Australia

Mayr's zoogeographic analysis of the Australian avifauna (1944e, 1944k), still valid, revealed endemic families, genera, species, and subspecies. Most immigrants came from Asia and the region of the Malay Archipelago across ocean barriers (see also

1972c, 1990b). Thus most of the Australian fauna developed through "continued single origin colonization," that is the steady and continuous colonization from the northwest (1965q). Corresponding to their differentiation and relative age, Mayr distinguished five major layers of colonists between which there are, of course, no clear cut discontinuities (1944k):

(a) Strongly endemic families and subfamilies, about 15,

(b) Less isolated indigenous families and subfamilies, about 8,

(c) Endemic genera of non-endemic families, about 30,

(d) Endemic species of non-endemic genera, about 60,

(e) Recently immigrated species, not differentiated at all, or only subspecifically, about 40.

The oldest layer comprising, besides emu and cassowary, the Megapodiidae, Anseranas, Pedionomus and perhaps Stictonetta, probably represents Gondwana elements, less than 3% of the bird fauna (Mayr 1990b). Australia split from Antarctica about 50 million years ago. At that time Australia must have had a fairly rich Gondwana bird fauna, but owing to circumstances that are not yet fully explained, this older fauna became almost completely extinct and was replaced during the Tertiary by immigrants from Asia. The ancestors of the Corvida immigrated into Australia ca. 30 million years ago and radiated into the characteristic families of Australian songbirds.

An island archipelago probably existed between eastern Asia and Australia during the Mesozoic at least since the early Cretaceous and during the entire Cenozoic (Tertiary-Quaternary). This archipelago probably included small drifting continental plates (split off northern Gondwanaland) which today form portions of Burma, Malaya, Borneo, Sumatra, Java and western Sulawesi; portions of what later became Timor and New Guinea were located off northern Australia (Audley-Charles 1987). The islands of this early Southeast Asian archipelago probably served as stepping stones for Asian immigrants into Australia as well as an area of intensive speciation and faunal differentiation during the Cretaceous and Tertiary. Many animal groups originated here which today form part of the rich fauna of the Malay Archipelago and the New Guinea region. The last faunal exchanges between Asia, the Malay Archipelago and Australia took place during several periods of low sea-level stand during the Pleistocene, when large portions of the shelf regions of the world became dry. As an example, the distance Timor-Australia during periods of lowered sea-level was only 50 miles instead of 300 miles today. This greatly facilitated the exchange of certain members of the bird faunas, mostly species of open savanna vegetation that was widespread on these shelf regions during cold (glacial) periods of lowered sea-level (Mayr 1944e, 1944k).

There is no question about the Asian origin of nearly all of the ancestors of Australian birds, as Mayr's analysis had revealed. Recent authors speculate that the ancestors of songbirds (Oscines) which reached Australia from Asia, also had originated in Gondwana land. How they may have arrived in the northern hemisphere (via contact of "Greater India" with Asia during the Eocene?) remains open (Cracraft 2002).

Tree runners (Neositta)

Fig. 4.14. Secondary hybrid belts among tree runners (Neositta) of Australia. The arrows indicate expansion from Pleistocene aridity refuges. Wherever former isolates have met, they have formed hybrid belts (indicated by hatching). R, subspecies with red wing bar; W, subspecies with white wing bar. From Mayr (1963b: 373)

Tree runners (Neositta)

Fig. 4.14. Secondary hybrid belts among tree runners (Neositta) of Australia. The arrows indicate expansion from Pleistocene aridity refuges. Wherever former isolates have met, they have formed hybrid belts (indicated by hatching). R, subspecies with red wing bar; W, subspecies with white wing bar. From Mayr (1963b: 373)

As to the differentiation of Australian birds at low taxonomic levels Mayr (1950b) conceived a theory of speciation and subspeciation in tree-runners (Neositta): During a Pleistocene or post-Pleistocene arid period (or several periods) a previously widespread ancestral species had been separated into a number of moist refuges around the periphery of the continent, where these isolated populations acquired the diagnostic characters of the taxa of this genus now recognized (Fig. 4.14). After the return of more humid conditions the previous isolates spread out from the refuges together with the expanding vegetation zones, hybridizing in the areas where they came in contact with one another. Keast (1961) and Short et al. (1983) strongly supported this interpretation. It was the first detailed presentation of the refuge theory of Pleistocene speciation for birds of the southern hemisphere. Zoo-geographers had proposed similar models for the differentiation of European birds and other animals which were assumed to have survived in humid Mediterranean refuges during the generally dry glacial periods.

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