Drosophila Melanogaster and Evolution by hcj

VIEWS: 129 PAGES: 15

									Drosophila Melanogaster and Evolution: An Examination of Evolutionary Forces Acting on
                         Populations of the Common Fruit Fly




                                                                       Matthew Gasbarre
                                                                 Biology 220W Section 4
                                                                    TA: Mark Darlington
                                                                       Due Date: 3-2-04
 Drosophila Melanogaster and Evolution: An Examination of Evolutionary Forces Acting on
                          Populations of the Common Fruit Fly

Introduction
      Everyday we walk around on this Earth we see variations among various plants and animals.

From the variation among trees, such as the oak tree and maple tree to the different colors of skin

we as humans have, there is an abundance of physical characteristics that vary from organism to

organism. Also, these variations are always changing over time and seem to never really stay

constant. In order to understand why physical characteristics change over time, we must understand

the concept of biological evolution.

       Biological evolution is simply the changes of genetic composition of a population with the

passage of each generation (Minesky Lecture 1 and 2). It would be very difficult to examine certain

organisms from generation to generation because certain organisms such as ourselves have a long

time period between each generation. That is why we use Drosophila melanogaster or the common

fruit fly to examine physical and genetic variation over time. The fruit fly has an incubation period

of roughly two weeks between each generation (Darlington Bio 220W TA). In addition, these

organisms are very cost effective and produce mass numbers of offspring that can be examined

using a microscope. Therefore, the fruit fly is an ideal organism for examining genetic variation

over a certain amount of time.

       The main purpose of this experiment was to examine the evolutionary process in Drosophila

melanogaster in two different populations (Population A and B) by examining their allele

frequencies. Population A consisted of two subpopulations, one that was large and another one that

started off with ten fruit flies from the larger population. The fruit flies in this population were

examined for various types of eye characteristics. Those eye features were wild-type eyes,

sparkling eyes, and an eyeless feature. Population B had two different wing characteristics that

included curly wings versus wild type straight wings. It was through the application of the Hardy
Weinberg Theorem that we were able to determine if evolution did occur in these two separate

populations. The Hardy Weinberg Theorem simply states that an evolving population shows

changes in allele frequency over time while a non-evolving population will have constant allele and

genotype frequencies from one generation to the next (Lab Manual Exercise 2 pg 2). A population

that is non-evolving is said to be in Hardy Weinberg Equilibrium. Hardy Weinberg Equilibrium is

met only when a population is large, has random mating, no mutation, no migration, and no natural

selection (Lab Manual Exercise 2 pg.3). In this particular experiment, the fruit flies were examined

to see if their populations were in constant Hardy Weinberg Equilibrium.

       The hypothesis for Population A was that the frequencies of the alleles would not change

from generation to generation if both the large and small populations were in Hardy Weinberg

Equilibrium. The hypothesis for Population B is the same as Population A in that the allele

frequencies would not change over time. Therefore, the overall null hypothesis is that each

population will be in Hardy-Weinberg Equilibrium. The way these hypotheses were tested was by

scoring flies for their various physical characteristics over a period of three generations for a five

week span.

Materials and Methods

       (The Materials and Methods were taken from the Biology Lab Manuel 220W Spring 2004

Edition Exercise 2 and can be found there.) The data for this experiment was collected each week

through observing the phenotypes of each fly subpopulation. For Population A, the eye

characteristics that were previously mentioned were examined under a microscope for each of the

three generations. In this population, the independent variable was the passage in time or each

generation and the dependant variable was the frequency of the p allele for the small population.

Also, the frequency of the large population served as a control for Population A. For Population B

the phenotypes were examined with the human eye to see if there were curly wings or straight
wings. After these phenotypes were recorded each week, for each population, they were analyzed

by using the genotypes of these know phenotype quantities. In this population, the passage of time

or number of generations served as the independent variable while the frequency of the Cy allele

served as the dependant variable.

        The genotype crosses had three different allele combinations for Population A. They were

ey/ey for eyeless, ey/sp for wild-type eyes, and sp/sp for sparkling eyes. In this population the ey

was dominant over the sp allele. Population B only had two different combinations for their

genotype. They were Cy/Cy+ for curly wings and Cy+/Cy+ for wild-type wings. In this case the

Cy allele was dominant over the Cy+ allele. These various allele combinations in each population

were then examined through various forms of comparison such as: formulas, graphs, and tables. A

conclusion was then made as to whether or not allele frequencies in each population changed over

time.

Results

        After scoring the flies for three generations and obtaining raw data for the phenotype

characteristics of Population A and B, several tables were set up using this data. In tables 1-3, the

each genotype for each eye trait is shown along with the frequency of the “ey” and “sp” for the

entire classes’ populations both large and small. In table 4-5, the data that my group obtained

through scoring the flies for Population A is shown over three generations. This data as compared

to class data in the first three tables shows some interesting trends. First, our small Population A for

some reason did not reproduce for the second and third generation. Therefore, we had no small

population A for generations two and three. It is because of this that the chi-squared test that was

administered for week 5 for small Population A yields statistically insignificant results. Also, that is

why Figure 1 of the graph of the frequency of the “ey” allele goes to zero after the first generation.

The large population for my group’s Population A yielded similar frequencies throughout the three
generations. In Figure 1, the graph shows that the frequencies were as high as .51 and as low .39. It

is because of the lack of reproduction in my group’s small population that I administered a chi-

squared test for the large population. For weeks 1 and 5 the chi-squared result for the large

population had values of 2.29 and .03 respectively for my group’s large Population A. These values

when corresponded to p-values in the goodness-of-fit test were both lower then the value of 3.814

with respect to a degrees of freedom value of one. Therefore, both generations one and three

accepted the null hypothesis that they were in Hardy Weinberg Equilibrium. (Biology Lab Manuel

Appendix C)

       In Population B, the overall data shown in Table 6 showed that the population grew from the

first to second generation. However, the population dropped in all groups from generation two to

three. Also, in Table 7 the general trend was that the frequency of the dominant Cy allele decreased

over time while the recessive Cy+ allele actually increased. This was evident in my data set

because the starting frequency of Cy and Cy+ were .31 and .69 in the first generation but was .19

and .81 in the third generation. In order to prove that this trend was not just a chance occurrence for

these three groups, a simulation was administered using a Populus 5.3 program. In this simulation

in Figure 3, the initial frequency was set at .31, which matched my starting frequency, and the

fitness level of the homozygous dominant genotype was set at 0 because this genotype is lethal in

Drosophila melanogaster (Biology Lab Manuel Exercise 2 pg.16). The results showed that as

generation after generation reproduced, the dominant “p” allele decreased in frequency over time.

Discussion

       The hypotheses for this experiment were that both Population A and Population B would

maintain Hardy-Weinberg Equilibrium over time. In small population A, the population had a

bottleneck effect because of some unknown reason. A bottleneck effect is some event that kills off

individuals and causes the population to become small (Biology Lab Manuel Exercise 2 pg 3). In
this case it not only killed off individuals but killed off all the females, therefore, allowing for no

reproduction in further generations. In large population A, the chi-squared test revealed that both

the first and third generations were statistically significant. This means that both generations were

in Hardy-Weinberg Equilibrium. Therefore, their allele frequencies did change over time but not

enough to make the population come out of Hardy-Weinberg Equilibrium. The reason that this

population may have stayed in equilibrium over generations is because it had a large population and

random mating was occurring. These two factors are two reasons why Hardy-Weinberg

Equilibrium does occur. (Biology Lab Manuel Exercise 2 pg 3).

       Population B showed that the population did not stay in Hardy-Weinberg Equilibrium

because the frequencies of the dominant Cy allele and recessive Cy+ allele fluctuated over time. In

Hardy-Weinberg Equilibrium the two alleles at a given locus should have the same frequency or

close to it (Minesky Lecture 3). The reason that this did not occur was because the homozygous

genotype of Cy/Cy is lethal as previously mentioned. Therefore, this will decrease the overall

number of Cy alleles in a population and decrease the frequency of those alleles. This trend was

evident in the Populus 5.3 simulation in Figure 3 that extended the number of generations beyond

just three. It showed that the lethal homozygous genotype does decrease the Cy allele over time.

This idea of a lethal genotype combination goes against the Hardy-Weinberg Equilibrium rule of no

natural selection ((Biology Lab Manuel Exercise 2 pg 3). Therefore, Hardy-Weinberg Equilibrium

will never be achieved in this type of population.

       Overall this experiment had two main sources of error. First, our small population did not

reproduce after the first generation which caused problems for administering the chi-squared test.

Second, because of a school cancellation and pressing time issues the incubator for the Drosophila

melanogaster for the last generation was turned up above its normal standards for incubation. This

resulted in the fruit flies that had the eyeless phenotype to not be as recognized as easily. This
could have flawed the scoring of the last generation for large Population A. A minor error was

human error in telling the differences between the different phenotypes for both Population A and

Population B. However, despite all these flaws the experiment did run fairly smoothly.

        If I were to do further experimentation and research with this experiment, I would first

observe the fruit flies for more generations other than just three. Then I would try to look at other

factors that affect evolution. One of these factors would be to examine how the male and female

Drosophila melanogaster interact with one another. According to Sawby and Hughes, certain male

genotypes actually affect female longevity and mating habits (Sawby, Hughes). Therefore, maybe

that factor may somehow affect the frequency of certain alleles and cause a population to not be in

Hardy-Weinberg Equilibrium. That factor as well as many others would be investigated to

determine just why evolution occurs at some instances but at other times it does not.

        In conclusion, the overall purpose of this experiment was to see how evolution works in

organisms. Through this five week expedition into the world of the fruit fly, it is safe to say that

evolution occurs in all organisms. From small organisms such as the Drosophila melanogaster to

complicated beings like ourselves change is always occurring both physically and genetically. If we

completely understand evolution to the fullest, then we can understand why certain diseases get

passed on in humans or how certain organisms survive climatic change. Evolution is what has kept

certain species on this Earth for hundreds of years. It is through understanding evolution and its

applications that we as a human race will be able to adapt to environmental changes in the years to

come.
        Table 1: Group Data for Week 1 Fly Lab Observations and Calculations for Population A



                                                                    No. of
                                                         No. of
                          No. of     No. of                       sparkling-   Frequency
                 No. of                         Total   eyeless                                Frequency of
Subpopulation             wild-    sparkling-                      polished    of ey alleles
                eyeless                          (N)      (ey)                                 sp alleles (q)
                           type     polished                         (sp)           (p)
                                                        alleles
                                                                    alleles
small #1             1         7           2      10         9           11            0.45              0.55
small #2             1         0           9      10         2           18            0.10              0.90
small #3             0         5           5      10         5           15            0.25              0.75
small #4             3         4           3      10        10           10            0.50              0.50
small #5             1         6           3      10         8           12            0.40              0.60
large #1             9        51          12      72        69           75            0.48              0.52
large #2            13        88          34     135       114          156            0.42              0.58
large #3            11        26          19      56        48           64            0.43              0.57
large #4             0        40          17      57        40           74            0.35              0.65
large #5             0        35          15      50        35           65            0.35              0.65

Total               39       262         119     420       340          500            0.40              0.60




        Table 2: Group Data for Week 3 Fly Lab Observations and Calculations for Population A



                          No. of     No. of              No. of                Frequency
                 No. of                         Total             No. of sp                    Frequency of
Subpopulation             wild-    sparkling-           eyeless                of ey alleles
                eyeless                          (N)               alleles                     sp alleles (q)
                           type     polished             allele                     (p)
small #1             1        57          42     100        59          141            0.30              0.71
small #2            55       103          34     192       213          171            0.55              0.45
small #3             6       116          69     191       128          254            0.34              0.66
small #4             0         0           0       0         0            0            0.00              0.00
small #5            19        60          29     108        98          118            0.45              0.55
large #1             1        68          31     100        70          130            0.35              0.65
large #2            50       139          62     251       239          263            0.48              0.52
large #3            16        65          55     136        97          175            0.36              0.64
large #4            59        51          55     165       169          161            0.51              0.49
large #5            15        58          33     106        88          124            0.42              0.58

Total              222       717         410    1349      1161        1537             0.43              0.57
        Table 3: Group Data for Week 5 Fly Lab Observations and Calculations for Population A




                                                                    No. of
                                                         No. of
                          No. of     No. of                       sparkling-   Frequency
                 No. of                         Total   eyeless                                Frequency of
Subpopulation             wild-    sparkling-                      polished    of ey alleles
                eyeless                          (N)      (ey)                                 sp alleles (q)
                           type     polished                         (sp)           (p)
                                                        alleles
                                                                    alleles
small #1             3        32           7      42        38           46            0.45              0.55
small #2             6        79          15     100        91          109            0.46              0.55
small #3             0         0           0       0         0            0            0.00              0.00
small #4            15        30          16      61        60           62            0.49              0.51
small #5             3        81          52     136        87          185            0.32              0.68
large #1             5        31          17      53        41           65            0.39              0.61
large #2            15        76          32     123       106          140            0.43              0.57
large #3            24        28          26      78        76           80            0.49              0.51
large #4            27        57          60     144       111          177            0.39              0.61
large #5             0        35          21      56        35           77            0.31              0.69

Total               98       449         246     793       645          941            0.41              0.59




*Note Bold Faced Items in Tables 1-3 are my subpopulation Data
Table 4: Raw Data for Small and Large Populations of Population A Weeks 1-5 for my
population

                                                    No. of
            Sub-           No. of      No. of
Week                                              sparkling-           Total (N)
          Population      eyeless     wild-type
                                                   polished
  1         small             3           4            3                   10
  3         small             0           0            0                    0
  5         small             0           0            0                    0
  1         large            11          26           19                   56
  3         large            59          51           55                  165
  5         large            27          57           60                  144


  Table 5: Genotype and Allele Frequencies for Population A Weeks 1-5 for my population

                               Genotype Frequencies                     Allele Frequencies
            Sub-          freq. of   freq. of     freq.                                      freq.
Week                                                                   freq. ey=p
          Population       ey/ey      ey/sp      sp/sp                                       sp=q
  1         small           0.30       0.40       0.30                    0.50               0.50
  3         small           0.00       0.00       0.00                    0.00               0.00
  5         small           0.00       0.00       0.00                    0.00               0.00
  1         large           0.20       0.46       0.34                    0.43               0.57
  3         large           0.36       0.31       0.33                    0.51               0.49
  5         large           0.19       0.40       0.42                    0.39               0.61
         Chi Squared Test for Small Subpopulation A Week 1

                  Observed (0)        Expected (E)         (0-E)^2/E          X^2 Value
    Sub        # of ey   # of sp   # of ey   # of sp   # of ey   # of sp
                                                                           Sum of (O-E)^2/E
 Population     alleles  alleles   alleles   alleles   alleles   alleles
small A           10       10        10         10        0         0             0
   p-value >   0.05




         Chi Squared Test for Small Subpopulation A Week 5

                  Observed (0)        Expected (E)         (0-E)^2/E          X^2 Value
    Sub        # of ey   # of sp   # of ey   # of sp   # of ey   # of sp
                                                                           Sum of (O-E)^2/E
 Population     alleles  alleles   alleles   alleles   alleles   alleles
small A            0        0         0         0         0         0             0
   p-value >   0.05




         Chi Squared Test for Large Subpopulation A Week 1

                  Observed (0)        Expected (E)         (0-E)^2/E          X^2 Value
    Sub        # of ey   # of sp   # of ey   # of sp   # of ey   # of sp
                                                                           Sum of (O-E)^2/E
 Population     alleles  alleles   alleles   alleles   alleles   alleles
small A           48       64        56         56      1.14       1.14          2.29
   p-value >   0.05




         Chi Squared Test for Large Subpopulation A Week 5

                  Observed (0)        Expected (E)         (0-E)^2/E          X^2 Value
    Sub        # of ey   # of sp   # of ey   # of sp   # of ey   # of sp
                                                                           Sum of (O-E)^2/E
 Population     alleles  alleles   alleles   alleles   alleles   alleles
small A          111      177      112.32    175.68     0.02       0.01          0.03
   p-value >   0.05
       Table 6: Group Raw Data for Population B Weeks 1-5

                      Number     Number of      Total
 Week       Group
                      of Curly   Wild Type       (N)
   1          1         127          80         207
   1          2         144          43         187
   1          3         105          51         156
   3          1         178         139         318
   3          2         187         137         323
   3          3          55          45         100
   5          1          17          28          45
   5          2          60          52         112
   5          3          80          43         123

*Note: Group 1 is my data colleted for Populatin B


Table 7: Genotype and allele Frequencies for Population B Weeks 1-5 for Entire Class

                             Genotype
                                                Allele Frequencies
                           Frequencies
            Group     freq. of     freq. of     freq
 Week                                                     freqCy+=q
           in Class   Cy/Cy+      Cy+/Cy+       Cy=p
   1           1        0.61         0.39       0.31         0.69
   1           2        0.77         0.23       0.39         0.61
   1           3        0.67         0.33       0.34         0.66
   3           1        0.56         0.44       0.28         0.72
   3           2        0.58         0.42       0.29         0.71
   3           3        0.55         0.45       0.28         0.73
   5           1        0.38         0.62       0.19         0.81
   5           2        0.54         0.46       0.27         0.73
   5           3        0.65         0.35       0.33         0.67
                                        Figure 1: Graph of “p” allele for Small and Large Population A for Weeks 1, 3, 5
                                                        (weeks 1, 3, 5 correspond to generations 1, 2, 3)



                                          Allele Frequency for (p) Allele for Small and Large Population for
                                                                     Weeks 1, 3, 5

                              0.70
                              0.60
Frequency of (p)




                              0.50
                              0.40                                                                                     Small
                                                                                                                       Population A
                              0.30
                                                                                                                       Large
                              0.20
                                                                                                                       Population A
                              0.10
                              0.00
                                            0            1          2          3          4            5        6
                                                                   Time passage in weeks




                                                Figure 2: Graph of Cy (p) Allele for Population B over Weeks 1, 3, 5
                                                          (weeks 1, 3, 5 correspond to generations 1, 2, 3)



                                                Frequeny of Cy Allele (p) over Weeks 1, 3, 5 for
                                                               Population B
              Frequency of (p) allele




                                        0.35
                                        0.30
                                        0.25
                                        0.20
                                        0.15
                                        0.10
                                        0.05
                                        0.00
                                                0            1            2           3            4            5             6
                                                                         Time passage in weeks
      Population B Simulation Using Populus 5.3

  Figure 3: Selection on a Diallelic Autosomal Locus
Beginning Frequency of .31 and Fitness of Homozygote 0
                                         Bibliography

1. Biology 220W Lab Manuel Spring 2004 Exercise 2 pgs 1-19 Written by: Leicht, B. G. 1995.
   Department of Biology, The Pennsylvania State University, University Park, PA.


2. Biology 220W Lab Manuel Spring 2004 Appendix C pgs 1-2, 5 Written by: Leicht, B. G. 1995.
   Department of Biology, The Pennsylvania State University, University Park, PA.


3. Darlington, Mark Teaching Assistant for Biology 220W Lab at Pennsylvania State University
   Week 2 of Biology 220W Lab “Population Genetics Fly Exercise” January 20, 2004.


4. Minesky, John Dr. Lecturer for Biology 220W class at Pennsylvania State University,
   University Park, PA. “Introduction to Ecology and Evolution” Lectures 1 and 2. January 12,
   2004 and January 14, 2004.

5. Minesky, John Dr. Lecturer for Biology 220W class at Pennsylvania State University,
   University Park, PA. “Variation, Evolution, and Natural Selection” Lectures 1 and 2. January
   12, 2004 and January 16, 2004.

 6. Sawby, R. and K.A. Hughes. 2001 Male genotype affects female longevity in Drosophila
    melanogaster. Evolution 55: 834-839. From Hughes Laboratory University of Illinois website,
    http://www.life.uiuc.edu/kahughes/index.htm

								
To top