Final answer:
The most significant factor that accounts for the differences in species between the Sahara Desert and Southeast Asia is Climate and Habitat.
Step-by-step explanation:
The most significant factor that accounts for the differences in species between the Sahara Desert and Southeast Asia is A) Climate and Habitat. The Sahara Desert's arid climate and extreme temperatures create specific adaptations in species, while Southeast Asia's tropical climate supports a diverse range of habitats.