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{{Short description|Line separating Asian and Australian fauna}}
{{For|the line in triangle geometry|Simson line}}

[[File:Map_of_Sunda_and_Sahul.svg|thumb|Wallace's Line delineates Australian and Southeast Asian fauna. The probable extent of land at the time of the [[Last Glacial Maximum]], when the sea level was more than {{cvt|110|m}} lower than today, is shown in grey. The deep water of the [[Lombok Strait]] between Bali and Lombok formed a water barrier even when lower sea levels linked the now-separated islands and landmasses on either side.]]

The '''Wallace Line''' or '''Wallace's Line''' is a faunal boundary line drawn in 1859 by the British naturalist [[Alfred Russel Wallace]] and named by the English biologist [[Thomas Henry Huxley]].

It separates the [[biogeographic realm]]s of [[Asia]] and '[[Wallacea]]', a transitional zone between Asia and [[Australia (continent)|Australia]] formerly also called the [[Malay Archipelago]] and the Indo-Australian Archipelago (Present day [[Indonesia]]). To the west of the line are found organisms related to Asiatic species; to the east, a mixture of species of Asian and Australian origins is present. Wallace noticed this clear division in both land mammals and birds during his travels through the [[East Indies]] in the 19th&nbsp;century.

The line runs through [[Indonesia]], such as [[Makassar Strait]] between [[Borneo]] and [[Sulawesi]] (Celebes), and through the [[Lombok Strait]] between [[Bali]] and [[Lombok]], where the distance is strikingly small, only about 35&nbsp;kilometers (22&nbsp;mi), but enough for a contrast in species present on each island. 

The complex biogeography of the Indo-Australian Archipelago is a result of its location at the merging point of four major tectonic plates and other semi-isolated microplates in combination with ancient sea levels. Those caused the isolation of different [[taxonomic groups]] on islands at present relatively close to each other. Wallace's Line is one of the many boundaries drawn by naturalists and biologists since the mid-1800s intended to delineate constraints on the distribution of the fauna and flora of the archipelago.<ref name="Ali-Heaney-2021">{{cite journal |last1=Ali |first1=Jason R. |last2=Heaney |first2=Lawrence R. |date=June 2021 |title=Wallace's line, Wallacea , and associated divides and areas: History of a tortuous tangle of ideas and labels |journal=Biological Reviews |volume=96 |issue=3 |pages=922–942 |doi=10.1111/brv.12683 |pmid=33502095 |s2cid=231764849 |issn=1464-7931 |url=https://onlinelibrary.wiley.com/doi/10.1111/brv.12683}}</ref>

== Historical context ==
[[File:Wallace-line1.jpg|thumb|upright=1.9|The original drawing of the line in Wallace's paper]]

One of the earliest descriptions of the biodiversity in the Indo-Australian Archipelago dates back to 1521 when Venetian explorer [[Antonio Pigafetta|Pigafetta]] recorded the biological contrasts between the Philippines and the [[Maluku Islands]] (Spice Islands) (on opposite sides of Wallace's Line) during the continuation of the voyage of [[Ferdinand Magellan]], after Magellan had been killed on [[Mactan]]. Later on, the English navigator [[George Windsor Earl|G. W. Earl]] published his observations in faunal differences between the islands in the Indo-Australian archipelago. 

In 1845, G. W. Earl described how shallow seas connected islands on the west ([[Sumatra]], [[Java]], etc.) with the Asian continent and with similar wildlife, and islands on the east such as [[New Guinea]] were connected to Australia and were characterized by the presence of [[marsupial]]s.<ref>{{cite book <!--it was a pamphlet-->|last=Earl |first=G.W. |author-link=George Windsor Earl |date=1845 |title=On the Physical Geography of South-Eastern Asia and Australia }}</ref>

These early investigations assisted Wallace in developing his theories about the biogeography which he stated publicly in his 1859 paper after extensively traveling the region.<ref name="Ali-Heaney-2021" /> He proposed a line to the east of [[Bali]] since "all the islands eastward of [[Borneo]] and [[Java]] formed part of an Australian or Pacific continent, from which they were separated."<ref name="Wallace-1863">{{harvnb|Wallace|1863|pp=217–234}}</ref>

The proposal of the line, however, was not the main objective of Wallace's endeavours: His primary purpose was in fact to understand the geological phenomena and the colonization events that caused the boundaries in faunal distribution in the region through the development of his theories of evolution and biogeography.<ref name="Ali-Heaney-2021" /> Wallace's studies in [[Indonesia]] demonstrated the emerging [[theory of evolution]], at about the same time as [[J. D. Hooker]] and [[Asa Gray]] published essays also supporting [[Charles Darwin|Darwin]]'s hypothesis.<ref>{{cite book |last=Bowler |first=P.J. |author-link=Peter J. Bowler |date=1989 |title=Evolution: The history of an idea |edition=Rev. |location=Berkeley, California |publisher=University of California Press |oclc=17841313 |isbn= 0-520-06385-6 |url= https://www.worldcat.org/oclc/17841313}}</ref> On the other hand, the lack of knowledge of [[tectonic plate]]s, and the uncertainty about biodiversity in the [[Philippines]], left Wallace with some contradicting points he had to deal with concerning his theory on biogeography.<ref name="Ali-Heaney-2021" /> 

In fact, Wallace did not include the Philippines in his 1859 paper, and the determination of a line in honor of his name was suggested by [[T. H. Huxley]] (1868).<ref>{{cite journal |last=Huxley |first=Thomas Henry |author-link=Thomas Henry Huxley |date=January 1868 |title=December 10, 1868 |journal=[[Proceedings of the Zoological Society of London]] |volume=36 |issue=1 |pages=622–637 |doi=10.1111/j.1469-7998.1868.tb00455.x |issn=0370-2774}}</ref><ref>{{cite journal |last=Camerini |first=J.R. |date= December 1993 |title=Evolution, biogeography, and maps: An early history of Wallace's line |journal=[[Isis (journal)|Isis]] |volume=84 |issue=4 |pages=700–727 |doi=10.1086/356637 |pmid=8307726 |s2cid=15219070 |issn=0021-1753 |url= https://www.journals.uchicago.edu/doi/10.1086/356637}}</ref> Huxley studied the distribution of [[gallinaceous birds]] in the archipelago, and noticed that species in the Philippines were remarkably distinct from those in the Asiatic realm. Based on that, he redrew Wallace's boundary placing it to the west of the Philippines and named it "Wallace's Line", although Wallace himself had refused to include the Philippines on the east side of the line.<ref name="Ali-Heaney-2021" />

Because of the complexity of the geographical landscape and the differences in diversity of organisms around the archipelago, continuous attempts to characterize faunal and botanical boundaries were carried out after Wallace. Some of them are

* [[Sclater]] line (1894)
* [[Richard Lydekker|Lydekker's]] line (1896)
* [[Paul Pelseneer|Pelseneer]]'s faunal balance (1904), also known as the [[Max Carl Wilhelm Weber|Weber line]]<ref name="Pelseneer 1904">{{cite journal |last1=Pelseneer |first1=Paul |title=La ligne de Weber, limite zoologique de l'Asie et de l'Australie |journal=Bulletin de la Classe des Sciences Académie Royale de Belgique |date=1904 |volume=1904 |pages=1001–1022}}</ref>
* [[Ernst Mayr|Mayr]]'s faunal balance (1944)

In addition, several smaller transition sub-regional boundaries were also proposed.<ref name="Ali-Heaney-2021" />

More recent work assessing biodiversity assemblages, phylogeny, and using computer-based geospatial tools to analyze previous boundaries have led to patterns of division similar to those proposed through the 19th&nbsp;century, although some special cases not explained before are reinforced by these modern analyses.<ref name="Rueda-2013"> {{cite journal |author1=Rueda, M.|author2=Rodríguez, M. Á.|author3=Hawkins, B. A.|date=2013| title=Identifying global zoogeographical regions: lessons from Wallace|journal=Journal of Biogeography|volume=40|issue=12|pages=2215–2225|ref=Rueda}}</ref>

{{full citation|date=May 2023}} for instance, evaluated the distribution of land mammals, birds, and amphibians in Wallace's realms and concluded that the boundaries suggested by Wallace remain valid. Ali ''et al''. (2020), in a different attempt, studied the fauna of [[Christmas Island]] and indicated that most of the ancestral colonizers of the island's land mammals and amphibians disappeared from the [[Lombok Strait]]. Therefore, they propose a re-conformation of Wallace's Line so that Christmas Island would be sited on the Australasian side of the biogeographical divide, instead of the oriental side.<ref name="Ali-Heaney-2021" />

==Biogeography==
[[File:Wallacea.png|thumb|upright=1.4|The [[Wallacea]] region situated between Wallace's Line (after [[Ernst Mayr]] or [[Thomas Henry Huxley]]) and the [[Lydekker Line]] ]]

Understanding of the [[biogeography]] of the region centers on the relationship of ancient [[sea level]]s to the [[continental shelves]]. Wallace's Line is visible geographically when the continental shelf contours are examined. It figures as a deep-water channel that separates the southeastern edge of the [[Sunda Shelf]] from the [[Sahul Shelf]]. The Sunda Shelf links Borneo, Bali, Java, and Sumatra underwater to the mainland of southeastern Asia, while the Sahul Shelf connects Australia to New Guinea and their adjacent islands.<ref name="Newton-1874">{{cite book |last=Newton |first=Alfred |date=1874 |title=Zoology |place=London, UK |publisher=Society for Promoting Christian Knowledge }}<br/>{{cite journal |title=''Zoology'' by Alfred Newton, M.A., F.R.S. ... 1874 |department=Bibliographical notice |type=book review |date=April 1875 |journal=Annals and Magazine of Natural History |series=4 |volume=15 |issue=88 |page=285 |issn=0374-5481 |doi=10.1080/00222937508681077 }}</ref>

During the Pleistocene, when the [[:File:Sea level temp 140ky.gif|ocean levels were up to 120 metres (390 ft) lower]], islands became connected, but never uniting Asia with Australia. Consequently, for over 50&nbsp;million years, deep water between those two large continental shelf areas created a barrier that kept the flora and fauna of Australia separated from those of Asia. It can reasonably be concluded it was an ocean barrier preventing species migration because the physical aspects of the separated islands are very similar.<ref name="Newton-1874" /> Thus, the ''[[Wallacea]]'' region consists of islands that remained isolated on their respective continental land masses, with only those organisms capable of crossing the straits between islands populating them.<ref name="Mayr-1944">{{cite journal |last=Mayr |first=Ernst |author-link=Ernst Mayr |date=March 1944 |title=Wallace's Line in the Light of Recent Zoogeographic Studies |journal=[[The Quarterly Review of Biology]] |volume=19 |issue=1 |pages=1–14 |doi=10.1086/394684 |s2cid=33245177 |issn=0033-5770 |url=https://www.journals.uchicago.edu/doi/10.1086/394684}}</ref>

Alternatively, "[[Max Carl Wilhelm Weber|Weber's line]]" runs through this transitional area (to the east of center), at the tipping point between Asian species against those with Australian origins.<ref name="Mayr-1944" />

== Zoogeography ==
The distributions of many bird species follow the limits of the line, since many birds do not cross even the shortest stretches of open ocean water. Among mammals, [[bat]]s have distributions that can cross the line, but larger terrestrial mammals are generally limited to one side or the other. On the Australian side, many species of marsupials are present, and some monotremes, alongside native rodents—although the occurrence of rodents in this case is derived from more recent colonization events.<ref name="Ali-Heaney-2021" />

By contrast, to the Asian side, marsupials are excluded, and placental mammals such as apes, cats, elephants, monkeys, rhinoceroses, and other species are found.<ref name="Ali-Heaney-2021" /> Exceptions to this include [[macaque]]s, pigs, and [[tarsier]]s on Sulawesi.

Other groups of plants and animals show differing patterns, but the overall pattern is striking and reasonably consistent. [[Flora]] do not follow the Wallace Line to the same extent as [[fauna]], since their colonization events differ in their ability to spread across bodies of water. One genus of plants that does not cross the line is the Australasian genus ''[[Eucalyptus]]'', except for one species, ''[[Eucalyptus deglupta|E. deglupta]]'', which naturally occurs on the island of [[Mindanao]] in the Philippines.<ref name="Wallace-1863" />

==See also==
* [[Geography of Indonesia]]
* {{Annotated link|Australasian realm}}

== References ==
{{Reflist|25em}}

== Sources ==
* {{cite journal |last=Wallace |first=A.R. |author-link=Alfred Russel Wallace |date=1862–1863 |title=On the physical geography of the Malay archipelago |journal=Proceedings of the Royal Geographical Society of London |volume=7 |issue=5 |pages=205–212 |jstor=179906 |issn=1478-615X |doi=10.2307/1799061 |doi-access=free}}
* {{cite journal |last=Wallace |first=A.R. |author-link=Alfred Russel Wallace |date=1863 |title=On the physical geography of the Malay archipelago |journal=[[Journal of the Royal Geographical Society of London]] |volume=33 |pages=217–234 |doi=10.2307/1798448 |doi-access=free |jstor=1798448 |issn=0266-6235 |url=https://archive.org/stream/proceedingsmonth07royauoft#page/n217/mode/2up |via=Internet Archive (archive.org)}}

==Further reading==
* {{cite thesis |author=Abdullah, M.T. |date=2003 |title=Biogeography and variation of ''Cynopterus brachyotis'' in Southeast Asia |degree=Ph.D. |publisher=[[The University of Queensland]] |place=St.&nbsp;Lucia, Australia |ref=none}}
* {{cite book |last=Dawkins |first=Richard |author-link=Richard Dawkins |date=2004 |chapter=Chapter&nbsp;14 &emsp; Marsupials |title=The Ancestor's Tale |title-link=The Ancestor's Tale |publisher=Weidenfeld & Nicolson |ISBN=0-7538-1996-1 |ref=none}}
* {{cite journal |author1=Hall, L.S. |author2=Grigg, Gordon G. |author3=Moritz, Craig |author4=Ketol, Besar |author5=Sait, Isa |author6=Marni, Wahab |author7=Abdullah, M.T. |date=2004 |title=Biogeography of fruit bats in Southeast Asia |journal=[[Sarawak Museum]] Journal |volume=LX |issue=81 |pages=191–284 |ref=none}}
* {{cite journal |author-link=George Gaylord Simpson |author=Simpson, G.G. |date=29 April 1977 |title=Too many lines: The limits of the oriental and Australian zoogeographic regions |journal=[[Proceedings of the American Philosophical Society]] |volume=121 |issue=2 |pages=107–120 |jstor=986523 |ref=none}}
* {{cite book |author=van&nbsp;Oosterzee, Penny |date=1997 |title=Where Worlds Collide: Wallace line |ref=none}}
* {{cite book |author1=Wilson, D.E. |author2=Reeder, D.M. |date=2005 |title=Mammal Species of the World |place=Washington, DC |publisher=Smithsonian Institution Press |ref=none}}

==External links==
{{Commons category}}
* {{cite web |title=Wallacea Research Group |type=main |url=http://www.wallacea.info/ |access-date=2010-06-11 |archive-date=2009-09-05 |archive-url=https://web.archive.org/web/20090905165443/http://www.wallacea.info/ |url-status=dead }}
* {{cite web |title=Map of Wallace's, Weber's, and Lydekker's lines |website=Starfish |department=Diving |place=Switzerland |url=http://www.starfish.ch/dive/Wallacea.html}}
* {{cite web |title=Pleistocene sea level maps |website=[[Field Museum]] |place=Chicago, Illinois |url=http://www.fieldmuseum.org/explore/pleistocene-sea-level-maps}}
* {{cite web |title=Wallacea – a transition zone from Asia to Australia, specially rich in marine life and on land |website=Starfish |department=Diving |place=Switzerland |url=http://www.starfish.ch/dive/Wallacea.html}}

{{Biomes|state=collapsed}}
{{Portal bar|Indonesia|Evolutionary biology|Geography}}

[[Category:Biota of the Australasian realm]]
[[Category:Biogeography]]
[[Category:Celebes Sea]]
[[Category:Ecoregions of Indonesia]]
[[Category:Biota of the Indomalayan realm]]
[[Category:Natural history of Indonesia]]
[[Category:Regions of Southeast Asia]]
[[Category:Wallacea]]