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# Unraveling the Mysteries of Biogeography: Wallace's Line Explained

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Chapter 1: Understanding Biogeography

Biogeography, the exploration of how living organisms are distributed across different regions, is a fascinating and intricate field. It encompasses both current distributions and the historical factors that have influenced them over time, intertwining the present with the distant past.

This poem reflects on the concept of boundaries as drawn by human beings:

"All maps are lines created by humans, defining borders between nations, coastlines that remain unyielding despite natural disasters, and territories laden with conflict. These lines wrap around the globe, existing only in ink, not in solid form. On this ancient map, a line traverses the strait separating Bali from Lombok and continues north between Borneo and Sulawesi. Scholars debate its trajectory—some extend it through Mindoro Strait while others steer it south of Mindanao. To the west, tigers roam freely; to the east, tree-kangaroos thrive. This line, a human construct, follows a reality far older than nations. It delineates the ancient land of Sunda, where islands once lay connected by dry land to the west and deep, unbridged waters to the east. This line confines tigers and organizes tree-kangaroos, at least for now, until geological forces reshape our world with boundaries of stone instead of ink."

A map illustrating Wallace's Line and its significance

Chapter 2: The Legacy of Wallace

Wallace’s Line, named after Alfred Russel Wallace, is a pivotal concept in biogeography. Wallace's extensive two-volume work, The Geographical Distribution of Animals (1876), marks the first comprehensive systematic investigation of this field. This line helps biogeographers comprehend how geological structures influence the distribution of fauna across Southeast Asia and Australia.

The original map from Wallace’s work showcasing various biogeographic lines

The line aligns with the boundary of the Sunda landmass, which once emerged during periods of lower sea levels, connecting the islands of Java, Sumatra, and Borneo. To the west lies a deep strait that restricts many species from migrating to smaller islands and ultimately to New Guinea, the Aru Islands, and Australia. The size of these islands acts as a natural filter, leading to reduced species diversity. Of those that make it, only a few thrive.

In 1895, Richard Lydekker introduced a corresponding line along the Sahul Shelf, marking the boundary of the Australian continent. Together, these two lines encapsulate an area rich in geological diversity, comprising both ancient and modern islands. Known as Wallacea, this region is home to a unique blend of Asian and Australian wildlife, including numerous endemic species.

These delineations of Wallace and Lydekker, while insightful, are not flawless. They reflect the current state of biodiversity, while acknowledging historical context. For example, songbirds originated in Australia, spread eastward, diversified, and returned in new forms. Some rodent species arrived in Australia millions of years ago, with some found in New Guinea, while others are uniquely Australian.

Section 2.1: Wallace's Insights

In 1855, long before his major works, Wallace penned a paper titled "On the Law Which Has Regulated the Introduction of New Species." While in Sarawak on Borneo, he articulated the Sarawak Law, outlining how new species arise in relation to geographic and geological factors. He concluded that each species emerges in tandem with a closely related species in both space and time.

Regarding Sumatra, Java, and Borneo, which he believed were once interconnected, he noted:

"The organic results we observe in the significant number of species common to some or all of these regions, alongside numerous closely related species unique to each, indicate that a considerable period has passed since their separation. The facts of geographical distribution and geology may mutually elucidate one another when the principles I have proposed are clearly established."

Three years later, Wallace and Charles Darwin co-authored a pivotal paper, "On the Tendency of Species to Form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection," published in the Journal of the Proceedings of the Linnean Society in London. In 1859, Darwin's groundbreaking On the Origin of Species by Natural Selection was released, coinciding with Wallace introducing the eponymous line that bears his name, a contribution communicated to the Linnean Society by Darwin.

The significance of their collaborative work reverberates through history.

Chapter 3: The Invisible Line

The video titled "The Invisible LINE That No Animal, Bird or Fish Can Cross" delves deeper into the concept of Wallace's Line, exploring its implications for animal distribution and the geological forces at play. Through visual illustrations and detailed explanations, it sheds light on the invisible barriers that shape the natural world.

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