Evolution__3rd_Edition

494 PART 4 / Evolution and Diversity
Major faunal ...
. . . and floral ...
. . . regions of the globe are
recognized, ...
. . . and are quantitatively described
biogeography of higher taxa. The distributions of the higher taxonomic levels are, obviously,
more widespread than those of species, but some taxonomically isolated higher
groups, with small numbers of species (they are usually examples of living fossils, see
Section 21.5, p. 606), have localized distributions. For example, the tuatara Sphenodon
punctatus is the only surviving species of a whole order of reptiles (or almost the only
survivor a there may be more than one surviving species of Sphenodon). Of about 20
orders of reptiles, 16 are completely extinct and only four have living survivors. Of
those four, three contain the turtles and tortoises, lizards and snakes, and crocodiles,
respectively. The fourth has only Sphenodon, which is now confined to some rocky
islands off New Zealand.
When the biogeographers of the nineteenth century looked at the distributions of
large numbers of species on the globe, they saw that different species often lived in the
same broad areas. They suggested that there are large-scale faunal regions on the Earth.
The first map of these faunal regions was drawn for birds by the British ornithologist
Philip Lutley Sclater (1829–1913), and Alfred Russel Wallace soon generalized Sclater’s
regions to other groups of animals. The Earth was thus divided up into six main biogeographic
regions (Figure 17.2a). The regions are mainly defined by the distribution
of birds and mammals, and might not have been recognized if other groups had been
used. Botanists, for instance, tend to draw different lines on the map: they usually combine
the Nearctic and Palearctic regions into one larger region called the Boreal or
Holarctic, and recognize a separate floral region, called the Cape, in Southern Africa
(Figure 17.2b). Figure 17.2, therefore, does not illustrate a set of hard and fast facts as
the regions are approximate. The regional terms a like Nearctic and Neotropical a are
often used in biogeographic discussion.
The division into regions was made according to the degree of similarity between
the lists of the species living in the various places. Biogeographic similarity can be
quantified by various indexes of similarity. One of the simplest indexes is Simpson’s
index. If N 1 is the number of taxa in the area with the smaller number of taxa, and N 2 is
the number of taxa in the other area, and C is the number of taxa in common between
the two regions, then Simpson’s index of similarity between the two areas is:
C/N 1
Table 17.1 gives the faunal similarities, for mammalian species, between several regions
(they are expressed as percentages: that is, (C/N 1 ) × 100). The indexes in the table show
some of the justification for the division of the Earth into faunal regions as in Figure
17.2. For example, the faunas of Australia and New Guinea are 93% similar, whereas
those of New Guinea and the Philippines are only 64% similar. The Philippines are
more similar to Africa than to New Guinea. This Indonesian discontinuity, which can
be seen in Figure 17.2a, is known as Wallace’s line. It was not properly understood until
the discovery of plate tectonics.
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