The Evolution of Populations
Balancing selection
*Balancing selection* occurs when natural selection maintains stable frequencies of 2 or more phenotypic forms in a population Balancing selection includes: - heterozygote advantage - frequency-dependent selection
Heterozygote advantage
*Heterozygote advantage*: occurs when heterozygotes have a higher fitness than do both homozygotes - Natural selection will tend to maintain two or more alleles at that locus - Heterozygote advantage can result from stabilizing or directional selection
2 examples of genetic drift that cause significant impact on a population
*The Founder Effect* - genetic drift that occurs when a few individuals become isolated from a larger population - allele frequencies in the small founder population can be different from those in the larger parent population - ex) when a few members of a population are blown by a storm to a new island *The Bottleneck Effect* - a sudden reduction in population size due to a change in the environment. - the resulting gene pool may no longer be reflective of the original population's gene pool - if the population remains small, it may be further affected by genetic drift - ex) a fire or flood
What is the frequency of the A1A2 genotype in a population composed of 20 A1A1 individuals, 80 A1A2 individuals, and 100 A2A2 individuals?
- 0.4 -- The calculation to determine the frequency of the A1A2 genotype is: 80 A1A2 individuals / (20 + 80 + 100) total individuals = 0.4, the frequency of the A1A2 genotype.
A large population of laboratory animals has been allowed to breed randomly for a number of generations. After several generations, 25% of the animals display a recessive trait (aa), the same percentage as at the beginning of the breeding program. The rest of the animals show the dominant phenotype, with heterozygotes indistinguishable from the homozygous dominants. What is the estimated frequency of allele A in the gene pool?
- 0.50
A large population of laboratory animals has been allowed to breed randomly for a number of generations. After several generations, 25% of the animals display a recessive trait (aa), the same percentage as at the beginning of the breeding program. The rest of the animals show the dominant phenotype, with heterozygotes indistinguishable from the homozygous dominants. What proportion of the population is probably heterozygous (Aa) for this trait?
- 0.50
Arrange the following in order from most general to most specific. 1 natural selection 2. microevolution 3. intrasexual selection 4. evolution 5. sexual selection
- 4, 2, 1, 5, 3
Rank the following one-base point mutations (from most likely to least likely) with respect to their likelihood of affecting the structure of the corresponding polypeptide. 1. insertion mutation deep within an intron 2. substitution mutation at the third position of an exonic codon 3. substitution mutation at the second position of an exonic codon 4. deletion mutation within the first exon of the gene
- 4, 3, 2, 1
Suppose 64% of a remote mountain village can taste phenylthiocarbamide (PTC) and must, therefore, have at least one copy of the dominant PTC taster allele. If this population conforms to Hardy-Weinberg expectations for this gene, what percentage of the population must be heterozygous for this trait?
- 48% frequency of T = p frequency of t = q p + q = 1 frequency of TT = p^2 (p squared) frequency of Tt = 2pq frequency of tt = q^2 p^2 + 2pq + q^2 = 1 64% of the population are tasters, therefore 36% must be non-tasters. Anyone with the non-taster phenotype MUST have the homozygous recessive genotype (tt). frequency of tt = 0.36 q^2 = 0.36 q = 0.6 p + q = 1 p = 1 - q p = 1 - 0.6 p = 0.4 frequency of Tt = 2pq = 2 x 0.6 x 0.4 = 0.48 48% of the population must be heterozygous for this trait. Just to check our answer is correct: frequency of TT = p^2 = 0.4^2 = 0.16 Check that p^2 + 2pq + q^2 = 1 0.16 + 0.48 + 0.36 = 1
If, on average, 46% of the loci in a species' gene pool are heterozygous, then the average homozygosity of the species should be _____.
- 54%
Which of these individuals is a homozygous genotype? Which of these individuals is a homozygous genotype? AG Gg Aa AA aG
- AA -- When both of the alleles of a given gene pair are the same, the individual is homozygous for that gene.
Refer to the figure above. In their investigation of natural selection on Mc1r alleles (the gene that determines coat color) in Arizona pocket mice, Hoekstra et al. determined the frequency of the D and d alleles in each population. They also determined the frequency of alleles for two neutral mitochondrial DNA genes (genes that do not affect and are not linked to coat color). Why did the researchers include the mitochondrial DNA genes as part of their experimental design?
- Allele change for the neutral mitochondrial genes serves as a control and gives information on any general background genetic difference among these populations.
Every few years a giant axe chops off the head of every person who is over 6 feet tall. How will this affect the human population?
- Alleles that promote "tallness" will decrease in frequency. --A reduction in "tallness" alleles is the change expected as a result of this type of selection.
Refer to the accompanying figure. Which one of the following is *NOT* a plausible hypothesis to explain the differences in caterpillar appearance observed in this population? - The longer day lengths of summer trigger the development of twig-like caterpillars. - Differences in diet trigger the development of different types of caterpillars. - Differences in air pressure, due to differences in elevation, trigger the development of different types of caterpillars. - The cooler temperatures of spring trigger the development of flowerlike caterpillars.
- Differences in air pressure, due to differences in elevation, trigger the development of different types of caterpillars.
Refer to the accompanying figure. In every case, caterpillars that feed on oak flowers look like oak flowers. In every case, caterpillars that were raised on oak leaves looked like twigs. These results support which of the following hypotheses? - Differences in diet trigger the development of different types of caterpillars. - The longer day lengths of summer trigger the development of twig-like caterpillars. - Differences in air pressure, due to elevation, trigger the development of different types of caterpillars. - The differences are genetic. A female will either produce all flowerlike caterpillars or all twig-like caterpillars.
- Differences in diet trigger the development of different types of caterpillars.
Which of the following are basic components of the Hardy-Weinberg model? - Allele frequencies in a subset of the population - Allele frequencies, phenotype frequencies - Allele frequencies, number of individuals in the population - Frequencies of two alleles in a gene pool before and after many random matings
- Frequencies of two alleles in a gene pool before and after many random matings -- Hardy and Weinberg were trying to determine how and whether allele frequencies in a population change from one generation to the next.
HIV Paragraph
- HIV's genome of RNA includes the code for *reverse transcriptase (RT)*, an enzyme that acts early in infection to synthesize a DNA genome off of an RNA template. - The HIV genome also codes for *protease (PR)*, an enzyme that acts later in infection by cutting long viral polyproteins into smaller, functional proteins. - Both RT and PR represent potential targets for antiretroviral drugs. - Drugs called *nucleoside analogs (NA)* act against RT, whereas drugs called *protease inhibitors (PI)* act against PR.
Swine are vulnerable to infection by bird flu virus and human flu virus, which can both be present in an individual pig at the same time. When this occurs, it is possible for genes from bird flu virus and human flu virus to be combined. If the human flu virus contributes a gene for Tamiflu resistance (Tamiflu is an antiviral drug) to the new virus, and if the new virus is introduced to an environment lacking Tamiflu, then what is most likely to occur?
- If the Tamiflu-resistance gene involves a cost, it will experience directional selection leading to reduction in its frequency.
How does diploidy help to preserve genetic variation?
- It allows recessive alleles that may not be favored in the current environment to be preserved in the gene pool by propagation in heterozygotes. -- Natural selection tends to decrease genetic variation. Recessive alleles are shielded from natural selection in heterozygotes.
What is true of microevolution?
- It is the kind of evolution that can change the relative abundance of a particular allele in a gene pool over the course of generations. --Microevolution is a generation-to-generation change in allele frequency.
The restriction enzymes of bacteria protect the bacteria from successful attack by bacteriophages, whose genomes can be degraded by the restriction enzymes. The bacterial genomes are not vulnerable to these restriction enzymes because bacterial DNA is methylated. This situation selects for bacteriophages whose genomes are also methylated. As new strains of resistant bacteriophages become more prevalent, this in turn selects for bacteria whose genomes are not methylated and whose restriction enzymes instead degrade methylated DNA. Over the course of evolutionary time, what should occur?
- Methylated and nonmethylated strains should be maintained among both bacteria and bacteriophages, with ratios that vary over time.
In 1986, a nuclear power accident in Chernobyl, USSR (now Ukraine), led to high radiation levels for miles surrounding the plant. The high levels of radiation caused elevated mutation rates in the surviving organisms, and evolutionary biologists have been studying rodent populations in the Chernobyl area ever since. Based on your understanding of evolutionary mechanisms, which of the following most likely occurred in the rodent populations following the accident?
- Mutation led to increased genetic variation.
Women often have complications during labor while giving birth to very large babies, whereas very small babies tend to be underdeveloped. Which kind of selection is most likely at work regarding the birth weight of babies? Disruptive selection Directional selection Stabilizing selection
- Stabilizing selection
Refer to the accompanying figure. Recall that eggs from the same female were exposed to each of the eight treatments used. This aspect of the experimental design tested which of the following hypotheses? - Differences in diet trigger the development of different types of caterpillars. - Differences in air pressure, due to elevation, trigger the development of different types of caterpillars. - The longer day lengths of summer trigger the development of twig-like caterpillars. - The differences are genetic. A female will either produce all flowerlike caterpillars or all twig-like caterpillars.
- The differences are genetic. A female will either produce all flowerlike caterpillars or all twig-like caterpillars.
What genotype frequencies are expected under Hardy-Weinberg equilibrium for a population with allele frequencies of p = 0.8 and q = 0.2 for a particular gene?
- The expected genotype frequencies are 0.64, 0.32, and 0.04 for A1A1, A1A2, and A2A2, respectively. -- The expected frequency of the A1A1 genotype is p2 = (0.8)(0.8) = 0.64 - the expected frequency of the A1A2 genotype is 2pq = 2(0.8)(0.2) = 0.32 - the expected frequency of the A2A2 genotype is q2 = (0.2)(0.2) = 0.04. - To verify your calculations, confirm that the three frequencies add up to one.
Which of the following statements is NOT a part of the Hardy-Weinberg principle? - Even if allele A1 is dominant to allele A2, it does not increase in frequency. - The genotype frequencies in the offspring generation must add up to two. - When alleles are transmitted according to the rules of Mendelian inheritance, their frequencies do not change over time. - If allele frequencies in a population are given by p and q, then genotype frequencies will be given by p2, 2pq, and q2 for generation after generation.
- The genotype frequencies in the offspring generation must add up to two. --This statement is not true; the genotype frequencies in the offspring generation must add up to one.
A large population of laboratory animals has been allowed to breed randomly for a number of generations. After several generations, 25% of the animals display a recessive trait (aa), the same percentage as at the beginning of the breeding program. The rest of the animals show the dominant phenotype, with heterozygotes indistinguishable from the homozygous dominants. What is the most reasonable conclusion that can be drawn from the fact that the frequency of the recessive trait (aa) has not changed over time?
- The two phenotypes are about equally adaptive under laboratory conditions.
In a bell-shaped curve, the x-axis (horizontal direction) of the graph represents which of the following?
- The value of a particular characteristic; characteristics of an organism can include such traits as size and color. -- The value of the characteristic increases from left to right.
The figure above shows the distribution of pocket-mouse coat colors in several Arizona populations found either on light-colored granite substrate or on dark volcanic rock (dark substrate). The Melanocortin-1 receptor (Mc1r) alleles, D and d, differ by four amino acids. Mice with DD and Dd genotypes have dark coats, whereas mice with the dd genotype are light colored. What sort of genotype frequencies might you expect to find in the Xmas, Mid, and O'Neill populations?
- Xmas-high dd frequency; Mid-high DD frequency, O'Neill-high Dd frequency
Refer to the paragraph on dark-eyed junco birds. The UCSD campus male junco population tails were, on average, 36% white, whereas the tails of males from nearby mountain populations averaged 4045% white. If this observed trait difference were due to a difference in the original colonizing population, it would most likely be due to _____.
- a founder effect
An earthquake decimates a ground-squirrel population, killing 98% of the squirrels. The surviving population happens to have broader stripes, on average, than the initial population. If broadness of stripes is genetically determined, what effect has the ground-squirrel population experienced during the earthquake?
- a genetic bottleneck
a TRUE statement about variation
- all new alleles are the result of nucleotide variability.
You sample a population of butterflies and find that 56% are heterozygous at a particular locus. What should be the frequency of the recessive allele in this population?
- allele frequency cannot be determined from this information.
The variable likely to undergo the largest change in value resulting from a mutation that introduces a new allele into a population at a locus for which all individuals formerly had been fully homozygous?
- average heterozygosity
In evolutionary terms, an organism's fitness is measured by its _____.
- contribution to the gene pool of the next generation -- Those organisms with adaptations best suited to the current environment will make the most significant contribution to the next generation.
You are maintaining a small population of fruit flies in the laboratory by transferring the flies to a new culture bottle after each generation. After several generations, you notice that the viability of the flies has decreased greatly. Recognizing that small population size is likely to be linked to decreased viability, the best way to reverse this trend is to _____.
- cross your flies with flies from another lab
Currently the only predators of Galápagos marine iguanas are Galápagos hawks. Iguana body size is not correlated with risk of hawk predation, although small iguanas can sprint faster than large iguanas. If predators (for example, cats) that preferably catch and eat slower iguanas are introduced to the island, iguana body size is likely to _____ in the absence of other factors; the iguanas would then be under _____ selection.
- decrease; directional
Which of the following is the most predictable outcome of increased gene flow between two populations?
- decreased genetic difference between the two populations
Which of the following is the best modern definition of evolution? - descent with modification - change in the number of genes in a population over time - inheritance of acquired characters - survival of the fittest
- descent with modification
If there is no gene flow, the curve shifts to the left or to the right, and the population size consequently increases over successive generations, which of the following is most likely occurring?
- directional selection
Long necks make it easier for giraffes to reach leaves high on trees, while also making them better fighters in "neck wrestling" contests. In both cases, which kind of selection appears to have made giraffes the long-necked creatures they are today? Disruptive selection Directional selection Stabilizing selection
- directional selection -- Directional selection drives the average of the population in one direction, in this case, toward longer necks.
Blue poppies native to China were grown at a plant-breeding center in California. The plants with the thickest leaves were most likely to survive and reproduce in the drier climate. After several generations, the percentage of thick-leaved plants had increased by 42%. This adaptation of the poppies to their new environment is due to _____.
- directional selection -- The characteristics of the poppies shifted in response to the changed environment.
In those parts of equatorial Africa where the malaria parasite is most common, the sickle-cell allele constitutes 20% of the β hemoglobin alleles in the human gene pool. In the United States, the parasite that causes malaria is not present, but African-Americans whose ancestors were from equatorial Africa are present. What should be happening to the sickle-cell allele in the United States, and what should be happening to it in equatorial Africa?
- directional selection; stabilizing selection
A biologist doing a long-term study on a wild spider population observes increased variation in silk thickness. Which of the following could the spider population be experiencing?
- disruptive selection
Three-spined stickleback fish (Gasterosteus aculeatus) show substantial heritable variation in gill-raker length related to differences in their diets. Longer gill rakers appear to function better for capturing open-water prey, while shorter gill rakers function better for capturing shallow-water prey. Which of the following types of selection is most likely to be found in a large lake (open water in the middle and shallow water around the sides) with a high density of these fish?
- disruptive selection
Black-bellied seedcrackers have either small beaks (better for eating soft seeds) or large beaks (better for hard seeds). There are no seeds of intermediate hardness; therefore, which kind of selection acts on beak size in seedcrackers? Disruptive selection Directional selection Stabilizing selection
- disruptive selection -- Disruptive selection causes both extreme phenotypes (large and small beaks) to be favored over the intermediate phenotypes.
Which type of selection tends to increase genetic variation? Directional selection Disruptive selection Stabilizing selection
- disruptive selection -- Disruptive selection eliminates phenotypes near the average and favors the extreme phenotypes, resulting in increased genetic variation in a population.
Mutation is the only evolutionary mechanism that _____.
- does little to change allele frequencies
a TRUE statement about the beak size of finches on the island of Daphne Major during prolonged drought
- each bird's survival was strongly influenced by the depth and strength of its beak as the drought persisted.
What situation most likely explains the occasional high frequency of certain inherited disorders among human populations established by a small population?
- founder effect -- The founder effect is genetic drift attributable to colonization by a limited number of individuals from a parent population.
The Dunkers are a religious group that moved from Germany to Pennsylvania in the mid-1700s. They do not marry with members outside their own immediate community. Today, the Dunkers are genetically unique and differ in gene frequencies, at many loci, from all other populations including those in their original homeland. Which of the following likely explains the genetic uniqueness of this population?
- founder effect and genetic drift
The restriction enzymes of bacteria protect the bacteria from successful attack by bacteriophages, whose genomes can be degraded by the restriction enzymes. The bacterial genomes are not vulnerable to these restriction enzymes because bacterial DNA is methylated. This situation selects for bacteriophages whose genomes are also methylated. As new strains of resistant bacteriophages become more prevalent, this in turn selects for bacteria whose genomes are not methylated and whose restriction enzymes instead degrade methylated DNA. The outcome of the conflict between bacteria and bacteriophage at any point in time results from _____.
- frequency-dependent selection
When imbalances occur in the sex ratio of sexual species that have two sexes (that is, other than a 50:50 ratio), the members of the minority sex often receive a greater proportion of care and resources from parents than do the offspring of the majority sex. This is most clearly an example of _____.
- frequency-dependent selection
Over time, the movement of people on Earth has steadily increased. This has altered the course of human evolution by increasing _____.
- gene flow
The ease with which humans travel across the globe is likely to increase _____.
- gene flow
All the genes in a population are that population's _____.
- gene pool -- All the alleles of all the genes of a population make up a gene pool.
Modern travel along with migration reduces the probability of _____ having an effect on the evolution of humans.
- genetic drift -- Modern travel and migration decreases the chances of there being a small, isolated population.
In the formula for determining a population's genotype frequencies, the *"2" in the term 2pq* is necessary because _____.
- heterozygotes can come about in two ways
In the formula for determining a populations genotype frequencies, the *"pq" in the term 2pq* is necessary because _____.
- heterozygotes have two alleles
On the Bahamian island of Andros, mosquitofish populations live in various, now-isolated, freshwater ponds that were once united. Currently, some predator-rich ponds have mosquitofish that can swim in short, fast bursts; other predator-poor ponds have mosquitofish that can swim continuously for a long time. When placed together in the same body of water, the two kinds of female mosquitofish exhibit exclusive breeding preferences. If one builds a canal linking a predator-rich pond to a predator-poor pond, then what type(s) of selection should subsequently be most expected among the mosquitofish in the original predator-rich pond, and what type(s) should be most expected among the mosquitofish in the formerly predator-poor pond?
- less-intense directional selection; more-intense directional selection
Homologous pairs of chromosomes are lined up independently of other such pairs during _____.
- metaphase I
Generation-to-generation change in the allele frequencies in a population is _____.
- microevolution -- Generation-to-generation change in the allele frequencies in a population is the definition of microevolution.
Genetic variation _____. - tends to be reduced by when diploid organisms produce gametes - is created by the direct action of natural selection - must be present in a population before natural selection can act upon the population - arises in response to changes in the environment
- must be present in a population before natural selection can act upon the population
Which of the following evolutionary forces could create new genetic information in a population?
- mutation -- Mutations, which are changes in a cell's DNA, can introduce new genetic information in a population.
The original source of all genetic variation is _____.
- mutation --Mutation is, in itself, very important to evolution because it is the original source of the genetic variation that serves as the raw material for evolution.
Refer to the paragraph on dark-eyed junco birds. The UCSD campus male junco population tails are about 36% white, whereas the tails of males from nearby mountain populations are about 4045% white. The founding stock of UCSD birds was likely from the nearby mountain populations because some of those birds overwinter on the UCSD campus each year. Population sizes on the UCSD campus have been reasonably large, and there are significant habitat differences between the UCSD campus and the mountain coniferous forests; UCSD campus has a more open environment (making birds more visible) and a lower junco density (decreasing intraspecific competition) than that in the mountain forests. Given this information, which of the following evolutionary mechanisms do you think is most likely responsible for the difference between the UCSD and mountain populations?
- natural selection
What is the only evolutionary mechanism that consistently leads to adaptive evolution?
- natural selection -- Because natural selection has a "sorting" effect, it consistently increases the frequency of alleles that improve the match between an organism and its environment.
A proficient engineer can easily design skeletal structures that are more functional than those currently found in the forelimbs of such diverse mammals as horses, whales, and bats. The actual forelimbs of these mammals do not seem to be optimally arranged because _____.
- natural selection is generally limited to modifying structures that were present in previous generations and in previous species
For a biologist studying a small fish population in the lab, which Hardy-Weinberg condition is easiest to meet?
- no gene flow
For biologists studying a large flatworm population in the lab, which Hardy-Weinberg condition is most difficult to meet?
- no mutation
Refer to the figure above. Is this population in Hardy-Weinberg equilibrium?
- no; there are more homozygotes than expected
The higher the proportion of loci that are "fixed" in a population, the lower are that population's _____.
- nucleotide variability and average heterozygosity
Use HIV paragraph Every HIV particle contains two RNA molecules. If two genes from one RNA molecule become detached and then, as a unit, get attached to one end of the other RNA molecule within a single HIV particle, which of these is true? - There are now fewer genes within the viral particle. - A point substitution mutation has occurred in the retroviral genome. - One of the RNA molecules has experienced gene duplication as the result of translocation. - There are now more genes within the viral particle.
- one of the RNA molecules has experienced gene duplication as the result of translocation.
Which of the following evolutionary forces results in adaptive changes in allele frequencies?
- selection --Selection is the only evolutionary force that consistently results in adaptation. Mutation without selection and genetic drift are random processes that may lead to adaptive, maladaptive, or neutral effects on populations.
Adult male humans generally have deeper voices than do adult female humans, which is the direct result of higher levels of testosterone causing growth of the larynx. If the fossil records of apes and humans alike show a trend toward decreasing larynx size in adult females and increasing larynx size in adult males, then _____.
- sexual dimorphism was evolving over time in these species
According to the Hardy-Weinberg theorem, the frequencies of alleles in a population will remain constant if _____ is the only process that affects the gene pool.
- sexual reproduction -- Sexual reproduction does not change the frequency of alleles in the gene pool.
Examine the figure above. What type of selection for body size appears to be occurring in these marine iguanas?
- stabilizing selection
Most Swiss starlings produce four to five eggs in each clutch. Starlings producing fewer or more than this have reduced fitness. Which of the following terms best describes this situation?
- stabilizing selection
Small Aristelliger lizards have difficulty defending territories, but large lizards are more likely to be preyed upon by owls. Which kind of selection acts on the adult body size of these lizards? Disruptive selection Stabilizing selection Directional selection
- stabilizing selection -- Stabilizing selection causes no change in the average of the population; extreme phenotypes (large and small lizards) become less common.
Whenever diploid populations are in *Hardy-Weinberg equilibrium* at a particular locus, _____.
- the allele's frequency should not change from one generation to the next
Soon after the island of Hawaii rose above the sea surface (somewhat less than one million years ago), the evolution of life on this new island should have been most strongly influenced by _____.
- the founder effect
The inability of organisms to evolve anything that could be an advantage reflects _____.
- the limits of historical constraints
The evolutionary effects of genetic drift are greatest when _____.
- the population size is small -- Changes in the gene pool of a small population are frequently due to genetic drift.
Genetic drift is a process based on _____.
- the role of chance
A mutation occurs when _____.
- there is a change in the DNA sequence of a gene
Use HIV paragraph Which of the following represents the treatment option *most likely to avoid the evolution of drug-resistant HIV* (assuming no drug interactions or side effects)? - using moderate doses of NA and two different PIs at the same time for several months - using a series of NAs, one at a time, and changed about once a week - using a single PI, but slowly increasing the dosage over the course of a week - using high doses of NA and a PI at the same time for a period not to exceed one day
- using moderate doses of NA and two different PIs at the same time for several months
In some jacana species, males take care of the eggs and young, and females compete among themselves for territories that contain one to several males. Female jacanas are significantly larger than males. Which of these statements would you predict to be true of this bird species? 1. Male jacana fitness is primarily limited by ability to take care of eggs and raise young. 2. Female jacana fitness is limited by the number of males in her territory with which a female mates. 3. Variation in reproductive success should be greater in male jacanas than in females. 4. Variation in reproductive success should be greater in female jacanas than in males. 5. Males and females have equal variation in reproductive success.
1, 2, and 4
In human gamete production there is an average of _____ crossover events per chromosome pair.
2-3 -- These crossover events increase the genetic variation among gametes.
Cystic fibrosis is a genetic disorder in homozygous recessives that causes death during the teenage years. If 9 in 10,000 newborn babies have the disease, what are the expected frequencies of the dominant (A1) and recessive (A2) alleles according to the Hardy-Weinberg model?
9/10,000 = 0.0009 = q^2 sqrt(0.0009)= 0.03 = q p + q = 1 p + 0.03 = 1 p = 0.97 f(A1) = 0.9700 f(A2) = 0.0300
True or false? Heterozygote advantage refers to the tendency for heterozygous individuals to have better fitness than homozygous individuals. This higher fitness results in less genetic variation in the population.
FALSE -- Heterozygote advantage results in more genetic variation in the population.
Paragraph on dark-eyed junco birds
In 1983 a population of dark-eyed junco birds became established on the campus of the University of California, San Diego (UCSD), which is located many miles from the junco's normal habitat in the mixed-coniferous temperate forests in the mountains. Juncos have white outer tail feathers that the males display during aggressive interactions and during courtship displays. Males with more white in their tail are more likely to win aggressive interactions, and females prefer to mate with males with more white in their tails. Females have less white in their tails than do males, and display it less often. (Pamela J. Yeh. 2004. Rapid evolution of a sexually selected trait following population establishment in a novel habitat. Evolution 58[1]:166-74.)
3 Modes of Selection
Natural selection can alter the frequency distribution of heritable traits in 3 ways, depending on which phenotypes in a population are favored: - *directional selection:* favors individuals at one extreme end of the phenotypic range - *disruptive selection:* favors individuals at both extremes of the phenotypic range - *stabilizing selection:* favors intermediate variants and acts against extreme phenotypes
What is the frequency of the A1 allele in a population composed of 20 A1A1 individuals, 80 A1A2 individuals, and 100 A2A2 individuals?
The frequency of the A1 allele is 0.3. --The frequency of the A1 allele is p = (number of A1 alleles) / (total of all alleles) = [(2 ( 20) + 80] / [(2 × 20) + (2 × 80) + (2 × 100)] = 0.3.
Which of the following is a fitness trade-off (compromise)?
Turtle shells provide protection but are heavy and burdensome when moving.
In a Hardy-Weinberg population with two alleles, A and a, that are in equilibrium, the frequency of the allele a is 0.3. What is the frequency of individuals that are homozygous for this allele?
aa = q^2 = 0.3^2 = 0.09
Crossing over, resulting in an increase in genetic variation, occurs between _____.
nonsister chromatids of homologous chromosomes -- This process produces chromosomes containing genes inherited from both parents.
In a Hardy-Weinberg population with two alleles, A and a, that are in equilibrium, the frequency of allele a is 0.2. What is the frequency of individuals that are heterozygous for this allele?
p + q = 1 p + 0.2 = 1 p = 0.8 Aa = 2pq = 2(0.8)(0.2) = 0.32
In a Hardy-Weinberg population with two alleles, A and a, that are in equilibrium, the frequency of allele a is 0.1. What is the frequency of individuals with AA genotype?
q(a) = 0.1 p(A) = 1 - 0.1 = 0.9 the frequency of individuals with AA genotype: p^2(AA) = (0.9)^2 = 0.81
In peas, a gene controls flower color such that R = purple and r = white. In an isolated pea patch, there are 36 purple-flowering plants and 64 white-flowering plants. Assuming Hardy-Weinberg equilibrium, what is the value of q for this population?
q(pp) = 0.64. q= sqrt(0.64) = 0.8. - 0.80
True or false? The Hardy-Weinberg model makes the following assumptions: no selection at the gene in question; no genetic drift; no gene flow; no mutation; random mating.
true -- These are the five assumptions of the Hardy-Weinberg model.
