The forces of evolution act to create diversity of life on this planet. Evolution is defined as a change in characteristics that are inherited from generation to generation within a population of an organism. Natural selection provides the catalyst that drives adaptive changes in characteristics (Darwin, 1859). Any change in characteristic that increases either survival or reproductive fitness will accumulate in the population because other changes that reduce survival or reproductive fitness fail to be passed on to the next generation. Biologists generally agree that this theory of evolution is correct and that it explains much of the variation in life we see on earth. Nevertheless, scientists continue to test evolution and natural selection by formulating hypotheses and testing these with scientific experiments and observations.
While natural selection is the only known cause of adaptation, it is not the only force driving evolution. The characteristics referred to above are now known to be due to the alleles (or genes) that make up an organism's genetic material. Genetic drift (Masel, 2011) can occur by a process of “random sampling”. When selection is not acting on an allele, it may be passed on to the next generation at a higher or lower frequency just by random chance. In this case, the allele may drift in frequency to fixation, which means that some alleles disappear while others become the only allele for that gene in the population. The process of fixation occurs more frequently in a small population because the fewer random chance events have to happen to take an allele frequency to fixation. Taken to the extreme of considering an infinitely large population, the Hardy-Weinberg principle says that in the absence of any selective pressure, alleles frequency will remain constant. For this to be true not only does there have to be no selective pressure but there also must be no mutation and no individuals migrating into or out of the population.
The prerequisites for the Hardy-Weinberg principle in reality never exist in nature. For example, mutations are always occurring, populations are not infinitely large, and there is natural selection of one kind or another always at work. Consequently, one powerful tool to test evolutionary theory is computer simulations of changing biological populations where some of these factors can be held constant. In the experiments described in this paper, we used a computer simulation of three populations of beetles carrying the color alleles red, yellow and orange to test the effects of genetic drift and natural selection. In the first two experiments, the prey preference is set to be equal for each color of beetle and they reproduce at the same rate. In this way, natural selection is not influencing allele frequency. The only difference between these two experiments is the size of the beetle population. In the first case, the beetle populations start out with 10 beetles of each color. In the second experiment, the beetle population has 40 beetles of each color. The hypothesis being tested is that genetic drift will have a bigger effect on the smaller populations of beetles. I expect that the alleles will become fixed more quickly in the small population, meaning that some color alleles will quickly go extinct leaving only one color allele remaining and that this will happen more slowly or not at all in the larger population. In the last experiment, we will use the larger beetle population, such that genetic drift has a small effect, and introduce a prey dietary preference for the different colored beetles to examine how natural selection affects allele frequency. In this experiment, we hypothesize that the color of beetles preferred by the predators will go extinct and the least preferred color will become fixed in the population.
The initial beetle population is set to 10 yellow, 10 red, and 10 orange beetles, the prey diet ration is set to 3:3:3 (yellow:red:orange) for the first experiment (1a). After each generation, for 20 generations, the number of yellow, red, and orange beetles is counted (Figure 1A). This experiment is repeated 3 times (Figure 1B, C, D). In the first trial (Figure 1A), the yellow allele goes to fixation at generation 10 with the red and orange alleles going extinct at generation 8 and 10, respectively. In trial 2 (Figure 1B), no color alleles go to fixation, although the yellow allele nears fixation. The red allele goes extinct at generation 7 and the orange allele nears extinction but is still present in the population. Trial 3 shows the red allele going to fixation at generation 17, whereas the orange and yellow alleles go extinct at generation 17 and 7, respectively. Finally, in the last trial, the red allele goes to fixation at generation 17, and the yellow and orange alleles go extinct at generation 8 and 17, respectively.
The second experiment (1b), is set up very similarly to the first experiment. The only difference is that each color beetle population is set at 40 instead of 10. In all four trials, no alleles go to fixation and no alleles go extinct (Figure 2A-D). There is much less variability in beetle color than that seen in the first experiment.
The final experiment uses the initial population of 40 beetles but then the prey diet ration is set at 3:2:1 (red:orange:yellow). The results of four trials are shown in Figure 3 as the average count and standard error of the mean for each color beetle over 20 generations. In this experiment, the red allele went extinct at generation 5, the orange allele went extinct at generation 9, and the yellow allele reached fixation at generation 9.
Natural selection and genetic drift are two forces that drive evolution. These forces act on alleles that give individuals in a population their adaptive, maladaptive, and neutral characteristics. In nature, the process of random mutation creates different alleles, alleles then migrate with individuals between populations and are passed on from one generation to the next at some frequency that is primarily determined by the processes of natural selection and genetic drift. The Hardy-Weinberg principle says that in the absence of evolutionary agents, such as natural selection, allele frequency will remain the same if the population size is large enough and there is no mutation and no migration of alleles in or out of the population. Since this condition never exists in nature, we tested some of the agents of evolutionary change in computer simulations where other factors could be excluded. Our results are consistent with these theories.
We tested three different experimental conditions in a computer-simulated beetle population. In the first experiment, the beetle population started off small. The theory of genetic drift would suggest that allele frequencies in a small population would change quickly with one allele going to fixation through the process of random sampling. This result is indeed what we found. In each three of the four trials (Figure 1), one allele went to fixation before generation 20. However, when we started with a population that was four times larger (Figure 2), no alleles went to fixation in any of the four trials. Allele frequencies remained more stable with less variation. The results of the first and second experiments are exactly what would be predicted by genetic drift and the Hardy-Weinberg principle.
In the last experiment, a bias in survival was introduced by setting the prey diet ratio at 3:2:1 (red:orange:yellow) in a starting population of 40 beetles carrying red, orange, or yellow alleles. This had a dramatic and consistent effect across all four trials (Figure 3). The red beetles (red allele) quickly went extinct in all 4 trials by generation 7 followed shortly by the orange beetles (orange allele) at generation 9. The yellow beetles (yellow allele) reached fixation at generation 9. The consistency of this effect of predator diet preferences demonstrates that natural selection “pushes” advantages alleles to fixation and drives maladaptive alleles to extinction in a population. On the other hand, the inconsistent change seen for allele frequencies in the absence of natural selection demonstrates the random nature of genetic drift giving rise to allele fixation. The results obtained in these experiments are consistent with the predictions of genetic drift and natural selection as agents of evolutionary change.
Darwin, C. (1859). On the origins of species. London: John Murray.
Masel, J. (2012). Genetic drift. Current Biology, 21, R837-R838.
(Appendix omitted for preview. Available via download)