Encyclopedia of Evolution.pdf - Online Reading Center

0 island biogeography
Protostomes v. deuterostomes. The bilaterally symmetrical
animals diverged into two lineages that differ primarily in
the manner in which their embryos develop. In protostomes,
the early embryo cell divisions occur in a spiral fashion,
whereas in deuterostomes they occur in a radial fashion. In
protostomes, the developmental fate of each cell is determined
from the beginning. In deuterostomes, the fate of the cells is
not determined until about the eight-cell stage. Therefore, in
deuterostomes, one fertilized egg cell can become up to eight
identical offspring. Deuterostomes, unlike protostomes, can
have identical twins, triplets, etc.
Protostomes: Ecdysozoans v. others. On the basis of
DNA evidence, the protostomes are divided into two groups.
The ecdysozoans secrete an external skeleton. As they grow,
they split open the skeleton and squirm out of it, secreting
a new one. This process, called ecdysis, characterizes arthropods
and nematodes.
• Arthropods (Phylum Arthropoda) have hard external skeletons
and jointed appendages. Appendages are legs or other
leglike structures (such as the antennae of insects). Arthropods
were among the earliest phyla of animals to have a
burst of evolutionary diversification, in the Cambrian period,
including the trilobites, which evolved into numerous
species, until becoming extinct at the end of the Permian
period (see Permian extinction). Today, the major
arthropod groups are the crustaceans (including crabs, lobsters,
shrimps, and terrestrial pillbugs); the centipedes and
millipedes; the chelicerates (including spiders, scorpions,
mites, and ticks); and the insects. In terms of sheer weight
and number, as well as species diversity, arthropods rule the
animal world (see biodiversity).
• Nematodes (Phylum Nematoda) are the roundworms. They
are incredibly abundant inhabitants of the soil, especially near
plant roots and in wetlands. Experts say that if everything in
the world except nematodes vanished, one would still be able
to see the Earth as a shimmering sphere of nematodes.
Among the other protostomes that are not ecdysozoans are:
• Flatworms (Phylum Platyhelminthes) include aquatic and
parasitic forms. The parasitic forms may have extremely
complex life cycles.
• Mollusks and brachiopods. Mollusks (Phylum Mollusca)
have a mantle and guts over a single foot. Bivalve mollusks
(e.g., clams) secrete two hard shells hinged together. They
filter the water in which they live to obtain food particles.
Cephalopod mollusks have a head (cephalo-) over a foot (pod),
with a beaked mouth in the middle of the foot, as in
octopus and squid. Gastropod (“stomach-foot”) mollusks
include snails and slugs. Despite their superficial similarity
to bivalves, the brachiopod “lampshells” are a distinct phylum,
because unlike bivalves they have a specialized feeding
apparatus called a lophophore. The present-day abundance
of bivalves and relative scarcity of brachiopods may be a
leftover effect of the Permian extinction, which nearly
destroyed the brachiopods.
• Annelids (Phylum Annelida) are the segmented worms,
such as many of the marine worms, the earthworms, and
the leeches. Because annelids and insects both have segments,
they were formerly thought to share a relatively
recent common ancestor, but DNA analysis has forced a
reevaluation of this idea.
Deuterostomes. The two phyla of deuterostomes are the
echinoderms and the chordates.
• Adult echinoderms (Phylum Echinodermata, which means
“spiny skin”) have spiny or knobby outer layers and move
along the rocks of the seashore by means of water-powered
tube-feet. They include starfish and sea urchins. The larvae
of echinoderms are bilaterally symmetrical, even though
the adults look superficially radial.
• The chordates have a rod, or notochord, along the back,
which strengthens the body and the main nerve cord. Invertebrate
chordates include the lancelet, which looks superficially
like a headless fish. The vertebrates evolved from an ancestor
that resembled a lancelet (see fishes, evolution of).
Almost all of the animal phyla are represented in the
Cambrian period in at least a rudimentary form. It is not
clear how many of them were present before the Cambrian
explosion. Some of the Ediacaran organisms from the
period just before the Cambrian resemble jellyfish, while others
resemble colonial cnidarians. All identifications of the
Ediacarans, however, are controversial. Fossils of animal
embryos are known from the Ediacaran period. DNA studies
suggest that some of the earliest evolutionary divergences
of animals occurred from 700 million to 1 billion years ago,
although the fossil evidence for these divergences is scarce.
Most invertebrate phyla are completely aquatic. However,
like the vertebrate lineage within the chordate phylum,
several arthropod lineages have diversified into all the habitats
of dry land.
Further Reading
Bottjer, David J. “The early evolution of animals.” Scientific American,
August 2005, 42–47.
Knoll, Andrew H., and Sean B. Carroll. “Early animal evolution:
Emerging views from comparative biology and geology. Science
284 (1999): 2,129–2,137.
Museum of Paleontology, University of California, Berkeley. “Introduction
to the Placozoa: The simplest of all known animals.” Available
online. URL: http://www.ucmp.berkeley.edu/phyla/placozoa/placozoa.
html. Accessed April 18, 2005.
island biogeography See biogeography.
isolating mechanisms Isolating mechanisms are biological
processes that cause reproductive isolation, in which populations
cannot successfully interbreed if they are in contact.
If two populations could interbreed if they were in contact
but do not do so because they live in different locations, the
populations are geographically isolated. Geographic isolation
is not an isolating mechanism. Isolating mechanisms keep
populations from interbreeding even if they live in the same
place. Evolutionary biologists consider two populations that