Adaptive Radiation
Intro: Darwin and the Galapagos Finches
The Adaptive Radiation of Darwin’s Finches:
In 1835, a young Charles Darwin visits the obscure Galapagos Islands on the
Beagle’s voyage around the world. Among the specimens he collects are the birds
of the Galapagos – a dozen different species. The differences in size and beak
shape reflect different foraging strategies (ecological niches). They also have
different habitats.
Darwin assumed that he’d collected many different families of birds – blackbirds,
grosbeaks and finches. In 1837, ornithologist John Gould studied the birds and
came to a conclusion: despite the remarkable variation, they all represented a
single lineage.
Recent work confirms this idea: all of the finches come from a single common
ancestor. The insect-eaters like the warbler-finches seem to be primitive, and
more specialised forms evolved from them.
Their closest relatives are tanagers from South America (not finches) – the
implication is that a flock of birds from South America got lost/was blown offshore
by a storm, landed on the Galapagos Islands and began to evolve. Since there
were no insect-eaters, some ate insects, since there were no hummingbirds, some
ate the available nectar, since there were no grosbeaks, some specialised on
large seeds, and since there were many different habitats, some specialised on
different habitat types.
Specialisation drives them apart: generalists may compete well if resources are
highly unpredictable year to year/season to season, but specialists do well when
resources are stable year to year/season to season.
Rapid radiation: after colonisation, the Galapagos finches rapidly occupied new
niches. The oldest of the Galapagos Islands is only 3.5 million years old, which
implies that this ecological diversity and all 15 species arose since then (an
average of 1 species every 240,000 years). This is assumes that the radiation is
as old as the oldest island and that no species have gone extinct. If the finches
arrived more recently and some have gone extinct, we are looking at much higher
speciation rates.
Adaptive Radiation: the finches that Darwin collected would later be recognised as
an example of adaptive radiation, which shows 3 main features:
High diversity
Wide range of niches occupied (wide range of diets, feeding adaptations,
behaviours, body sizes, habitats, etc.)
Occurs over a geologically short period
Colonisation Radiations: Finches, The Silversword Alliance, Drosophila &
others
Hawaii: the Hawaiian Islands are a larger land mass (twice the area of the
Galapagos), much lusher (higher rainfall) and older (oldest island is 5 million years
old). With higher productivity and area to support more diversity, and a longer
time to play out, Hawaii has seen even more adaptive radiation.
Honeycreeper-finches: similar to Darwin’s finches in having nectar-feeders,
insectivores, woodpeckers and seed-eaters, but they are far more specialised for
these lifestyles than Darwin’s finches. They were also far more diverse – there are
56 known species, although most of these were wiped out by introduced pests,
disease and habitat destruction. Most species are known only as skins/fossil bones
in museums.
Intro: Darwin and the Galapagos Finches
The Adaptive Radiation of Darwin’s Finches:
In 1835, a young Charles Darwin visits the obscure Galapagos Islands on the
Beagle’s voyage around the world. Among the specimens he collects are the birds
of the Galapagos – a dozen different species. The differences in size and beak
shape reflect different foraging strategies (ecological niches). They also have
different habitats.
Darwin assumed that he’d collected many different families of birds – blackbirds,
grosbeaks and finches. In 1837, ornithologist John Gould studied the birds and
came to a conclusion: despite the remarkable variation, they all represented a
single lineage.
Recent work confirms this idea: all of the finches come from a single common
ancestor. The insect-eaters like the warbler-finches seem to be primitive, and
more specialised forms evolved from them.
Their closest relatives are tanagers from South America (not finches) – the
implication is that a flock of birds from South America got lost/was blown offshore
by a storm, landed on the Galapagos Islands and began to evolve. Since there
were no insect-eaters, some ate insects, since there were no hummingbirds, some
ate the available nectar, since there were no grosbeaks, some specialised on
large seeds, and since there were many different habitats, some specialised on
different habitat types.
Specialisation drives them apart: generalists may compete well if resources are
highly unpredictable year to year/season to season, but specialists do well when
resources are stable year to year/season to season.
Rapid radiation: after colonisation, the Galapagos finches rapidly occupied new
niches. The oldest of the Galapagos Islands is only 3.5 million years old, which
implies that this ecological diversity and all 15 species arose since then (an
average of 1 species every 240,000 years). This is assumes that the radiation is
as old as the oldest island and that no species have gone extinct. If the finches
arrived more recently and some have gone extinct, we are looking at much higher
speciation rates.
Adaptive Radiation: the finches that Darwin collected would later be recognised as
an example of adaptive radiation, which shows 3 main features:
High diversity
Wide range of niches occupied (wide range of diets, feeding adaptations,
behaviours, body sizes, habitats, etc.)
Occurs over a geologically short period
Colonisation Radiations: Finches, The Silversword Alliance, Drosophila &
others
Hawaii: the Hawaiian Islands are a larger land mass (twice the area of the
Galapagos), much lusher (higher rainfall) and older (oldest island is 5 million years
old). With higher productivity and area to support more diversity, and a longer
time to play out, Hawaii has seen even more adaptive radiation.
Honeycreeper-finches: similar to Darwin’s finches in having nectar-feeders,
insectivores, woodpeckers and seed-eaters, but they are far more specialised for
these lifestyles than Darwin’s finches. They were also far more diverse – there are
56 known species, although most of these were wiped out by introduced pests,
disease and habitat destruction. Most species are known only as skins/fossil bones
in museums.
