University of Michigan - Department of Astronomy

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Evolution by Natural Selection

Probably all organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed. There is grandeur in this view of life that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved.


--Charles Darwin, The Origin of Species

Evolution is a theme that runs through all areas of study in astronomy, from the evolution of the solar system and stars all the way to the evolution of the universe. Understanding how physical systems change and react is the driving force behind all of science. With the advent of the field of astrobiology, astronomers are now beginning to study the evolution of environments that permit and sustain the evolution of life. One of the defining features of the evolution of life is natural selection. A basic understanding of the process of natural selection is necessary if we are ever to understand under what circumstances life evolves and hence how to define ‘habitable planet'.

 

In this experiment, we will be studying natural selection by modeling the evolution of several generations of predators and prey.  Beans will be the prey, and you, using various implements to capture the prey, will be the predators.

 

Background

An allele is any one of a number of alternative forms of the same gene occupying a particular position on a chromosome. An example is the gene for blossom color in many species of flower - a single gene controls the color of the petals, but there may be several different versions of the gene. One version might result in red petals, while another might result in white petals.  

Often one allele is "dominant" (denoted with a capital letter) and the other is "recessive" (denoted with a lower-case letter) - the "dominant" allele will generally determine what trait is expressed. The different combinations of alleles, e.g., AA, Aa, aa are called genotypes. A phenotype is the expression of a genotype.  For example, in the case of blossom color, if the "red" allele (A) is dominant to the "white" allele (a), a flower with the red alleles will have genotype AA and phenotype red petals, a flower with genotype aa will have phenotype white petals, and a flower with one red and one white allele will have genotype Aa, and the phenotype of red petals. In sexual reproduction one allele is inherited from the female and one from the male.

 

 

 

Some alleles may be co-dominant, in which case  the alleles are represented by capital letters (usually A and B).  There are 3 possible genotypes: AA , BB, or AB.  For example, if the red allele A and white allele B are co-dominant the genotype AA would result in red petals, BB would result in white petals, and AB may result in pink petals.

For a population of individuals it is the frequency of the phenotype and genotype that survive to adulthood and reproduce that determines which traits are passed onto the next generation. If the expression of the gene (allele combination) is either advantageous or neutral, it will be passed on with a higher frequency. If the expression of the gene is deleterious or lethal it will not disappear from the population but its frequency will decline.

The phenotype frequency is the number of individuals in a population expressing a particular trait: how many flowers have red petals, how many white? The genotype frequency is the percentage of the population with a particular allele combination or gene (AA, Aa, or aa). The allele frequency measures the frequency of A or a, which determine the makeup of the next generation.  Essentially, it is the fraction of all of the “A”s or “a”s occurring in a population.  For example:

Allele frequency of A = p = [(2 x (# of AA)) + (1 x (# of Aa))] /  (total # of  alleles)

Where the total # of alleles = 2 x (total # of individuals) (because each individual has 2 alleles)

A punnett square is used to shoe the relationship between the genotypes, allele frequency and genotype frequency.  See appendix A for more background information and an example of the calculations.

Note: Group size for this lab must be 5 – 9 people.

Part 1: Prey

Set Up

You'll need to clear off the table or work on the floor for this.  Clear your area and get some paper from your GSI.  Spread the paper on the table and record what kind of paper it is (classified, comics or plain white paper) on the worksheet.

The beans are your prey.  The average weight of one black bean is 0.18g, one mottled bean is 0.32g, and one white bean is 0.30g. You will start with a population of 100 black beans (18.0 g), 100 white beans (30.0 g) and 200 mottled beans (64.0 g). This gives you a total of 400 beans, which means there are 800 alleles: 400 A's (all the black and half the mottled alleles) and 400 B's (all the white and half the mottled alleles).  The frequency is 0.5 for both alleles. 

Place a cup on the scale and zero it (push tare).  Fill it with 18.0 g of black beans (or as close as you can get).  Use a new cup to measure the white beans then the pinto beans.  Give the three cups to one member of the group, who will control the hunting.

Foraging

You will “forage” for prey for 5 seconds. The person with the beans will start and stop the foraging.  While foraging make sure that you follow these rules:

1.     You may collect beans ONLY with your thumb and index fingers

2.     You may only pick up ONE bean at a time

3.     Place the beans you pick up in your cup.

4.     Your cup must be off the ground at ALL times

5.     You can not knock over or take from another predator's cup

The predators should each take a cup and arrange themselves around the table facing AWAY from the table or the space on the floor.  The controller will spread the beans on the paper. When everyone is ready, the controller should give the signal to begin foraging.  The predators should turn around and gather as many beans as they can while the controller counts off 5 seconds.  All foraging must stop after 5 seconds.

Reproduction

Now you need to determine the makeup of the next generation prey. 

1.     Count the number of prey of each type that were eaten.  Enter the totals in the table on the worksheet.

2.     Record the number of each type of prey surviving by subtracting the number eaten from the number at the beginning of the round.

3.     Calculate the total number of prey remaining and the number of alleles remaining.

4.     Calculate the allele frequency of the remaining prey (see the last section under background.)

5.     Enter the allele frequencies in the Punnett Square and calculate the genotype frequencies (see the appendix for details.)  Then enter the genotype frequencies in the spaces below the Punnett Square (remember AB is the same as BA).

6.     Each surviving bean gets to reproduce, so the number of prey in the next generation is 2*number of survivors.  Calculate the number of prey for generation 2 and the total number of alleles.  Enter these values on the worksheet at the beginning of generation 2. 

7.     Multiply the total number of beans by the genotype frequencies to determine the number of each bean for generation 2.

8.     Determine how many of each type of bean you need to add to the foraging area by subtracting the number you had at the end of the last round from the number of prey you need.  Add this number of beans to the foraging area by weighing them out on the scale (see “Set Up” for weights.)

Once you have completed your calculations, repeat the Foraging and Reproduction steps for generation 2 and 3.  After foraging generation 3, do the calculations to determine what generation 4 will look like (# of prey and alleles, number of each phenotype and final genotype frequency).

Fill in the paper types and 4th generation table up on the chalkboard.

Return your beans to the proper containers and the paper to your GSI.

You can go on to Part 2 at this point, however the questions from part 1 require information from the chalkboard (so don't forget to go back and do them!)

Part 2: Predators

Set Up

In this section, the prey will all be the same however there will be two types of predators: ones with fingers (A) and ones with utensils (a).  Since A is dominant over a, all predators with AA or Aa genotype have the “fingers” phenotype.  The allele frequency is about 0.5 to begin.

Get a set of utensils and beans from your GSI.  Record your utensil type on the worksheet.  Pick one person to spread the beans and control the hunting.

Record the total number of predators and alleles for generation 1.  Since the allele frequency is 0.5, 1/4 of the predators should be geneotype AA, 1/4 genotype aa and 1/2 genotype Aa. Assign a genotype to each predator, rounding where necessary with preference to type aa (e.g. if there are 6 people in a group, 1 is AA, 2 are aa and 3 are Aa.)  Give all predators with genotype aa a utensil.  Record your genotype below the table (or check the space to say you were the controller),  and the actual number of predators with each genotype in the table.  Note that because of the limited size of your groups, your predator populations will be artificially limited!

Foraging

You will “forage” for prey for 5 seconds.  The controller will start and stop the foraging.  While foraging make sure that you follow these rules:

1      You may collect beans ONLY with your thumb and index fingers OR your utensil (not both)

2      Pick up as many beans as you can with your foraging tool (i.e. you may not scoop up the beans with your whole hand, but you can scoop up several if your tool is a spoon)

3      Place the beans you pick up in your cup

4      Your cup must be off the ground at ALL times

5      You may not knock over or take from another predator's cup

The predators should each take a cup and arrange themselves around the table facing AWAY from the table or the space on the floor.  The controller will spread the beans on the paper. When everyone is ready, the controller should give the signal to begin foraging.  The predators should turn around and gather as many beans as they can while the controller counts off 5 seconds.  All foraging must stop after 5 seconds.

Record how many beans you “ate.”

Reproduction

Now you need to determine the makeup of the next generation predators. 

1.     Determine who “ate” the most.  Record the number of beans “eaten” by this predator.  Any predators that ate less than half of this number starve.  However: at least 2 predators must survive to create the next generation, and (due to the artificial constraints of the lab) no more than half can survive.  So, if one person collects significantly more beans than anyone else, the best 2 predators survive.  If everyone collects close to the same number, the best half of the group survives.  You can round up and have the controller join the next generation if necessary. 

2.     Record the number of survivors of each geneotype in the table.

3.     Add the survivors and record the total number of survivors and their remaining alleles.  Calculate their allele frequency. 

4.     Enter the allele frequencies in the Punnett Square and calculate the genotype frequencies.  Then enter the genotype and phenotype frequencies in the following table.

5.     Each surviving predator gets to reproduce, so the number of predators in the next generation is 2*number of survivors.  Calculate the number of predators for generation 2 and their number of alleles.  Multiply the number of survivors by the genotype frequencies to determine the genotype number for generation 2.  Remember you must have a whole number for each type of predator!

6.     Reassign the genotypes to the predators and hand out utensils to all the aa genotypes.

Repeat the Foraging and Reproduction steps for generations 2 and 3.  After foraging with generation 3, figure out what generation 4 will look like (i.e. stop before you reassign the utensils and forage with generation 4)

Fill in the utensil types and 4th generation table up on the chalkboard.

Return your beans to the proper containers and the utensils to your GSI.

Make sure you answer all the questions, and turn in your worksheet.


Appendix A

1. Quantitative Natural Selection: measuring the probability that a trait will be passed to the next generation

For a population in equilibrium[1] allele frequencies are expressed mathematically from 0 to 1. An allele frequency of 1 means that all individuals in the population express that allele. Conversely, an allele frequency of 0 means that no individual expresses the allele.  More commonly alleles will have different frequencies. Also, within a population the allele frequencies can differ between dominant and recessive alleles. The frequency of alleles in a gene pool is given by a binomial distribution:

(p + q)2 = p2 + 2pq + q2  = 1                               (Eq. 1)                                              

 p = the frequency of allele A
q = the frequency allele a.

For example, alleles in a gene pool of a population might have an allele frequency (p) of 0.80 for A, and allele frequency (q) of 0.20 for a. Since A has a frequency of 0.80, when it combines with another A of 0.80, the genotype frequency for AA will be 0.64 (p*p = 0.80*0.80 = 0.64). A genotype frequency can be defined as the percentage of a population with a particular allele combination (or gene).   The frequency of genotypes will be:

p2 + 2pq + q2  = 1
or
0.64 + 2(0.16) + 0.40 = 1.00

If an allele is deleterious (lethal) its frequency will gradually decrease although it will never disappear from the population (2).

A Punnett Square is a table which shows the relationship between alleles, allele frequencies and genotypes.  An example of the above situation is shown below.  As stated above, in sexual reproduction one allele is inherited from the female and one from the male.  So in the Punnet  Square, the row might represent the father's possible genetic contribution and the column the mother's possible genetic contribution.

Table 1: Punnett square for genotypes with possible alleles A and a.  The frequency of genotypes AA, Aa and aa are given, based on the allele frequencies of A (p) = 0.80 and a (q) = 0.20 in an equilibrium population.

p = 0.80
Allele freq. of A

q = 0.20
Allele freq. of a

p = 0.80
Allele freq. of A

AA
0.64

Aa
0.16

q = 0.20
Allele freq. of a

Aa
0.16

aa
0.04

 

2. Additional Background Information:

Definitions:

Allele: One member of a pair or series of genes that occupy a specific position on a specific chromosome. Alleles are often referred to by the letter ‘A' or ‘a', with the capital letter indicating dominance. Possible allele pairings (pairings of different versions of a gene, called genotypes) are AA, Aa, and aa.

Chromosome: Long, stringy aggregate of genes that carry heredity information.

Gene: Unit of heredity information that consist of DNA located on chromosomes. Genes can exist in alternative forms called alleles.

Genotype: This is the "internally coded, inheritable information" carried by all living organisms. These instructions are found within almost all cells (the "internal" part), they are written in a coded language (the genetic code), they are copied at the time of cell division or reproduction and are passed from one generation to the next ("inheritable"). These instructions are intimately involved with all aspects of the life of a cell or an organism. They control everything from the formation of protein macromolecules, to the regulation of metabolism and synthesis (3). Genotypes are identified as the different combinations of  alleles, e.g., AA, Aa, aa.

Genotype Frequency: percentages or proportions of the genotypes in a  population, usually denoted by the letters, P, Q and R.  The sum of the genotype frequencies is always 100%. Frequencies are measured simply by observing and counting the numbers of each type of organism in the population, and dividing by the total number of organisms in the population (the population size).

Phenotype: The expression of a genotype. For example: In the case of blossom color, if the "red" allele (A) is dominant to the "white" allele (a), in a flower with one red and one white allele  (Aa), the petals will be red. The expression of the genotype, in this case that the flower has red petals, is called a phenotype. So, in this example there are two possible phenotypes (red or white) that are expressed from three possible genotypes (AA => red, Aa => red, aa => white) which are the result of the combinations of two alleles (A or a). Examples of common phenotypes are eye color, hair color, skin color in humans. Surface pattern/color in beans (3).

Punnett Square: A table listing the possible genotype, or allele combination frequencies. Each combination of alleles results in a genotype. In the example below AA, Aa and aa are the possible genotypes. Genotypes result in the manifestation of a physical trait, like eye color in humans, or petal color in flowers. The physical manifestation of a genotype is called a phenotype. Human eye color phenotypes include blue, brown, green. Flower petal phenotypes include red, white, yellow, and more.
Phenotype and Genotype using blood type in humans as an example.

Table 2: The ABO Blood Groups

Blood Group Phenotype

Blood Group Genotype

A

IA/IA or IA/i

B

IB/IB or IB/i

AB

IA/IB

O

i/i


Table 3
: ABO Blood Groups

Population

Identification

Total

Tested

Total of Each Group Frequency of Each Group
O A B AB O A B AB

American Indian:
Cherokee, NC, USA

166

157

6

3

0

0.95

0.04

0.02    

0.00

Pawnee,
OK, USA

80

46

32

2

0

0.58

0.40

0.03    

0.00

Bengali:       
Calcutta, India

10,032

3045

2561

3766

660

0.30

0.26

0.36    

0.07

Blacks:  
 St Louis, MO, USA

1395

713

353

269

60

0.51

0.25

0.19    

0.04

Chinese:            
Hong Kong

4648

2025

1127

1233

263

0.44

0.24

0.27    

0.06

Dunkers:
PA, USA

228

81

135

7

5

0.36

0.59

0.03    

0.02

Whites:
IA, USA

6313

2892

2626

568

227

0.46

0.42

0.09    

0.04

Eskimo:     
Greenland

1156

546

522

68

20

0.47

0.45

0.06    

0.02

French Canadians:
Quebec

9689

3832

4549

895

413

0.40

0.47

0.09    

0.04

Japanese:
Nikappu

355

109

136

77

33

0.31

0.38

0.22    

0.09

Jews:
Israel

465

177

187

74

27

0.38

0.40

0.16    

0.06

Palestinians:   
Kuwait

4165

1383

1641

802

337

0.33

0.39

0.19    

0.08

Raw data from Mourant, A. E., Kipec, Ada C., and Domanjewska-Sobezak, Kazimiera, The Distribution of Human Blood Groups, Oxford University Press, London, 1976. (3)

Table 2 above illustrates phenotype vs. genotype for human blood groups. The genotype is determined from the combination of alleles, Here there are three alleles: IA, IB and i.  Alleles IA and IB are dominant to i, but show no dominance with respect to each other. The ABO blood groups are inherited. The genotypes and their corresponding phenotypes (blood groups) are shown in Table 3.

 

Citations:

1)     Wikipedia (http://en.wikipedia.org/wiki/Allele)

2)     Evolution by Natural Selection: Historical Perspective & Hardy-Weinberg http://faculty.uca.edu/~march/bio2/evolutionlabs/lab2intro_sp99.htm

3)     Serology Information from : The Teacher's manual of Carolina Blood Typing from the Aseptic Bulk Blood Cells Set (https://www2.carolina.com/webapp/wcs/stores/servlet/ProductDisplay?memberId=-1002&productId=5361&langId=-1&storeId=10151&catalogId=10101). 

4)     Campbell, Reece, Mitchell,  Biology Concepts & Connections, 5th edition, Pearson, Benjamin Cummings, p. 1112

5)     Activity taken/modified from: iweb.tntech.edu/cabrown/Ecology/ Ecology%20Lab/Selection%20Lab.doc

6)     Bennet, Shostak, Jakosky,  Life in the Universe, Addison Wesley 2003, p. 57.

 



[1]If individuals in a population mate randomly, without regard to genetic constitution, the offspring can have any combinations of genes or alleles from the gene pool. That is, whatever alleles are present in the population will have an equal chance of appearing from generation to generation. The population is said to be in equilibrium. This generalization is known as the Hardy-Weinberg principle. It operates only when there is random mating, no introduction of new alleles by mutation or migration of alleles into the population (gene flow). If mating is not random, or mutation or migration of alleles occurs, then inheritable changes result. In other words, evolution occurs.

 


updated: 7/15/08 by SAM Original by BE 2003

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