Encyclopedia of Evolution.pdf - Online Reading Center

Ardipithecus
Archaeopteryx is an almost perfect intermediate between
modern birds and coelurosaurian dinosaurs, with a mixture of
features characteristic of both. Its modern bird features include:
• Feathers
• Large brain
• Large eyes
• Clavicles fused into a furcula (wishbone)
Its reptilian features include:
• Long tail of vertebrae. Tails of modern birds are reduced to
a small pygostyle from which tail feathers radiate.
• Digits in forearm. Wings of modern birds are not reinforced
by long fingers.
• Conical reptilian teeth. Modern birds are toothless; the
beaks of modern birds are hardened lips.
• Small sternum (breastbone). Modern birds have large
keeled breastbones that allow attachment of flight muscles.
Archaeopteryx seems like something of a patchwork,
since the features listed above are mostly either bird or reptilian,
rather than intermediate. It is the organism as a whole that
is intermediate between birds and reptiles. This is an example
of mosaic evolution, in which different parts of an organism
evolve at different rates. In particular, the feathers appear
fully modern. A considerable amount of bird evolution must
have preceded Archaeopteryx. Paleontologists have found
other feathered dinosaurs, including a tyrannosaur discovered
in 2004 that had been covered with feathers that appear to
have a more primitive structure (see birds, evolution of).
Because birds with much more modern features existed just a
few million years later than Archaeopteryx, it is unlikely that
Archaeopteryx was the actual ancestor of modern birds. It is
more likely to have been a side branch of bird evolution. It
clearly indicates the evolutionary transition that was occurring
in several related branches of reptiles at that time. There were
feathered dinosaurs running around all over the place during
the Jurassic period; Archaeopteryx is simply the first to be
discovered, the most famous, and still the oldest.
Archaeopteryx had fully modern flight feathers but a
small sternum. Scientists conclude that Archaeopteryx could
fly but was not a strong flier. This was part of the reason that
evolutionary scientists debated about whether bird flight had
begun from the ground up (small motile dinosaurs running
along the ground and eventually launching into the air) or
from the treetops down (small dinosaurs gliding from treetops).
The fact that the dinosaur group that contained feathered
species was known for rapid pursuit of prey on the
ground (as in Velociraptor) suggests the former possibility,
while the weak flight of Archaeopteryx suggests the latter.
Further Reading
Mayr, Gerald, Burkhard Pohl, and D. Stefan Peters. “A well-preserved
Archaeopteryx specimen with theropod features.” Science
310 (2005): 1,483–1,486.
Nedin, Chris. “All about Archaeopteryx.” Available online. URL:
http://www.talkorigins.org/faqs/archaeopteryx/info.html. Accessed
July 11, 2005.
Shipman, Pat. Taking Wing: Archaeopteryx and the Evolution of
Bird Flight. New York: Simon and Schuster, 1998.
Ardipithecus See australopithecines.
arthropods See invertebrates, evolution of.
artificial selection Artificial selection refers to genetic
changes in populations that result from human choice. Artificial
selection resembles natural selection in these ways:
• It is a process that occurs in populations.
• The population must have genetic variability (see population
genetics) from which selection, natural or artificial,
can choose.
• As a result of their genetic characteristics, some individuals
in the population reproduce more than other individuals.
• As a result of differential reproduction of individuals, the
characteristics of the population change.
In natural selection, the individuals that reproduce the
best are those that are best suited to the natural conditions of
their habitat, and the social conditions of their populations.
In artificial selection, success depends entirely on human will.
Charles Darwin (see Darwin, Charles) used artificial
selection as a model for natural selection. Some writers have
claimed that artificial selection was just a metaphor for natural
selection, but it was more. Natural populations contain
genetic variability, on which either natural or artificial selection
can act. It was artificial selection that allowed Darwin to
demonstrate that this variability did, in fact, exist. His most
striking example was the domestic pigeon. Different breeds
of domestic pigeon had different outlandish characteristics:
Some had inflatable throats; some had ornate feathers; others
could tumble in the air while flying. Darwin cited the historical
fact that all of these pigeons had been bred, within recent
centuries, from wild rock doves (Columba livia). Crossing
any two of the domesticated breeds of pigeons produced offspring
that resembled the wild rock dove. Artificial selection
proved what extreme changes were possible, starting with
just the genetic variability that exists in wild populations.
Artificial selection can have practical value or appear
almost whimsical. Artificial selection was entirely responsible
for producing the many species of crop plants and livestock
animals from wild ancestors (see agriculture, evolution
of). Silviculturists have bred trees that rapidly produce
strong wood, and horticulturists have bred garden flowers,
bushes, and fruit and ornamental trees that are very useful
in the human landscape. Some crop and garden plants have
been so altered by artificial selection that they cannot survive
in the wild. Garden tomatoes, for example, cannot even
hold their heavy fruits off of the ground, where they rot if
left unattended. Seedless fruits, from oranges to watermelons,
can reproduce only through cuttings, which would seldom
occur without human intervention. Some livestock animals,
and many pets, have been bred into outlandish forms. Tiny
hairless dogs and cats are an extreme example. A few centuries
ago, breeders produced sheep whose tails were so fat
that they had to drag a little cart behind them as they walked.