Encyclopedia of Evolution.pdf - Online Reading Center

out in competition with the species that was already present
on the island.
4. On many islands, mammals are nearly absent. On the
Galápagos Islands, large turtles have taken the place in
the economy of nature that grazing mammals would have
filled; in New Zealand, it is gigantic wingless birds that
did this. The exception proves the rule: On many islands,
the only native mammals are bats! Amphibians are also
frequently absent from islands. In both cases, it is not
because the animals cannot survive on the islands—continental
mammals and amphibians thrive when humans
introduce them to islands—but because they cannot disperse
well over great distances. The theory of creation
gives us no reason why islands should be so deficient in
amphibians and in large mammals. Some naturalists say
that oceanic islands have not existed long enough for
mammals to have been created. Although some volcanic
islands are young, many are old, and yet they do not have
mammals either. [Darwin was responding to the idea of
progressive creationism, rather than the six-day creationism
espoused by many modern creationists, who could not
possibly have raised this point.]
5. On many islands, plants whose mainland relatives are herbaceous
grow into trees (as in the tree-sunflowers of St.
Helena). This has occurred because herbaceous plants are
frequently much better at dispersing over wide distances
than are trees; once they have dispersed to islands, however,
some of these herbaceous plants find it advantageous
to evolve into trees.
These patterns make no sense if the species of each island
were independently created.
Many facts of biogeography make sense when we consider
the effects of recent Ice Age glaciations, as revealed by
the research of Louis Agassiz (see Agassiz, Louis; ice ages):
1. Mountaintops within a continent, for example within
North America, often have the same alpine tundra species
upon them, even though these species cannot grow in
the intervening lowlands. They are also very similar to the
northern arctic tundra species of the same continent. How
could the same species travel among the arctic tundra and
the alpine tundras of these different mountaintops? They
got there because they retreated both northward and up
the mountains, where the alpine plants are now stranded,
as the weather became warmer and the glaciers retreated.
So far, this has little to do with evolution. But here is the
fact that can be explained only by evolution: Each continent
has a different set of alpine tundra species, each most
closely resembling the arctic tundra species of its own continent.
Mountains are [ecological] islands of cool climate
surrounded by warm lowland, just as geological islands
are surrounded by water.
2. The forest tree species are different on the different continents,
although related: For example, there are oak
forests in Europe, Asia, and North America, but each of
these places has its own species of oaks (see adaptive
appendix 431
radiation). How did oaks originally get to these three
continents? The arctic regions of the world form a nearly
continuous landmass, separated only by narrow channels
(e.g., on either side of Greenland, and the Bering Strait).
In earlier periods, when the weather was warmer, oaks
and other forest trees grew far north of where they are
now found and could have dispersed freely among the
three continents. After the glaciations began, however, the
oaks of the three continents have been separated and have
evolved into different species on each continent. [Scientists
would now add that the continents have also drifted
apart.]
3. Some species similarities among distant regions in the
Southern Hemisphere may be explained by dispersal
through Antarctica, which today is covered with ice but in
earlier ages was warmer and had forest species, the fossils
of which we have found.
In order to understand geographical distribution we
must understand dispersal—the ways that organisms get from
one place (as I believe, their point of origin) to new locations.
Among the factors that influence dispersal are the following:
1. Islands that are now separated by oceanic waters may
not always have been so in the past. When ocean levels
were lower, some regions that are now separate islands
(the islands separated by what are now shallow seas) were
mountains in a plain, allowing the organisms to freely
travel among them. However, other islands (the ones now
separated by deep oceans) have been isolated for long
periods. We would expect, by this theory, that the organisms
found on islands separated by shallow seas should
more closely resemble one another than the organisms
found on islands separated by deep oceans. This is in fact
the case. A very good example [and still the most famous]
is provided by Mr. Wallace from the Malay Archipelago:
The islands near Asia, separated from Asia and from one
another by shallow seas, have Asian mammals, all placental;
the islands near Australia, separated from Australia
and from one another by shallow seas (but from the Asian
islands by a deep trench) have Australian mammals, many
of them marsupials. Also, species that are found in widely
separated places may have once had a more continuous
range, and the individuals in the intervening areas have
become extinct.
2. Organisms may have astonishing powers of dispersal. My
own experiments have shown that many species of seeds
(provided their fruits are mature) can survive in, and
float upon, saltwater for weeks—which is long enough
to allow them to disperse to distant islands on ocean currents.
Other seeds, which would otherwise die in ocean
water, occasionally disperse to islands within clods of dirt
lodged in driftwood. Others can germinate after being carried
in, and expelled from, the crops or the intestines of
birds—which have frequently been known to fly to distant
islands. Waterfowl could easily carry seeds in the mud on
their feet, thus dispersing freshwater plant seeds between