Ch 21: The Evolution of Populations Flashcards
In a population with two alleles, B and b, the allele frequency of b is 0.4. What would be the frequency of heterozygotes if the population is in Hardy-Weinberg equilibrium?
0.48
( 2 x 0.4 (b) x 0.6 (B) )
(- heterozygotes = 2pq)
Selection that acts over evolutionary time to preserve traits that increase an individual’s ability to mate is known as __________.
sexual selection
Genetic analysis of a large population of mink inhabiting an island in Michigan revealed an unusual number of loci where one allele was fixed. Which of the following is the most probable explanation for this genetic homogeneity?
A very small number of mink may have colonized this island, and this founder effect and subsequent genetic drift could have fixed many alleles.
An increase in the number of antibiotic-resistant individuals in a population of bacteria is an example of __________.
directional selection
In the context of a population, how is evolution defined?
Evolution is a change in a population’s allele frequencies over generations.
Assume a population is in Hardy-Weinberg equilibrium for a character with these genotypic frequencies: AA = 0.25, Aa = 0.50, and aa = 0.25. If you remove all the homozygous dominants and allow the remaining population to reproduce (again under Hardy-Weinberg conditions), what will be the frequency of homozygous dominants in the next generation?
0.11
(Since 2 x 0.25 = 0.50, one = 0.
AA (q) = 0, Aa (2pq) = 2/3 aa (p) = 1/3.
p^2 = 0.11)
A population of squirrels is preyed on by small hawks at maturity. The smaller adult squirrels can escape into burrows. The larger adult squirrels can fight off the hawks. After several generations, the squirrels in the area tend to be very small or very large. This is an example of __________.
disruptive selection
A population of 15 birds inhabits a small island. Ten of the birds are dark brown, and five of them are light brown. By chance, two of the dark brown birds and three of the light brown birds die before producing any offspring. All of the birds in the next generation are dark brown. This change in phenotypic frequency can be attributed to __________.
genetic drift
The average heterozygosity of Drosophila melanogaster is estimated to be about 14%, which means that __________.
on average, 14% of a fruit fly’s gene loci are heterozygous
In a particular central African population, 4% are born with sickle-cell disease (they are homozygous recessive at a hemoglobin loci). If this population is in Hardy-Weinberg equilibrium, what percentage of the population is heterozygous at the relevant allele and has the selective advantage of being more resistant to malaria?
32%
(4% = 1/population = 0.04
sqrt 0.04 = 0.2
1-0.2 = 0.8
2 x 0.2 x 0.8 = 0.32 = 32%)
Which of the following mechanisms can alter allele frequencies?
All of the listed responses are correct.
Birds with average-size wings survived a severe storm more successfully than other birds in the same population with longer or shorter wings. If severe storms occur regularly, then over time, one should expect these storms to bring about __________.
stabilizing selection
Which of the following sets of conditions is required for Hardy-Weinberg equilibrium?
Random mating, no natural selection, and a large population
An earthquake hits a small island. All but a few individuals of a lizard species are eliminated in this catastrophe, and an allele is lost from the population. This is an instance of __________.
the bottleneck effect
Which of the following provides most of the genetic variation found among individuals in plant and animal populations?
Sexual reproduction
A scientist observes that the height of a certain species of asters decreases as the altitude on a mountainside increases. She gathers seeds from samples at various altitudes, plants them in a uniform environment, and measures the height of the new plants. All of her experimental asters grow to approximately the same height. From this she concludes __________.
the height variation she initially observed was an example of environmental influence
Which of the following is a true statement concerning sexual selection?
Sexual selection can result in sexual dimorphism.
In a large population of randomly breeding organisms, the frequency of a recessive allele is initially 0.3. There is no migration and no selection. Humans enter this ecosystem and selectively hunt individuals showing the dominant trait. When the gene frequency is reexamined at the end of the year, __________.
the frequency of the homozygous dominants will go down, the frequency of the heterozygous genotype will go down, and the frequency of the homozygous recessives will go up
A plant population is found in an area that is becoming more arid. The average surface area of leaves has been decreasing over the generations. This trend is an example of __________.
directional selection
Which of the following is the best example of gene flow?
Wind blows pollen from one population of plants to another and cross-fertilization occurs.
Sexual recombination includes the shuffling of chromosomes in __________ and fertilization.
meiosis
Approximately 1 out of every 2,500 Caucasians in the United States is born with the recessive disease cystic fibrosis. According to the Hardy-Weinberg equilibrium equation, approximately what percentage of people are carriers?
4%
(1/2500)
A fruit fly population has a gene with two alleles, A1 and A2. Tests show that 70% of the 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?
0.42
(70% in allele which means p or q.
If p = 0.7, q = 0.3, 2pq (2 x 0.7 x 0.3) = 0.42)
For several years, scientists have warned doctors of the danger of overprescribing antibiotics, such as penicillin. Scientists are concerned because __________.
natural selection can lead to strains of microorganisms resistant to these drugs
In a population that is in Hardy-Weinberg equilibrium for two alleles, C and c, for a character determined by complete dominance, 16% of the population show a recessive trait. Assuming C is dominant to c, what percent of individuals show the dominant trait?
84%
(dominant = p, recessive = q,
1 - 0.16 = 0.84)
Three main mechanisms that cause allele frequency change
– Natural selection
– Genetic drift
– Gene flow
Sexual reproduction can shuffle existing alleles into new
combinations through three mechanisms
– Crossing over
– Independent assortment
– Random fertilization
a group of individuals that live in the
same area and interbreed, producing fertile offspring
population
consists of all the alleles for all loci in a
population
gene pool
A population in which mating is random and none of the
mechanisms of evolution are acting is in
Hardy-Weinberg
equilibrium
Hardy- Weinberg formula
p squared + 2pq + q squared = 1
How to find q for Hardy- Weinberg
1/ number of population, then sq rt of that
How to find p for Hardy- Weinberg
1-q
Five Conditions for Hardy-Weinberg Equilibrium
- no mutations
- random mating
- no natural selection
- Extremely large population size
- no gene flow
occurs when a few individuals
become isolated from a larger population
founder effect
can result from a drastic reduction
in population size due to a sudden environmental change
bottleneck effect
consists of the movement of alleles among
populations
Gene flow
There are three modes of natural selection
- directional selection
- disruptive selection
- stabilizing selection
occurs when conditions favor
individuals at one end of the phenotypic range
Directional selection
occurs when conditions favor
individuals at both extremes of the phenotypic range
Disruptive selection
occurs when conditions favor
intermediate variants and act against extreme
phenotypes
Stabilizing selection
occurs when natural selection
maintains stable frequencies of two or more phenotypic
forms in a population
Balancing selection
Balancing selection includes
– Heterozygote advantage
– Frequency-dependent selection
occurs when heterozygotes
have a higher fitness than do both homozygotes
Heterozygote advantage
the fitness of a
phenotype depends on how common it is in the
population
frequency-dependent selection,
