In the 19th century, Alfred Russel Wallace noticed a shift in the composition of organisms between Asia and Australia.

• About Wallace line: : Wallace proposed an invisible barrier in the ocean, later termed the Wallace Line, running between Bali and Lombok, north between Borneo and Sulawesi, and curving south of Mindanao. The line separates the distinct biodiversity of Asia and Australia.

o The line acted as a biogeographic boundary, explaining the separate greatness of both continents' biodiversity.

• The Wallace Line is linked to evolutionary theory, showing a sharp species transition over a narrow region.
• Geological and Evolutionary Insights:

o The Malay Archipelago is geologically complex, with over 25,000 islands.

o Wallace proposed that some islands were once connected to Asia and later drifted apart, isolating species.

o Over millions of years, Australia moved northward, forming Indonesia’s volcanic islands.

o Sea-level changes, monsoons, and aridity influenced species’ adaptation and diversification.

• Species Dispersal Patterns: Research on 20,000 species found that Malay islands remained warm and wet, aiding Asian fauna migration to Australia.

o Australian species, adapted to cooler climates, struggled to migrate to Asia.

Northern route (rainforest-rich) helped Asian species migrate, whereas southern route (via Timor) was challenging for Australian species