THE GEoGRAPHy of EvoluTion 479
Organisms sometimes disperse long distances over unsuitable habitat, resulting
in rare, even unlikely, colonization. Often a colonizing species gives rise to diverse
descendant species, sometimes forming an adaptive radiation. The ancestor of the
Galápagos finches (see Figure 2.2) in the Galápagos Islands was a member of a
group of South American species known as grassquits. Many insects and nonfly-
ing animals such as lizards, as well as seeds of diverse plants, are carried to oceanic
islands by masses of vegetation that have been swept to sea by floods; some of
these species are transported as water-resistant eggs. A group of South American
rodents that includes porcupines and chinchillas stems from a species that arrived
by transoceanic dispersal from Africa in the Miocene, as did the ancestor of South
American primates. At least 110 genera of plants that occur on both sides of the
tropical Atlantic are too young to have occupied pre-rifting Gondwana, and have
dispersed across the ocean, most by floating [36]. Even some individual species
have dispersed from South America to Africa, such as the kapok tree (Ceiba pentan-
dra), a giant of tropical rainforests (FIGURE 18.11B) [10].
Biogeographers have studied species in the Hawaiian Islands extensively,
because the islands are so remote and have an interesting geological history. The
islands have formed as a tectonic plate has moved northwestward, like a conveyor
belt, over a “hot spot,” causing the sequential formation of volcanic peaks. This
process has been going on for tens of millions of years, and a string of submerged
volcanoes that once projected above the ocean surface lies northwest of the present
islands. Of the current islands, Kauai, at the northwestern end of the archipelago,
is about 5.1 My old; the southeasternmost island, the “Big Island” of Hawaii, is the
youngest and is less than 500,000 years old (FIGURE 18.12A).
Given the geological history of the archipelago, the simplest phylogeny expected
of a group of Hawaiian species would be a “comb,” in which the most basal lin-
eages occupy Kauai and the youngest lineages occupy Hawaii. This pattern wouldma Kirk patrick Evolution, 4e
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Evolution4e_1811.ai Date 11-02-2016
(A)(B)FIGURE 18.11 Some disjunct distributions have resulted
from dispersal. (A) The family Malpighiaceae, represented
here by the South American Malpighia glabra (left) and the
African Acridocarpus natalitius (right), moved by progres-
sive dispersal from tropical America through North America
and Europe to Africa. (B) The giant kapok tree (Ceiba
pentandra) emerges above the canopy of rainforests in
both tropical America and western Africa.18_EVOL4E_CH18.indd 479 3/22/17 1:39 PM