Bsci110b Chapter 23 Concept Check

1. Explain why genetic variation within a population is a prerequisite for evolution. Within a population, genetic differences among individuals provide the raw material on which and other mechanisms can act. Without such differences, could not change over time, and hence the population could not .
What factors can produce genetic variation among populations.Genetic variation among populations can arise by natural selection if selection favors different in different ; this might occur, for example, if the different populations experienced different . Genetic variation among populations can also arise by when the genetic differences between populations are selectively .

Genetic variation within a population is a prerequisite for evolution because it provides the raw material on which natural selection and other mechanisms can act. Without genetic differences among individuals, there would be no variation for natural selection to act upon, and therefore no potential for the population to evolve over time. Factors such as natural selection, allele frequencies, environmental conditions, genetic drift, and neutral mutations can all contribute to genetic variation among populations.

Explanation

Genetic variation within a population is a prerequisite for evolution because it provides the raw material on which natural selection and other mechanisms can act. Without genetic differences among individuals, there would be no variation for natural selection to act upon, and therefore no potential for the population to evolve over time. Factors such as natural selection, allele frequencies, environmental conditions, genetic drift, and neutral mutations can all contribute to genetic variation among populations.

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2. 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 cells that do not produce gametes and 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 can act. Others have a harmful effect and are thus unlikely to increase in frequency because they decrease the reproductive success of their bearers.

Only a small fraction of mutations become widespread among the population's members because many mutations occur in somatic cells that do not produce gametes and are lost when the organism dies. Additionally, mutations that occur in cell lines that produce gametes may not have a phenotypic effect on which natural selection can act. Furthermore, mutations that have a harmful effect decrease the reproductive success of their bearers and are therefore unlikely to increase in frequency.

Explanation

Only a small fraction of mutations become widespread among the population's members because many mutations occur in somatic cells that do not produce gametes and are lost when the organism dies. Additionally, mutations that occur in cell lines that produce gametes may not have a phenotypic effect on which natural selection can act. Furthermore, mutations that have a harmful effect decrease the reproductive success of their bearers and are therefore unlikely to increase in frequency.

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3. If a population stopped reproducing sexually (but still reproduced asexually), how would its genetic variation be affected over time? Its genetic variation, whether measured at the level of the gene or at the level of nucleotide sequences, would probably over time. During meiosis, and of chromosomes produce many new combinations of alleles. In addition, a population contains a vast number of possible mating combinations, and fertilization brings together the gametes of individuals with different genetic backgrounds. Thus, via crossing over, independent assortment of chromosomes, and fertilization, sexual reproduction reshuffles alleles into fresh combinations each generation. Without , new sources of genetic variation would be reduced, causing the overall amount of genetic variation to drop.

If a population stopped reproducing sexually, its genetic variation would likely decrease over time. This is because sexual reproduction introduces new combinations of alleles through processes like crossing over and independent assortment of chromosomes. These processes create fresh combinations of alleles each generation. Without sexual reproduction, these sources of genetic variation would be reduced, leading to a decrease in overall genetic variation in the population.

Explanation

If a population stopped reproducing sexually, its genetic variation would likely decrease over time. This is because sexual reproduction introduces new combinations of alleles through processes like crossing over and independent assortment of chromosomes. These processes create fresh combinations of alleles each generation. Without sexual reproduction, these sources of genetic variation would be reduced, leading to a decrease in overall genetic variation in the population.

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4. What are the three sources of alleles to produce fresh combinations each generation.

Crossing over refers to the exchange of genetic material between homologous chromosomes during meiosis, resulting in new combinations of alleles. Independent assortment occurs when homologous chromosomes align randomly during meiosis, leading to the distribution of alleles into gametes in various combinations. Fertilization is the fusion of gametes from two different individuals, bringing together different combinations of alleles and further increasing genetic diversity. These three processes collectively contribute to the production of fresh combinations of alleles in each generation.

Explanation

Crossing over refers to the exchange of genetic material between homologous chromosomes during meiosis, resulting in new combinations of alleles. Independent assortment occurs when homologous chromosomes align randomly during meiosis, leading to the distribution of alleles into gametes in various combinations. Fertilization is the fusion of gametes from two different individuals, bringing together different combinations of alleles and further increasing genetic diversity. These three processes collectively contribute to the production of fresh combinations of alleles in each generation.

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5. In what sense is natural selection more “predictable” than genetic drift?
Natural selection is more “predictable” in that it alters allele frequencies in a way: It tends to the frequency of alleles that increase the organism’s reproductive success in its environment and the frequency of alleles that decrease the organism’s reproductive success. Alleles subject to genetic drift increase or decrease in frequency by alone, whether or not they are advantageous.

Natural selection is more "predictable" than genetic drift because it is nonrandom. Natural selection tends to increase the frequency of alleles that increase an organism's reproductive success in its environment and decrease the frequency of alleles that decrease reproductive success. On the other hand, alleles subject to genetic drift can increase or decrease in frequency by chance alone, regardless of whether they are advantageous or not.

Explanation

Natural selection is more "predictable" than genetic drift because it is nonrandom. Natural selection tends to increase the frequency of alleles that increase an organism's reproductive success in its environment and decrease the frequency of alleles that decrease reproductive success. On the other hand, alleles subject to genetic drift can increase or decrease in frequency by chance alone, regardless of whether they are advantageous or not.

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6. Distinguish genetic drift from gene flow in terms of (a) how they occur and (b) their implications for future genetic variation in a population. results from chance events that cause allele frequencies to fluctuate at random from generation to generation; within a population, this process tends to genetic variation over time. is the exchange of alleles between populations; a process that can introduce new alleles to a population and hence may its genetic variation.

Genetic drift occurs due to chance events causing random fluctuations in allele frequencies over generations, leading to a decrease in genetic variation within a population over time. On the other hand, gene flow refers to the exchange of alleles between populations, introducing new alleles and increasing the genetic variation within a population.

Explanation

Genetic drift occurs due to chance events causing random fluctuations in allele frequencies over generations, leading to a decrease in genetic variation within a population over time. On the other hand, gene flow refers to the exchange of alleles between populations, introducing new alleles and increasing the genetic variation within a population.

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7. Explain why natural selection is the only evolutionary mechanism that consistently leads to adaptive evolution.
Although both and can increase the frequency of advantageous alleles in a population, they can also the frequency of advantageous alleles or the frequency of harmful alleles. Only consistently results in an increase in the frequency of alleles that enhance survival or reproduction. Thus, is the only mechanism that consistently causes adaptive evolution.

Natural selection is the only evolutionary mechanism that consistently leads to adaptive evolution because it is the only process that consistently increases the frequency of alleles that enhance survival or reproduction. Gene flow and genetic drift can both increase the frequency of advantageous alleles, but they can also decrease the frequency of advantageous 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. Therefore, natural selection is the only mechanism that consistently causes adaptive evolution.

Explanation

Natural selection is the only evolutionary mechanism that consistently leads to adaptive evolution because it is the only process that consistently increases the frequency of alleles that enhance survival or reproduction. Gene flow and genetic drift can both increase the frequency of advantageous alleles, but they can also decrease the frequency of advantageous 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. Therefore, natural selection is the only mechanism that consistently causes adaptive evolution.

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