Mechanisms of Evolution. Genetic
Drift and Natural Selection
Lab 2
A
population is a group of individuals of the same species living in the same
geographical area that inbreeds freely.
The sum of all the alleles of all the members of the population is its
gene pool. For each gene, every
individual has only two alleles, but there may be more than two alleles in the
gene pool, each with its own frequency.
Evolution is frequently defined genetically as a change in the frequency
of one or more alleles in the gene pool from one generation to the next. As the
frequency of an allele refers to its-frequency in the population, evolution
occurs in the population as a whole, and not in individuals. The conditions that cause change in allele
frequencies are:
1.
mutations
2.
gene
flow
3.
nonrandom mating
4.
genetic
drift
5.
natural
selection
This
exercise focuses on genetic drift and natural selection.
Genetic drift refers to
changes in allele frequencies due to random chance alone. The founder effect occurs when a few individuals
start a new colony and only a fraction of the genetic diversity of the original
gene pool is
present in these individuals. The
new population is likely to have much different allele frequencies than the
parental population.
Natural selection occurs when a new
phenotype appears in a few members of a population that allows them to better
utilize the available resources. These
individuals are more likely to survive and have more offspring than
these individuals that lack the
new phenotype. In subsequent generations more members of the population will
have the new phenotype, and eventually, many members of the population will
have the phenotype and be better adapted to the environment.
The Hardy-Weinberg Law
The Hardy-Weinberg law, which is used to calculate the frequencies of alleles in a gene pool, provides a baseline by which to judge if evolution has occurred or not. Hardy and Weinberg independently demonstrated that sexual reproduction alone will do nothing to change the frequencies of alleles in a gene pool provided that certain conditions are met. These conditions are :
Provided these conditions are perfectly met, the population will not evolve and the allele frequency will not change from one generation to the next. However if an outside evolutionary agent, such as genetic drift, natural selection, nonrandom mating, mutation, or gene flow is operating, allele frequencies will change and the population will evolve.
The frequency of the recessive and dominant alleles, the three genotypes-., (homozygous dominant, heterozygous, and homozygous recessive), and, the two phenotypes (dominant and recessive) can be calculated using the HardyWeinberg law, which is outlined below.
p2 + 2pq + q2=1
where p2 = %
homozygous dominant individuals
p
= frequency of the dominant allele
q2= % homozygous recessive individuals
q = frequency of the recessive allele,
2pq= % heterozygous individuals
Also, p
+ q =1 (there are only two alleles for a particular gene in an individual), and
p2 + 2pq + q2 =1 (these are the only genotypes).
Establishing a Baseline
In this exercise, the class constitutes a population and we will be following the allelic and genotypic frequencies for PTC tasting and a type of earlobe.
PTC (phenylthiocarbamide) is an antithyroid drug that prevents the incorporation of iodine by the thyroid gland into thyroid hormone.; The ability to taste PTC is an autosomal trait; tasting (T) is dominant to nontasting (t). Whether earlobes are attached or free is also an autosomal trait. Unattached (E) is dominant to attached (e) earlobes.
To get the baseline (control) values for the class
follow the steps below and enter the appropriate values in table 1.
1. Taste a piece of PTC
paper Can you taste it? What is
your, genotype?
2.
Have a class member examine your earlobes- and determine whether they are
attached or unattached. Do you have attached or unattached earlobes?
What is your genotype?
3.
Determination of homozygous recessive frequency (q2):
Number
of students with attached earlobes=
___________________=q2
Number
of students in the class
4. Fill in the first column of table 1.
5.
Determine the frequency of the recessive allele (q) by calculating the square
root of q2. Fill in the second column of table 1.
6.
Determine the frequency of the dominant allele (p) using the formula p + q =1,
that is, p --1 q and fill in the third column of table 1
7. Determine the frequency of the homozygous dominant genotype (p) by
multiplying p times p and fill in the fourth column of
table 1.
8. Determine the frequency of the heterozygous genotype (2pq) by multiplying 2
times p times q and complete column five of table 1.
Table 1
Trait |
q2 |
q |
p |
p2 |
2pq |
PTC Tasting |
|
|
|
|
|
Earlobes |
|
|
|
|
|
9. Complete table 2 by filling in the "Genetic Frequencies" column with the numbers calculated in steps 3, 7 and 8. Then complete the "Number of Students" columns by first filling in the number of students who have the recessive phenotype (q) from step 3. Then calculate the number of students with the homozygous dominant genotype by multiplying p2 times the number of students in the class, and the number of students with the heterozygous genotype by multiplying 2pq times the number of students in the class.
Table 2 Present Generation
Genotypes |
Genotypic Frequencies |
|
Number of Students |
|
|
PTC Tasting |
Earlobes |
PTC Tasting |
Earlobes |
Homozygous Recessive ( q2 ) |
|
|
|
|
Homozygous Dominant ( p2 ) |
|
|
|
|
Heterozygous (2pq) |
|
|
|
|
Testing
the Hardy: Weinberg Law
Your instructor will select one of the two traits further testing., Given the data in table 1, predict the genotypic frequencies m the next generation if the Hardy-Weinberg law applies.
q2 = |
p2 = |
2pq = |
q. = |
p. = |
1. Using the frequencies from table 2 as a guide,
your instructor will assign you a genotype for the trait being
followed. (Students who know their phenotype should use their own genotype.)
2. Write
down your initial parental genotype.
3. To ensure random mating you must behave completely
uninhibitedly and choose anyone in your class (male or, female) as a
mate. This person cannot refuse you.
4 Each
couple will have two offspring. Each-member contributes one allele to each
offspring, and is determined by- flipping a coin. If the coin comes
up beads, the left allele is passed on, and tails means the right allele is
passed on. Now fill in Table 3.
Table 3 F1 Generation
Offspring |
Your
Contribution |
Partner’s
Contribution |
1 |
|
|
2 |
|
|
5. Select one of the two offspring; its genotype
now becomes yours (the
parent's have died).
Record your new F1
genotype
6.
Find
a new partner and repeat the procedure outlined above, recording your new
genotype each time, until you have completed five generations:
F2___________ F3________________
F4____________________ F5____________________
7.
Now
fill in. table 4, using; the F5 genotype information. You will need to complete
the number of students column first and the genotypic-frequencies, column:, second.
Table 4 F5 Generation
Genotypes |
Genotypic
Frequencies |
Number of
Students |
Homozygous
recessive ( q2 ) |
|
|
Homozygous
Dominant ( p2 ) |
|
|
Heterozygous
(2pq) |
|
|
8. Compare the results in table 2 and table 4. Do these results match the predictions you made earlier? Why or why not?
B. Genetic Drift
Genetic
drift refers to chance, or random, changes in the allele frequency of a gene
pool. These changes cannot be predicted nor can organisms adapted to
prevent these random changes. Examples
of events that cause genetic drift are natural disasters like fires and floods
that randomly eliminate members of a population that are "in the wrong
place at the wrong time". The remaining members of the population
reproduce more prolifically than they normally would (less competition for
resources and mates) and the alleles they carry will be present in the gene
pool of subsequent generations at frequencies greater than those of the individuals who
were eliminated.
The founder effect occurs when a small number of
individuals from a large population colonize a new habitat. The smaller the
founding population, the less likely the founding gene pool will reflect the
gene pool of the original population.
Consequently, the allele frequencies in the subsequent generations of
the new colony will be much different from those of the original population.
1.
To model the founder effect, the instructor will divide the class (the original
population) into two smaller populations.
How many individuals in each new population?_____________ Each member begins with their F5
genotype from the previous section.
2.
Calculate the genotypic and allele frequencies for your new population and complete
table 5.
Table 5. Parental Generation
Genotypes |
Genotypic Frequencies |
Number of Students |
||
Homozygous
recessive (q) |
|
|
|
|
Homozygous
dominant (p |
|
|
|
|
Heterozygous
(2pq) |
|
|
3. Follow the same mating procedure as before, but population, until you have completed five generations.
Initial (P) generation __________ F1____ F2_____
F3 ___________F4_______ F5________
Fill in Table -6 using the F5 generation data for your population.
Table 6. F5 Generation
Genotypes |
Genotypic Frequencies |
Number of Students |
Homozygous
recessive (q) |
|
|
Homozygous
dominant (p |
|
|
Heterozygous
(2pq) |
|
|
Compare Table 5 and Table 6. Do the results
suggest that genetic drift has occurred? Explain.
C. Natural Selection
Populations contain varied individuals. These differences
affect their ability to successfully leave behind offspring. The differential
success in reproduction was described by
Antibiotic Selection of Bacteria,
Most bacteria that are grown in the presence of small amounts of antibiotics die, although some survive. Those bacteria that survive contain mutations that neutralize the effect of the antibiotics and confer antibiotic resistance to the bacterium. Each surviving bacterium then gives rise to a colony of bacterial cells, each containing the mutant gene that gives resistance to the antibiotic.
Laboratory.
Questions
1.
List the conditions that must exist for the
Hardy-Weinberg law to apply.
2. What evidence would
you look for to indicate that a population is evolving?
3
Assume Hardy Weinberg equilibrium
a. If p= 0.8 for a population, calculate the
values of q, p2, q2 . Show all your work
b. What would be the gene pool frequencies in
the next generation if evolution does not occur?
4. How does genetic drift differ from natural
selection
5. How does the end result of genetic drift
differ from the end result of natural selection?
6.
If 49% of a population showing Hardy-Weinberg equilibrium has
recessive phenotype for a trait, what is the value
of p?