IB Biology Chapter 23
In the Hardy-Weinberg equation, p2 represents __________.
the expected frequency of homozygous dominant individuals in the population
In the Hardy-Weinberg equation, 2pq represents __________.
the expected frequency of the heterozygous genotype
The human genome consists of approximately 3 billion base pairs. If humans typically differ from one another by about 3 million base pairs, what is the nucleotide variability of Homo sapiens?
0.1%
Assume a population in Hardy-Weinberg equilibrium 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 The new frequencies of the three genotypes are: AA = 0, Aa = 0.66, and aa = 0.33. If there were 100 individuals (carrying 200 alleles), 66/200 alleles would be A and p = 0.33. The frequency of homozygous dominant individuals would then be p2, or 0.11
In a population that is in Hardy-Weinberg equilibrium, 64% of the individuals express the recessive phenotype for a particular gene locus. What is the expected frequency of the dominant allele in this population?
0.2
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
In a population that is in Hardy-Weinberg equilibrium, 64% of the individuals express the recessive phenotype for a particular gene locus. What is the expected frequency of the recessive allele in this population?
0.8
In a population in Hardy-Weinberg equilibrium, 1% of the individuals in a population show the recessive trait of a certain characteristic. In this situation, what is the value of p?
0.9 In this equation q2 = .01, q = 0.1, 1 − 0.1 = p = 0.9.
In a large population of bonobos, the frequency of the recessive allele is initially 0.1. There is no migration and no selection. What is the frequency of the dominant allele? Assume that there are two alleles of this gene
0.9 p (the frequency of the dominant allele) = 1 − q (the frequency of the recessive allele), or 0.9.
In a certain group of African people, 4% are born with sickle-cell disease (homozygous recessive). If this group is in Hardy-Weinberg equilibrium, what percentage of the group has the selective advantage of being more resistant to malaria (heterozygous) than those individuals who are homozygous for normal hemoglobin or for sickle-cell disease?
32% The 4% born with sickle-cell disease are homozygous for the sickle hemoglobin allele, so q2 = 0.04 and q = 0.2. Because p = 1 − q, p = 0.8. The frequency of heterozygotes would be equal to 2pq, or 0.32, which could be expressed as 32%.
Describe a scenario that would most likely result in the microevolution of a population of humans?
A colony of humans on the moon is isolated from Earth. Genetic drift (through the founder effect) could occur in such a population.
intersexual selection
A form of natural selection in which individuals of one sex (usually the females) are choosy in selecting their mates from the other sex; also called mate choice.
intrasexual selection
A form of natural selection in which there is direct competition among individuals of one sex for mates of the opposite sex.
genetic drift
A process in which chance events cause unpredictable fluctuations in allele frequencies from one generation to the next. Effects of genetic drift are most pronounced in small populations.
sexual selection
A process in which individuals with certain inherited characteristics are more likely than other individuals of the same sex to obtain mates.
Name a statement that correctly describes a population in Hardy-Weinberg equilibrium?
Allele and genotype frequencies in the population will remain constant from generation to generation.
What does Hardy Weinberg do say about allele frequencies for populations in equilibrium?
Allele frequency must remain the same
Which type of selection maintains stable frequencies of two or more phenotypic forms in a population?
Balancing selection. Balancing selection preserves variation through many mechanisms, such as heterozygote advantage and frequency-dependent selection.
sexual dimorphism
Differences between the secondary sex characteristics of males and females of the same species.
How does natural selection fashion organisms?
Chance and the environment interact with natural selection, so that the best available traits are selected for.
microevolution
Evolutionary change below the species level; change in the allele frequencies in a population over generations.
genetic variation
Differences among individuals in the composition of their genes or other DNA segments.
What is the difference between directional, stabilizing and disruptive selection?
Directional: Favors individuals at one extreme end of the phenotypic range of the population Stabilizing: Favors the intermediate forms of the phenotype and acts against the extremes Disruptive: Favors the extreme forms of the phenotype (can lead to a creation of new species)
A population of squirrels is preyed on by small hawks. The smaller squirrels can escape into burrows. The larger squirrels can fight off the hawks. After several generations, the squirrels in the area tend to be very small or very large. What process is responsible for this outcome?
Disruptive selection
In the context of populations, how do we define evolution?
Evolution is a change in a population's allele frequencies over generations.
adaptive evolution
Evolution that results in a better match between organisms and their environment.
Which of the following statements explains why male peacocks with brightly colored feathers are more prevalent than those with plain colors?
Female peacocks choose the showiest males as mates, causing this trait to be more prevalent in the population.
What is the relationship between 'fixed' loci and nucleotide variability?
Fixed loci= Homozygous Nucleotide variability= Heterozygous
founder effect
Genetic drift that occurs when a few individuals become isolated from a larger population and form a new population whose gene pool composition is not reflective of that of the original population.
bottleneck effect
Genetic drift that occurs when the size of a population is reduced, as by a natural disaster or human actions. Typically, the surviving population is no longer genetically representative of the original population.
How does genetic drift differ from natural selection?
Genetic drift: Random events cause changes in gene frequencies Natural Selection: Environment allows certain genes to survive and reproduce (not random)
neutral variation
Genetic variation that does not provide a selective advantage or disadvantage.
heterozygote advantage
Greater reproductive success of heterozygous individuals compared with homozygotes; tends to preserve variation in a gene pool.
For heterozygous individuals, why do we use the formula 2pq and not just pq?
Heterozygous individuals could be dominant x recessive or recessive x dominant
The lethal genetic disorder Tay-Sachs disease occurs in individuals with two copies of a recessive allele of the responsible gene. What would this cause?
Heterozygous individuals will survive and may pass the recessive allele on to their offspring.
Although the sickle cell gene is lethal for homozygous individuals, the sickle cell gene still persists in areas where there is malaria. Explain why.
Heterozygous protection-- The heterozygous individuals are able to survive malaria. However, this also means that there is a chance their offspring will be homozygous individuals.
What is the difference between intrasexual selection and sexual selection?
Intrasexual: Within the same sex (same sex competing for mates; mostly between males) Sexual: Between different sexes (one sex chooses their mate based off of defining characteristics; mostly with females)
Which of the following statements correctly describe(s) characteristics of genetic drift?
It can lead to a loss of genetic variation in a population. It is significant in small populations. It can cause allele frequencies to change at random. It can cause harmful alleles to become fixed in a population.
Describe what drought does to the beak size in Galapagos finches.
Makes the beak size bigger-- only large seeds were left after the drought so a big beak was needed to crack open the large seeds
A change in gene frequency within a population is considered microevolution or macroevolution?
Microevolution
What is the difference between microevolution and macroevolution?
Microevolution: Evolutionary change that occurs below the species level Macroevolution: Evolutionary change that occurs above the species level
__________ and __________ generate variation, whereas __________ results in an adaptation to the environment.
Mutation ... sexual recombination ... natural selection
directional selection
Natural selection in which individuals at one end of the phenotypic range survive or reproduce more successfully than do other individuals.
disruptive selection
Natural selection in which individuals on both extremes of a phenotypic range survive or reproduce more successfully than do individuals with intermediate phenotypes.
stabilizing selection
Natural selection in which intermediate phenotypes survive or reproduce more successfully than do extreme phenotypes.
balancing selection
Natural selection that maintains two or more phenotypic forms in a population.
What is the importance of neutral variation in evolution?
Neutral variation increases genetic variation, allowing a population to carry more alleles that may help it respond to environmental change.
Do all changes in DNA cause a change in phenotype? Explain.
No, they may be a mutation in an intron or the mutation may code for the same amino acid, resulting in the same protein
What does p and q represent?
P= dominant gene q= recessive gene
What usually causes a bottleneck effect?
Random catastrophic events
Which of the following sets of conditions is required for Hardy-Weinberg equilibrium?
Random mating, no natural selection, and a large population
frequency-dependent selection
Selection in which the fitness of a phenotype depends on how common the phenotype is in a population.
Create a true statement about sexual selection?
Sexual selection can result in sexual dimorphism—marked differences between the sexes in secondary sexual characteristics that are not associated directly with reproduction.
Genetic drift affects what kind of population the most?
Small populations
gene pool
The aggregate of all copies of every type of allele at all loci in every individual in a population. The term is also used in a more restricted sense as the aggregate of alleles for just one or a few loci in a population.
relative fitness
The contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals in the population.
Define gene flow.
The movement of alleles between populations as individuals and their gametes move
Describe founder's effect.
The new population has gene frequencies that are different from the original population
Hardy-Weinberg Equilibrium
The state of a population in which frequencies of alleles and genotypes remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at work.
gene flow
The transfer of alleles from one population to another, resulting from the movement of fertile individuals or their gametes.
If two distinct populations have significant gene flow, what is the predicted outcome?
They will come back into one species
At what level does gene variability quantify genetic variation?
Whole-gene
Name an example of gene flow
Wind blows pollen from one population of plants to another and cross-fertilization occurs.
A number of mosquito populations today are resistant to insecticides that were once quite effective. Biologists think that insecticide resistance evolved in mosquitoes because __________.
a few mosquitoes were probably resistant to the insecticide before it was ever used, and these individuals were more likely to survive and reproduce in the presence of insecticide
what mechanism can form entirely new alleles?
mutation
Consider the following mutations to the nucleotide sequence of a gene. Which of these mutations would not alter the reading frame of the gene's genetic message?
a single nucleotide-pair substitution
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?
about 4% If q2 equals 1/2,500, then q equals .02, p equals .98, and 2pq equals approximately .04, or 4%.
The gene pool of a population consists of __________.
all copies of every type of allele at every locus in all members of the population
Antibiotic resistance in bacteria is an example of what?
directional selection
A population of squirrels is preyed on by small hawks. The smaller squirrels can escape into burrows. The larger squirrels can fight off the hawks. After several generations, the squirrels in the area tend to be very small or very large. What process is responsible for this outcome?
disruptive selection
Which type of mutation plays the most important role in increasing the number of genes in the gene pool?
duplication
Stabilizing selection __________.
favors intermediate variants in a population
Allele frequencies can be altered by __________.
gene flow, genetic drift, and natural selection
If the nucleotide variability of a locus equals 0%, what is the gene variability and number of alleles at that locus?
gene variability = 0%; number of alleles = 1
A population of 15 birds inhabits a fairly new 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
There are 25 individuals in population 1, all with genotype AA, and there are 40 individuals in population 2, all with genotype aa. Assume that these populations are located far from each other and that their environmental conditions are very similar. Based on the information given here, the observed genetic variation is most likely an example of
genetic drift
What most accurately measures an organism's fitness?
how many fertile offspring it produces. Fitness is related to the number of fertile, viable offspring an organism produces.
Name an example of neutral variation?
human fingerprints
What would seem to be an example of neutral variation?
human fingerprints
Sexual recombination includes the shuffling of chromosomes in __________ and fertilization.
meiosis
Name a condition that the Hardy Weinberg does not need fulfilled
natural selection In a population that is not evolving, allele and genotype frequencies will remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at work.
Point mutations in noncoding regions of DNA result in __________.
neutral variation
What is the formula for phenotypically dominant individuals in a population?
p^2 + 2pq
If 84% of a population can taste PTC - If the population is in Hardy-Weinberg equilibrium, what % are heterozygous for the trait?
p^2 + 2pq= 84%, q^2= 16% so q= .4, p=.6, 2pq= 2(.4) (.6)= .48 which turns into 48%
Name a set of conditions that are required for Hardy-Weinberg equilibrium?
random mating, no natural selection, and a large population
Selection that acts over evolutionary time to preserve traits that increase an individual's ability to mate is known as __________.
sexual selection In sexual selection, secondary sexual characteristics that may not seem to be adaptive in the general sense may give an individual an advantage in gaining a mate and producing fertile, viable offspring.
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
For several years, scientists have warned doctors of the danger of overprescribing antibiotics such as penicillin. Scientists are concerned because __________.
strains of microorganisms that are resistant to these drugs will be selected for. A beneficial mutation can increase in frequency in a bacterial population very rapidly. If a single individual in the population harbors a mutation that renders it resistant to an antibiotic, there may be millions of resistant bacteria within several hours of antibiotic treatment.
An earthquake hits a small island. All but a small group of closely related lizards are eliminated, and the survivors spread out over the island. This is an instance of __________.
the Bottleneck effect
A hurricane hits a small island, killing all but a few members of a bird population. This is an example of __________.
the bottleneck effect
In the Hardy-Weinberg equation, q2 represents __________.
the expected frequency of the homozygous recessive genotype
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
Consider a gene that exists in two allelic forms in a simple Mendelian dominant/recessive pair. 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. (The reason why the heterozygous genotype will decrease is because they display the dominant trait)
The sickle-cell allele, which is recessive, causes anemia but confers resistance to malaria in individuals who possess it. However, homozygous recessive individuals often die from anemia but not from malaria, and homozygous dominant individuals do not have anemia but could die from malaria. Heterozygous individuals have the highest relative fitness. This is an example of __________.
the heterozygote advantage
No two people are genetically identical, except for identical twins. The main source of genetic variation among human individuals is __________.
the reshuffling of alleles in sexual reproduction
In the equation for Hardy-Weinberg equilibrium, 1 represents __________.
the sum of the frequencies of the genotypes for a particular gene locus
What is sexual dimorphism?
when different sexes have different forms/appearances (peacock)
