2. Chapter 23 Reading Quiz
1. What is all of the genes in a population
called?
2. In the Hardy-Weinberg equation, what
symbolizes the dominant allele?
3. Name one way in which natural populations
do not fit Hardy-Weinberg equilibrium.
4. Any change in the allele frequencies of a
population is called _____.
5. 10,000 years ago, cheetahs went through
what type of genetic drift?

3. 1. Very large population size
2. Isolation from other populations (no
migration in or out of group)
3. No net mutations
4. Random mating
5. No natural selection  all genotypes
are equal in survival and reproductive
success 

4. 1. Explain what is meant by the “modern
synthesis”.
 Comprehensive theory integrating discoveries
from different fields (paleontology, taxonomy,
biogeography, and population genetics)
 Emphasized  the importance of populations
as units of evolution
 central role of natural selection as the
primary mechanism of evolutionary change
 gradualism as the explanation of how large
changes can result from an accumulation of
small changes over long periods of time 

6. 2. Explain how microevolutionary change can
affect a gene pool.
 Gene pool  the total aggregate of genes in
a population at any one time
 Microevolution  small scale evolutionary
change represented by a generational shift in
a population’s relative allelic frequencies 

8. 3. In your own words, state the Hardy-
Weinberg theorem.
 The frequencies of alleles in the gene pool
will remain constant unless acted upon by
other agents
 Describes the genetic structure of non-
evolving populations 

11. 4. Write the general Hardy-Weinberg equation
and use it to calculate allele and genotype
frequencies.
P2
+ 2pq + q2
= 1
 The sum of frequencies must = 100%
(p + q = 1)
 When 2 alleles exist, only the frequency of
one must be known since the other is derived
 1 – p = q OR 1 – q = p 

12. #4 example
Ex: One individual in every ten thousand has ‘phenylketonuria’, a
deficiency which does not allow the body to process the amino acid
phenylalanine. What percent of the population are carriers for this
recessive disease?
1 in 10, 000 ; recessive = q2
; q2
= .0001 ; q = .01
p = 1 – q ; p = 1 - .01 ; p = .99
p2
+ 2pq + q2
= 1 ; 2pq = 2(.99)(.01) = .0198
therefore: 2% of the population are carriers for phenylketonuria 

13. More examples…
1. .01% of the Caucasian population has
cystic fibrosis. In a sample population
of 100,000 white people, how many
would be expected to carry the
disease?

14. About 1 in 17,000 kids in the UK are born with
albinism. This is a recessive disorder. On
average, what % of the population would be
carriers for albinism?

15. In humans, Huntington’s disease is lethal in utero if
both dominant alleles are inherited. The
disease, however, is dominant. Therefore, for
an individual to inherit the disease, they must
be heterozygous for the condition. 7 in 100,000
people have Huntington’s. What are the
dominant and recessive frequencies for this
disorder? How many people in the 100,000
would be recessive for this condition?

16. 5. Explain the consequences of Hardy-Weinberg
equilibrium.
 It provides a baseline from which
evolutionary departures take place
 It provides a reference point with which to
compare the frequencies of alleles and
genotypes of natural populations whose gene
pools may be changing 

17. 6. Demonstrate, with a simple example, that a
disequilibrium population requires only one generation
of random mating to establish Hardy-Weinberg
equilibrium.
 Continued sexual reproduction with segregation,
recombination, and random mating would not alter
the frequencies of alleles and the gene pool would be
in Hardy-Weinberg equilibrium
 If the gene pool was originally in disequilibrium, only
one generation would be necessary for equilibrium to
be established (as long as random mating is occurring
in the population) 

18. 7. List the conditions a population must meet in
order to maintain Hardy-Weinberg equilibrium.
1. Very large population size
2. Isolation from other populations (no
migration in or out of group)
3. No net mutations
4. Random mating
5. No natural selection  all genotypes are
equal in survival and reproductive success 

19. 8. Explain how genetic drift, gene flow,
mutation, nonrandom mating and natural
selection can cause microevolution.
 These all cause microevolution because each
of these conditions is a deviation from the
criteria for Hardy-Weinberg equilibrium 

20. Genetic Drift

21. 9. Explain the role of population size in genetic
drift.
 Genetic drift  changes in the gene pool of a
small population due to chance
 if a population is small, its existing gene
pool may not be accurately represented in
the next generation due to sampling error
 chance events may cause the frequencies
of alleles to drift randomly from generation
to generation 

22. 10. Distinguish between the bottleneck effect
and the founder effect.
 Bottleneck  genetic drift which results
from drastic reduction in population size
- reduces overall genetic variability in a
population since some alleles may be gone
 Founder  when a few individuals colonize a
new habitat and genetic drift occurs
- inherited diseases are obvious examples 

24. 11. Explain why mutation has little quantitative
effect on a large population.
 Mutation itself has little quantitative effect
on large populations in a single generation
since mutation at any given locus is very rare


25. 12. Describe how inbreeding and assortative
mating affect a population’s allele frequencies
and genotype frequencies.
 Inbreeding  results in relative genotypic
frequencies that deviate from the frequencies
predicted for Hardy-Weinberg equilibrium, but does
not alter frequencies of alleles (p & q) in the gene
pool
 Assortative mating  type of nonrandom mating
which results when individuals mate with partners
that are like themselves in certain phenotypic
characters
ex: toads commonly mate with those of the same size
ex: snow geese (blue with blue, white with white)
- results in less heterozygotes than Hardy-Weinberg
predicts 

26. 13. Explain, in your own words, what is meant by the
statement that natural selection is the only agent of
microevolution which is adaptive.
 It is the only agent which is adaptive, since it
accumulates and maintains favorable genotypes
 environmental change would result in selection
favoring genotypes present in the population which
can survive the new conditions
 variability in the population makes it possible for
natural selection to occur 

27. 14. Describe the technique of electrophoresis and
explain how it has been used to measure genetic
variation within and between populations.
 The technique allows researchers to identify
variations in protein products of specific
gene loci
 Within  the Drosophila population gene pool has 2
or more alleles for about 30% of the loci examined –
bottom line: any two flies will differ in genotype at
about 25% of their loci
 Between  geographical variation in allele
frequencies exists among populations of most species
– due to  natural selection, genetic drift, localized
inbreeding 

28. 15. List some factors that can produce
geographical variation among closely related
populations.
1. Natural selection: environmental factors
differ among locals
2. Genetic drift: causes chance variations
among different populations
3. Localized inbreeding: subpopulations can
appear resulting from a ‘patchy’
environment
4. Cline: one type of geographical variation
that is a graded change in some trait along
a geographic transect 

30. 16. Explain why even though mutation can be a source of
genetic variability, it contributes a negligible amount to
genetic variation in a population.
 Mutations produce new alleles  they are
random and rare events which usually occur in
somatic cells and are thus not inheritable
- only mutations that occur in cell lines which
will produce gametes can be passed to the
next generation
- estimate only 1 or 2 mutations occur in each
human gamete-producing cell line 

31. 17. Give the cause of nearly all genetic variation
in a population.
 Sexual recombination provides almost
entirely the genetic variation which makes
adaptation possible
 Due to  gametes vary extensively from
crossing over and random segregation during
meiosis 

32. 18. Explain how genetic variation may be
preserved in a natural population.
 Diploidy  hides much genetic variation
from selection by the presence of recessive
alleles in heterozygotes (not expressed and
not selected against)
 Balanced polymorphism  the ability of
natural selection to maintain diversity in a
population
 Heterozygote advantage  have greater
reproductive success (ex: sickle cell &
anemia) 

34. 19. In your own words, briefly describe the neutral
theory of molecular evolution and explain how changes
in gene frequency may be nonadaptive.
 It states that many variant alleles at a locus
may confer no selective advantage or
disadvantage
 variation in DNA which does not code for
proteins may be nonadaptive 

35. 20. Explain what is meant by “selfish” DNA.
 Noncoding DNA has resulted from the
inherent capacity for DNA to replicate
itself and has expanded to the
tolerance limits of each species
 The entire genome could exist as a
consequence of self-replication rather
than providing an adaptive advantage to
the organism - “selfish DNA” 

36. 21. Explain the concept of relative fitness and
its role in adaptive evolution.
 Relative fitness  the contribution of a genotype to
the next generation compared to the contributions
of alternative genotypes for the same locus
 every aspect of survival and fecundity
(reproductive success) are components of fitness
Ex: pink flowers (AA & Aa) produce more offspring
than white (aa), therefore AA & Aa genotypes have a
higher relative fitness 

37. 22. Explain why the rate of decline for a deleterious
allele depends upon whether the allele is dominant or
recessive to the more successful allele.
 Deleterious recessives are normally
protected from elimination by heterozygote
protection
 Selection against harmful dominant alleles is
faster since they are expressed in
heterozygotes 

38. 23. Describe what selection acts on and what factors
contribute to the overall fitness of a genotype.
 Selection acts on phenotypes, indirectly
adapting a population to its environment
by increasing or maintaining favorable
genotypes in the gene pool
 An organism is an integrated composite
of many phenotypic features and the
fitness of a genotype at any one locus
depends upon the entire genetic
context 

39. 24. Give examples of how an organism’s
phenotype may be influenced by the
environment.
 physical traits, metabolism, physiology,
and behavior are all exposed to the
environment and may be selected upon


40. 25. Distinguish among stabilizing selection, directional
selection and diversifying selection.
 Stabilizing selection  favors intermediate
(average) variants by selecting against extreme
phenotypes
 Ex: spider size  large spiders are more easily found
by predators; small spiders have difficulty finding
and getting food; average size spiders can hide and
find food

41. #25 continued…
 Directional selection  favors variants of one
extreme – shifts frequency curve for phenotypic
variations in one direction toward rare variants
which deviate from the average
 Ex: woodpecker beaks – the long beak is always
selected over the average and short lengths

42. #25 continued…
 Diversifying selection  opposite phenotypic
extremes are favored over intermediate phenotypes
 Ex: limpets (shelled animals) in a tidal area that is
dark and light, with no in-between color
- dark and light limpets are selected, not the
intermediate colors

44. 26. Define sexual dimorphism and explain how it
can influence evolutionary change.
 It is the distinction between the secondary
sexual characteristics of males and females
 Ex: size, plumage, lion manes, deer antlers,
etc…
 Separate selection process –
- have no other adaptive advantage other
than attracting mates
- showier can contribute more to gene pool 

46. 27. Give at least four reasons why natural
selection cannot breed perfect organisms.
1. Organisms are locked into historical constraints
(descent with modification)
2. Adaptations are often compromises
- must be versatile
3. Not all evolution is adaptive
- genetic drift; alleles become fixed in small populations
4. Selection can only edit variations that exist
- these variations may not represent ideal characteristics
- new genes are not formed by mutation on demand 