Conditionss On The Allele Frequency Of Drosophila Melanogaster Populations Essay, Research Paper

Introduction

The interaction between familial fluctuation and natural choice is one of the most of import constructs in modern biological science. The merchandise of this interaction, development, which is a alteration in a population? s allele frequence, is responsible for the great complexness and diverseness of life seen on Earth today.Allele frequences of a non-evolving population ( one in which the allelomorph frequences are non altering ) can be elegantly modeled utilizing the Hardy-Weinberg theorem. For a population to be at equilibrium, five conditions must be met: 1 ) big population size, 2 ) random coupling, 3 ) no mutant, 4 ) no migration, and 5 ) no natural choice. Misdemeanor of these conditions can bring forth a alteration in the allele frequences of the population under survey. Our intent is to analyze the effects of little population size and of natural choice. In this experiment, we will utilize populations of Drosophila melanogaster to pattern the Hardy-Weinberg theorem. Drosophila populations are ideal for patterning evolutionary kineticss for two grounds: they are cheap, to both bargain and maintain, and, more of import, have a brief generative rhythm, leting several coevalss to be studied in a short length of clip. We will be working with two populations of Drosophila, designated A and B. Population A will be used to analyze the effects of a little population ( the laminitis consequence ) . Natural choice will be studied utilizing population B. We hypothesize that the misdemeanor of a individual Hardy-Weinberg equilibrium status nowadays in each population will give rise to a specific alteration in allele frequence in that population. We hypothesize that the allele frequences of the two A sub-populations will diverge over clip, due to familial impetus magnified by trying mistake in the A-Small population. We hypothesize that the frequence of the Cy allelomorph will diminish in population B, due to natural choice. In order to prove our hypotheses, we will be hiting Drosophila for several coevalss ( F2, F3, and F4 ) . We will so be able to cipher allele frequences within the different populations. These frequences will be compared to find if the populations are in Hardy-Weinberg equilibrium and if their allelomorph frequences are altering.

Materials and Methods

For a complete listing of Materials and Methods ( including the processs for anaesthetizing and placing Drosophila ) , see the Biology 220W Lab Manuel.

Population A is comprised of wild-type Drosophila and those marked with one of two mutant traits: eyeless ( ey ) or sparkling-polished ( sp ) . Each mutant is declarative of a Drosophila that is homozygous for that allelomorph. Heterozygous flies show the wild-type phenotype. The Drosophila in population A are first indiscriminately divided in two groups: a little group, incorporating 10 flies, and a big group, incorporating all other flies from population A. Note that the population A-Small must incorporate at least 2 flies from each sex. The figure of Drosophila of each phenotype is determined for both the A-Small and the A-Large. The 2nd and 3rd coevalss are besides scored in this manner.Population B is comprised of the F2 coevals of a cross between wild type

and curly winged (Cy) Drosophila. All flies with the curly winged phenotype are assumed to be heterozygous, as the Cy mutation is homozygous lethal. The number of flies of each phenotype is determined; this is done for each of three generations.In both parts of the experiment, phenotype frequencies are used to determine allele frequencies. Allele frequencies are used to determine if the populations are in Hardy-Weinberg equilibrium. They are also compared across generations to determine if they are changing. In both parts of the experiment, the independent variable is the small population size (i.e. the violation of Hardy-Weinberg equilibrium) and the dependent variable is the change in allele frequencies.ResultsTable 1 presents all data for sub-populations A-Small and A-Large. It shows a total number of Wild-type flies in week 5 as 666. Table 2 presents the results of x2 tests performed to test for the existence of Hardy-Weinberg equilibrium within the Drosophila populations. It shows most populations to be far out of Hardy-Weinberg equilibrium. However, all Small sub-populations are in Hardy-Weinberg equilibrium in week 1, at a 0.005 level of confidence. Table 3 shows the tests for shifts in phenotype (which is indicative of shifts in allele frequencies) across generations. x2 tests show changes in phenotype in almost every population. Graphs 1 and 2 show Populous simulations for sub-populations A-Small and A-Large. The are indicative of the possible result of running this experiment out to 100 generations. Notice that the allele frequencies of sub-population A-Small fluctuate greatly before fixation, and that the frequencies of A-Large shift only gradually.Table 4 shows the raw data for population B. It shows a steadily decrease in the frequency of Cy and a steady increase in the frequency of Cy+. Table 4 shows the results of the x2 test on this change in frequency. At the 0.05 level of confidence, it shows a change in frequency in all but one population. Graph 3 show a Populous simulation representative of population B, if the experiment had been taken out to 100 generations. Note the quick drop in the frequency of the Cy allele, but that it was always maintained at a low frequency.Discussion In general, the data from population A supports our hypotheses. Both A sub-populations showed a shift in allele frequencies. They were both gradual though, probably due to the fact that A-Small was constrained to 10 flies for only the first experimental cross. A procedure more indicative of the effects of small population size might have culled the population back to 10 flies at each generation. Of note is the fact that the original samplings in the Small sub-population were at Hardy-Weinberg equilibrium. This is probably due to sampling error. The data from population B also supports our hypothesis. There is an obvious drop in the frequency of the Cy allele. Natural selection constrains the expression of the Cy allele (i.e. homozygous genotype results in death) so Cy alleles are weeded out of the population.Unfortunately, our results are not particularly meaningful. With only 3 generations to look at, it is difficult to infer patterns in shifts in allele frequencies. An obvious extension of this experiment would be one in which the fly populations were followed for many generations.