Encyclopedia of Evolution.pdf - Online Reading Center

ern continents, and they remained largely separate from one
another. By about a million years ago, the isthmus of Panama
formed a land bridge over which placental mammals dispersed
from the Nearctic realm into South America, and marsupial
mammals in the opposite direction. Plants of temperate
climates, not adapted to tropical conditions and less mobile
than animals, did not disperse across this land bridge. As it
turned out, placental mammals frequently drove marsupial
mammals toward extinction by competition. The placental
mammals of South America, now dominant, are the descendants
of invaders from North America. Most of the marsupials
that moved north into North America became extinct,
except for the opossum. Meanwhile, Australia and New
Guinea remained separate and the realm of marsupials. Few
placental mammals made the journey into the Australasian
realm. One exception was humans. Homo erectus ventured
into the islands of Indonesia, becoming Java man, but there
is no clear evidence that they traveled to Australia. Some of
them became isolated on the island of Flores (see Flores
Island people). Modern humans dispersed to Australia at
least 50,000 years ago and apparently brought dogs with
them. For thousands of years, humans and dogs were the
main placental mammals of Australia. Australian marsupials
included many species that were similar to placental species,
but which had evolved separately (see convergence). Europeans
brought placental mammals, such as rodents and livestock,
with them. Once again the placental mammals proved
to be the superior competitors. Most of the Australian marsupials,
except various species of kangaroo, have become rare
or extinct, largely as a result of competition with the placental
invaders.
When a continental fragment separates, it may sometimes
retain a collection of species that closely resemble
their ancestors, because they are not exposed to many of the
newly evolved continental species. This appears to be what
happened with the island of New Caledonia. The forests
consist largely of conifers more similar to those that dominated
Mesozoic forests than those that dominate modern forests.
This is why film crews who want a realistic backdrop
to dinosaur movies choose New Caledonia for their film site.
The species of flowering plant that most closely resembles the
presumed ancestor of all flowering plants, Amborella trichopoda,
is found only on New Caledonia (see angiosperms,
evolution of).
Some distribution patterns remain mysterious. One
example is the plant family Empetraceae (crowberries), which
is found in the cool regions of North America and Eurasia,
but also the cool tip of South America, and in the warm climate
of Florida. This is difficult to explain in terms of continental
movements or dispersal to another habitat with similar
climatic conditions.
Islands
Islands, starting with Darwin’s experience in the Galápagos
Islands, have proven to be evolutionary showcases. When a
new island forms, it receives only a genetic subset of organisms
from the nearby mainland. In particular:
biogeography
• In any species some of whose members disperse to a new
island, only a small portion of the mainland genetic variability
may be represented on the island. This founder
effect may be followed by genetic drift if the island population
remains small.
• The newly arrived species may have fewer parasites, and
perhaps no predators or competitors. This may, if the species
happens to be well suited to the physical environment
of the island, allow a population explosion, leading to evolutionary
change and diversification.
• Many small mammals remain small partly because their
small size allows rapid population growth, giving them an
advantage in competition; but on the island, with fewer or
no competitors, the small mammals may evolve into larger
forms. The same thing appears to happen in plants. Many
small, weedy plants have an advantage due to rapid growth
and early reproduction. In this way, they avoid competition
with large trees. When the plants disperse to an island
and there are no trees, the weedy plants may evolve into
a tree form. The plant family Asteraceae consists mostly
of weedy herbaceous plants, except on some islands (such
as the Channel Islands of California and St. Helena in the
Atlantic) where they have evolved into small trees. The nettle
family Urticaceae consists mostly of weedy herbaceous
plants, but on Hawaii there is a bush nettle.
• Many large mammals remain large partly because their size
allows them to resist the attacks of predators; but on the
island, with no predators, the large mammals may evolve
into smaller forms. Mammoths on Wrangel Island, northeast
of Siberia, and on Santa Rosa Island off the coast of
California evolved into smaller forms. Another example
is the Flores Island people, Homo floresiensis. Some large
mammals evolved into smaller forms on this island at the
same time that the Flores Island folk lived there; the predator
release explanation, however, is unlikely to be true for
the small size of these people. The explanation of the Flores
Island people remains a mystery.
• Many island species lose their defenses. Since defenses
(spines or chemicals in plants, behaviors in animals) can
be expensive, natural selection favors their loss in conditions
where they are not useful. Many island plants have
lost their defenses. For example, not only is the Hawaiian
nettle unusual in its family for being a bush but also for
its lack of stinging hairs. Numerous island animal species
(reptiles, birds, and mammals) are notorious for their lack
of fear of humans, with whom they have had little contact
until recently.
• In order to get to the island in the first place, plants and
animals needed to have superior dispersal abilities. Once
on the island many of them have lost their dispersal abilities.
One extreme example is the flightless cormorant of the
Galápagos Islands. On some islands, plant families with
small seeds have produced species with seeds larger than is
normal for the family.
Consider new islands, such as the volcanic islands of
Hawaii, as opposed to islands like New Caledonia that are
fragments of continents. The new islands accumulate all of