Hardy-Weinberg Activity Microevolution in the Eastern Gray Squirrel

Gray Variant (wild type)

Black Variant (mutant)

Squirrels competing for limited resources in their environment

INTRODUCTION

What do porcupines, flying squirrels, beavers, mice, and naked mole rats all have in common? They belong to one of the most diverse and successful of all mammals – the rodents. In fact, forty percent of all species of mammals are rodents. Living worldwide in extremely varied habitats, from tropical sea shores to icy mountain ranges, rodents demonstrate the adaptability of life. But how did these chisel-toothed creatures adapt to such extremes?

Evolution, on a genetic level, is a change in the frequency of alleles in a population over time. The Eastern Gray Squirrel (Sciurus carolinensis) demonstrates evolution in action. This familiar species, commonly seen pillaging bird feeders and scampering about trees in neighborhoods around Acton, has a geographical range that extends from Florida up through Canada and into the Midwest. Surprising to many, the “gray” squirrel has a black variant that can be born from normal gray-furred parents. Such offspring contain a mutation that causes more melanin to be concentrated in their hairs, giving these squirrels a black appearance. Offspring born from such mutants are likely to inherent this trait since it is a dominant allele.

PURPOSE In this activity you will investigate a mystery: black squirrels, usually very rare, are common in concentrated populations in some parks throughout Eastern North America, especially in colder city parks around the Great Lakes of America and Canada. What biological process or mechanism might account for this? Can we model how this might work?

BACKGROUND As we have seen with these squirrels, some alleles may be more common than others in a gene pool. For example, let’s suppose that if we counted all the squirrels around the ABRHS campus, we found the population to be 20% black and 80% gray. The gene frequency in a population for the Hardy-Weinberg Equilibrium is written as:

Key Genotypes: Black phenotype homo dom. = hetero. =

Gray phenotype homo rec. =

pp : 2pq : qq

or p2 + 2pq + q2 = 1 where p = the frequency of the dominant allele (black) , and q = the frequency of the recessive allele (gray). It follows that p + q = 100% of all the genes in the gene pool. For this population, qq = .80. Therefore, the square root of .80 = .89, or 89% of the genes in this small gene pool. So, the frequency of the dominant allele must be 11% of the total genes for fur color. In other words, squirrels which are homozygous for the dominant gene make up about 1% (.11 x .11) of the total population. While 19% (2pq = 2 x .11 x .89) are the heterozygotes, and 80% are the homozygous recessive individuals. MATERIALS (per group) Beaker of grayish beans Beaker black beans 1 plastic bag 3 empty beakers Masking tape PROCEDURE 1. Get into a group of 4-5 people and get the materials listed above (6 groups in a class). 2. The black beans represent the allele for black fur, and the grayish beans represent the allele for gray fur. The plastic bag represents the environment in eastern North America where the squirrels randomly mate. 3. Label one beaker “Black Fur, FF” for the homozygous dominant genotype. Label a second beaker “Black Fur, Ff” for the heterozygous condition. Label the third beaker “Gray Fur, ff” for those squirrels with the homozygous recessive genotype. (See below):

Black Fur FF

Black Fur Ff

Gray Fur ff

Beakers:

4. Each group will start with 25 black and 25 gray beans. Put the fifty beans (representing alleles) into the plastic bag and shake it up (represents a mixing of alleles via reproduction between squirrels). 5. The six lab groups will now be assigned to a scenario: “Scenario #1 – the Hardy-Weinberg Equilibrium,” “Scenario #2 – Natural Selection,” and “Scenario #3 – Genetic Drift.” Notice that since there are six lab groups, two groups will be doing one of these three situations. Scenario #1 – Hardy-Weinberg Equilibrium: a) Without looking at the beans, select two at a time, and record the results on the data form on the following page - next to "Generation 1." For instance, if you draw one black and one gray bean, place a mark in the chart under "Number of Ff individuals." Continue drawing pairs of beans and recording the results in your chart until all beans have been selected and sorted. (Please note that the total number of individuals will be half the total number of beans because each squirrel requires two alleles.) b) For this simulation, count the F and f alleles (beans) that were placed in each of the beakers for "black squirrels" in the first round and record the number in the chart in the columns labeled "Number of F Alleles" and "Number of f Alleles." Repeat this step for the “gray squirrels.” Total the number of F alleles and f alleles for the first generation and record this number in the column labeled "Total Number of Alleles." Below is a sample of how your results might look:

Generation

Number of FF

Individuals

Number of Ff

Individuals

Number of ff

Individuals

Number of F Alleles

Number of f Alleles

Total Number of

Alleles

Gene Frequency

of F

Gene Frequency

of f 1 xxxxxx

xxxxxxxx xxxx

xxxxxxx

24

26

50

c) Place the alleles of the squirrels (which have grown, survived and reached reproductive age) back into the plastic bag and mate them (shake bag) again to get the next generation. d) Repeat steps “a” through “c” to obtain generations two through five. Try to make sure everyone in your group has a chance to either select the beans or record the results. e) Determine the gene frequency of F and f for each generation and record them in the chart in the columns labeled "Gene Frequency F" and "Gene Frequency f." To find the gene frequency of F, divide the number of F by the total, and to find the gene frequency of f, divide the number of f by the total. Express results in decimal form. The sum of the frequency of F and f should equal one for each generation.

DATA – SCENARIO #1 (HARDY-WEINBERG EQUILIBRIUM) Generation Number

of FF Individuals

Number of Ff

Individuals

Number of ff

Individuals

Number of F Alleles

Number of f

Alleles

Total Number

of Alleles

Gene Frequency

of F

Gene Frequency

of f 1 2 3 4 5 Scenario #2 – Natural Selection: a) As with the Hardy-Weinberg scenario, your group will start with 25 black and 25 gray beans. Put the fifty beans (representing alleles) into the plastic bag and shake it up (represents a mixing of alleles via reproduction between squirrels). b) Select two beans (alleles) at a time from the bag without looking, and record the results on the data form next to "Generation 1." For instance, if you draw one black and one gray bean, place a mark in the chart under "Number of Ff individuals." Continue drawing pairs of beans and recording the results in your chart until all beans have been selected and sorted. Place the "squirrels" into the appropriate dish: FF, Ff, or ff. c) The FF and Ff squirrels are born with shiny black fur. Unlike the Hardy-Weinberg situation above, squirrels with black fur living in a wooded environment stand out against the dull gray/brown background more than their gray-furred relatives (see photo on the first page for an example). This is especially true in the colder months once deciduous trees have dropped their leaves, creating a landscape full of grayish trees and a forest floor covered by brown, dried leaves. The shinny, black-coated squirrels easily stand out in this environment, especially in large forest tracts where red-tailed hawks abound. These keen-eyed raptors spot the conspicuous black squirrels and swoop down upon them often before they can escape. Therefore, the black variants are less likely to reach reproductive age and pass on their genes. Place half the beans from the FF and Ff containers aside before beginning the next round. d) Once half of the beans have been removed from the homozygous dominant and heterozygous beakers, you may now count the remaining F alleles (beans) in each container. Do the same for the f alleles. Total the number of F alleles and f alleles for the first generation and record this number in the column labeled "Total Number of Alleles." Below is a sample of how your results might look:

Generation

Number of FF

Individuals

Number of Ff

Individuals

Number of ff

Individuals

Number of F Alleles

Number of f Alleles

Total Number of

Alleles

Gene Frequency

of F

Gene Frequency

of f 1 xxx

xxxxxx

xxxxxxx

12

20

32

e) Place the alleles of the surviving squirrels (which have grown and reached reproductive age) back into the container and mate them again to get the next generation. f) Repeat steps “a” through “e” to obtain generations two through five. Make sure everyone in your group has a chance to either select the beans or record the results. g) Determine the gene frequency of F and f for each generation and record them in the chart in the columns labeled "Gene Frequency F" and "Gene Frequency f." To find the gene frequency of F, divide the number of F by the total, and to find the gene frequency of f, divide the number of f by the total. Express results in decimal form. The sum of the frequency of F and f should equal one for each generation.

DATA – SCENARIO #2 (NATURAL SELECTION) Generation Number

of FF Individuals

Number of Ff

Individuals

Number of ff

Individuals

Number of F Alleles

Number of f

Alleles

Total Number

of Alleles

Gene Frequency

of F

Gene Frequency

of f 1 2 3 4 5 Scenario #3 – Genetic Drift: a) Unlike the Hardy-Weinberg scenario, your group will start with 40 black and 10 gray beans. The cause of this imbalance is the result of the founder effect. A professor at a college in Ohio studied the black squirrel variety in her laboratory and a few of her graduate students accidentally released twenty black individuals onto the campus, flooding the gene pool with the dominant allele. Put the fifty beans (representing alleles) into the plastic bag and shake it up (represents a mixing of alleles via reproduction between squirrels). b) Without looking at the beans, select two at a time, and record the results on the data form on the following page - next to "Generation 1." For instance, if you draw one black and one gray bean, place a mark in the chart under "Number of Ff individuals." Continue drawing pairs of beans and recording the results in your chart until all beans have been selected and sorted. (Please note that the total number of individuals will be half the total number of beans because each squirrel requires two alleles.) c) For this simulation, count the F and f alleles (beans) that were placed in each of the beakers for "black squirrels" in the first round and record the number in the chart in the columns labeled "Number of F Alleles" and "Number of f Alleles." Repeat this step for the “gray squirrels.” Total the number of F alleles and f alleles for the first generation and record this number in the column labeled "Total Number of Alleles." On the next page is a sample of how your results might look:

Generation Number of

FF Individuals

Number of Ff

Individuals

Number of ff

Individuals

Number of F Alleles

Number of f Alleles

Total Number of

Alleles

Gene Frequency

of F

Gene Frequency

of f 1 xxxxxx

xxx xxxxxxxx xxxxxx

xx

32

18

50

d) Place the alleles of the squirrels (which have grown, survived and reached reproductive age) back into the plastic bag and mate them (shake bag) again to get the next generation. e) Repeat steps “a” through “d” to obtain generations two through five. Try to make sure everyone in your group has a chance to either select the beans or record the results. e) Determine the gene frequency of F and f for each generation and record them in the chart in the columns labeled "Gene Frequency F" and "Gene Frequency f." To find the gene frequency of F, divide the number of F by the total, and to find the gene frequency of f, divide the number of f by the total. Express results in decimal form. The sum of the frequency of F and f should equal one for each generation.

DATA – SCENARIO #3 (GENETIC DRIFT) Generation Number

of FF Individuals

Number of Ff

Individuals

Number of ff

Individuals

Number of F Alleles

Number of f

Alleles

Total Number

of Alleles

Gene Frequency

of F

Gene Frequency

of f 1 2 3 4 5

CLASS DISCUSSION

1) Someone from each group should go the board and make a bar graph of their data in the following manner:

Scenario: _____________ KEY:

= F allele

= f allele

Allele Frequency

Gen. 1 Gen. 2 Gen. 3 Gen. 4 Gen. 5 2) How does the Hardy-Weinberg provide a baseline for identifying how populations evolve, as a function of changes in their allele frequencies? Reconsider the criteria of the Hardy-Weinberg Equilibrium:

- Large population. The population must be large to minimize random sampling errors. - Random mating. There is no mating preference. For example an AA male does not prefer an aa female. - No mutation. The alleles must not change. - No migration. Exchange of genes between the population and another population must not occur. - No natural selection. Natural selection must not favor any individual.

3) As you have seen in this activity, gray squirrels predominate due to the fact that they are camouflaged better than their black-coated relatives. However, biologists have measured that black squirrels have 18% lower heat loss in temperatures below -10 degrees Celcius, along with a 20% lower basal metabolic rate, and therefore shiver less (by 11%) compared to the gray variety. How does this study answer why concentrated populations of black “gray” squirrels are found commonly in some northern city parks?

More Fun with Black “Gray” Squirrel Mutants (Check out these cooky websites): http://www.roadsideamerica.com/set/squirrelsblack.html

http://www.victoria-park.com/ksu.htm

SOURCES This lab has been modified from a lab entitled Breeding Bunnies, by WGBH Educational Foundation and

Clear Blue Sky Productions (2001), a lab originally written by Joseph Lapiana (1994). The genetics of these squirrels came from: http://www.woodrow.org/teachedrs/bi/1994/find.html