Page 76 - Environment: The Science Behind the Stories

Common Rosefinch other islands, founding populations that
(most closely related to
the honeycreepers) each adapted to local conditions and
Poouli might eventually evolve into separate
(diverged when
Kaua`i formed) species.
Maui Creeper Because the age of each island is
known, Lerner’s team could calibrate
Kaua`i Creeper rates of evolutionary change in the DNA
The common ancestor sequences of the birds, and thus meas-
of the rosefinch and Palila ure the age of each divergence. That is,
the honeycreepers (diverged when
lived 7.2 mya O`ahu formed) they could tell how “old” each bird spe-
Nihoa Finch cies is. They found that the rosefinch-
like ancestor arrived by 5.7 mya, about
Laysan Finch the time that the oldest of today’s main
islands (Kaua‘i and Ni‘ihau) were form-
`I`iwi
The ancestor of ing. After O‘ahu emerged 4.0–3.7 mya,
the honeycreepers the speciation process went into over-
arrived in Hawai`i Akohekohe
about 5.7 mya (diverged when drive, giving rise to many new species
Maui formed) with distinctively different colors, bill
`Apapane
shapes, and habits. By the time Maui
arose 2.4–1.9 mya, most of the major
`Akiapōlā`au
differences in body form and appear-
ance had evolved (Figure 1).
Maui Parrotbill Thus, most major innovations arose
Most speciation
took place after midway through the process, when O‘ahu
O`ahu was formed Anianiau and Kaua‘i were the main islands in the
chain. After this burst of innovation, major
Hawai`i Creeper
changes were fewer, perhaps because
most possibilities had been explored, or
Kaua`i `Ākepa
perhaps because the newer islands of
Maui and Hawai‘i were too close together
`Ākepa
to isolate populations adequately.
The team’s data show that the age
Kaua`i `Amakihi
of each honeycreeper species does not
neatly match the age of the island(s)
O`ahu `Amakihi
it inhabits today. Instead, the island-
hopping process was complex, with
Maui `Amakihi
some birds hopping “backwards” from
newer islands to older ones. Moreover,
Hawai`i `Amakihi CHAPTER 3 • Ev ol u T i on, Bi odiv ER si T y, A nd Po P ul AT i on E C ology
(diverged when within each island there is great variation
8 7 6 5 4 3 2 1 0 Hawai`i formed) in climate, topography, and vegetation,

KAUA`I NI`IHAU O`AHU MAUI HAWAI`I as windward slopes catch moisture from
trade winds over the ocean and become
Islands were formed lush and green, whereas leeward slopes
in sequence, from in the rainshadow (p. 118) are arid. The
Kaua`i and Ni`ihau varied habitats and rugged topography
(4.9–5.7 mya) to Pacific Ocean create barriers that can lead to specia-
Hawai`i (0.4–0.7 mya)
tion within islands.
For all these reasons, the “natural
Figure 1 using gene sequences, researchers generated this phylogenetic tree showing
relationships among the hawaiian honeycreepers. They then matched the history of the laboratory” of Hawai‘i still has much to
birds’ diversification with the known geologic history of the islands’ formation. Adapted teach us about how the honeycreepers
from: Lerner, H.R.L., et al. 2011. Multilocus resolution of phylogeny and timescale in the extant adaptive and other groups have evolved, and
radiation of Hawaiian honeycreepers. Curr Biol. 21: 1838–1844. how new species are formed.

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