in Linnaeus' day (early and middle 1700's) people thought that the animals they knew where they lived were more or less like those elsewhere ... Europeans knew about some African animals but more or less as fables ... when Linnaeus began his catalogues of the plants and animals of the world he assumed that species in North America and elsewhere were similar to those in Sweden

with increased exploration (and more scientists along for the ride) it became apparent that some animals elsewhere in the world had no close counterparts in Europe ... specimens sent home on Darwin's voyage around the world made this point clear ... even the vertebrate fossils he collected in Argentina were more similar to animals still living there than to European animals

Alfred Russel Wallace, a naturalist about 14 years younger than Darwin, explored the Amazon basin (but lost all his specimens and notes, and almost his life, when his ship burned and sank in the middle of the Atlantic on his return) ... undaunted he then explored the "Malay Archipelago" (now mostly Indonesia) ... he first emphasized the patterns of biogeography

Wallace's Line divides one set of species in Australia, New Guinea, and nearby islands from another set in southeastern Asia and the large islands of Indonesia ... for example, marsupial mammals, birds-of-paradise, and elapid snakes (cobras and relatives) occur mostly east of this line (a few species and genera of cobras occur in southeast Asia and Africa)

similar relatively narrow transitions between different sets of species, genera, and families of vertebrates occur elsewhere . . . there are six major zoogeographic (or biogeographic) regions . . .

• Neotropic region (New World tropics) from southern Mexico southward
• Nearctic region (North America from central Mexico northward)
• Palearctic region (Europe and Asia north of the Sahara and Himalayas)
• Nearctic and Palearctic often combined as the Holarctic region because these two regions are less differentiated than others
• Ethiopian region (Africa south of the Sahara)
• Oriental region (Asia south of the Himalayas ... India, southern China, southeast Asia east to Wallace's Line)
• Australasian region (Australia, New Guinea)

there are some important smaller regions . . .

• Madigascar, New Zealand, Polynesia

none of these regions corresponds exactly to one of the continents ... these regions represent regions where plants and animals have evolved for long periods with little interchange ... they have been separated by . . .

• barriers (desert belt from the Sahara through central Asia to Mongolia, high mountains like the Himalayas, oceans)
• continental drift over the past 250 million years

distributions of some vertebrate groups suggest vicariance by continental drift (evolution in Gondwana and subsequent isolation in Australia, South America, and Africa) ... variance occurs when populations are separated by geological or climatic events . . .

• ratite birds (emu, rhea, ostrich) in Australia, Africa, and South America
  (also extinct moas in New Zealand)
• leptodactylid frogs (many species in South America and Australia)

distributions of land vertebrates more often require explanations that include episodes of . . .

• vicariance by continental drift
• dispersal across barriers
• extinction in places of origin

primates

descendants of early primates (prosimians including lemurs, lorises, galagos) occur today in Madagascar, Africa, and southern Asia ... true primates occur in Africa and southern Asia and in South America but not in Australia ... so primates evolved after Africa separated from Gondwana

prosimians separated from other mammals well after Madagascar separated from Africa (evidence from DNA sequences used as a "molecular clock") ... an ancestral population of lemurs must have colonized Madagascar across a then-narrower channel of the Indian Ocean ... they then became extinct in Africa as more advanced primates evolved

South American primates separated from those in Africa in middle Cenozoic (about 35 MYA) ... well after Africa had separated from South America ... an ancestral population must have crossed a then-narrower Atlantic Ocean to reach South America ... South American primates spread northward as far as southern Mexico after the Panamanian land-bridge formed (10 MYA)

African primates went on to evolve into many existing genera in Africa and Asia and to give rise to the great apes and to humans

so the distribution of primates today is a result of a couple of long-distance dispersals across narrow oceans and dispersal across a land-bridge

marsupials

marsupials first appear as fossils in the late Mesozoic in the northern continents ... they must have colonized South America across a water gap and then reached Australia before Gondwana finally split into South America, Antarctica, and Australia ... subsequently the South American and Australia/New Guinea marsupials have remained isolated (vicariants)

marsupials became extinct in the northern continents as they were replaced by placental mammals ... but then a few species (like the familiar Virginia Opossum) recolonized the north after the Panamanian isthmus formed

so the distribution of marsupials today is a result of dispersal across a water barrier, vicariance as Gondwana split, extinction at its origin, and dispersal across a land-bridge

horses (and camels)

horses and camels first appear in the fossil record in North America, where both lineages diversified into many genera and species ... some colonized South America when the Panamanian isthmus formed (ancestors of the llama, alpaca, guanaco, and vicuna) at a time when many other northern mammals invaded South America (cats, raccoons, deer, peccaries, cricetid rodents)

horses and camels also colonized Asia during low sea-levels early in the Pleistocene and then spread across Asia and Africa (ancestors of existing dromedary and bactrian camels, zebras, wild asses, and the domestic horse)

horses and camels in North America all became extinct shortly after the appearance of humans at the end of the last ice age (about 15,000 years ago) ... like most other large mammals and many other vertebrates they succumbed directly or indirectly to efficient human hunting

horses were re-introduced into the Americas by Spanish colonists ... native Americans acquired horses after the Pueblo Revolt in New Mexico in 1680 ... in the late 1700's and early 1800's horses radically altered the nature of warfare, hunting, society, and distributions of tribes on the Great Plains ... it took about a century for horses to reach the northern Great Plains ... at the time of Custer's Last Stand in 1876 native tribes on the northern Great Plains had had horses for no more than a century

so the distribution of wild horses and camels today is a result of dispersal across land-bridges, extinction at their origins, and human introductions

passeriform (perching) birds

of the 10,000 species of birds in the world, over half (about 5,700) belong to one order (Passeriformes, perching birds) ... these separated into two groups near the end of the Mesozoic ... the Passeri (songbirds, about 4,500 species) and the Tyranni (suboscines)

the Tyranni radiated almost entirely in South America (about 1,100 species) ... many also spread into Central America after the Panamanian isthmus formed ... and some have spread as far as Canada (our flycatchers, including Eastern Phoebe and Eastern Wood-Pewee) ... a few Tyranni (about 55 species) occur in Africa, southern Asia, Madagascar, and New Zealand ... the present distribution of the Tyranni might thus reflect Gondwana vicariance and a radiation in South America

the origin of the Passeri is not clear ... but one large group (the "crow assemblage") radiated in Australasia into about 18 families (about 540 species) of diverse form and ecology ... the greatest adaptive radiation of vertebrates ever ... some 14 families of this assemblage now occur outside Australasia (worldwide, including smaller radiations in Africa, Madagascar, and New Zealand)

evolutionary relationships of the rest of the Passeres are still being worked out ... their current zoogeography might involve Gondwana vicariance, multiple dispersals, and multiple extinctions!