Chapter 23: Population Genetics Flashcards
Define gene pool
The gene pool refers to all of the alleles for every gene in a population.
In a population of flowers there are 3 red-flowered plants (CRCR), 2 pink-flowered plants (CRCW), and 1 white-flowered plant (CWCW) what is the frequency of the CR
allele in this
population?
Allele Frequency= Number of copies of a specific allele in a population/total number of all alleles for that gene in a population
There are 8 total CR alleles (there are 3 red plants and each red plant has 2 CR alleles; there are 2 pink plants and each pink plant has 1 CR allele).
There are 6 plants, so there are 12 total alleles in the population because each plant has 2 alleles.
Therefore, CR allele frequency = 8/12 = 2/3
What is the frequency of the CRCW genotype in the above population?
Genotype Frequency= Number of individuals with a particular genotype in a population/total number of individuals in a population
The number of individuals with the CRCW genotype is 2
The total number of individuals is 6
Therefore, the frequency of the CRCW genotype is 2/6 = 1/3
Write out the Hardy-Weinburg equilibrium. What do each of the following terms represent:
p, q, p2, 2pq, q2
The Hardy-Weinburg equation, p2 + 2pq + q2 = 1, relates allele frequencies and genotype frequencies in a population. If you know the allele frequencies, you can use the equation to calculate the genotype frequencies and vice versa.
p = allele frequency of the dominant allele
q = allele frequency of the recessive allele
p2 = frequency of the homozygous dominant genotype
2pq = frequency of the heterozygous genotype
q2 = frequency of the homozygous recessive genotype
Define microevolution
Changes in allele frequencies that occur in a population from one generation to the next.
What type of traits does natural selection select for and why?
Natural selection selects for traits that enhance reproductive success because such traits ensure that genes are passed to the next generation. Traits that enhance reproductive success are traits that provide a survival advantage as well as traits that directly enhance reproduction (e.g. brightly colored plumage on a male bird may enhance reproduction by making him more attractive to females).
Define fitness. Will genotypes with higher fitness become more, or less, prevalent in the population?
Fitness is the relative likelihood that a particular genotype will contribute genes to the next generation relative to other genotypes. Genotypes that have the highest fitness are naturally selected for and become more prevalent in a population.
How do the following types of natural selection work:
- Directional selection
- Stabilizing selection
- Disruptive/Diversifying selection
- Balancing selection due to the heterozygote advantage
- Directional selection: favors individuals at one extreme, so the phenotype moves from
one direction to another. - Stabilizing selection: selects for individuals of intermediate phenotype and against
individuals at each extreme. - Disruptive/Diversifying selection: selects for the survival of 2 or more phenotypes within
a population. - Balancing selection due to the heterozygote advantage: maintains both alleles in a population because the heterozygous genotype has a survival advantage over either of
the homozygous genotypes.
In the graphs below, the red dashed line shows the phenotype of an original population, and the blue solid line shows the phenotype of the population after natural selection. What type of natural selection (directional, stabilizing or disruptive/diversifying) is illustrated by graphs A
– C?
Graph A – stabilizing selection
Graph B – directional selection
Graph C – diversifying/disruptive selection
The mean (average) beak depth of a finch species is 15 millimeters. Assuming the trait is heritable, what would happen to mean beak depth after several generations of stabilizing selection? What about after several generations of directional selection?
After stabilizing selection, the mean beak depth would remain at 15 mm because stabilizing selection selects for the intermediate phenotype, which is the mean phenotype. However, stabilizing selection would result in less spread around the mean, so the peak would become
narrower.
After several generations of directional selection, the mean beak depth would either decrease or increase depending on the direction of selection. If birds with small beaks have a survival advantage then the mean beak size would decrease. If birds with large beaks have a survival advantage then the mean beak size would increase.
Hemoglobin caries oxygen around the body. It has a normal allele (H), and an affected allele (h) that causes sickle cell anemia when present in homozygous recessive form (hh). In areas where malaria is endemic, the heterozygous genotype (Hh) is more resistant than either homozygous genotype (HH and hh) to malaria. What type of selection does malaria have on
the hemoglobin alleles?
Balancing selection due to the heterozygote advantage. The heterozygous genotype (Hh) has a survival advantage over the 2 homozygous genotypes, so both alleles will be maintained in populations where malaria is endemic.
Male Anolis lizards use dewlap displays (flap of skin on the neck) to attract females. In a
single species, males with darker colored dewlaps tend to display in open habitats and males with lighter colored dewlaps tend to display in more forested habitats resulting in the selection of two different phenotypes within the population. What type of selection is occurring?
Diversifying selection because 2 different phenotypes are being selected for in a heterogeneous (diverse) environment. There is selection for the males that have the darkest dewlaps that are most successful in the open habitats, and there is also selection for males that have the lightest colored dewlaps that are most successful in the forested habitats.
Define genetic drift. Does it have a greater effect on smaller or larger population sizes?
Genetic drift refers to changes in allele frequencies that occur due to random chance.
It affects small populations more than large populations.
For example, say we have 2 alleles, A and a, present at equal frequency and there are 10 heterozygous individuals in the population.
By chance, all individuals may pass on the A allele and no individuals pass on the a allele, or vice versa. This is much less likely to occur in a larger population of 1000 heterozygous
individuals.
In 2 or three sentences for each, describe the bottleneck effect and founder effect and explain how each can cause genetic drift.
- Bottleneck effect: a population is dramatically decreased in size, due to an event such as a natural disaster or destruction of habitat. Death or survival is not based on genotype but is due to chance. The surviving population may have allele/genotype frequencies that differ
from the starting population. The population size may rebound over time, but the
allele/genotype frequencies are forever changed. - Founder effect: a small group of individuals (founder population) migrate from a larger
population to a new area. By chance, the founder population may have different allele
frequencies than the original population from which they separated from.
Define random mating. Name two types of non-random mating discussed in class.
Random mating means that a mating partner is chosen randomly, not based on any genotype or phenotype.
Two types of non-random mating discussed in class are:
- Assortative mating: Individuals prefer a mating partner with a similar trait to themself (which is positive assortative mating) or a dissimilar trait to themself (which is negative assortative mating). Assortative mating can alter allele/genotype frequencies in a population.
- Inbreeding: mating between closely genetically related individuals. It tends to concentrate rare, recessive alleles in a population and therefore increases the frequency of individuals homozygous for rare recessive genotypes (i.e., recessively inherited genetic conditions).
