Testing the Principles of Evolutionary Change in Beetle Populations

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Introduction

Evolution is responsible for the amazing diversity of life that exists on this planet. In the broadest sense, evolution is the change in inherited characteristics within populations from one generation to the next. These changes occur at all levels of the organism, from DNA to biochemical processes, to morphological traits. Charles Darwin was the first to come up with the theory of evolution by natural selection (Darwin 1859). This theory rests on the assumptions that more offspring are produced than will survive, that individuals have different traits leading to differences in survival and reproduction, and that these traits can be passed on to their offspring. This theory has withstood the test of time and is generally accepted by the scientific community.

We now know that the traits Charles Darwin was referring to are determined by the genetic makeup of each organism and that variations in these traits arise from variations in DNA. Each gene, also called an allele, encodes for a protein and contributes to an organism’s traits. In the absence of any evolutionary pressure, the alleles in a population will remain unchanged. This statement is the basis of the Hardy-Weinberg principle (Masel 2012). For this to be true mating must be random, there must be no latitudinal migration, mutation, or selection, and the population size must be very large.

The experiments in this paper are designed to test aspects of the Hardy-Weinberg principle and evolution in an artificial population of beetles. We will look at three colors of beetles, orange, yellow, and red. Each of these colors breeds true, meaning that each color of beetles only gives rise to the same color offspring (each population is dominant and homozygous for the allele determining color), and there is no interbreeding between each population of beetle. In the first experiment (1a), we will look at three small populations of beetle to test whether the allele frequencies (number of beetles of each color) remains the same or changes through successive generations. We will assume each population has the same number of offspring each generation, that there is no preference for beetle predators based on color, and we will follow each beetle population for 20 generations. We hypothesize that small population size may result in random changes in allele frequency due to genetic drift. In the second experiment (1b), we will increase the population size to test the hypothesis that genetic drift will be less pronounced in large populations. In the third experiment (2), we will keep the population size large and introduce a prey preference for beetle color. In this case, we expect that the population with the unlucky color of choice for beetle predators will decrease in number more rapidly than the other beetle populations.

Results

In experiment 1a, the initial beetle population is set to 10 yellow, 10 red, and 10 orange beetles. The predator diet ration was set at 3:3:3. The numbers of each color of beetle were counted after each generation for 20 generations and the data are presented in Figure 1. Four replicas were performed and the data is shown in Figure 1A, B, C, and D. In replica 1 (Figure 1A), the yellow and orange beetles go extinct at generation 13 and 17, respectively. The red beetles continue to increase in number until reaching a maximum of 30 beetles at generation 17. In contrast, replica 2 (Figure 1B) shows the red beetles going extinct at generation 15, whereas the yellow beetles continue to increase in number for all 20 generations and the orange beetles maintain their population until generation 13 at which time their population begins to decline. In replicas 3 and 4, the yellow beetles go extinct prior to generation 10 while the orange beetles go extinct at generation 15, and the red beetles continue to increase in number until reaching a maximum of 30 beetles at approximately generation 17.

Experiment 1b is set up exactly as described in experiment 1 only the starting number of beetles is 40. The results are shown in Figure 2. In this experiment, no beetle populations went extinct in four replications of the experiment and there was less variation in the allele frequency (beetle color) than that seen in experiment 1a.

The experiment 2 set up is exactly as described for experiment 1b except that the predator diet preference changes such that the predatory ratio is 3:2:1 (red, orange, yellow). Four replicas of the experiment were performed. The data is shown in Figure 3 as the average and standard deviation from the mean for each generation. Under this scenario, red beetles go extinct after 5 generations and orange beetles after 10 generations. Yellow beetles increase their population until reaching a maximum of 120 beetles after 10 generations.

Figures

(Figure 1 omitted for preview. Available via download)

Allele frequency changes over 20 generations in 3 small beetle populations. The initial number of beetles in each population is 10 and the beetle predators have no preference for beetle color. Data is shown for the number of beetles in the red, orange, and yellow populations over 20 generations. Four replicas were performed and the data is shown in A, B, C, and D. The red, orange and yellow lines correspond to the red, orange and yellow beetle populations, respectively.

(Figure 2 omitted for preview. Available via download)

Allele frequency changes over 20 generations in 3 large beetle populations. The initial number of beetles in each population is 40 and the beetle predators have no preference for beetle color. Data is shown for the number of beetles in the red, orange, and yellow populations over 20 generations. Four replicas were performed and the data is shown in A, B, C, and D. The red, orange and yellow lines correspond to the red, orange and yellow beetle populations, respectively.

(Figure 3 omitted for preview. Available via download)

Allele frequency changes over 20 generations in 3 large beetle populations with predatory preferences. The initial number of beetles in each population is 40 and the predation diet ration is 3:2:1 for red, orange, and yellow beetles, respectively. Data is shown for the number of beetles in the red, orange, and yellow populations over 20 generations subject to the predator diet preferences. Four replicas were performed and the data is shown as the mean +/- Standard error of the mean for each generation. The red, orange and yellow lines correspond to the red, orange and yellow beetle populations, respectively.

Discussion

Evolution is the major shaping force for the diversity of life on this planet. Underlying principles involved are that organisms within a population have variable traits, that these traits are passed from one generation to the other, and that some of these traits confer an advantage on the survival and reproductive fitness of an organism. Today we understand that these traits are due to the genetic makeup of each organism. Alleles that confer beneficial traits and improved reproductive success are passed on to the next generation. The Hardy-Weinberg principle states that alleles will remain constant in a population in the absence of any evolutionary forces if the population size is large enough. The experiments in this paper were designed to test aspects of this principle in an artificial population of beetles. By analyzing three populations of beetle differing only by color, over 20 generations and three different experimental conditions, we were able to obtain evidence that is consistent with the Hardy-Weinberg principle and modern theories of evolution.

In the first experiment, we examined what would happen to the allele frequency (color) in a small population of beetles over time. We found that allele frequency changed dramatically over the course of four replicates of the experiment (Figure 1). This finding is consistent with the concept of genetic drift and the idea that random sampling effects can alter allele frequency in small populations when reproductive rates and survival rates are held constant. In the second experiment, we changed only the population size from 10 beetles per population to 40 beetles per population. The results from four replicas of this experiment demonstrated that overall the allele frequencies remained far more constant than in experiment 1 (Figure 2). This result supports the Hardy-Weinberg principle and suggests that if we tested a much larger population that the allele frequencies would remain even more constant. In the third experiment (2), we kept the population size large, 40 beetles for each color, and introduce a prey preference for beetle color. In this case, we found that predatory preference and survival rates had a very large effect on allele frequency (Figure 3) with the color allele most preferred by the predator going extinct within 5 generations, the second preferred color allele going extinct at 10 generations and the least preferred color allele being the only allele surviving after 10 generations. We also found that this effect was remarkably consistent over four replications of the experiment. In summary, the results presented here support the basic premises of evolution and the Hardy-Weinberg principle.

References

Darwin, C. (1859). On the origins of species. London, England: John Murray.

Masel, J. (2012). Rethinking Hardy-Weinberg and genetic drift in undergraduate biology. Bioessays 34: 701-710.