An Origin of Species
Introduction:
What do these birds have in common? Very little, judging from their specialized beaks,
body types, and behaviors.
Yet close examination and molecular analyses reveal otherwise. Each is a member of a
family of birds called honeycreepers, and they all share the same common ancestor: a
single finchlike species that scientists estimate arrived on the Hawaiian Islands about 5
million years ago. At one time there were as many as 57 species of honeycreepers.
This is certainly not the first time living creatures have taken the dramatic evolutionary
pathway known as adaptive radiation. The Species Gallery highlights some of the most
extraordinary outcomes of this very common process.
How can one species evolve in so many different directions?
Comparing the Islands
Windsor Island
Description A small island approximately five
miles off the mainland coast.
Warwick Archipelago
A chain of four variously sized islands approximately
600 miles northeast of the mainland.
Habitat
Wetter, more lush, and more diverse
than the mainland.
High diversity relative to the mainland. Individual
islands are very different from one another, each
boasting a wide range of habitat types, rainfall, and
elevation.
Niches
Very few available jobs or niches, due Many jobs or niches available upon pollenpeepers'
to the large numbers and high
arrival, due to the island chain's distance from the
diversity of bird species in and around mainland.
the pollenpeepers' habitat.
As you begin to investigate the evolution through time of the pollenpeepers on their new island
homes, consider some important factors that drive evolution.
Habitat:
Habitat strongly directs the path of evolution. Change the habitat, or put an organism into a new
habitat, and you can expect evolutionary change. Often, new habitats are ripe with opportunity
because they've not had millions of years to become occupied by a diverse range of creatures. Niches
or jobs remain open and free for the taking to any species able to use them.
Food:
Food is arguably the most important piece of a creature's habitat, driving the process of evolution,
especially when its availability changes. Where food sources are diverse and plentiful, species can
afford to eat a wide variety of foods. A reduction in food supply, however, forces a population to find
new food resources. Individuals best adapted for utilizing the resources that are available have a
selective advantage over others in the population.
Competition:
Competition may lead to divergence (the splitting of a species into different species). Populations
often take different paths in ways that allow them to use different resources. This happens because
individuals that differ a great deal from their competitors in behavior or the way they look can use
different sets of resources better and thereby survive and reproduce better than individuals that have
more in common with their competitors. Over generations, populations that live in the same place
diverge (take different paths, like on a classification tree) in their patterns of resource, use and
competition is reduced.
Predators:
Predators put limits on their prey by restricting their movements and forcing populations to evolve
defense strategies. When you take away predation pressures, you eliminate many limitations on how a
species can evolve. In the absence of predators, unique traits can develop freely and persist, if they
offer advantages to their owners. For example, flashy coloration may allow an individual to be more
obvious to a potential mate. The result: a more widely diverse group of organisms in predator-free
environments than would otherwise be possible.
As you will see, pollenpeeper populations on Windsor Island and Warwick Island will show relatively
minor changes over time. Populations will continue to occupy niches or jobs similar to those they
occupied before the storm, since there are few opportunities to move into other places. Follow along
to explore what happens to the pollenpeepers.
Windsor Island: 5 Million Years Ago
Highlight:
 A new wave of immigrants is blown in by storm.
Pollenpeeper changes:
 The pollenpeeper population increases 400 percent -- from 50 to 200 -- as birds
are blown from the mainland to Windsor Island, five miles to the east.

Immediately following the storm, immigrants assimilate into the local population;
individuals from each population are indistinguishable from one another.
As the population grows, pollenpeepers slowly begin to expand their range inland from
their coastal habitat.
Habitat:
 The island receives more rainfall than the coastal mainland; consequently, it is
much more lush.
Food:

Seeds
Some seeds available

Insects
Many insects available

Flowers
Few nectar flowers available
Competition:

When the newest pollenpeepers arrive on Windsor Island, there are few other
species present to compete with them for resources. Competition remains low
during this time period.
Predators:
 Many predators threaten these pollenpeepers
Windsor Island: 4 Million Years Ago
Highlight:
 Two new species of finch arrive.
Pollenpeeper changes:
 There is very little change in the appearance of the beaks of Windsor
pollenpeepers.
o Head plumage of Windsor pollenpeepers slowly becomes redder than
that of mainland peepers.
o Occasional immigrants -- four or five per year -- arrive on Windsor
Island from the coastal mainland.
Habitat:
 The habitat remains wet and lush across the island during this time period.
Food:

Seeds
Some seeds available

Insects
Many insects available

Flowers
Few nectar flowers available
Competition:

Two new species of finch arrive on the island during this time period and begin to
compete with pollenpeepers for food resources. This only provides significant
pressure infrequently, in years when food availability is low.
Predators:

Many predators threaten these pollenpeepers
Windsor Island: 3 Million Years Ago
Highlight:

Birds develop redder heads and move inland.
Pollenpeeper changes:


The head plumage of the pollenpeepers, especially the males, continues to grow
redder.
There is no apparent change in beak size and shape during this time period.
Habitat:

Overall, pollenpeeper habitat does not change significantly. Pollenpeepers begin
to use their habitat differently, however, as a portion of the population moves
from the coast into the wetter, denser island interior.
Food:

Seeds
Some seeds available

Insects
Many insects available

Flowers
Few nectar flowers available
Competition:

Pollenpeepers that move inland feed on the same insects as two other species of
insect-eating birds. Insect larvae are plentiful, though, and there is still very little
competition among the birds.
Predators:

Many predators threaten these pollenpeepers
Windsor Island: 2 Million Years Ago
Highlight:

Some residents move back to the mainland.
Pollenpeeper changes:



There is little change in the morphology of Windsor pollenpeepers during this
period.
Population density of pollenpeepers along the coastal habitats on Windsor has
reached a peak.
Each year, at least a dozen or more pollenpeepers leave Windsor Island, bound
for the mainland coast. Nearly as many disperse from the mainland to Windsor,
though, so the population remains stable.
Habitat:

The habitat remains lush and tropical during this period.
Food:

Seeds
Some seeds available

Insects
Many insects available

Flowers
Few nectar flowers available
Competition:

Competition among pollenpeepers for foraging and nesting resources, especially
along the coast, reaches a peak due to the highest population density to date.
Predators:

Many predators threaten these pollenpeepers
Windsor Island: 1 Million Years Ago
Highlight:

A new wave of immigrants from the mainland invade Windsor.
Pollenpeeper changes:


Pollenpeepers from the mainland's northern coast disperse to Windsor Island in
increasing numbers. The new immigrants have slightly longer, narrower beaks
than the native Windsor pollenpeepers.
Movement and interbreeding between Windsor Island and mainland pollenpeeper
populations becomes more frequent. The mainland birds with their longer bills
are better adapted and more successful at finding food on Windsor Island;
through interbreeding they pass their genes and longer, narrower beaks on to the
population as a whole.
Habitat:

The habitat remains lush and tropical. Seed-producing grasses are increasingly
displaced by trees and shrubs during this time period.
Food:

Seeds
Few seeds available

Insects
Many insects available

Flowers
Few nectar flowers available
Competition:

Competition among birds along the coast becomes intense, favoring individuals
with longer, narrower beaks.
Predators:

Many predators threaten these pollenpeepers
Windsor Island: Present
Highlight:

Short, blunt beaks become obsolete
Pollenpeeper changes:


Pollenpeepers move regularly between Windsor Island and the mainland; the two
populations interbreed.
There is a trend toward longer, narrower beaks within both populations.
Habitat:

Grass seeds become rare and unreliable as a food resource. The tropical
environment supports an abundance of insects. Pollenpeepers become better
adapted at finding and using this rich food resource.
Food:

Seeds
Few seeds available

Insects
Many insects available

Flowers
Few nectar flowers available
Competition:
 The reduction in seed availability and the presence of birds that are better adapted
for insect-eating pushes short, blunt beaks from the population.
Predators:

Many predators threaten these pollenpeepers
As you will see, on the Warwick Archipelago, the founding population will diversify into
several distinct species in a manner consistent with the adaptive radiation history of the
Hawaiian honeycreepers and similar groups elsewhere in the world. The diversity of
habitat on the islands in this archipelago provides more opportunity for divergence than
does the less diverse mainland. Also, the distance from the mainland makes immigration
difficult, leaving dozens of niches available when the pollenpeepers arrive. Follow along
to explore what happens to the Warwick Archipelago pollenpeepers.
Warwick Archipelago: 5 Million Years Ago
Highlight:
 Nearly 150 pollenpeepers arrive on largest island.
Pollenpeeper changes:


As the hurricane loses its intensity, nearly 150 pollenpeepers are carried by the
storm to the largest island of the Warwick Archipelago, an island chain 600 miles
northeast of the mainland.
The group settles on the north side of the island, where the habitat is most
similar to the pollenpeepers' mainland home.
Habitat:


A very high ridge divides the large island the pollenpeepers now occupy.
The north side of the island has a lush, wet habitat on the coast; the island's south
side is desertlike, receiving less than half the rainfall of the north side.
Food:

Seeds
Many seeds available

Insects
Few insects available

Flowers
No nectar flowers available
Competition:

There is virtually no competition for the new immigrants when they arrive on the
island. The only other bird on the archipelago is a species of hummingbird that
lives in the high alpine forest of another island in the Warwick chain.
Predators:

Very few predators threaten these pollenpeepers
Warwick Archipelago: 4 Million Years Ago
Highlight:

The population grows quickly and pollenpeepers begin dispersing.
Pollenpeeper changes:



The population on the north side of the largest island grows gradually.
Dispersal from the large island to other islands in the chain is rare, though,
due to the low population density on the large island, the small size of other
islands in the chain, and the distance between the islands.
There is very little change in the appearance of pollenpeepers on the island
chain.
Habitat:


Initially, the pollenpeepers settle along the coast of each island -- the coastal
habitat is most like that of their original mainland home.
As individual populations grow, small groups expand their ranges, dispersing
slowly into other areas.
Food:

Seeds
Many seeds available

Insects
Some insects available

Flowers
Few nectar flowers available
Competition:

There is very little competition among pollenpeepers. When competition does
arise, some members of the population move to another area or to a slightly
different habitat.
Predators:

Very few predators threaten these pollenpeepers
Warwick Archipelago: 3 Million Years Ago
Highlight:

Isolation on high mountains causes rapid change in one population.
Pollenpeeper changes:

The first pollenpeeper group to become significantly isolated and
differentiated from the others is a population occupying the highest alpine
mountains of one of the middle islands in the chain.
Habitat:

The alpine mountain habitat of the newly isolated population is very high, cool,
and wet, with a short growing season. Flowers are plentiful seasonally, offering
pollen and nectar to insects and birds.
Food:

Seeds
Some seeds available

Insects
Some insects available

Flowers
Some nectar flowers available
Competition:

Competition among different populations of pollenpeepers remains low
throughout the archipelago.
Predators:

Very few predators threaten these pollenpeepers
Warwick Archipelago: 2 Million Years Ago
Highlight:

Populations diversify rapidly in response to new conditions.
Pollenpeeper changes:





Populations of pollenpeepers become established and relatively isolated in each of
the archipelago habitat types: coastal scrub, desert, lowland forest, and high alpine
forest.
The coastal pollenpeepers develop short, wide beaks, perfectly suited to eating
seeds. They remain fairly drab; males develop a muted reddish breast when they
reach maturity.
Desert pollenpeepers develop short, wide beaks similar to those of the coastal
birds. Their plumage, however, begins to distinguish them from the coastal
birds: Both males and females begin to show dramatic red, yellow, and black
plumage.
Lowland forest pollenpeepers develop beaks specialized for picking insects
out of tree bark. Their feet and tails become well adapted to this type of
foraging, too, allowing them to cling to the trunks of trees. Both males and
females develop very dark body plumage, accented by brilliant white wing
patches. Some males begin to show the beginnings of a red head crest, which
females highly prefer.
The beaks of alpine forest pollenpeepers become longer and narrower,
allowing the birds to probe inside flowers for insects and, increasingly, flower
nectar. The plumage of the males is becoming a brilliant iridescent green.
Habitat:

The island chain continues to offer an extremely diverse set of habitats, from low,
dry desert to high-elevation alpine forest. Each habitat type has its own best food
source for pollenpeepers.
Food:

Seeds
Many seeds available

Insects
Many insects available

Flowers
Many nectar flowers available
Competition:

Not long after the pollenpeepers arrived on the archipelago, there were countless
available habitats for groups to move into when competition in a preferred habitat
got too intense. Now, available niches are dwindling.
Predators:

Very few predators threaten these pollenpeepers
Warwick Archipelago: 1 Million Years Ago
Highlight:

Lowland forest population begins feeding on nectar.
Pollenpeeper changes:





Coastal pollenpeepers specialize in cracking the shells of seeds quickly and
efficiently using their wide, heavy beaks.
The beaks of desert pollenpeepers look very much like those of the coastal birds,
only slightly heavier and shorter. The desert birds, however, distinguish
themselves with their flashy yellow, red, and black plumage.
Lowland forest pollenpeepers develop a long, sharp, and slightly downcurved beak that allows them to most efficiently find and retrieve beetle larvae
and moths under the bark of trees. This beak also allows the birds to feed on
flower nectar where flowers are plentiful. Their plumage becomes darker, except
for the brilliant white wing patches and the males' red crest.
Alpine forest pollenpeepers do well with their long, sharp, sickle-shaped beak. It
is perfectly suited for obtaining nourishment from one of the island's most
plentiful flowers, a relative of the hibiscus that hides its nectar deep within
curved, tubular flowers. The brilliant iridescent green of the males is also highly
selected by females; over successive generations, it becomes more intense.
Some of the lowland forest pollenpeepers expand their range to higher elevations
as their preferred habitat creeps higher. They are different enough in color and
size from other pollenpeepers that as they encounter alpine forest
pollenpeepers, they do not interbreed with them. They do, however, compete
with them for food.
Habitat:

A climate change on one of the islands causes the boundary between two
populations' habitats to shift. Shrubs from the lowlands encroach on and outcompete some of the lowermost alpine forests, resulting in a replacement and
mixing of habitat types.
Food:

Seeds
Many seeds available

Insects
Many insects available

Flowers
Many nectar flowers available
Competition:

As lowland forest pollenpeepers expand into the habitat of the alpine
pollenpeepers, the two populations begin competing with one another. Lowland
forest birds now feed preferentially on nectar in this habitat.
Predators:

Very few predators threaten these pollenpeepers
Warwick Archipelago: Present
Highlight:

Six distinct populations occupy the archipelago.
Pollenpeeper changes:





There are now six distinct populations of pollenpeepers on the archipelago and
four clearly defined species. The four species have become specialized in the
way they find food and other resources. They are now genetically isolated from
one another, meaning that they do not interbreed successfully with any of the
other species. The distinct populations that are not yet distinct species continue to
diversify from one another and will likely soon be.
The pollenpeepers that most recently moved from the lowland forest into alpine
pollenpeeper habitat maintain a tenuous hold on survival given the competition
of the more established birds. In response to the competition, both the newest
arrivals and alpine species are diverging further from one another, eliminating
overlap in their behaviors and the resources they use.
Alpine birds now have extremely long down-curved beaks, which allow them
to obtain nectar from the deepest flowers -- an adaptation they did not have
before the birds from the lowland forest arrived.
The new arrivals from the lowland still feed on flower nectar they can reach with
their relatively short beaks. These flowers contain less nectar than the deeper
flowers, but they also attract pollinating insects, for which the lowland birds
have no competition.
A portion of the desert population occupying the east end of the archipelago
moves to a new island that has formed. They are best suited to the hot, dry,
sparsely vegetated environment they encounter there.
Habitat:

A new island has formed on the east end of the archipelago. For several hundred
years, the island sits quietly barren. Periodically seeds are carried ashore by
pollenpeepers and ocean currents, and plants begin to take hold across the island.
Food:

Seeds
Many seeds available

Insects
Many insects available

Flowers
Many nectar flowers available
Competition:

Competition has been reduced temporarily. Cohabitating populations like the
lowland and alpine pollenpeeper species have become more distinct from one
another, specializing to a point where the overlap between them is reduced.
Predators:

Very few predators threaten these pollenpeepers