Paul baltes believes that natural selection has not weeded out many harmful conditions and nonadaptive characteristics that appear among older adults because those conditions are not tied to

Reproductive fitness.

Explanation:

The natural selection may be defined as one of the most important factors that plays the main role in evolution. Darwin explains the theory of the natural selection.

The organisms must adapt to their environment and reproduce in the large numbers. The reproductive fitness  helps the individual to effectively transfer their genes to the next generation. The successfully adapted and evolved population must show high reproductive fitness. The natural selection increase the chance of individual to survive in harmful environmental conditions and do not remove the conditions.

Thus, the answer is reproductive fitness.

b exhibiting favoritism toward the kikuyu people.

sheep is a herbivore animal whereas grasshopper belongs to the class chordates but both of them feed on greens.  

if grasshopper and sheep feeding on a pasture land, i do not think both of them will have any impact on each other. however, the number of the species can make difference. since sheep feed on greens, there is a possibility there left no pasture land for the grasshoppers or they may get extinct.