Evolution__3rd_Edition

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CHAPTER 17 / Evolutionary Biogeography 515
Table 17.2
Pattern of faunal exchange between North and South America in different time periods. The table gives the total numbers of genera
of South or North American origin in each region (these are the numbers plotted in Figure 17.15), and breaks down the immigrant
genera according to whether they were “primary” (that genus itself immigrated) or “secondary” (that genus descended from a
primary immigrant genus, e.g., a secondary immigrant in North America evolved in there but came from a genus that itself evolved in
South America). The total of the immigrant genera in the bottom two rows equals the number of alien genera in the “number of
genera” row above. Note: (i) the similar proportions of primary immigrant genera moving in each direction, and (ii) the much
greater numbers of secondary immigrants in South America than in the north. Modified from Marshall et al. (1982).
South America North America
Time period
(Myr BP): 9–5 5–3 3–2 2–1 1–0.3 0.3–Recent 9.5–4.5 4.5–2 2–0.7 0.7–Recent
Duration (Myr) 4 2 1 1 0.7 0.3 5 2.5 1.3 0.7
Number of genera
North American 1 4 10 29 49 61 128 99 90 102
South American 72 68 62 55 58 59 3 8 11 12
Total 73 72 72 84 107 120 131 107 101 114
Number of
immigrant genera
Primary 1 1 2 10 18 20 2 6 8 9
Secondary 0 3 8 19 31 41 1 2 3 3
North America contained more
species to begin with
Patterns of speciation differed
the total number is higher in the north, perhaps because of the continent’s greater area.
(It is an important principle of island biogeography that a larger area supports a
larger number of species.) Marshall et al. (1982) then counted the numbers of North
American mammals moving south, and vice versa, and expressed them both as
proportions of the total pool. They found that the proportions are about the same.
Approximately 10% of available North American mammals invaded the south. (For
instance, Table 17.2 shows that North America had about 100 endemic mammalian
genera. Around 10 of them migrated south, with the number of primary immigrants
going up from one to 10 between 3 and 1 million years ago.) Likewise, about 10% of
South American mammals moved north. (In Table 17.2, there were 60 or so South
American mammalian genera 3 million years ago. The number of South American genera
in the north increased by about six between 4.5 and 1 million years ago.) The greater
absolute numbers moving south is mainly due to the larger number of mammals in the
north to begin with.
The pattern of primary immigration is thus similar in both directions. Something
like 10% of the genera from each side successfully invaded the other. But when we look
at the subsequent proliferation of the immigrants, the pattern diverged markedly
(Table 17.2). By the Recent period, a total of 12 (the nine in Table 17.2 is the number
alive a three others had arrived and then gone extinct) immigrant southern mammal
genera had produced only three new genera, while the 21 immigrant northern mammalian
genera in the south produced 49 genera. In the Recent period the trend has