The scenario that would most likely reduce the number of inheritable traits in a population, causing the individuals to remain very similar to one another, is:
• A desert that has existed for millennia.
A long-existing desert environment would likely result in a stable habitat with limited resources and environmental pressures, leading to the possibility of a smaller gene pool due to isolation and less variability among individuals adapted to those specific conditions. This can contribute to reduced genetic diversity. In contrast, the other scenarios (swamp draining, forest conversion to farmland, and a drying lake) involve environmental changes that could lead to habitat loss, population fragmentation, and possibly increased genetic diversity as populations adapt to new conditions.