Evolution__3rd_Edition

502 PART 4 / Evolution and Diversity
Lizards have radiated in the
Caribbean ...
. . . with a similar range of
ecological forms on each island
The lizards of the genus Anolis in the Caribbean islands, are a well studied example.
Species of Anolis have evolved to occupy various ecological niches, and have adaptations
appropriate to their ways of life. Some species live on twigs, others in the canopy,
and others in the grass. Species that live on twigs have long tails and short legs; species
that live in the grass have short tails; and species that live on low tree trunks have long
legs. Anolis are found on all the major islands of the Greater Antilles, and occupy a similar
range of habitats on each island. The species that live on twigs (for example) all look
similar, with long tails and short legs, whether they are the species from Cuba, Haiti,
Jamaica, or Puerto Rico. The other ecological types also show similarities among the
islands.
We can ask whether the twig-dwelling species on any one island shares a more recent
common ancestor with other twig-dwelling species on the other islands, or with the
ecologically different Anolis on the same island. That is, did the twig-dwelling ecological
type evolve only once, and spread to all the islands? Or did each ecological type
evolve independently on each island? Losos et al. (1998) answered the question by constructing
a molecular phylogeny of the species. They found that for the most part each
ecological type of lizard had evolved independently on each island (Figure 17.5).
Thus, each island tended to be colonized by one lizard population, which then radiated
into a common set of ecological types on every island. Some exceptions exist in
Figure 17.5. For instance, two sets of Cuban species are found in different parts of the
phylogeny, as if they evolved after separate colonizations. But for the most part, the
species are clustered by island rather than ecological type. The similarity between
species in characters such as tail length is homoplasious rather than homologous
(Section 15.3, p. 427). The force that drives the radiation is probably ecological competition.
The adaptive radiation of the Caribbean Anolis lizards would then be a
miniature example of Darwin’s “principle of divergence” (Section 16.8, p. 487).
Grass Hispaniola
Twig Hispaniola
Grass Hispaniola
Trunk Cuba
Grass Cuba
Trunk Jamaica
Twig Jamaica
Grass Puerto Rico
Trunk Puerto Rico
Twig Puerto Rico
Trunk Hispaniola
Grass Cuba
Twig Cuba
Figure 17.5
Phylogenetic relations of different ecological types of species
of lizards (Anolis) on four Caribbean islands. The full results
are for six ecological types, of which only three are shown here
(grass, twig, and trunk). The other three, however, illustrate
much the same pattern. Each ecological type tends to have
evolved independently on different islands: the phylogeny groups
species by island more than by ecological type. The phylogeny was
produced by parsimony, using mitochondrial DNA sequences
from the lizards. Modified from Losos et al. (1998).
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