Which of the following prevents the increase in fitness of a population?

Inbreeding can lead to a decrease in a population's fitness due to the increased likelihood of offspring inheriting deleterious recessive traits. When closely related individuals mate, the genetic diversity of the population is lowered, which can result in a higher occurrence of genetic disorders and reduced adaptability to environmental changes. This reduction in genetic variation can lead to inbreeding depression, where the overall health and survival of the population are compromised. Such genetic bottlenecks limit the potential for beneficial traits to emerge in future generations, ultimately hindering the population's ability to thrive and adapt, and thus preventing an increase in fitness.

In contrast, other factors listed, such as direct exploitation or changes in ecological balance due to environmental pressures, may not inherently reduce fitness on a genetic level but instead impact population numbers or dynamics. Migration can increase genetic diversity by introducing new alleles into the population, potentially enhancing fitness. Therefore, inbreeding stands out as a significant factor that directly undermines the genetic health and adaptability of a population.