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Anole lizard species that occur together (sympatrically) on certain Caribbean islands occupy different habitats: some live
only in the grass, some only on tree trunks, and some only on twigs. These
species also differ morphologically: grass dwellers are slender with long
tails, tree dwellers are stocky with long legs, twig dwellers are slender but
stubby-legged. What is striking about
these lizards is not that coexisting species differ in morphology and habitat
use (such differences are common among closely related sympatric species), but
that the same three types of habitat specialists occur on each of four islands:
Puerto Rico, Cuba,
Hispaniola, and Jamaica.
Moreover, the Puerto Rican twig species closely resembles the twig species of Cuba, Hispaniola, and Jamaica in morphology, habitat use,
and behavior. Likewise, the specialists for other habitats are similar across
the islands. The presence
of similar species on different islands could be variously explained.
An
ancestral species might have adapted to exploit a particular ecological niche
on one island and then traveled over water to colonize other islands.
Or this
ancestral species might have evolved at a time when the islands were connected,
which some of these islands may once have been. After the islands separated,
the isolated lizard populations would have become distinct species while also
retaining their ancestors’ niche adaptations. Both of these scenarios imply
that specialization to each niche occurred only once. Alternatively, each
specialist could have arisen independently on each of the islands.
If
each type of specialist evolved just once, then similar specialists on
different islands would be closely related. Conversely, if the specialists
evolved independently on each island, then a specialist on one island would be
more closely related to other types of anoles on the same island—regardless of
their ecological niches— than it would be to a similar specialist on a
different island. Biologists can infer how species are related evolutionarily
by comparing DNA sequences for the same genes in different species. Species
with similar DNA sequences for these genes are generally more closely related
to each other than to species with less-similar DNA sequences. DNA evidence
concerning the anoles led researchers to conclude that habitat specialists on
one island are not closely related to the same habitat specialists elsewhere,
indicating that specialists evolved independently on each island.
36. The passage suggests that if a grass-dwelling anole lizard species
evolved on one island and then traveled over water to colonize a second island,
the grass-dwelling anoles on the two islands would eventually
A.
develop very
different DNA sequences
B.
develop into
different species that are more distantly related to each other than to tree-
and twig-dwelling anoles on their own islands
C.
come to differ
significantly from one another in habitat use √
D. develop into different,
but closely related, species
evolve significant morphological differences
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