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Zoogeography 165
members of such groups colonized other continents or islands where they radiated
conspicuously (e.g., meliphagids in New Zealand, drepanidids in Hawaii); (3)
faunas are built up by waves of colonization from outside sources superimposed
on original stocks; (4) geographical speciation occurs in populations separated by
physical barriers (due to continental movements and climatic-vegetational fluctuations);
(5) secondary range expansion from the areas of origin led to sympatry of
representatives of different faunas.
Mayr (1982h) severely criticized an exclusive explanation of zoogeographic
patterns by the vicariance paradigm but agreed (1983e) that it is no longer sufficient
simply to develop an explanatory scenario. Hypotheses and predictions should be
tested. On the other hand, he pointed out that not all problems can be settled in this
manner, e.g., the placement of borders between biogeographical regions (Wallace’s
or Weber’s lines versus “Wallacea” in the Malay Archipelago). In quantitative
studies one must avoid treating all species of certain animal groups as exhibiting
uniform biogeographical characteristics. Island birds are not a random sample of
mainland birds with equal dispersal facilities. Among all kinds of animals there
are good and poor dispersers. Mayr (1983f) emphasized again that the elements of
faunas have had different histories and mean values are often meaningless. Mayr
and Diamond (2001g) discussed these aspects in great detail with respect to the
avifauna of northern Melanesia.
To explain discontinuous ranges, many authors favored an “either-or” choice,
either secondary (vicariance) or primary (dispersal across a barrier). However, it
is becoming clearer that in poor dispersers many if not most discontinuities are
secondary, while in good dispersers they are primary. Also, the extant pattern of
distribution was, at least in part, established in that geological period in which
a group had its major evolutionary development, hence earthworms or mayflies
have other patterns than mammals and birds. Single-track explanations are unable
to cope with this pluralism and are, therefore, bound to be misleading.
Summarizing, spatial events and climatic-vegetational fluctuations shape faunas
and species from their early history. Separation of populations through dispersal
and vicariance, secondary contact and overlap led to speciation and faunal differentiation.
The early history of faunal lineages was influenced by continental drift,
i.e., the break-up and contact of continental blocks–processes which had more
pronounced effects on some groups, like freshwater fishes, than others like birds.
First Steps
The ecology and distribution of birds fascinated Mayr already as a student (see
pp. 28, 35). In his dissertation prepared under Erwin Stresemann as thesis advisor,
Mayr analyzed a zoogeographical problem, the range expansion of the Serin
(Serinus serinus), a small greenish yellow finch which, spreading northward from
the Mediterranean region, had colonized large portions of Europe since the early
19th century (Mayr 1926e; see Vuilleumier 2005b). He described the process of
range expansion and discussed the ecology of this and other expanding species