Final answer:
The distinctive species on Madagascar are primarily the result of geographic isolation, allowing unique evolutionary paths due to separation from other land masses and limited human intervention.
Step-by-step explanation:
The primary reason for the distinctive plant and animal species found in Madagascar is A) Geographic isolation. Madagascar's separation from the mainland of Africa over millions of years has created conditions for unique species to evolve. The island's isolation has provided a habitat where endemic species could develop without the influence of species from other regions, leading to a high level of biodiversity and the presence of many organisms found nowhere else on Earth.
Madagascar's terrain varies from rain forests to dry regions, and this diversity of ecosystems supports an array of species that have adapted to these specific environments. Moreover, the lack of human presence until relatively recently has allowed these species to thrive in isolation. The concept of geographic isolation aligns with a scientific principle called allopatric speciation, where new species evolve in isolation due to separation from the main population, often driven by geological changes such as continental drift.