Biology Reference
In-Depth Information
geological history, is enabling evolutionary ecologists to piece together
the history of invasion and differentiation of species in many locations.
One of the best-studied cases involves land birds in the Lesser Antilles.
The island of Barbados probably appeared about 700,000 yr ago by
geological uplift. This island, with 11 species of land birds and one
recently extinct species, has the most depauperate fauna of any of the
larger islands of the Lesser Antilles. None of the land birds is considered
endemic, even at the subspecific level. Since Barbados has never been
connected to any other island or to South America, its bird fauna has
arrived by over-water dispersal.The nearest islands, about 175 km to the
west, are St. Vincent and St. Lucia. Volcanic in origin, these islands are
20-30 million yr older than Barbados.
Lovette et al. (1999) examined mitochondrial DNA (mtDNA) vari-
ability of eight bird species present on Barbados, St.Vincent, and St. Lucia.
Tw o species, the black-faced grassquit (
Tiaris bicolor
) and the common
ground dove (
Columbina passerina
), were not only common South Amer-
ican mainland birds but also showed very little divergence among the
three islands.Thus, it appears that they colonized Barbados from either St.
Vincent and/or St. Lucia quite recently. A second pair, the Caribbean
elaenia (
Elaenia martinica
) and the Lesser Antillean bullfinch (
Loxigilla noc-
tis
), are both endemic species of Caribbean islands.The degree of mtDNA
differentiation was about the same between all three pair combinations of
islands, suggesting that these species colonized all three islands at about
the same time, but earlier than the grassquit and ground dove.A third pair,
the black-whiskered vireo (
Vireo altiloquus
) and the bananaquit (
Coereba
flaveola
), showed strong mtDNA differentiation between St. Lucia and St.
Vincent, but only moderate differentiation between St. Lucia and Barba-
dos. These species may have colonized Barbados from St. Lucia at about
the time that the elaenia and bullfinch arrived. Finally, a fourth pair, the
Antillean crested hummingbird (
Orthorhynchus cristatus
) and the Carib
grackle (
Quiscalus lugubris
), showed little differentiation between St. Lucia
and St.Vincent but substantial differentiation between Barbados and these
other islands. Based on the molecular clock relationship of about 2%
divergence per million years (Lovette et al. 1999), the maximum diver-
gence that might be expected for birds on Barbados, assuming that they
came from St.Vincent or St. Lucia, is about 1.4%.The hummingbird, dif-
fering on Barbados by 1.1-1.2% from St.Vincent and St. Lucia, respec-
tively, may thus have been a very early invader. The grackle, differing by
3.4% from both other islands, is likely to have colonized Barbados from a
source area other than St. Vincent or St. Lucia. The mtDNA profiles of
