Evolution__3rd_Edition

514 PART 4 / Evolution and Diversity
North America
Myr BP
South America
Number of genera Percentage of genera
Percentage of genera Number of genera
130 100 50 0 100 50 0 0 50 100 0 50 100 170
Recent
0
South American
immigrant
North American
native
Figure 17.15
Numbers (and percentages) of genera of land mammals in the
last 9 million years in North and in South America. Immigrant
and native genera are distinguished in both places. Note the
Similar fractions of species moved
in both directions
1
2
3
4
5
6
7
8
9
North American
immigrant
South American
native
wave of immigration after about 3 million years ago. Redrawn,
by permission of the publisher, from Marshall et al. (1982).
© 1982 American Association of the Advancement of Science.
as the Great American Interchange, and popular biology still portrays it as a competitive
rout of the South American mammals by the superior northern forms. There is
some truth in that idea; but the increasing quantity of fossil evidence is allowing a more
detailed reconstruction of the events.
A study by Marshall and colleagues (1982) has examined the time course of the
Interchange in detail. They counted the number of mammalian genera in South and
North America at successive times and divided the genera according to where they
originally evolved. They then divided the immigrant genera into primary immigrants
(genera that evolved in the south and immigrated to the north or vice versa) and secondary
immigrants (genera descended from primary immigrants). They argued the
primary invasions were roughly equal in both directions, and that the takeover of the
south by northern mammals was partly a result of two other factors: weight of numbers
and different rates of speciation after arrival.
Figure 17.15 shows the numbers of genera, expressed both as absolute numbers
and as proportions, of mammals in North and South America. On both sides, after
2.5–3 million years ago, an increasing proportion of the mammal genera were immigrants
(or descendants of immigrants) from outside. At present, about 50% of South
American genera are descended from originally North American mammals. The proportion
of southern mammals in the north is much lower, at about 20%. The numbers
become more revealing when we break them down further (Table 17.2).
We can begin by counting the total number of genera before the Interchange;
..