Lab 4 – Natural Selection
Species and their environments change with time. To ecologists, the most profound changes are genetic. The theory of evolution broadly describes genetic change in populations. Many mechanisms can change the genetic makeup of populations, and our understanding of the relative importance of each mechanism is constantly being refined. Nevertheless, genetic change, and therefore, evolution, is universally accepted by ecologists. Events such as mutations (changes in the genetic message of a cell) and catastrophes (e.g., meteor showers, ice ages), all lead to some degree of genetic change. However, all modern evidence points to natural selection as the major force behind genetic change and evolution.
Charles Darwin first described the mechanics of natural selection. Darwin postulated that organisms that survive and reproduce successfully in a competitive environment must have traits better adapted for their environment than those of their competitors. In other words, adaptive traits increase organisms’ fitness, and these traits are passed more frequently to the next generation. If traits of the most fit individuals are transmitted to the next generation through increased reproduction, then the frequency of these traits will, after many generations, increase in the population. Subsequently, the population and its characteristics will gradually change. Darwin called this overall process natural selection and proposed it as a major force guiding genetic change and the formation of new species.
Natural selection in living populations over many generations is difficult to demonstrate in the lab. Therefore, in this exercise you will simulate reproducing populations with nonliving, colored beads representing organisms and their gametes. This artificial population quickly reveals genetic change over many generations. We reviewed the terms gene, allele, dominant alleles, recessive alleles, homozygous, and heterozygous in class. You will begin your experiments with a “stock population” of organisms consisting of a container of beads. Each bead represents a haploid gamete (having one set of chromosomes). Its color represents the allele it is carrying. An organism from this population is diploid (has two sets of chromosomes per nucleus) and is represented by two beads.
Frequency refers to the proportion of alleles, genotypes or phenotypes or phenotypes of a certain type relative to the total number considered. Frequency is a decimal proportion of the total alleles or genotypes in a population. For example if ¼ of the individuals of a population are genotype Bb, the genotypic frequency of Bb is 0.25. If ¾ of all alleles in a population are B, then the frequency of B is 0.75. Remember, by definition the frequencies of all possible alleles or genotypes of phenotypes will always total 1.0.
In the following procedures you will simulate evolutionary changes in allelic and genotypic frequencies in an artificial population.

• The trait is fur color
• A blue bead is a gamete with a dominant allele (complete dominance) for Black fur (B).
• A white bead is a gamete with a recessive allele for white fur (b).
• An individual is represented by two gametes (beads).
• Individuals with genotypes BB and Bb have black fur and those with bb have white fur.
  Part 1 – Establish a parental population

1. Obtain a “stock population” of organisms consisting of a container of blue and white beads.
2. Obtain an empty container marked “Parental population”
3. From the stock population select nine homozygous dominant individuals (BB) and place them in the container marked “Parental Population.” Remember each individual is represented by two blue beads.
4. From the stock population select 42 heterozygous individuals (Bb) and put them in the container marked “parental population.” Remember each individual is represented by a blue and a white bead.
5. From the stock population select 49 homozygous recessive individuals (bb) and put them in the container marked “Parental Population.” Remember each individual is represented by two white beads.
6. Calculate the total number of individuals and the total number of alleles in your newly established parental population. Use this information to calculate and record in the first table the correct genotypic frequencies for your parental population
   Questions for Part 1:
7. How many total beads are blue?
8. How many are white?
9. What color of fur do Bb individuals have?
10. How many beads represent the population of 100 organisms?

Table 1.
Genotypes Frequency Alleles Frequency Phenotype Frequency
BB ?? B ? Black Fur
Bb ?? b ? White fur
bb ??

Part 2 – The Hardy-Weinberg Principle
The Hardy Weinberg Principle enables us to calculate and predict allelic and genotypic frequencies. We can compare these predictions with actual changes that we observe in natural populations and learn about factors that influence gene frequencies.
This predictive model includes two simple equations first describes for stable populations by G.H. Hardy and W. Weinberg. Hardy-Weinberg (HW) equations (1) predict allelic and genotypic frequencies based on data for only one or two frequencies; and (2) establish theoretical gene frequencies that we can compare to frequencies from natural populations. For example, if we know the frequencies of B and BB, we can use the Hardy –Weinberg equations to calculate the frequencies of b, Bb, and bb. Then we can compare these frequencies with those of a natural population that we might be studying. IF we find variation for our predications we can study the reasons for this genetic change. For the Hardy-Weinberg equations, the frequency of the dominant allele of a pair is represented by the letter p, and that of the recessive allele by the letter q. Also, the genotypic frequencies of BB (homozygous dominant), Bb (heterozygous), and bb (homozygous recessive) are represented by p2, 2pq, and q2, respectively. Examine the frequencies in your first table and verify calculations of the HW equations:
p + q = 1
p2 + 2pq + q2 = 1.

The HW principle and its equations predict that frequencies of alleles and genotypes remain constant from generation to generation in stable populations. Therefore, these equations can be used to predict genetic frequencies through time. However, the HW prediction assumes that:

• The population is large enough to overcome random events.
• Choice of mates is random.
• Mutations do not occur.
• Individuals do not migrate into or out of the population.
• Natural or artificial selection pressures are not activing on the population
  Questions for Part 2:

1. Consider the HW equations. If the frequency of a recessive allele is 0.3, what is the frequency of the dominant allele?
2. If the frequency of the homozygous dominant genotype is 0.49, what is the frequency of the dominant allele?
3. If the frequency of the homozygous dominant genotype is 0.49, what is the frequency of the homozygous recessive genotype
4. Which HW equation relates the frequencies of the alleles at a particular gene locus?
5. Which HW equation relates the frequencies of the genotypes for a particular gen locus?
6. Which HW equation relates the frequencies of the phenotypes for a gene?

To verify the predictions of the HW Principle, use the following procedure to produce a generation of offspring from the parental population you created in the previous procedure. Remember, the fact that the genetic frequencies of various alleles, genotypes, and phenotypes total 1.0 is not a prediction of the HW Principle. The total of 1.0 is a mathematical fact. The prediction is that the relative frequencies will not change if all assumptions are met.
Part 3 – Verify the Hardy-Weinberg Principle.

1. Using your parental population described in Part 1, simulate random mating of individuals by mixing the population.
2. Reach into the parental container without looking, and randomly select two gametes. Determine their genotype. Keep a tally here as to what genotype you get, and return the beads to the container
   BB Bb bb
3. Repeat step two 100 times to simulate production of 100 offspring
4. Calculate the frequency of each genotype and allele, and record the frequencies in table 2. Besides each of these new-generation frequencies write (in parentheses) the original frequency of that specific genotype or allele from table 1.
   Table 2.
   Genotypes Frequency Alleles Frequency
   BB ?? B ?
   Bb ?? b ?
   bb ??

Questions for Part 3:

1. The HW principle predicts that genotypic frequencies of offspring will be the same as those of the parental generation. Where they the same in your simulation?
2. If the frequencies were different, then one of the assumptions of the HW Principle was probably violated. Which one?

Part 4 – Effect of a Selection Pressure
Selection is the differential reproduction of phenotypes – that is, some phenotypes (and their associated genes) are passed to the next generation more often than others. In positive selection, genotypes representing adaptive traits in an environment increase in frequency because their bearers survive and reproduce more. In negative selection, genotypes representing nonadaptive traits in an environment decrease in frequency because their bearers are less likely to survive and reproduce. Selection pressures are factors such as temperature and predation that result in selective reproduction of phenotypes. Some pressures may elicit 100% negative selection against a characteristic and eliminate all successful reproduction by individuals having that characteristic. For example, mice with white fur may be easy prey for a fox if they live on a black lava field. This dark environment is a negative selection pressure against white fur. If survival and reproduction of mice with white fur were eliminated (i.e., if there is 100% negative selection), would the frequency of white mice in the population decrease with subsequent generations? To test this, use the following procedure to randomly mate members of the original parental population to produce 100 offspring.

1. Using the same parental population before, simulate the production of an offspring from this population by randomly withdrawing two gametes to represent an individual offspring. If that offspring is BB or Bb place it in a new container for the accumulation of the “Next Generation” Record this occurrence on the tally sheet below.
2. If the offspring is bb, place this individual in a container for those that “cannot reproduce.” Individuals in this container should not be used to produce subsequent generations. Record the occurrence of this genotype on the tally as well.
3. Repeat steps 1 and 2 until the parental population is depleted, thus completing the first generation.
   BB Bb bb
4. Calculate the frequencies of each of the three genotypes recorded on the separate sheet and record these frequencies for the first generation in table 3. Individuals in each generation will serve as the parental population for each subsequent generation.
5. Repeat steps 1-3 to produce a second, third, fourth and fifth generation, record you results in table 3.
   BB Bb bb

Table 3.
Generation
Genotypes First Second Third Fourth Fifth
BB ??
Bb ??
bb ??
Total 1 1 1 1 1

1. Graph your date from table 3. Generation is the independent variable on the x axis and Genotype is the dependent variable on the y axis. Graph three curves, one for each genotype.

Because some members of each generation (i.e., the bb that you removed ) cannot reproduce, the number of offspring from each successive generation of your population will decrease. However, the frequency of each genotype, not the number of offspring, is the important value.
Questions for part 4:

1. Did the frequency of white individuals decrease with successive generations? Explain why?
2. Was the decrease of white individuals from the first to second generation the same as the decrease from the second to the third generation? From the third to the fourth generation? Why or why not?
3. How many generations would be necessary to eliminate the allele for white fur?

Most natural selective pressure do not completely eliminate reproduction by the affected individuals. Instead, their reproductive capacity is reduced by a small proportion to show this use the below steps to eliminate only 20% of the bb offspring from the reproducing population.
Part 5 – Simulating 20% negative selection pressure.

1. Mix your beads together to start with your original parental population
2. Simulate the production of an offspring from this population by randomly withdrawing two gametes to represent an individual offspring.
3. If the offspring is BB or Bb, place it in a container for production of the “Next Generation.” Record the occurrence of this genotype on the tally below.
4. If the offspring is bb, place every fifth individual (20%) in a separate container for those that “Cannot Reproduce.” Individuals in this container should not be used to produce subsequent generations. Place the other 80% of the homozygous recessives in the container for the “Next Generation.” Record this tally below.
   BB Bb bb
5. Calculate the frequencies of each of the three genotypes recorded on the separate sheet and record these frequencies for the first generation in table 3. Individuals in each generation will serve as the parental population for each subsequent generation.
6. Repeat steps 1-4 to produce a second, third, fourth and fifth generation, record you results in table 4.
   BB Bb bb

Table 4. 20% negative selection
Generation
Genotypes First Second Third Fourth Fifth
BB ??
Bb ??
bb ??
Total 1 1 1 1 1

1. Graph your date from table 4. Generation is the independent variable on the x axis and Genotype is the dependent variable on the y axis. Graph three curves, one for each genotype.

Because some members of each generation (i.e., the bb that you removed) cannot reproduce, the number of offspring from each successive generation of your population will decrease. However, the frequency of each genotype, not the number of offspring, is the important value.
Questions for part 5:

1. Did the frequency of white individuals decrease with successive generations?
2. Was the rate of decrease for 20% negative selection similar to the rate for 100% negative selection? If not how did the rates differ?

Part 6 – Adaptations
Natural selection has shaped available genetic variation and the results are adaptations. Over many generations, characteristics with no adaptive advantage for survival and reproduction may decrease in frequency and those with significant advantage become prominent and frequent. A widely studied example of subtle variation of an adaptation involves the beaks and feeding ecology of Darwin’s finches of the Galapagos Islands. When the parent populations of finches arrived on the Galapagos, the birds became isolated as subpopulations on the islands. With time, speciation occurred, and subpopulations evolved beaks adapted to particular food items in the varied island environments. Food availability and competition were selective pressures that shaped beak morphologies, allowing each species to exploit a particular food.
In the following exercise you will be in groups of four and be given a tool analogous to the beak of a feeding bird. That beak represents an adaptation to gather food items of a particular size or shape. Some adaptations (beaks) are more advantageous than others at gather food of a particular size. In a competitive environment, the organism with the best adaptive morphologies will gather more food and will therefore be more fit. The four students will simultaneously feed from the same resource, and their success at gather food will measure the effectiveness of the “beak” adaptations.

1. Each group will get a chance at each of the feeding stations, you will have 4 – 20 second feeding sessions at each station. Each group member will “feed” from the same container placed in the middle of the table, so it should be equidistant from each member. Items will be placed in the cup to represent the stomach, you are not allowed to use your hands only “beak” otherwise that food item does not count. Cups should be kept on the table in front of the group member, but at the edge of the table.
2. Examine your food item, and hypothesize which tool is best adapted to gather the food available.
3. Listen to instructor for start times, feed for 20 seconds, and when the instructor tells you time is up, count the food items obtained and returned to the central container.
4. You will repeat this process a total of 4 times for each food item. Those values should be record on table 5.
5. Repeat this procedure at each of the food items recording your group’s data on the table.

Adaptations
Food Item A Beak 1 Beak 2 Beak 3 Beak 4
Feeding session 1
Feeding session 2
Feeding session 3
Feeding session 4
Mean items per session

Food Item B Beak 1 Beak 2 Beak 3 Beak 4
Feeding session 1
Feeding session 2
Feeding session 3
Feeding session 4
Mean items per session

Food Item C Beak 1 Beak 2 Beak 3 Beak 4
Feeding session 1
Feeding session 2
Feeding session 3
Feeding session 4
Mean items per session

Food Item D Beak 1 Beak 2 Beak 3 Beak 4
Feeding session 1
Feeding session 2
Feeding session 3
Feeding session 4
Mean items per session

Food Item E Beak 1 Beak 2 Beak 3 Beak 4
Feeding session 1
Feeding session 2
Feeding session 3
Feeding session 4
Mean items per session

Food Item F Beak 1 Beak 2 Beak 3 Beak 4
Feeding session 1
Feeding session 2
Feeding session 3
Feeding session 4
Mean items per session

Questions for Part 6:

1. Which beak is best adapted for Food item A?
2. Food item B?
3. Food item C?
4. Food item D?
5. Food item E?
6. Food item F?
7. Food item G?
8. Would a mixture of food sizes be more realistic of a natural situation?
9. Is competition a factor in the success of adaptations? Why or why not?
10. Does the success of a beak depend on which organism wields that beak? What is your evidence?
11. Would a mixture of food sizes amplify or diminish the differences among the success of adaptations?
12. Were there beaks that allowed you to survive with multiple food sources? Do you think in these cases, you would be a generalist or a specialist?

Sample Solution