The scenario that would likely reduce the number of inheritable traits in a population, causing the individuals in that population to remain very similar to one another, is:
A swamp that is being drained in order to build houses.
This activity usually leads to habitat destruction, which can dramatically reduce the population sizes of species living in that ecosystem. When populations decline significantly, genetic diversity is often lost, leading to an increase in similarity among the remaining individuals due to inbreeding and a limited gene pool.