Chapter 23 practice problems / case study problems Flashcards
Explain why genetic variation within a population is a prerequisite for evolution.
Genetic variation among a population allows for natural selection and other mechanisms to act. Since natural selection only favors heritable traits and alleles already in the population, if a population does not have variation, the population won’t evolve.
Of all the mutations that occur in a population, why do only a small fraction become widespread?
Natural selection favors advantageous traits, so many mutations that don’t code for advantageous traits are thusly unfavored and are removed from the population.
Mutations are mainly deleterious, so many of these traits are lost due to the mechanism of natural selection.
Additionally, most mutations occur in somatic cells, which don’t pass genetic info to gametes.
If a population stopped reproducing sexually (but still reproduced asexually), how would genetic variation be affected over time? Explain.
Genetic variation would decrease because crossing over, independent assortment, and random fertilization are all modes that cause further genetic variance and are only available with sexual reproduction.
A population has 700 individuals, 85 individuals of genotype AA, 320 of genotype Aa, and 295 of genotype aa. What are the frequencies of A and a alleles?
Freq(A) = 0.35
Freq (B) = 0.65
The frequency of allele b is 0.45 in a population in HW equilibrium. What are the expected genotype frequencies?
BB = 0.3025
Bb = 0.495
bb = 0.2025
A locus that affects susceptibility to a degenerative brain disease has two alleles, V and v. In a population, 16 people have genotype VV, 92 have Vv, and 12 have genotype vv. Is the
population evolving? Explain.
The population is evolving. Remember when you get your p2, 2pq, and q2 values multiply them by the population number to compare! If the numbers are the same, then the population isn’t evolving. If they’re not the same, then it is evolving!
In what sense is natural selection more predictable than genetic drift?
Natural selection enhances a population of organisms over time by selecting advantageous alleles. Genetic drift selects for random alleles in a population, with no pattern/mechanism.
Genetic drift vs. gene flow
Genetic drift is an event that depreciates the population, thus selecting at random different alleles in surviving organisms. Since this event deteriorates the population, the genetic variance is low, and further offspring will have low genetic variance.
Gene flow is when new alleles are introduced to a population by migrating populations and thus increase genetic variance.
Suppose two plant populations exchange pollen and seeds. In one population, individuals of genotype AA are most common (9,000 AA, 900 Aa, and 100 aa), while the opposite is try in the other populations (100 AA, 900 Aa, and 9,000 aa). If neither allele has a selective advantage, what will happen over time to the allele and genotype frequencies of these populations?
if neither allele provides a selective advantage, the frequency will end up balancing between populations as pollen and seeds are exchanged and gene flow occurs between them.
What is the relative fitness of a sterile mule? Explain?
The relative fitness of the mule is 0, because part of relative fitness is fertility and contributing to the next generation.
Explain why natural selection is the only evolutionary mechanism that consistently leads to adaptive evolution.
Although genetic drift and gene flow can both cause advantageous alleles in a population, they can also cause an increase in harmful alleles in a population because they are random. Natural selection is the only evolutionary mechanism that consistently selects for advantageous alleles, thus allowing organisms who benefit from these mechanisms to adapt to their surroundings.
Consider a population in which heterozygotes at a certain locus have an extreme phenotype (such as being larger than homozygotes) that confers a selective advantage. Does this situation represent directional, disruptive, or stabilizing selection? Explain your answer.
Three modes of natural selection (disruptive, directional, and/or stabilizing) refer to phenotype, not genotype. Because of this, the phenotype of the heterozygotes is extreme, and therefore this would be a directional selection mode
