Final answer:
Drought may lead to natural selection favoring smaller midges less prone to desiccation, potentially resulting in a population trend towards smaller body size over time. Genetic drift can also play a role, especially in small populations, where the loss of large-bodied individuals can have a more pronounced effect on the gene pool.
Step-by-step explanation:
During a drought, the larger midges in a population might face increased risks of desiccation due to arid conditions, despite their generally higher reproductive success. Given that body size is largely heritable and controlled by multiple genes, natural selection may favor smaller midges that are less susceptible to drying out. Over time, if the drought persists, the population could evolve towards smaller body size, as these individuals would have better survival rates and pass on their genes for smaller size more effectively than their larger counterparts. This scenario represents an example of natural selection where environmental pressures influence the genetic distribution in a population.
Moreover, in small populations, the force of genetic drift can influence the gene pool significantly. For instance, if a large-bodied individual dies before reproducing in a small population, the alleles for larger body size could be rapidly reduced, whereas in a larger population, the impact would be less severe due to a more diverse gene pool. Therefore, drought conditions can potentiate the selective and random effects that, together, shape the evolutionary trajectory of the midge population.