Answers to Analyzing Data 12.1 Questions
1. A total of 18 plant populations were established in this experiment. In each of these populations, the initial frequency of each plant genotype was 1/27 = 0.037 or 3.7%.
2.
If evolution had not occurred in the control populations, we would have expected all 27 plant genotypes to survive and their frequencies to change little from their initial values of 3.7% for each genotype. This was not the case: many genotypes did not survive (and hence had a final frequency of 0%), while others increased dramatically in frequency. Genotype 6, for example, reached a frequency of 42.3% when grown in the control environment. Genotype 6 may have been particularly well suited to the growing conditions experienced in the control populations, where plant genotypes were grown at high densities and in soil that may have differed from the soil of their home environments. Such changes in environmental conditions could have caused natural selection to occur in the control populations.3. Plant genotype frequencies also changed in populations exposed to aphid herbivores, with many genotypes being driven to extinction while others increased dramatically in frequency. Hence, evolution occurred in these populations as well. Plant populations exposed to aphid herbivores could have experienced multiple sources of selection, such as novel environmental conditions (e.g., high plant densities and different soil from those found in their home environments) as well as the consequences of feeding by aphid herbivores.
4. In the B. brassicae treatment, 75% of the surviving plants encoded 4C defensive compounds; one of these, genotype 25, was the most common surviving genotype (67.4% of the surviving plants had this genotype). In contrast, in the L. erysimi treatment, 83% of the surviving plants had 3C genotypes, the most common of which was genotype 9, at 63.2%. Although a few genotypes performed reasonably well in both treatments (e.g., genotypes 9 and 25), overall, the outcome of selection differed considerably between treatments. These results suggest that natural selection by different herbivore species can favor different plant genotypes.