Evolution and Population Genetics

Variation

Natural selection operates on populations that have variable characteristics. If some of the variants are "better" than others, they are more likely to allow individuals that possess them to survive and/or reproduce. These better variants will then increase in frequency in subsequent generations.

We will explore variation in the size of plant seeds to illustrate the concept of variation.

1) Choose one of the following kinds of seeds for measuring: maple samaras (seeds; they look like helicopters when they fall), acorns, dried beans, any other large seed. Dried beans can be purchased at a grocery store. Try to use a large variety because they are easier to measure.

Which of the above kinds of seeds are you going to use for this lab?_____________

Measure the length of 50 of these seeds to the nearest millimeter and record the lengths in the table below. If you choose maple samaras, measure the entire length, including the wing.

2) Write the lengths from above in the table below beginning with the smallest seed and ending with the largest seed.

Length

Length

Group the data above so that approximately 5 to 8 categories are formed. For example, if your smallest seed was 7 mm and your largest seed was 25 mm, you could have 5 categories. Seeds that are 7 mm to 10 mm could be the smallest category, 11 mm to 14 mm would be the next category. These categories are listed below.

  7 - 10 mm
11 - 14 mm
15 - 18 mm
19 - 22 mm
23 - 26 mm

Notice that each category covers the same span of measurements; in this case, it is 3 mm. 

Create 5 to 8 categories for your data in the left column of the table below. After your categories are listed, count the number of seeds in each category and enter that information in the column on the right.

Category List the smallest and largest in each category	Number of Seeds

3) In this step, you will use a graphing program to create a bar graph of the data in the table above. This can be done easily by going to the Create-A-Graph website. The Create-A-Graph website is designed for elementary and secondary school children so it is relatively easy to use and it makes nice graphs. A link to this website and a link to instructions for using the website are below. If you prefer, you may use any other graphing program or software. A link to instructions for using Microsoft Excel (2003 or earlier) to create graphs is below.

Instructions for using Create A Graph

Create A graph

Instructions for creating graphs using Excel

Use a computer graphing program such as Create A Graph or Microsoft Excel to create a bar graph of the data in the table above. The X-axis should be the category and the Y-axis should be the number of seeds. Go to the document titled "Submit Your Graph Here" to submit your graph.

4) Suppose that your seeds each have approximately 50 calories per mm. A seed that is 5 mm long therefore contains 250 calories and a seed that  is 30 mm contains 1,500 calories. Calculate the number of calories of energy that was required by the plant to produce the largest seed that you measured. (Note- If you do not have a calculator, use the one on the computer. Click Start, Programs, Accessories, Calculator).

5) Calculate the number of calories of energy that was required by the plant to produce the smallest seed that you measured.

6) Suppose that an average plant has 100,000 calories available to produce seeds. How many of the smallest seeds can it produce?

7) How many of the largest seeds can it produce?

8) If all the seeds survive and germinate in a warm climate, which plant will have the greater reproductive output, the one that produces small seeds or the one that produces large seeds?

9) Suppose that during a cold year, large seeds survive and germinate better than small seeds because they have more stored energy available for germination. Suppose also that the climate changes and becomes very cold such that the smallest seeds have a 30% survival rate and the largest ones have an 95% survival rate. If an average plant has 100,000 calories available, calculate the reproductive output in a cold climate for the plant that produces the smallest seeds. (First calculate how many seeds that this plant will produce, then calculate how many of these will survive.)

10) Calculate the reproductive output for the plant that produces the largest seeds.

11) Which plant will have the greatest reproductive output in a cold climate? Which plant will have the greatest reproductive output in a warm climate? (See your answer to the answer to #5 above.)

12) How will natural selection act on the size of seeds in areas where the climate is slowly changing from warm to cold?

13) Suppose that in extremely cold conditions such as are found in arctic areas, only the very largest seeds are capable of survival. What would likely happen to a population in in the arctic if there were no variation in the weight of seeds; all seeds medium in size and weighed the about the same? 

Population Genetics

Gene Frequencies in Populations

Model Population

In this exercise, we will explore what happens to the gene frequency of a population from one generation to the next.

The model population will consist of diploid, sexually-reproducing insects. Each individual in the model population will be represented by two squares that are attached to each other as shown in the diagram below.

These two squares represent one individual in the population.

Although each individual has many thousands of genes, we will let each square represent a gene for the color of the insect. In this species there are two colors: black and white. Black squares will be used to represent the gene (allele) for black-colored individuals. We will also use the letter "A" to represent this gene. White squares will be used to represent the gene for white-colored individuals. The letter "a" will also be used to represent the white gene. Each individual must have two squares because animals are diploid. 

The population below contains 18 individuals of the genotypes indicated in the table (36 squares total).

Number of   Individuals	Genotype
2	AA
8	Aa
8	aa

Click here to view the initial population for this exercise.

Gene frequency refers to the proportion of alleles that are of a particular type. For example, if 60% of the alleles in a population are "a" and 40% are "A", then the gene frequency of "a" is 0.6 and the gene frequency of "A" is 0.4. We will let p = frequency of "A" and q = frequency of "a".

14) Calculate the frequency of "A" in the initial population. This can be done by counting the number of black squares and converting this count to a proportion. The number of black squares can be converted to a proportion by dividing it by the total number of squares (36). Round your answer to two decimal places. For example, when rounding, the number 0.787 becomes 0.79.

15) Calculate the frequency of "a" in the initial population by counting white squares and dividing this by 36. Round your answer to two decimal places.

Gametes

Normally, males produce sperm and females produce eggs. In order to simplify our model, we will ignore the sex of the individual. Assume that any individual can mate with any other. Also for simplicity, we will have each individual produce four gametes (sperm or eggs) when it reproduces.

16) How many "A" gametes (single black squares) will an "AA" individual produce?

17) How many "a" gametes (single white squares) will an "AA" individual produce?

18) On average, how many "A" gametes will an "Aa" individual produce? 

19) How many "a" gametes will an "Aa" individual produce?

20) How many "A" gametes will an "aa" individual produce? 

21) How many "a" gametes will an "aa" individual produce?

Go to the gametes page, print it, and use scissors to cut out squares to represent the gametes.

Gametes produced by each individual will be placed in a container. Each of the two "AA" individuals in the population above produces 4 gametes for a total of 8 gametes. Place 8 single black squares in the container to represent the gametes produced by these individuals. 

Each "Aa" individual in the population above produces 4 gametes each for a total of 32 gametes. Place 16 black and 16 white squares in the container to represent reproduction of "Aa" individuals.

Each "aa" individual produces 4 gametes for a total of 32 gametes. Place 32 white squares in the container to represent reproduction by "aa" individuals.

Mating

Mate selection in this population is approximately random. You will simulate random mating by mixing all of the squares in the container.

22) Randomly select two squares from the container to represent a sperm and an egg. Place these side-by-side on a table to represent a new diploid individual produced by a sperm fertilizing an egg. Put a tally mark in the table below next to the genotype of this individual. Continue removing the squares two at a time and placing them next to each other. Tally each individual formed in the table below.

Convert the number in the tally column to a proportion by dividing by the total number of individuals.

Genotype	Tally or Count
AA (two black squares)
Aa (one black and one white)
aa (two white squares)

23) The gene frequency of "A" and "a" in the second generation can be calculated from the table above. Calculate these frequencies and record your answers in the table below.

	Initial Generation	2nd Generation
Frequency of "A"	0.33
Frequency of "a"	0.67

24) What happened to the gene frequency from the initial generation to the next generation?

25) If each individual produced 1,000 gametes instead of 4, do you think that the gene frequency would change?

26) If this experiment were carried out for another generation, what do you predict would happen to the gene frequency? If you are unsure of the answer to this question, continue the experiment for another generation.

Natural Selection

In this exercise, you will examine what happens to the gene frequency of an allele when natural selection acts to change the frequency.

We will use the same initial population that we used above. This population is summarized in the table below.

Number of    Individuals	Genotype
2	AA
8	Aa
8	aa

Suppose that the white (aa) insects produce fewer offspring than black (AA and Aa) insects. This could be due to a number of reasons. Perhaps white insects have lower survival because their predators can see them easier. Maybe they are not as efficient at finding food or nesting sites. Whatever the reason, each "aa" individual produces only 2 gametes (2 offspring). The other two genotypes (AA and Aa) produce 4 gametes each.

When this population reproduces, each of the two AA individuals will produce 4 gametes for a total of 8 gametes. Place 8 black squares in an empty container to represent these gametes.

When each Aa individual reproduces, 4 gametes will be produced, two black and two white. A total of 16 black and 16 white squares should be added to the container to represent the gametes produced by these 8 individuals.

Each aa individual produces only 2 gametes. A total of 16 white squares should be added to the container.

27) Draw the squares two at a time to represent diploid offspring and tally their genotype in the table below.

Genotype	Tally or Count
AA (two black squares)
Aa (one black and one white)
aa (two white squares)

28) Calculate the gene frequencies of "A" and "a" from the table above and record your answers in the table below..

	Initial Generation	2nd Generation
Frequency of "A"	0.33
Frequency of "a"	0.67

29) What was the effect of natural selection on the frequency of "A"?

30) What was the effect of natural selection on the frequency of "a"?

The Founder Effect and Migration

The initial population for this part will be the same one that was used in the previous two parts above (2 AA, 8 Aa, and 8aa).

Use scissors to cut out each of the 18 individuals in the initial population and place them in an empty container. Keep each of the two squares attached to each other.

Mix the individuals and then randomly select two individuals to start a "new" population. These two individuals represent emigrants to a new area.

31) Enter the number of each genotype in the new population. The new population that you enter below will contain only two individuals.

	Initial Population	New Population
Number of AA	2
Number of Aa	8
Number of aa	8

32) Record the gene frequency of the new population in the table below.

	Initial Population	New Population
Frequency of "A"	0.33
Frequency of "a"	0.67

Starting a new population from a few individuals is called the founder effect.

33) Did the founder effect result in a change in gene frequency when compared to that of the original population?

The gene frequency of the initial population also changed when the emigrants left.

34) Count the number that remained in the initial population after emigration. Enter this number in the table below. 

	Initial Population	Number Remaining
Number of AA	2
Number of Aa	8
Number of aa	8

35) Calculate the gene frequency of the initial population after the emigrants left and record it in the table below.

	Initial Population	Initial Population  After Emigration
Frequency of "A"	0.33
Frequency of "a"	0.67

36) Did emigration change the gene frequency? Explain.

Lab Report

Go to the document "Questions- Evolution and Population Genetics Lab" to submit the answers to these questions.