Biology Reference
In-Depth Information
twentieth century and considered critically endangered today. Another, related hummingbird
found on the island, the green-backed firecrown, arrived in the nineteenth century. Unlike its
close relative, the green-backed is widespread in its range, being common across Argentina
and Chile, and it is easily twice as numerous as the endemic hummingbird on Robinson Cru-
soe Island. Among the 330 species of hummingbirds in the world, though, these are a rare
brace, the only two to have reached an oceanic island, reproduced, and gone on to live such a
remote life.
Early nineteenth-century naturalists surely read Defoe's classic novel. And the most fam-
ous, Charles Darwin, passed near what is now Robinson Crusoe Island as the HMS
Beagle
set sail for the Galápagos Islands from coastal Chile. If Darwin had visited, it's hard to ima-
gine his not mentioning with interest the presence of hummingbirds on an oceanic island, so
far from the mainland. The modern explanation of why one hummingbird species is found
only on a dot in the South Pacific while another, closely related species has a much broader
range and is much more abundant is straightforward. The endemic red-backed species, the
Juan Fernández firecrown, evolved over time into the distinct species it is on the islands,
whereas the green-backed is too recent an arrival on the island for it to have diverged from its
mainland relatives. For many other closely related species, however, the riddle of why some
are common while others are rare remains to be answered.
Endemism epitomizes island life, especially along the equator. And among biologists,
tropical islands crowd the atlas of daydreams—not just for the idyllic scenery but also be-
cause the unusual, and often rare, plants and animals inhabiting such islands showcase the
arc of evolution. This scientific fascination has noteworthy milestones. Darwin's visit to the
Galápagos and Alfred Russel Wallace's journey to the Malay Archipelago two decades later
gave rise to these two men's vision of evolution by natural selection, the greatest organizing
principle of life on Earth and one that sheds much light on the issue of rarity. Natural selec-
tion, at its essence, is the evolutionary process that results in living creatures becoming well
suited, or adapted, to their locale. Darwin and Wallace's theory views evolution by natural
selection as the dynamic force by which individuals that possess certain advantageous fea-
tures, or traits, are more likely to survive to bestow these heritable traits, and duplicates of
the genes that code for them, on future generations. The sum of these traits increases what
is called the reproductive fitness of an individual, and the metric biologists use to assess this
quality is the offspring one leaves behind and their ability to survive and breed.
Biologists still marvel over the explanatory power of the theory of evolution by natural se-
lection, a lens through which to view every aspect of the natural world, from the habitats wild
species select to the foods they eat, the mates they choose, the places they sleep, and their
responses to predators. Yet few are aware that its coinventor Charles Darwin was also one
of the first scientists to highlight rarity in nature. “Rarity,” he stated in a neglected sentence
from
On the Origin of Species
in 1859, “is the attribute of a vast number of species of all
