Encyclopedia of Evolution.pdf - Online Reading Center

The flowering plants (angiosperms) are a monophyletic group that
consists of all of the species descended from a common ancestor; within
the angiosperms, the monocots are a monophyletic group.
• “Monkey” is a paraphyletic category. The new world monkeys
diverged from the old world lineage that includes old
world monkeys and all of the apes (see primates). To a
cladist, either a human is a monkey or else there is no such
thing as a monkey.
• “Reptile” is a paraphyletic category. Terrestrial vertebrates
are traditionally classified into reptiles, birds, and
mammals. Reptiles are cold-blooded and have scales;
birds are warm-blooded and have feathers; mammals are
warm-blooded and have hair and milk. This sounds like
a straightforward classification based upon characteristics:
Classes Reptilia, Aves, and Mammalia. Cladists point
out that several lineages of reptiles branched off from one
another, producing separate lineages such as turtles, lizards,
tuataras, more than one branch of dinosaurs, and
therapsids. Mammals evolved from therapsids, and birds
from dinosaurs. Reptiles, birds, and mammals are not separate
clades, or branches. “Reptile” is therefore a paraphyletic
group that contains some, but not all, of the species
descended from the group’s common ancestor. Strict cladists
insist that either the term reptile should not be used at
all, or else the term reptile should include birds and mammals.
To a cladist, the term dinosaur includes birds, and
birds are dinosaurs. One can always tell a cladist because
he or she will refer to the big lumbering beasts of the
Mesozoic era as “nonavian dinosaurs” rather than simply
as dinosaurs. To a cladist, either a human is a reptile,
or else there is no such thing as a reptile.
• “Fish” is a paraphyletic category. Vertebrates include fishes,
amphibians, reptiles, mammals, and birds (see fishes,
evolution of; amphibians, evolution of). The reptile
clade (which includes birds and mammals) and the amphibian
clade share a common ancestor. But the amphibian-reptile
clade branched off from the lobe-finned fishes, which is
just one branch of several branches that are usually called
“fishes”; the others are the ray-finned fishes, cartilaginous
fishes, hagfishes, and lampreys. “Fishes” is therefore also a
paraphyletic group. Cladistically speaking, then, there is no
such thing as a fish, unless by “fish” you include amphibians
and reptiles (which includes birds and mammals). To
cladistics
a cladist, either a human is a fish, or else there is no such
thing as a fish.
Some cladists, in fact, insist that scientists should not
even bother with the Linnaean system. Instead of kingdoms,
phyla, classes, orders, families, and genera scientists should
refer only to clades. This approach is called the PhyloCode.
Most people, from common experience and also from the
biology classes they took in school, tend to think of the oldfashioned
classifications such as fish, amphibian, reptile, bird,
and mammal. The new classifications are unfamiliar and confusing
to most laypeople. What many science teachers do is
accept the evolutionary insights of cladistic analysis but do
not attempt cladistic consistency in terminology.
Cladistics confirms evolution
The grouping patterns produced by one cladistic analysis
often closely resemble the grouping patterns produced by
another. For example, a cladistic analysis using visible characteristics
usually comes out almost the same as a cladistic
analysis using DNA. This is highly unlikely to happen by
chance and almost certainly means that the resulting classification
represents what really happened in the evolutionary
history of the organisms.
Moreover, the grouping patterns produced by cladistic
analyses usually bear a close resemblance to the grouping patterns
that had been proposed previous to the development of
cladistics. One example is within the walnuts (genus Juglans).
For at least a century, botanists have recognized four major
groups of trees in this genus, based upon leaf and fruit characteristics,
as well as upon the geography of where the trees grew:
• the black walnuts of North and South America, section
Rhysocaryon (16 species);
• the Asian butternuts, section Cardiocaryon (3 species);
• the butternut of eastern North America, section Trachycaryon
(1 species);
• the Eurasian walnut, section Dioscaryon (1 species).
Botanist Alice Stanford and colleagues performed cladistic
analyses upon the walnuts, using three different regions
of DNA: the matK gene, an ITS (internal transcribed spacer,
a type of noncoding DNA), and RFLP (restriction fragment
length polymorphism DNA). They did not base their results
on just one region of DNA but on a consensus of three
regions. The result was the same four groupings of walnut
species. Cladistics, therefore, often confirms that the evolutionary
classifications that had been previously produced
were not the mere product of evolutionists’ imaginations.
Computers, which have no imagination, produce the same
groupings.
This result is similar to what happened when Charles
Darwin published Origin of Species (see origin of species
[book]). Before Darwin’s theory, in fact before the acceptance
of any form of evolution, scientists had been classifying organisms
by the Linnaean system, based on what seemed to them
to be the most “natural” groupings. Darwin said that these
“natural” groupings were, in fact, the product of common