The scenario that would reduce the number of inheritable traits in a population, causing individuals in that population to remain very similar to one another, is:
A swamp that is being drained in order to build houses.
This scenario likely leads to habitat destruction, which can result in a reduction of biodiversity. As habitats are destroyed, fewer individuals and populations may survive, leading to a decrease in genetic variability. This can cause the remaining population to become more similar genetically, as fewer organisms reproduce and contribute to the gene pool.