Final answer:
The three main modes of selection are stabilizing, which decreases variation by favoring average phenotypes; directional, which shifts the population mean towards one extreme; and diversifying, which increases variation by favoring extreme phenotypes. The mode affecting gall size in flies depends on the environmental context.
Step-by-step explanation:
The three major modes of selection that shape population are stabilizing selection, directional selection, and diversifying selection. Stabilizing selection favors the average phenotypes and selects against the extremes, resulting in a decrease in population variation. The population's genetic variance decreases as the individuals with the mean trait values have higher survival and reproductive success compared to those with extreme traits.
Directional selection, on the other hand, selects for phenotypes at one end of the spectrum and can be triggered by environmental changes, leading to a shift in the population mean towards the fitter phenotype. One historical example of directional selection is the evolution of the peppered moth during the Industrial Revolution in England, where darker moths became more prevalent due to their increased survival in sooty environments.
Diversifying selection favors extreme phenotypes over intermediate ones. This can happen in environments with diverse ecological niches, resulting in an increase in genetic variance within the population. Frequency-dependent selection and sexual selection are other ways in which natural selection can also shape the genetic makeup of a population.
The mode of selection operating on gall size of gall-making flies would be deemed directional if the environment is selecting for a particular size, stabilizing if the average size is favored, or diversifying if both small and large galls were beneficial compared to average-sized galls. Without specific information regarding the environmental context, it cannot be determined which selection mode is at play.