Encyclopedia of Evolution.pdf - Online Reading Center

another, and this generally cannot occur in disjunct species.
If the populations of a disjunct species are genetically different,
the populations can often be described as subspecies. If
enough time passes, it is practically inevitable that the disjunct
populations will evolve into separate species, unless
they become extinct (see speciation).
Two ways are generally recognized in which populations
within a species could have become disjunct. The first
is that the populations are relictual, that is, they are relicts of
a common, ancestral population. Populations of the species
that were formerly present between the disjunct populations
have disappeared, leaving the relictual populations isolated
from one another. If, before or after the disappearance of the
intervening populations, the two relictual populations diverge
(perhaps becoming distinct subspecies), they are considered
the product of vicariance (see biogeography). The other
way is that one of the populations dispersed (as seeds or as
a small flock) from one population and settled down in a distant
location, an explanation called dispersal.
How can scientists determine which explanation, the
relictual versus the dispersal explanation, is correct? The best
evidence is if fossil or other evidence of the formerly intervening
populations can actually be discovered. In some cases,
evidence for populations of that particular species cannot be
found, but if it is found for other species with similar disjunct
distributions, then it can be inferred for the species in question
as well. For example, there are many species of wildflowers in
the alpine tundra of the high elevations of the Sierra Nevada,
Cascade, Olympic, Rocky, and Sangre de Cristo Mountains
of western North America that are very similar, in some cases
nearly identical, to those of the arctic tundra of northern
Alaska and Canada. Hundreds or thousands of miles separate
the disjunct arctic and alpine populations. These distributions
were, in fact, somewhat difficult to explain prior to Agassiz
and Darwin (see Agassiz, Louis; Darwin, Charles). Agassiz
explained the ice ages: At various times, much of North
America was, in fact, in the arctic tundra zone. As the glaciers
retreated, some tundra plants migrated north, where they are
now found as arctic tundra; others migrated up the mountains,
where they are now stranded as alpine tundra. Darwin
explained the subsequent vicariance of these populations by
evolution occurring separately in each. Researchers have not
found fossil evidence to confirm this for each of the wildflower
species. The evidence for the glaciation and the overall
patterns of vegetation change is overwhelming, and scientists
can infer that it is true for each of these disjunct species. This
is also the explanation of how disjunct populations of tundra
species such as the sedge Eriophorum vaginatum, and boreal
species such as the larch tree, came to be stranded in the bogs
of northern states in the United States such as Michigan.
The next best evidence comes from genetic comparisons.
If the disjunct populations can be shown to have several distinct
mutations within their DNA, mutations not shared
with one another, then they may be relictual (see DNA [evidence
for evolution]). If one of the populations has DNA
that is a subset of another population, then the first population
clearly dispersed from the second. The only problem
with this approach is that even if one population dispersed
disjunct species
from another, the new population can evolve its own distinct
mutations, which would make it appear as relictual. Genetic
studies are most useful to detect recently dispersed disjunct
populations.
Numerous examples of disjunct populations have been
found. One example is the seaside alder, Alnus maritima.
Many alders are large bushes that grow in swamps or next
to streams and rivers with rocky beds. The hazel alder, Alnus
serrulata, grows in this kind of streamside habitat throughout
much of the eastern United States. In contrast, the seaside alder
is a disjunct species, consisting entirely of three small disjunct
populations. One population lives on the Delmarva Peninsula
east of Chesapeake Bay. Another lives almost entirely within
Johnston County in central Oklahoma. The third lives in a single
swamp in Bartow County in northwest Georgia. Why is the
seaside alder so rare, despite its apparent similarity to the hazel
alder, which grows throughout eastern North America?
First, consider the vicariance explanation. Alders reproduce
in two ways. Their seeds, produced in the autumn,
grow best in sunny locations with moist mineral soil. Along
many rivers, locations that have moist mineral soil are often
shaded and not good for the growth of alder seedlings. In a
second reproductive mode, once a bush is established, it can
keep resprouting from the same roots even if floods wash
many of its branches away, or if it is subsequently shaded by
the growth of larger trees. Botanists Stanley Rice and J. Phil
Gibson have demonstrated that both species of alders prefer
sunny over shady conditions, but the hazel alders are found
in the shade more frequently than seaside alders.
In the past (perhaps after the most recent Ice Age), both
species of alders may have established themselves abundantly
in the new landscape of sunny, moist, mineral soil. Subsequent
regrowth of forests shaded the alder habitats, and the
hazel alders have tolerated these shaded conditions at least a
little better than the seaside alders. Hazel alders have therefore
persisted throughout eastern North America, whereas the
seaside alders have persisted in only three places. The seaside
alder survived in these three places by chance, not because the
three places were the most suitable habitat. The climatic conditions
in Oklahoma, Georgia, and Delaware are quite different,
and without doubt there are many places between these three
locations that are better for seaside alder growth but in which
the seaside alder by chance became extinct. Botanists James
Schrader and William Graves have found that separate mutations
have occurred in the three populations, enough to cause
noticeable differences in leaf shape. These three populations are
now classified as three distinct subspecies. There is no direct
evidence that earlier populations of the seaside alder lived in
places between Oklahoma, Georgia, and Delaware. Fossilized
leaves that are identical to modern seaside alder leaves have
been found in northwestern North America, which suggests
that this species once grew across the entire continent. The species
of alder that are most closely related to the seaside alder,
in fact, grow in eastern Asia. This is the vicariance explanation
for the disjunct distribution of the seaside alder.
Now consider the dispersal explanation. The seaside alder
might have originated in one location, perhaps Delmarva, and
subsequently dispersed to the other locations. How could this