PCB 4674 - Evolution Exam 2 - Questions with
Correct Answers (Verified Update)
67 questions with answers
Topic: PCB 4674 - Evolution Exam 2
1. Define these types of natural selection and be able to distinguish them from each
other: normalizing, directional, stabilizing, disruptive, and correlational selection.
Answer: - Normalizing selection: Selection in which individuals at both extremes are
eliminated as each generation passes by, selecting for the middle distribution of individuals
and reducing genetic variation in the population as a whole - Directional selection: Occurs
when individual fitness tends to increase or decrease with the value of phenotypic trait; can
result in steady evolutionary change in the mean value of the trait in the population -
Stabilizing selection: Occurs when individuals with intermediate values of a trait have higher
fitness; can result in reduced phenotypic variation in a population and can prevent evolution
in the mean value of the trait - Disruptive selection: Occurs when individuals with more
extreme values of a trait have higher fitness; can result in increased phenotypic variation in a
population - Correlational selection: Occurs when selection favors certain combinations of
traits; it can promote both genetic correlations and high levels of genetic variation;
occasionally favors a group of traits at the expense of others and if it does favor a particular
set of traits it will include the most used traits whose functional effectiveness is essential for
their ability to work together, and whose successful interaction is needed for the fitness of
the individual
2. Can genotypes differ in their probability of survival to maturity and yet still have
no differences in fitness? If not, why not?
Answer: No, there can only be a difference in survival probability if there is a difference in
fitness because fitness, by definition, is the contribution of an individual to the rate of
population growth, and by that logic it would be impossible for individuals to have the same
fitness but have a different probability of survival to maturity. However, their fitness might
also differ due to genetic drift, such as in the case of "dumb luck" in which fitness is
decreased just due to a chance event occurring
3. Would we expect a greater response to sustained directional selection in a larger
or smaller population? Justify your answer.
Answer: It will occur greater in larger population since there is greater genetic variance in
large populations. As a rule of thumb, alleles in larger populations are fixed more rapidly by
natural selection (in this case sustained directional selection) and alleles in smaller
populations are fixed more rapidly by genetic drift
4. Which species concept, if any, seems the best for recognizing species of asexually
reproducing bacteria?
Answer: The cohesion species concept, as this concept includes the act of conjugation
between bacteria in its criteria that a species must have the potential to exhange genetic
material with one another (which unlike the biological species concept, includes asexually
reproducing species)
5. Define what we mean by "reinforcement" in the study of species formation and
discuss why this is an important concept.
Answer: Reinforcement reduces the frequency of hybrids among populations. This occurs
through natural selection producing adaptations for distinct habitats, sexual selection
producing changes in the mating system, and genetic drift leading to fixation of alleles in
both populations that don't work well together in heterozygote form.
Page 1
, 6. --What do we mean by "the Bateson- Dobzhansky-Mueller hypothesis"? Which
critical data or result allows us to diagnose if this process is acting?
Answer: In their model, a new allele arises and becomes fixed at one point. In the other now
diverged lineage, a different allele arises at a different point on the gene. The two diverged
species are now incompatible with each other because they have two alleles that have never
been together in the same population. This changes the proteins that are produced during
transcription and translation, which could make the hybrid offspring inviable. However, each
lineage can still hypothetically reproduce with the ancestral population. If these new
mutations in the lineages are advantageous, eventually they will become permanent alleles
in each population and the ancestral population has split into two new species. Evidence is
shown when the fitness of the F2 generation is lower
7. You know that in a finite population the probability that a neutral allele will
achieve fixation from this point in time onward - or at any given point in time - is
simply its relative frequency in the population at that moment in time. Summon your
powers of deductive reasoning and tell us what is the probability that a brand new
neutral mutant will be fixed in a population of N haploid individuals.
Answer: The probability will be "1 / (2 * N)"
8. Distinguish "rooted" from "unrooted" phylogenetic trees.
Answer: - Rooted trees: cladogram or phylogenetic tree in which the position of the deepest
common ancestor is specified either a priori (via choice of outgroup) or by assumptions of
global or basal parsimony; each node with descendants represents the inferred most recent
common ancestor of the descendants, and the edge lengths in some trees may be
interpreted as time estimates. - Unrooted trees: cladogram or phylogenetic tree that does not
specify the position of the deepest common ancestor; illustrate only the relatedness of the
leaf nodes, and do not require the ancestral root to be known or inferred.
9. A scientist was studying the relationship between fitness and adult tail length in a
local species of lizard. She found that for tail lengths in the range from 50 mm to 100
mm, fitness increased linearly, but from 101 mm to about 130 mm, which was the
longest tail she measured, fitness changed no further and was relatively constant.
Does this fit in our simple paradigm of "directional, stabilizing, disruptive, etc."?
How would you describe it?
Answer: It is stabilizing if and only if we know that the fitness begins to decrease as tail
length increases in specimen beyond 130 mm. In this case, with what we know it goes from a
directional selection from 50 mm to 100 mm and stabilizing up to 130 mm.
10. What are the specific genetic and phenotypic effects of inbreeding?
Answer: Genetically, inbreeding serves to increase the frequency of homozygotes compared
to what is expected under Hardy-Weinberg assumptions, however it does not change the
frequency of alleles and therefore cannot serve as a mechanism of evolution. Phenotypically,
this increase in homozygosity can have its most profound phenotypic effects with inbreeding
depression, or the reduced fitness of inbred individuals, which results usually from the
exposure of deleterious recessive alleles (often lethal alleles) that would otherwise be
present in heterozygotes but not expressed phenotypically.
11. What is Haldane's Rule?
Answer: If, in a species hybrid, only one sex is inviable or sterile, that sex is more likely to be
the heterogametic sex. The heterogametic sex is the one with two different sex
chromosomes; in mammals, for example, this is the male; "When in the F1 offspring of two
different animal races one sex is absent, rare, or sterile, that sex is the heterozygous sex
(heterogametic sex)."
Page 2
Correct Answers (Verified Update)
67 questions with answers
Topic: PCB 4674 - Evolution Exam 2
1. Define these types of natural selection and be able to distinguish them from each
other: normalizing, directional, stabilizing, disruptive, and correlational selection.
Answer: - Normalizing selection: Selection in which individuals at both extremes are
eliminated as each generation passes by, selecting for the middle distribution of individuals
and reducing genetic variation in the population as a whole - Directional selection: Occurs
when individual fitness tends to increase or decrease with the value of phenotypic trait; can
result in steady evolutionary change in the mean value of the trait in the population -
Stabilizing selection: Occurs when individuals with intermediate values of a trait have higher
fitness; can result in reduced phenotypic variation in a population and can prevent evolution
in the mean value of the trait - Disruptive selection: Occurs when individuals with more
extreme values of a trait have higher fitness; can result in increased phenotypic variation in a
population - Correlational selection: Occurs when selection favors certain combinations of
traits; it can promote both genetic correlations and high levels of genetic variation;
occasionally favors a group of traits at the expense of others and if it does favor a particular
set of traits it will include the most used traits whose functional effectiveness is essential for
their ability to work together, and whose successful interaction is needed for the fitness of
the individual
2. Can genotypes differ in their probability of survival to maturity and yet still have
no differences in fitness? If not, why not?
Answer: No, there can only be a difference in survival probability if there is a difference in
fitness because fitness, by definition, is the contribution of an individual to the rate of
population growth, and by that logic it would be impossible for individuals to have the same
fitness but have a different probability of survival to maturity. However, their fitness might
also differ due to genetic drift, such as in the case of "dumb luck" in which fitness is
decreased just due to a chance event occurring
3. Would we expect a greater response to sustained directional selection in a larger
or smaller population? Justify your answer.
Answer: It will occur greater in larger population since there is greater genetic variance in
large populations. As a rule of thumb, alleles in larger populations are fixed more rapidly by
natural selection (in this case sustained directional selection) and alleles in smaller
populations are fixed more rapidly by genetic drift
4. Which species concept, if any, seems the best for recognizing species of asexually
reproducing bacteria?
Answer: The cohesion species concept, as this concept includes the act of conjugation
between bacteria in its criteria that a species must have the potential to exhange genetic
material with one another (which unlike the biological species concept, includes asexually
reproducing species)
5. Define what we mean by "reinforcement" in the study of species formation and
discuss why this is an important concept.
Answer: Reinforcement reduces the frequency of hybrids among populations. This occurs
through natural selection producing adaptations for distinct habitats, sexual selection
producing changes in the mating system, and genetic drift leading to fixation of alleles in
both populations that don't work well together in heterozygote form.
Page 1
, 6. --What do we mean by "the Bateson- Dobzhansky-Mueller hypothesis"? Which
critical data or result allows us to diagnose if this process is acting?
Answer: In their model, a new allele arises and becomes fixed at one point. In the other now
diverged lineage, a different allele arises at a different point on the gene. The two diverged
species are now incompatible with each other because they have two alleles that have never
been together in the same population. This changes the proteins that are produced during
transcription and translation, which could make the hybrid offspring inviable. However, each
lineage can still hypothetically reproduce with the ancestral population. If these new
mutations in the lineages are advantageous, eventually they will become permanent alleles
in each population and the ancestral population has split into two new species. Evidence is
shown when the fitness of the F2 generation is lower
7. You know that in a finite population the probability that a neutral allele will
achieve fixation from this point in time onward - or at any given point in time - is
simply its relative frequency in the population at that moment in time. Summon your
powers of deductive reasoning and tell us what is the probability that a brand new
neutral mutant will be fixed in a population of N haploid individuals.
Answer: The probability will be "1 / (2 * N)"
8. Distinguish "rooted" from "unrooted" phylogenetic trees.
Answer: - Rooted trees: cladogram or phylogenetic tree in which the position of the deepest
common ancestor is specified either a priori (via choice of outgroup) or by assumptions of
global or basal parsimony; each node with descendants represents the inferred most recent
common ancestor of the descendants, and the edge lengths in some trees may be
interpreted as time estimates. - Unrooted trees: cladogram or phylogenetic tree that does not
specify the position of the deepest common ancestor; illustrate only the relatedness of the
leaf nodes, and do not require the ancestral root to be known or inferred.
9. A scientist was studying the relationship between fitness and adult tail length in a
local species of lizard. She found that for tail lengths in the range from 50 mm to 100
mm, fitness increased linearly, but from 101 mm to about 130 mm, which was the
longest tail she measured, fitness changed no further and was relatively constant.
Does this fit in our simple paradigm of "directional, stabilizing, disruptive, etc."?
How would you describe it?
Answer: It is stabilizing if and only if we know that the fitness begins to decrease as tail
length increases in specimen beyond 130 mm. In this case, with what we know it goes from a
directional selection from 50 mm to 100 mm and stabilizing up to 130 mm.
10. What are the specific genetic and phenotypic effects of inbreeding?
Answer: Genetically, inbreeding serves to increase the frequency of homozygotes compared
to what is expected under Hardy-Weinberg assumptions, however it does not change the
frequency of alleles and therefore cannot serve as a mechanism of evolution. Phenotypically,
this increase in homozygosity can have its most profound phenotypic effects with inbreeding
depression, or the reduced fitness of inbred individuals, which results usually from the
exposure of deleterious recessive alleles (often lethal alleles) that would otherwise be
present in heterozygotes but not expressed phenotypically.
11. What is Haldane's Rule?
Answer: If, in a species hybrid, only one sex is inviable or sterile, that sex is more likely to be
the heterogametic sex. The heterogametic sex is the one with two different sex
chromosomes; in mammals, for example, this is the male; "When in the F1 offspring of two
different animal races one sex is absent, rare, or sterile, that sex is the heterozygous sex
(heterogametic sex)."
Page 2
