Page 75 - Environment: The Science Behind the Stories

tHE sCIENCE BEHINd tHE stoRy

Hawaii: so diverse that researchers have long
The Hawaiian honeycreepers are
species Factory and puzzled over what type of bird gave rise
Lab of Evolution to their radiation—and whether there was
just one colonizing ancestor or many. In
For scientists who study how species 2011, to clarify how the honeycreeper
form, no place on Earth is more radiation took place, one research team
fascinating and informative than the combined genetic sequencing technol-
Hawaiian Islands, often called a “natural ogy with resources from museum collec-
laboratory of evolution.” tions and our knowledge of Hawaiian
The key to this laboratory lies in the geology.
process that drives Hawaii’s geologic Heather Lerner and five col-
history. Turn back to Figure 2.22 leagues first took tissue samples from
(Chapter 2, p. 59), and examine it closely. bird specimens in museum collec-
Deep beneath the Pacific Ocean, a dr. heather Lerner, of earlham tions. Working with Robert Fleischer
volcanic “hotspot” spurts magma as the college and Helen James at the Smithsonian
Pacific Plate slides across it in tectonic plants have spiky, silvery leaves and tall Institution, Lerner, now at Earlham
motion like a conveyor belt. Mountains stalks that explode into bloom with flow- College in Indiana, sampled 19 species
of lava accumulate underwater until ers once in the plant’s long life before it of honeycreepers plus 28 diverse types
eventually a volcano rises above the dies. Researchers have discovered that of finches from around the Pacific Rim
waves, building an island. As the tectonic Hawaii’s 28 species of silverswords all that experts had identified as possible
plate moves northwest, it carries each evolved from a modest tarweed plant ancestors.
newly formed island with it, creating a from California that reached Hawai‘i and Lerner’s team obtained data from
long chain, or archipelago. Over several diversified by island-hopping. University 13 genes and from mitochondrial
million years, each island gradually sub- of California–Berkeley botanist Bruce genomes by sequencing DNA (p. 47)
sides, erodes, and disappears beneath Baldwin and other researchers analyzed from each tissue sample. They ran the
the waves. As old islands disappear on genetic relationships to determine the data through computer programs to
the northwest end of the chain, new silverswords’ history of speciation, and analyze how the DNA sequences—and
islands are formed on the southeast end. learned that their radiation was rapid, tak- thus the birds—were related to one
Geologists analyzing radioisotopes ing place in just 5 million years. another, then produced phylogenetic
(p. 42) in the islands’ rocks have The best-understood radiation has trees (p. 72) showing the relationships.
determined that this process has been occurred with the Hawaiian fruit flies. They published their results in the jour-
going on for at least 85 million years. Some of these insects speciate within nal Current Biology.
They estimate that Kaua‘i was formed islands in kipukas (see Figure 3.5), but Lerner’s team found that the
about 5.1 million years ago (mya), and most have done so by island-hopping. Hawaiian honeycreepers apparently
the island of Hawai‘i just 0.43 mya. By combining genetic analysis and derive from one ancestor, and are most
The Hawaiian Islands comprise the geologic dating, researchers deter- related to the Eurasian rosefinches,
most remote archipelago in the world, mined that the process began 25 mya indicating that honeycreepers evolved
but over time a few plants and animals on islands that today are beneath the after some rosefinch-like bird arrived
found their way there, establishing popu- ocean. From a single original fruit fly from Asia. Today’s rosefinches are
lations that evolved into new species. As species, an estimated 1000 species partly nomadic; when food supplies
some individuals hopped to neighboring have evolved—fully one-sixth of all the crash, flocks fly long distances to find
islands, populations that were adequately world’s fruit fly species. food. Perhaps a wandering flock of
isolated evolved into further species. Other groups that have undergone ancestral rosefinches was caught up in
Such speciation by “island-hopping” has adaptive radiations on the Hawaiian a storm long ago and blown to Hawai‘i.
driven the radiation of Hawaiian honey- Islands include damselflies, crickets, Once this common ancestor of
creepers and many other organisms. mirid bugs, spiders, and multiple today’s rosefinches and honeycreepers
For instance, the barren and wind- families of plants. Scientists propose arrived on an ancient Hawaiian island, its
swept high volcanic slopes of Hawai‘i are that once a species colonizes an island, progeny adapted to conditions there by
graced by some of the most striking flow- it can often spread and evolve rapidly natural selection, resulting in modified bill
ering plants in the world, the silverswords because competitors are few and there shape, diet, and coloration. Every once in
(see Figure 3.1b). These spectacular tend to be unoccupied niches (p. 79). a great while, wandering birds colonized

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