Week 2: Evidence of Evolution: Population Genetics

Dmitry Brogun; Azure Faucette; Kristin Polizzotto; Farshad Tamari

Week 2: Evidence of Evolution: Population Genetics

Objectives

Describe evidence of evolution that is based on microevolution and population genetics.
Define and use the terminology of population genetics correctly.
Use the equations of the Hardy-Weinberg equilibrium to calculate allele and genotype frequencies.
Graph allele frequencies using Microsoft Excel and identify changes in allele frequencies.
Draw graphs and explain three types of selection.

This lab begins with a brief overview of the Hardy-Weinberg Theorem and then includes two activities in which you apply the theorem. In the final section of the lab, you will consider what type of selection was at work in Activity 2.

I. Hardy-Weinberg Theorem
II. Types of Selection

I. Hardy-Weinberg Theorem

Direct evidence of a population that is evolving is a change in allele frequencies. The Hardy-Weinberg Theorem is one way to determine whether allele or genotype frequencies are changing from generation to generation. It states that under the conditions listed below, alleles and genotype frequencies do not change from generation to generation. As you know, most of the conditions (assumptions) are not met in real populations, and therefore, populations evolve.

Changes in allele frequencies can be calculated based on the following equations:

p + q = 1

p = frequency of dominant alleles in a population
q = frequency of recessive alleles in a population

Squaring both sides (p + q)² = 1² will yield:

p² + 2pq + q² = 1

p² = frequency of homozygous dominant genotypes (individuals) in a population
q² = frequency of homozygous recessive genotypes (individuals) in a population
2pq = frequency of heterozygous genotypes (individuals) in a population

The conditions (assumptions) of the Hardy-Weinberg Equilibrium are:

No mutations
Random mating
No natural selection
Extremely large population size
No gene flow
No genetic drift

Key Terms

Term

Definition

Microevolution

Microevolution is a change in allele frequencies in a population over generations

Alleles

Alternative forms of a gene, or variations of a gene

Natural selection

Differential survival and reproduction of individuals in a population of a particular species

Gene flow

The movement of alleles among populations due to immigration and/or emigration

Speciation

The origin of new species

Mutation

Changes in the nucleotide sequence of DNA

Genetic drift

Fluctuations in allele frequencies from one generation to the next that are not based on selection

Bottleneck effect

Constriction of the gene pool due to natural disasters (this is one example of genetic drift)

Founder effect

Constriction of the gene pool when a few individuals become isolated from a larger population (another example of genetic drift)

Directional selection

Favors individuals at one end of the phenotypic range

Stabilizing selection

Favors intermediate variants and acts against extreme phenotypes at either end of the phenotypic range

Disruptive selection

Favors individuals at both extremes of the phenotypic range and acts against intermediate variants

Homozygous

Having two identical alleles at a particular locus

Heterozygous

Having two different alleles at a particular locus

Dominant

An allele that suppresses the expression of a recessive allele in a heterozygote

Recessive

An allele whose expression is suppressed if a dominant allele is present (as in heterozygotes), but expressed in the homozygous state

Pre-lab Activity

View the following videos on the effect of natural selection before coming to lab:

Activity 1: Selection on Prey by a Predator, White and Black Alleles

In this activity, you will apply the Hardy-Weinberg Theorem to determine how allele frequencies change from generation to generation of a prey species being hunted by a predator.

1. Use the figure below to simulate the selection of the prey by the predator. These circles represent the prey species, and their colors represent variations (such as the ability to hide or to run fast) that may help them avoid being hunted and eaten by a predator. There are two alleles for the color gene, white (p) and black (q). The gray individuals are heterozygous.

Row of 10 black circles, 2 rows of grey circles totaling 20 circles, and 10 white circles

2. Now, you will act as the predator and “hunt” the circles (prey) by drawing an X through 3 white circles, 2 gray circles, and 1 black circle according to the following directions:

a. Set 30 seconds on the timer on your phone, and then start the countdown and your hunt.

b. Place your first X through the white and count to 10 before drawing the next X (about 3 seconds). If you have crossed out 3 white, 2 gray, and 1 black and you still have time left on the clock, repeat the same pattern until your 30 seconds is up. But remember to leave a little time (3 seconds or so) between each “attack” (represented by drawing an X), or you will run out of prey before the 30 seconds is up.

3. The number of each color that you hunt (3 white, 2 gray, 1 black) represents the advantage that each prey variation has. You can find and eat more white than black because white is less well adapted than the black. Based on the instructions given here, which color do you think is the best adapted to avoid predation? Write a hypothesis about what will happen to the frequency of each allele over time.

4. When the 30-second hunt is over, see how many survivors are left (circles not crossed out are survivors). Use the number of survivors to calculate p and q as shown in the example below. Once you have p and q, you will calculate p², 2pq, and q² and use those numbers to predict how many circles of each color there will be in the next generation, as shown in the example below.

Example

Calculations for generation 1

In generation 1, the allele and genotype frequencies are as follows:

C^WC^W = Homozygous (white)
C^WC^B = Heterozygous (gray)
C^BC^B = Homozygous (black)

We will designate the white allele C^W as p, and the black allele C^B as q. There are 40 individuals in the original population, which means there are 80 alleles total since the individuals are diploid.

10 white circles
20 gray circles
10 black circles

p = [(2 x 10) + 20]/80 = 0.5

p + q = 1, therefore, q = 0.5

If you are not sure why p and q are calculated in this way, please review the videos, reading, or slides for this week before continuing.

Calculations for generation 2

Imagine that the following numbers of individuals in generation 1 survive the hunt.

4 white circles
14 gray circles
8 black circles

These numbers may differ from the numbers that survive your first hunt. The purpose of this example is to show you how to figure out the proportions of colored circles for generation 2.

First, we recalculate new p and q values as below. Remember that the white allele is p and the black allele is q.

p = [(2 x 4) + 14]/52 = 0.423, and q = 1 – 0.423 = 0.577

p² = (0.423)² = 0.18 (round to two decimal places)

q²= (0.577)² = 0.33 (round to two decimal places)

2pq = 2 * 0.423 * 0.577 = 0.49 (round to two decimal places)

And then we construct a new population with 50 individuals using the values of p², 2pq and q² calculated above. You will need to round up or down to the nearest whole number if the formula gives you a number such as 16.5.

50 * p² = 50 x 0.18 = 9, so 9 white circles

50 * q² = 50 x 0.33 = 17 (round up), so 17 black circles

50 * 2pq = 50 * 0.49 = 24 (round down), so 24 gray circles

5. Now draw generation 2 with the predicted values and do the hunt again. For the example above, we would use 9, 24, and 17 and draw the circles as shown below.

Rows of 17 black circles, 2 rows of grey circles totaling 24 circles, and 9 white circles

Remember, however, that you should draw circles representing the numbers you calculated from your own hunt rather than these example numbers.

6. Now hunt again, as you did before, and record the number of survivors in your lab report. Then calculate new allele and genotype frequencies.

7. Based on the new allele and genotype frequencies you calculated, construct a new population (generation 3) of 50 individuals.

8. Now hunt again, as you did before, and record the number of survivors in your lab report. Then calculate new allele and genotype frequencies.

9. If you have time, repeat the experiment two more times for a total of five generations. It may be difficult to see a trend and draw any conclusions about your hypothesis with only three generations.

10. Using Microsoft Excel (Kingsborough Community College students have free access to a web version of Excel) or drawing your graph by hand, plot allele frequencies (p and q) and copy and paste your graph into the lab report. Your graph may look similar to the graph in Figure 1 (below). In 3 or 4 sentences in your lab report, describe and interpret your graph. Do the results support your hypothesis?

Line graph describing the relationship between p and q values. As one goes up, the other goes down, proportionally. — Figure 1: P and q value relationship. By: F. Tamari

Activity 2: Selection on Prey by a Predator, White and Red Alleles

In General Biology I you learned about the genetics of the Four O’Clock plant, showing incomplete dominance with:

C^RC^R plants being red
C^RC^W plants being pink, and
C^WC^W plants being white

This genetics of this system of inheritance, called incomplete dominance provide a an easy way to distinguish between homozygous dominants (red) and heterozygotes (pink).

We will use a similar system, except that the genotypes will not represent different colors in the Four O’Clock plant with different colors, but will represent fur color in a prey species that also exhibits incomplete dominance. You will again apply the Hardy-Weinberg Theorem to determine how the allele frequencies change from generation to generation of this species.

1. Secure a tray containing the material that you need as depicted in the photograph below. Work in groups of at least four students per group. Tray showing materials need for in-person lab. A. Tray, B. pink beads, C. white beads, D. red beads, E. pink beads

2. Select 10 red beads (B), 20 pink beads (D), and 10 white beads (C). These beads represent the prey species with different colors. The colors represent variations (such as the ability to hide or to run fast) that may help them avoid being hunted and eaten by a predator. There are two alleles for the color gene, red (p, C^R) and white (q, C^W). The pink individuals are heterozygous (C^RC^W).

3. Place the 10 red beads (B), 20 pink beads (D), and 10 white beads (C) in the container with small white beads (A). Mix the content. Assume that you are a predator that sees the red beads as not camouflaged, the pink beads as less camouflaged, and the white beads as camouflaged. You will capture an individual prey every 3 seconds for 30 total seconds in the following manner (one bead per every 3 seconds):

a. 3 red beads (take one out, wait 3 seconds, take a second out, wait 3 seconds, take the third one out), then move to the next step

b. 2 pink beads (waiting 3 seconds between each removal)

c. 1 white bead

d. Repeat steps a–c for a total of 30 seconds (one bead per every 3 seconds).

First, write a hypothesis about what will happen to the frequency of each allele (p and q) over time, and then begin the 30-second hunt.

4. When the 30-second hunt is over, see how many survivors are left (represented by beads with different colors). Use the number of survivors to calculate p and q as shown in the example below. Once you have p and q, you will calculate p², 2pq, and q² and use those numbers to predict how many beads of each color there will be in the next generation, as shown in the example below.