Final answer:
The factor not used to define biomes is historical similarities; biomes are characterized by abiotic factors such as climate (precipitation and temperature), not by the historical context of the region.
Step-by-step explanation:
The factor not used to define biomes is historical similarities (C). Biomes are determined by abiotic factors, predominantly by climate, which includes precipitation levels and temperature. Factors like plant structures and leaf type also contribute to the characterization of biomes because they are influenced by the climatic conditions. An ecologist studying biomes would consider variations in elevation and rainfall when examining changes in biomes up a mountain. However, historical similarities, which can refer to the evolutionary or geographic history shared by species within a region, do not directly define a biome.