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tions of the mite harvestmen on a map of ancient Earth, she found that they were
all close to each other in the Southern Hemisphere.
The study of how biodiversity is spread around the world is known as bio-
geography. Mite harvestmen illustrate one of the most common patterns in bio-
geography, called vicariance: species become separated from each other when
geographical barriers emerge. Those barriers can be formed by oceans, as in the
case of the mite harvestmen; they can also be separated by rising mountains,
spreading deserts, and shifting rivers. The other major pattern in biogeography,
known as dispersal, occurs when species themselves spread away from their
place of origin. Birds can fly from one island to another, for example, and insects
can float on driftwood.
The biogeography of many groups of species is the result of both dispersal
and vicariance. Most living species of marsupials can be found today on Aus-
tralia and its surrounding islands. But marsupials originally evolved thousands
of kilometers away (
Figure 10.4). The oldest fossils of marsupial-like mammals,
dating back 150 million years, come from China. At the time, Asia was linked to
North America, and by 120 million years ago marsupials had spread there as
well. Many new lineages of marsupials evolved in North America over the next
55 million years. From there, some of these marsupials spread to Europe, even
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Modern
World
Late Jurassic
152 Mya
Figure 10.3 One
lineage of mite har-
vestmen can be
found on continents
and islands separated
by thousands of miles
of ocean. They
reached their present
locations thanks to
continental drift.
Around 150 million
years ago, the ranges
of these invertebrates
formed a continuous
belt. Later, the conti-
nents broke apart
and moved away, tak-
ing the mite harvest-
men with them.
(Adapted from Boyer
et al., 2007)
North
America
Africa
Asia
Europe
South
America
Antarctica
Australia
Late Jurassic–Early Cretaceous
(150–120 million years ago)
Late Cretaeous–Paleogene
(70–55 million years ago)
Paleogene
(40–25 million years ago)
Pliocene
(3 million years ago)
Figure 10.4 The fossil record sheds light on the spread of marsupial mammals around the world.
reaching as far as North Africa and Central Asia. All of these northern hemi-
sphere marsupials eventually died out in a series of extinctions between 30 and
25 million years ago.
But marsupials did not die out entirely. Another group of North American
marsupials dispersed to South America around 70 million years ago. From there,
they expanded into Antarctica and Australia, both of which were attached to
South America at the time. Marsupials arrived in Australia no later than 55 mil-
lion years ago, the age of the oldest marsupial fossils found there. Later, South
America, Antarctica, and Australia began to drift apart, each carrying with it a
population of marsupials. The fossil record shows that marsupials were still in
Antarctica 40 million years ago. But as the continent moved nearer to the South
Pole and became cold, these animals became extinct.
In South America, marsupials diversified into a wide range of different forms,
including cat-like marsupial sabertooths. These large carnivorous species
became extinct, along with many other unique South American marsupials,
when the continent reconnected to North America a few million years ago.
However, there are still many different species of small and medium-sized mar-
supials living in South America today. One South American marsupial, the
familiar Virginia opossum, even recolonized North America.
Australia, meanwhile, drifted in isolation for over 40 million years. The fossil
record of Australia is too patchy for paleontologists to say whether there were any
placental mammals in Australia at this time. Abundant Australian fossils date
back to about 25 million years ago, at which point all the mammals in Austrlia
were marsupials. They evolved into a spectacular range of forms, including kan-
garoos and koalas. It was not until 15 million years ago that Australia moved close
enough to Asia to allow placental mammals—rats and bats—to begin to colonize
the continent. These invaders diversified into many ecological niches, but they
don’t seem to have displaced any of the marsupial species that were already there.
Isolated islands have also allowed dispersing species to evolve into remarkable
new forms. The ancestors of Darwin’s finches colonized the Galápagos Islands
two to three million years ago, after which they evolved into 14 species that live
nowhere else on Earth. On some other islands, birds have become flightless. On
the island of Mauritius in the Indian Ocean, for example, there once lived a big
flightless bird called the dodo. It became extinct in the 1600s, but Beth Shapiro, a
biologist now at the Pennsylvania State University, was able to extract some DNA
from a dodo bone in a museum collection. Its DNA revealed that the dodo had a
close kinship with species of pigeons native to southeast Asia. Only after the
ancestors of the dodos diverged from flying pigeons and ended up on the island of
Mauritius did they lose their wings and become huge land-dwellers. A similar
transformation took place on Hawaii, where geese from Canada settled and
became large and flightless.
Hawaiian geese and dodos may have lost the ability to fly for the same reason.
The islands where their flying ancestors arrived lacked large predators that
would have menaced them. Instead of investing energy in flight muscles that
they never needed to use, the birds that had the greatest reproductive success
216
were the ones that were better at getting energy from the food that was available
on their new island homes.
The Pace of Evolution
Biodiversity forms patterns not just across space, but also across time. New species
emerge, old ones become extinct; rates of diversification speed up and slow down.
These long-term patterns in evolution get their start in the generation-to-
generation processes of natural selection, genetic drift, and reproductive isolation.
When a lineage of organisms evolves over a few million years, these processes
can potentially produce a wide range of patterns (see
Figure 10.5). Natural selec-
tion may produce a significant change in a trait such as body size, for example. On
the other hand, the average size of a species may not change significantly at all (a
pattern known as stasis). Stabilizing selection can produce stasis by eliminating
the genotypes that give rise to very big or very small sizes. It’s also possible for a
species to experience a lot of small changes that don’t add up to any significant
trend. (This type of pattern is known as a random walk, because it resembles the
path of someone who randomly chooses where to take each new step.)
At the same time, a species can split in two. The rate at which old species in a
lineage produce new ones can be fast or slow (see
Figure 10.5c). Over millions
of years, one lineage may split into a large number of new species, while a related
217
Time
TimeTime
Size Size
Stasis
High rate of
diversification
Low rate of
diversification
An early burst of
diversification
Random
walk
Directional
selection
Punctuational
change
Time
A
Directional selection
plus speciation
Punctuated
equilibria
Diversification without
adaptive radiaition
Diversification with
adaptive radiaition
B
D
C
Size Size Size Size Size Size
Figure 10.5 Over long periods of time, evolution can form many patterns. A: A trait, such as size, may be
constrained by stabilizing selection, undergo small changes that don’t add up to a significant shift, experience
long-term selection in one direction, or experience a brief punctuation of change. B: A lineage may also
branch into new species while experiencing different kinds of morphological change. C: The rate at which new
species evolve is different in different lineages. It can also change in a single lineage. D: In an adaptive
radiation, a lineage evolves new species and also evolves to occupy a wide range of niches.
lineage hardly speciates at all. It’s also possible for a lineage’s rate of speciation to
slow down or speed up.
Even as new species are evolving, however, others may become extinct. The
rate at which species become extinct may be low in one lineage and high in
another. It’s also possible for the rate of extinction to rise, only to drop again later.
All of these processes can also unfold at the same time, and so the range of
possible long-term patterns in evolution can be enormous. A lineage with a low
rate of speciation may end up enormously diverse because its rate of extinction
is even lower. On the other hand, a lineage that produces new species at a rapid
rate may still have relatively few species if those species become extinct quickly.
Evolutionary change may happen mainly within the lifetime of species, or it may
occur in bursts when new species evolve. A lineage may produce many species
that are all very similar to each other, or evolve a wide range of forms.
Any one of these patterns is plausible, given what biologists know about how
evolution works. Which of these patterns actually dominate the history of life is
a question that they can investigate by studying both living and extinct species.
Evolutionary Fits and Starts
One of the most influential studies of the pace of evolutionary change was pub-
lished in 1971 by two young paleontologists at the American Museum of Natural
History named Niles Eldredge and Stephen Jay Gould. They pointed out that the
fossils of a typical species showed few signs of change during its lifetime. New
species branching off from old ones had small but distinctive differences.
Eldredge carefully documented this stasis in trilobites, an extinct lineage of
armored arthropods. He counted the rows of columns in the eyes of each sub-
species. He found that they did not change over six million years.
Eldredge and Gould proposed that this pattern was the result of stasis punc-
tuated by relatively fast evolutionary change, a combination they dubbed punc-
tuated equilibria. They argued that natural selection might adapt populations
within a species to their local conditions, but overall the species experienced
very little change in its lifetime. Most change occurred when a small population
became isolated and branched off as a new species. Eldredge and Gould argued
that paleontologists could not find fossils from these branchings for two rea-
sons: the populations were small, and they evolved into new species in just thou-
sands of years—a geological blink of an eye.
This provocative argument has inspired practically an entire generation of
paleontologists to test it with new evidence. But testing punctuated equilibria
has turned out to be a challenge in itself. It demands dense fossil records that
chronicle the rise of new species. Scientists have also had to develop sophisti-
cated statistical tests to determine whether a pattern of change recorded in
those fossils is explained best as stasis, a random walk, or directional change.
Scientists now have a number of cases in which evolution appears to unfold in
fits and starts.
Figure 10.6 (top) comes from a study by Jeremy Jackson and Alan
218
Cheetham of bryozoans, small animals that grow in crustlike colonies on sub-
merged rocks and reefs. On the other hand, more gradual, directional patterns
of change have also emerged.
Figure 10.6 also charts the evolution of a diatom
called Rhizosolenia that left a fairly dense fossil record over the past few million
years. One structure on the diatom gradually changed shape as an ancestral
species split in two.
219
0.0 1.0 2.0 3.0 4.0 5.0
Height of hyaline area
15
10
5
0
tenue
auriculatum
colligatum
kugleri
chipolanum
micropora
lacrymosum
unguiculatum
n. sp. 10
n. sp. 9
n. sp. 5
n. sp. 6
n. sp. 7
n. sp. 3
Metrarabdotos
Rhizosolenia
n. sp. 4
n. sp. 2
n. sp. 1
n. sp. 8
20
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
Millions of years ago
Millions of years ago
Hyaline
area
Figure 10.6 Paleontologists have documented cases of punctuated change and gradual change in the fossil
record. Top: A lineage of bryozoans (Metrarabdotos) evolved rapidly into new species, but changed little once
those species were established. Bottom: A shell-building organism called Rhizosolenia changed over the course
of millions of years. This graph charts the size of a structure called the hyaline area. (Adapted from Benton, 2003)
220
At this point, paleontologists have found few well-documented cases that
match the original model of punctuated equilibra, with rapid change happening
only during speciation. But Eldredge and Gould’s ideas have led to some signifi-
cant changes in how paleontologists look at the fossil record. For example, Gene
Hunt, a paleontologist at the Smithsonian Institution, recently developed a
method for statistically analyzing patterns of change and used it to study 53 evo-
lutionary lineages ranging from mollusks to fishes and primates. In 2007, Hunt
concluded that only 5% of the fossil sequences showed signs of directional change.
The other 95% was about evenly split between random walks and stasis. Hunt did
not look for evidence of directional change during speciation, so he could not di-
rectly address the original model of punctuated equilibria. But Hunt’s 2007 study
does support the idea that stasis is a major feature of the history of life.
The Lifetime of a Species
Paleontologists estimate that 99% of all species that ever existed have vanished
from the planet. To understand the process of extinction, paleontologists have
measured the lifetime of species—especially species that leave lots of fossils
behind. Mollusks (a group of invertebrates that includes snails and clams) leave
some of the most complete fossil records of any animal.
Michael Foote, an evolutionary biologist at the University of Chicago, and his col-
leagues inventoried fossils of mollusks that lived in the ocean around New Zealand
over the past 43 million years. They cataloged every individual fossil from each
species, noting where and when it lived. Foote and his colleagues found that a typ-
ical mollusk species expanded its range over the course of a few million years and
then dwindled away.
Figure 10.7 shows a selection of the species they cataloged.
Some species lasted only 3 million years, while others lasted 25 million years.
Left: The dodo became
extinct in the late 1600s,
probably due to hunting and
rats that ate their eggs.
Right: The Carolina parakeet
became extinct in the early
1900s, due in part to log-
ging, which removed the
hollow logs in which it built
its nests.
11 11 11 16 6 7 1 22 15 22 6 13 3 326113 5 3 3
28 19 19 43 27 22 6 26 15 163521 51919 35 6 6 12
28 19 11 13 5 35 15 19 12 616 19 12 62219 43 25 12 2
19 19 37 22 15 6 1 28 15 16113 31310 16 6 12 27
11 35 11 19 6 22 6 19 10 326 43 34 37335 21 15 3
26 19 13 26 5 28 6 43 21 1535 26 15 121915 11 3 12 5
36 16 33 19 12 37 15 37 25 1526 43 34 31125 16 5 1 3
11 35 11 19 12 22 15 32 6 513 19 10 6191 16 6 19 3
35 26 19 4 1 4 1 7 1 311 19 12 61921 19 10 15 10
74316283363173319 11 3 61636 1 27 6
13 7 26 11 3 11 2 7 3 2537 37 27 16319 10 3 15
26 11 19 7 3 26 15 22 15 1526 7 3 1715 19 5 3 6
37 43 28 26 15 16 6 22 15 26113 62227 26 10 34 15
11 26 26 7 2 11 3 19 3 516 19 10 619319 10 15 3
Earliest fossils of
a species are 16
million years old
Last known
fossils are 1
million years old
Size of
species
range
16 1
Figure 10.7 These graphs chart the rise and fall of mollusk species over the past 43 mil-
lion years around New Zealand. The left number on each graph is the age of the earliest
fossil in a species (in millions of years), and the right number is the age of the youngest fos-
sil. The height of each graph represents the range over which fossils at each interval have
been found. As these graphs demonstrate, some species survive longer than others, but in
general they endured for a few million years. (Adapted from Foote, 2008)
To understand how species became extinct millions of years ago, biologists
can get clues from extinctions that have taken place over the past few centuries.
When Dutch explorers arrived on Mauritius in the 1600s, for example, they killed
dodos for food or sport. They also inadvertently introduced the first rats to Mau-
ritius, which then proceeded to eat the eggs of the dodos. As adult and young
dodos alike were killed, the population shrank until only a single dodo was left.
When it died, the species was gone forever.
Simply killing off individuals is not the only way to drive a species towards
extinction. Habitat loss—the destruction of a particular kind of environment
where a species can thrive—can also put a species at risk. The Carolina parakeet
once lived in huge numbers in the southeastern United States. Loggers probably
hastened its demise in the early 1900s by cutting down the old-growth forests
where the parakeets made their nests in hollow logs.
Habitat loss can turn a species into a few isolated populations. Their isolation
makes the species even more vulnerable to extinction. In small populations,
genetic drift can spread harmful mutations and slow down the spread of benefi-
cial ones. If the animals in an isolated population are wiped out by a hurricane,
their numbers cannot be replenished by immigrants. As isolated populations
wink out, one by one, the species as a whole faces the threat of extinction.
Cradles of Diversity
Understanding the long-term patterns of speciation and extinction may help sci-
entists answer some of the biggest questions about today’s patterns of bio diver-
sity—such as why the tropics are so diverse. David Jablonski, a paleontologist at
the University of Chicago, has tackled the question by analyzing the fossil record
of bivalves, noting where they were located, how large their ranges became, and
how long they endured.
Jablonski’s analysis of 3,599 species from the past 11 million years revealed a
striking pattern. Twice as many new genera of bivalves had emerged in the tropi-
cal oceans than had emerged in cooler waters. Jablonski found that once new
bivalve genera evolved in the tropics, they expanded towards the poles. In time,
however, the bivalves near the poles became extinct while their cousins near the
equator survived. From these results, Jablonski argued that the tropics are both a
cradle and a museum. New species can evolve rapidly in the tropics, and they
can accumulate to greater numbers because the extinction rate is lower there as
well. Together these factors lead to the high biodiversity of the tropics.
A similar pattern emerged when Bradford Hawkins, a biologist at the Univer-
sity of California, Irvine, studied the evolution of 7,520 species of birds. The
birds that live closer to the poles belong to younger lineages than the ones that
live in the tropics.
It’s possible that the tropics have low extinction rates because they offer a
more stable climate than regions closer to the poles. Ice ages, advancing and
retreating glaciers, swings between wet and dry climates—all of these may have
222
raised the risk of extinction in the cooler regions of the Earth. The changes that
occurred in the tropics were gentler, which made it easier for species to survive.
But the tropics also foster a higher rate of emergence of new species. Why the
tropics can sustain more species than other regions is not clear, however; it’s
possible that the extra energy the tropics receive somehow creates extra ecologi-
cal room for more species to live side by side.
Radiations
When biologists examine the history of a particular lineage, they discover a mix
of diversification and extinctions.
Figure 10.8 shows the history of one such
line age, that of a group of mammal species called mountain beavers. About 30
species of mountain beavers have evolved over the past 35 million years in the
223
30 25 20 15 10 5 0
Millions of years ago
Pterogaulus laevis
Pterogaulus barbarellae
Pterogaulus cambridgensis
Ceratogaulus rhinocerus
Ceratogaulus anectdotus
Ceratogaulus minor
Ceratogaulus hatcheri
Hesperogaulus wilsoni
Hesperogaulus gazini
Umbogaulus galushai
Umbogaulus monodon
Mylagaulus sesquipedalis
Alphagaulus douglassi
Mylagaulus kinseyi
Mylagaulus elassos
Alphagaulus tedfordi
Alphagaulus vetus
Alphagaulus pristinus
Galbreathia bettae
Galbreathia novellus
Mesogaulus ballensis
Mesogaulus paniensis
Mylagaulodon angulatus
Trilaccogaulus lemhiensis
Trilaccogaulus montanensis
Trilaccogaulus ovatus
Promylagaulus riggsi
Aplodontia rufa
Meniscomys hippodus
Allomys nitens
Deep River Alphagaulus
Figure 10.8 Over the past 35 million years, some 30 species of mountain beavers have
existed in western North America. A burst of new lineages evolved around 15 million years
ago. (Adapted from Barnovsky, 2008)
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