  9. What is mutation?
    A change in the nucleotide sequence of an organism's DNA.

    A mutation is like a shot in the dark - we cannot predict accurately which segments of DNA will be altered or in what way. In multicellular organisms, only mutations in cell lines that produce gametes can be passed to offspring.

    In plants and fungi, this is not as limiting as it may sound, since many different cell lines can produce gametes.

    In animals, most mutations occur in somatic cells and are lost when the individual dies.
  10. What is a point mutation?
    A change of as little as one base in a gene can have a significant impact on phenotype.

    Organisms reflect thousands of generations of past selection, and hence their phenotypes generally provide a close match to their environment. As a result, it's unlikely that a new mutation that alters a pheotype will improve it but most such mutations are at least slighly harmful. Much of the DNA in eukaryotic genomes do not code for protein products, and point mutations in these noncoding regions are often harmless.

    On rare occasions, a mutant allele may actually make its bearer better suited to the environment, enhancing reproductive success.
  11. (T/F) Chromosomal changes that delete, disrupt, or rearrange many loci at once are almost certain to be harmful. However, when such large-scale mutations leave gene intact, their effects on organisms may be neutral. In rare cases, chromosomal rearrangements may even be beneficial.
    True.

    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.

    Gene duplications that do not have severe effects can persist over generations, allowing mutations to accumulate. The result is an expanded genome with new loci that may take on new functions. Such beneficial increases in gene number appear to have played a major role in evolution.

    For instance, the remote ancestors of mammals carried a single gene for detecting oders that has been duplicated many times. As a result, humans today have about 1,000 olfactory receptor genes, and mice have 1,300.
  12. (T/F) Mutation rates tend to be low in plants and animals, averaging about one mutation in every 100,000 genes per generation, and they are often even lower in prokaryotes.

    However, prokaryotes typically have short generation spans, so mutations can quickly generate genetic variation in these organisms. The same is true for viruses.
    True.
  13. (T/F) Sexual reproduction is a method for gene variation.
    True.

    In organisms that reproduce sexually, most of the genetic variation in a population results from the unique combination of alleles that each individual receives. Of course, at the nucleotide level, all the differences among these alleles have originated from past mutations. But it is the mechanism of sexual reproduction that shuffles existing alleles and deals them at random to determine individual genotypes.
  14. What three mechanisms contribute to this shuffling in sexual reproduction?
    Crossing over; independent assortment of chromosomes and fertilization.

    During meiosis, homologous chromosomes, on inherited from each parent, trade some of their alleles by crossing over. These homologous chromosomes and the alleles they carry are then distributed at random into gametes.

    Then, because myriad possible mating combinations exist in a population, fertilization brings together gametes of individuals that are likely to have different genetic backgrounds.

    The combined effects of these three mechanisms ensure that sexual reproduction rearranges existing alleles into fresh combination each generation, providing much of the genetic variation that makes evolution possible.
  15. a) Explain why genetic variation within a population is a prerequisite for evolution. b) What factors can produce genetic variation among populations?
    a) Within a population, genetic differences among individuals provide the raw material on which natural selection and other mechanisms can act. Without such differences, allele frequencies could not change over time - and hence the population could not evolve.

    b) Genetic variation among populations can arise by natural selection if selection favors different alleles in different populations; this might occur, for example, if the different populations experienced different environmental conditions. Genetic variation among populations can arise by genetic drift when the genetic differences between populations are selectively neutral.
  16. Of all the mutations that occur in a population, why do only a small fraction become widespread among the population's members?
    Many mutations occur in somatic cells that do not produce gametes and are so are lost when the organism dies. Of mutations that do occur in cell lines that produce gametes, many do not have a phenotypic effect on which natural selection can act. Others have a harmful effect and thus unlikely to increase in frequency because they decrease the reproductive sucessess of their bearers.
  17. What is the equation used to test whether a population is evolving?
    The Hardy-Weinberg equation.

    p^2 + 2pq + q^2 = 1
  18. (T/F) The presence of genetic variation does not guarantee that a population will evolve.
    True
  19. What is a population?
    A group of individuals of the same species that live in the same area and interbreed, producing fertile offspring.

    Different populations of a single species may be isolated geographically from one another, thus exchanging genetic material only rarely. Such isolation is common for species that live on widely separated islands or in different lakes. But not all populations are isolated, nor must populations have sharp boundaries.

    Still, members of a population typically breed with one another and thus on average are more closely related to each other than to members of other populations.
  20. What is a gene pool?
    A gene pool is the aggregate of all of the alleles for all of the loci in all individuals in a population. The term gene pool is also used in a more restricted sense as the aggregate of alleles for just one or a few loci in a population.

    If only one allele exists for a particular locus in a population, that allele is said to be fixed in the gene pool, and all individuals are homozygous for that allele. But if there are two or more alleles for a particular locus in a population, individuals may be either homozygous or geterozygous.
  21. Each allele has a frequency (propotion) in the population.

    For example, in a population of 500 wildflower plants with two alleles, C^R and C^W, for a certain locus that codes for flower pigment. These alleles show incomplete dominance where plants homozygous for the C^R allele produce red pigment and have red flowers; plants homozygous for the C^W allele produce no red pigment and have white flowers. Heterozygotes (C^R C^W) produce what color flowers?

    a) red
    b) pink
    c) white
    B.

    The flowers exhibit incomplete dominance meaning they would exhibit both of the phenotypes brought on by the alleles, red and white. This translates to a pink flower.
  22. Each allele has a frequency (propotion) in the population.

    For example, in a population of 500 wildflower plants with two alleles, C^R and C^W, for a certain locus that codes for flower pigment. These alleles show incomplete dominance where plants homozygous for the C^R allele produce red pigment and have red flowers; plants homozygous for the C^W allele produce no red pigment and have white flowers.
    Heterozygotes (C^R C^W) produce pink flowers.

    In our population, suppose there are:
    320 plants with red flowers
    160 plants with pink flowers
    20 plants with white flowers

    In the population of 500 plants, how many copies of genes are present for flower color?

    hint: These are diploid organisms
    1000 copies of genes for flower color in the population of 500 plants.
  23. Each allele has a frequency (propotion) in the population.

    For example, in a population of 500 wildflower plants with two alleles, C^R and C^W, for a certain locus that codes for flower pigment. These alleles show incomplete dominance where plants homozygous for the C^R allele produce red pigment and have red flowers; plants homozygous for the C^W allele produce no red pigment and have white flowers.
    Heterozygotes (C^R C^W) produce pink flowers.

    In our population, suppose there are:
    320 plants with red flowers
    160 plants with pink flowers
    20 plants with white flowers

    In the population of 500 plants, how many C^R alleles are present? How many C^W alleles are present?
    • p is the frequency of allele C^R: 800
    • q is the frequency of allele C^W: 200

    For loci that have more than two alleles, the sum of all allele frequencies must equal 1.

    These numbers are used for the Hardy-Weinberg Principle.

    p^2 + 2pq + q^2 = 1

    • p^2 = expected frequency of C^R C^R genotype
    • q^2 = expected frequency of C^W C^W genotype
    • 2pq = expected frequency of C^W C^R (same as C^R C^W) genotype
  24. What is the Hardy-Weinberg Equilibrium?
    The principle that states the freqency 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.

    p^2 + 2pq + q^2 = 1

    • p^2 = expected frequency of C^R C^R genotype
    • q^2 = expected frequency of C^W C^W genotype
    • 2pq = expected frequency of C^W C^R (same as C^R C^W) genotype

  25. What are the five conditions that must be met for the Hardy-Weinberg Equilibrium to be valid? How might these five conditions be summarized?
    The Hardy-Weinberg Equilibrium assumes that the population at equilibrium - meaning no evolution.

    The conditions are as follows:

    1) No mutations - By altering alleles or (in large-scale changes) deleting or duplicating entire genes, mutations modify the gene pool.

    2) Random mating - If individuals mate preferentially within a subset of the population, such as their close relatives (inbreeding), random mixing of gametes does not occur, and genotype frequencies change.

    3) No natural selection - Differences in the survival and reproductive success of individuals carrying different genotypes can alter allele frequencies.

    4) Extremely large population size - The smaller the population, the more likely it is that allele frequencies will fluctuate by chance from one generation to the next (genetic drift)

    5) No gene flow - By moving alleles into or out of populations, gene flow can alter allele frequencies.
  26. What does departure from any of the five conditions below results in?

    1) Mutations
    2) Nonrandom mating
    3) Natural selection
    4) Small population size
    5) Gene flow
    evolution

    It is typical that if one of these conditions occurs that evolution will result.
  27. What equation is oftenly used to test if evolution is occuring in a population?
    The Hardy-Weinberg equation.
  28. If all conditions are met for the Hardy-Weinberg principle and the frequency of individuals in a population with a exhibiting a particular recessive disease is 1 out of 10,000, what is the frequency of carriers (heterozygous people who do not have the disease but pass the disease allele to offspring)?
    0.0198 or 2% of the population.

    q^2 = 1/10000 => q = 0.01

    • frequency of the dominant allele is
    • p = 1 - q = 1 - 0.01 = 0.99

    • The frequency of carriers is
    • 2pq = 2 x 0.99 x 0.01 = 0.0198
  29. Suppose a population of organisms with 500 loci is fixed at half of these loci and has two alleles at each of the other loci. How many different alleles are found in its entire gene pool? Explain.
    750.

    Half of the loci (250) are fixed, meaning only one allele exists for each locus: 250 x 1 = 250.

    There are two alleles each for the other loci: 250 x 2 = 500.

    250 + 500 = 750.
  30. If P is the frequency of allele A, which parts of the Hardy-Weinberg equation correspond to the frequency of individuals that have at least one A allele?
    • p^2 + 2pq
    • p^2 represents homozygotes with two A alleles
    • 2pq represents heterozygotes with one A allele
  31. (T/F) Natural selection, genetic drift, and gene flow can alter allele frequencies in a population.
    True.

    New mutations can alter allele frequencies but because mutations are rare, the change from one generation to the next is likely to be very small. Nevertheless, mutation ultimately can have a large effect on allele frequencies when it produces new alleles that strongly influence fitness in a positive or negative way.

    Nonrandom mating can affect the frequencies of homozygous and heterozygous genotypes but by itself usually has no effect on allele frequencies in the gene pool.

    The three mechanisms that alter allele frequencies directly and cause most evolution change are natural selection, genetic drift, and gene flow.
  32. (T/F) The three mechanisms that alter allele frequencies directly and cause most evolution change are natural selection, genetic drift, and gene flow.
    True.

    mutations are rare and nonrandom mating usually has little to no effect on allele frequencies in the gene pool itself.
  33. Consistently favoring some alleles over others, natural selection can cause adaptive evolution. What is adaptive evolution?
    Adaptive evolution is evolution that results in a better match between organisms and their environment.
  34. What is genetic drift?
    A process in which chance events cause unpredictable fluctuations in allele frequencies from one generation to the next.

    Effects of genetic drift are most pronounced in small populations.
  35. What is the founder effect?
    When a few individuals become isolated from a larger population, this smaller group may establish a new population whose gene pool differs from the source population.

    The founder effect might occur, for example, when a few members of a population are blown by a storm to a new island. Genetic drift - in which chance events alter allele frequencies - occurs in such a case because the storm indiscriminately transports some individuals (and their alleles), but not others, from the source population.
  36. What is the bottleneck effect?
    A sudden change in the environment, such as a fire or flood, may drastically reduce the size of a population. The severe drop in population size can cause the bottleneck effect, so named because the population has passed through a restrictive "bottleneck" in size

    Ongoing genetic drift is likely to have substantial effects on the gene pool until the population becomes large enough that chance events have less effect. But even if a population that has passed through a bottleneck ultimately recovers in size, it may have low levels of genetic variation for a long period of time - a legacy of the genetic drift that occured when the population was small.

    By chance alone, certain alleles may be overrepresented among the survivors, others may be underrepresented, and some may be absent altogether.
  37. (T/F) Genetic drift is significant in small populations.
    True.

    Chance events can cause an allele to be disproportionately over- or underrepresented in the next generation. Although chance events occur in populations of all sizes, they alter allele frequencies substantially only in small populations.
  38. (T/F) Genetic drift can cause allele frequencies to change at random.
    True.

    Because of genetic drift, an allele may increase in frequency one year, then decrease the next; the change from one year to the next is not predictable.

    Thus, unlike natural selection, which in a given environment consistently favors some alleles over others, genetic drift causes allele frequencies to change at random over time.
  39. (T/F) Genetic drift can lead to a loss of genetic variation within populations.
    True.

    By causing allele frequencies to fluctuate randomly over time, genetic drift can eliminate alleles from a population. Because evolution depends on genetic variation, such losses can influence how effectively a population can adapt to a change in the environment.
  40. (T/F) Genetic drift can cause harmful alleles to become fixed.
    True.

    Alleles can be lose or become fixed entirely by chance through genetic drift. Alleles fixed by genetic drift are as likely to be harmful to the organism as to be beneficial or of no effect. When drift leads to the fixation of harmful alleles, the survival of a small population can be threatened (as for the Illinois greater prairie chicken exhibiting the bottleneck effect).
  41. What is gene flow?
    The change of allele frequencies by the transfer of alleles into or out of a population due to the movement of fertile individuals or their gametes.
  42. In what sense is natural selection more "predictable" than genetic drift?
    Natural selection is more "predictable" in that it alters allele frequencies in a nonrandom way: It tends to increase the frequency of alleles that increase the organism's reproductive success in its environment and decrease the frequency of alleles that decrease the organism's reproductive success.

    Alleles subject to genetic drift increase or decrease in frequency by chance alone, whether or not they are advantageous.
  43. Distinguish genetic drift from gene flow in terms of (a) how they occur and (b) their implications for future genetic variation in a population.
    Genetic drift results from chance events that cause allele frequencies to fluctuate at random from generation to generation; within a population, this process tends to decrease genetic variation over time.

    Gene flow is the exchange of alleles between populations; a process that can introduce new alleles to a population and hence may increase its genetic variation (albeit slightly, since rates of gene flow are often low)
  44. (T/F) Adaptive advantage can lead to greater relative fitness.
    True.

    For example, a barnacle that is more efficient at collecting food than its neighbors may have greater stores of energy and hence be able to produce a larger number of eggs.
  45. What is relative fitness?
    The contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals.
  46. (T/F) The entity that is subjected to natural selection is the whole organism and acts more directly on the phenotype than on the genotype.
    True.

    Although we often refer to the relative fitness of a genotype, remember that the entity that is subjected to natural selection is the whole organism, not the underlying genotype. Thus, selection acts more directly on the phenotype than on the genotype; it acts on the genotype indirectly, via how the genotype affects the phenotype.
  47. What are the three modes of selection that natural selection can alter the frequency distribution of heritable traits?
    Directional selection - Natural selection in which individuals at one end of the phenotypic range survive or reproduce more successfully than do other individuals.

    Disruptive selection - Natural selection in which individuals on both extremes of a phenotypic range survive or reproduce more successfully than do individuals with intermediate phenotypes.

    Stablizing selection - Natural selection in which intermediate phenotypes survive or reproduce more successfully than do extreme phenotypes.
  48. What is directional selection?
    Directional selection - Natural selection in which individuals at one end of the phenotypic range survive or reproduce more successfully than do other individuals.
  49. What is disruptive selection?
    Disruptive selection - Natural selection in which individuals on both extremes of a phenotypic range survive or reproduce more successfully than do individuals with intermediate phenotypes.
  50. What is stabilizing selection?
    Stablizing selection - Natural selection in which intermediate phenotypes survive or reproduce more successfully than do extreme phenotypes.
  51. What is sexual selection?
    A form of natural selection in which individuals with certain inherited characteristics are more likely than other individuals to obtain mates.

    Sexual selection operates though intrasexual selection (selection within same sex) and intersexual selection (mate choice).

    Sexual selection can result in sexual dimorphism, marked differences between the two sexes in secondary sexual characteristics, which are not directly associated with reproduction or survival. These distinctions include differences in size, color, ornamentation, and behavior.
  52. What is intrasexual selection?
    A direct competition among individuals of one sex (usually males in vertebrates) for mates of the opposite sex.
  53. What is intersexual selection?
    Selection whereby individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex; also called mate choice.
  54. (T/F) Because most eukaryotes are diploid, a considerable amount of genetic variation is hidden from selection in the form of recessive alleles.

    Recessive alleles that are less favorable than their dominant counterparts, or even harmful in the current encironment, can persist by propagation in heterozygous individuals.
    True.

    This is one of the methods that genetic variation is preserved.
  55. What is balancing selection?
    Natural selection that maintains two or more phenotypic forms in a population.

    This type of selection include heterozygote advantage and frequency-dependent selection.
  56. What is heterozygote advantage?
    Greater reproductive success of heterozygous individuals compared with homozygotes; tends to preserve variation in a gene pool.
  57. What is frequency-dependent selection?
    A decline in the reproductive success of individuals that have a phenotype that has become too common in a population.

    This tends to preserve variation in a gene pool.
  58. What is neutral variation?
    Genetic variation that does not appear to provide a selective advantage or disadvantage.
  59. (T/F) Selection can act only on existing variations.
    True.

    Natural selection favors only the fittest phenotypes among those currently in the population, which may not be the ideal traits. New advantageous alleles do not arise on demand.
  60. (T/F) Evolution is limited by historical constraints.
    True.

    Each species has a legacy of descent with modification from ancestral forms. Evolution does not scrap the ancestral anatomy and build each new complex structure from scratch; rather, evolution co-opts existing structures and adapts them to new situations.

    We could imagine that if a terrestial animal were to adapt to an environment in which flight would be advantageous, it might be best just to grow an extra pair of limbs that would serve as wings.

    However, evolution does not work in this way, instead operating on the traits an organism already has. Thus, in birds and bats, an existing pair of limbs took on new functions for flight as these organisms evolved from walking ancestors.
  61. (T/F) Adaptations are often compromises.
    True.

    Each organism must do many different things. A seal spends part of its time on rocks; it could probably walk better if it had legs instead of flippers, but then it would not swim nearly as well.

    We humans owe much of our versatility and athleticism to our prehensile hands and flexible limbs, but these also make us prone to sprains, torn ligaments, and dislocations: Structural reinforcement has been compromised for agility.
  62. (T/F) Chance, natural selection, and the environment interact.
    True.

    Chance events can affect the subsequent evolutionary history of populations.

    For instance, when a storm blows insects or birds hundreds of kilometers over an ocean to an island, the wind does not necessarily transport those individuals that are best suited to the new environment. Thus, not all alleles present in the founding population's gene pool are better suited to the new environment than alleles that are "left behind".

    In addition, the environment at a particular location may change unpredictably from year to year, again limiting the extent to which adaptive evolution results in a close match between the organism and current environmental conditions.
  63. What is the relative fitness of a sterile mule? Explain
    Zero

    Fitness includes reproductive contribution to the next generation, and a sterile mule cannot produce offspring
  64. Explain why natural selection is the only evolutionary mechanism that consistently leads to adaptive evolution.
    Although both gene flow and genetic drift can increase the frequency of advantageous alleles in a population, they can also decrease the frequency of advantage alleles or increase the frequency of harmful alleles.

    Only natural selection consistently results in an increase in the frequency of alleles that enhance survival or reproduction. Thus, natural selection is the only mechanism that consistently causes adaptive evolution.
  65. A fruit fly population has a gene with two alleles, A1 and A2. Tests show that 70% of gametes produced in the population contain the A1 allele. If the population is in Hardy-Weinberg equilibrium, what proportion of the flies carry both A1 and A2.

    a) 0.7
    b) 0.49
    c) 0.21
    d) 0.42
    e) 0.09
    D

    Notice that 70% contain A1 gametes - this implies both that either p or q is equal to 0.7 (depending on your selection).

    • p+q = 1
    • p^2 + 2pq +q^2 = 1

    p = 0.7; q = 0.3

    heterozygotes (contain both A1 and A2) = 2pq = 0.42
  66. There are 40 individuals in population 1, all of which have genotype A1A1, and there are 25 individuals in population 2, all of genotype A2A2. Assume that these populations are located far from one another and that their environmental conditions are very similar. Based on the information given here, the observed genetic variation is most likely an example of

    a) genetic drift
    b) gene flow
    c) disruptive selection
    d) discrete variation
    e) directional selection.
    A

    Both populations are at the same conditions (thus not a form of natural selection), and far away (thus not gene flow).
  67. Natural selection changes allele frequencies because some _____ survive and reproduce more successfully than others.

    a) alleles
    b) loci
    c) gene pools
    d) species
    e) individuals
    E

    Natural selection: Individuals in a population exhibit variations in their heritable traits, and those with traits that are better suited to their environement tend to produce more offspring than those with traits that are less well suited.
  68. No two people are genetically identical, except for identical twins. The chief cause of genetic variation among human individuals is

    a) new mutations that occured in the preceding generation.
    b) the reshuffling of alleles in sexual reproduction
    c) genetic drift due to the small size of the population
    d) geographic variation within the population
    e) environmental effects
    B
  69. Sparrows with average-sized wings survive severe storms better than those with longer or shorter wings, illustrating
    a) the bottle neck effect
    b) stablizing selection
    c) frequency-dependent selection
    d) neutral variation
    e) disruptive selection
    B
  70. How is the process of evolution revealed by the imperfections of living organisms?

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