Encyclopedia of Evolution.pdf - Online Reading Center

Williams, George C. “Pleiotropy, natural selection and the evolution
of senescence.” Evolution 11 (1957): 398–411.
———. Sex and Evolution. Princeton, N.J.: Princeton University
Press, 1975.
Linnaean system The Linnaean system is used by biologists
to classify species of organisms into nested categories.
The system was invented by the Swedish botanist Karl Linné
(see Linnaeus, Carolus) in the 18th century. Prior to Linnaeus,
the classification of organisms was in disarray.
First, each species was usually identified by long Latin
descriptions. For example, the common ground cherry was
called Physalis amno ramosissime ramis angulosis glabris
foliis dentoserratis, which means, in part, that it had angled
stems and the leaves were hairless and had toothed margins.
Linnaeus shortened these long names to two-word names (in
this case, Physalis angulata, a name it still uses). Thus Linnaeus
established the practice of binomial nomenclature
(“naming system that uses two names”).
Second, the same species could be given different descriptive
names by different botanists. Linnaeus standardized the
names. Was Rosa sylvestris alba cum rubore, folio glabro the
same as Rosa sylvestris inodora seu canina? Linnaeus standardized
it to Rosa canina. Mistakes still happen, in which
the same species is accidentally given two different names,
but it is now much less common.
These innovations were accepted by the biological
community with a sense of relief, but this was not the most
important innovation of the Linnaean system. Before Linnaeus,
there was no recognizable basis for classification.
Some zoologists classified animals on the basis of terrestrial
vs. aquatic, others on the basis of big vs. small. French paleontologist
Georges Buffon (see Buffon, Georges) organized
animals according to their usefulness in the human economy.
Linnaeus brought order into this chaos. In classifying
plants, Linnaeus recognized that reproductive characteristics
were less variable than the characteristics of leaves and stems.
He therefore classified plants on the basis of the numbers
and condition of the stamens (male parts) and pistils (female
parts) of their flowers. Consistent with the male bias of science
and society, Linnaeus’s major axis of classification was
the number of stamens: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, and
many individual stamens; the grouping of the stamens (tetradidymous,
dididymous, monadelphous, diadelphous, polyadelphous);
the fusion of stamens to one another or to pistils;
plants with separate male and female flowers on the same tree
(monoecious) or different trees (dioecious) or a mixture of
arrangements. His last class, the cryptogams (crypto- means
hidden), consisted of plants that had no obvious flowers (see
seedless plants, evolution of). Within the male axis, he
divided plants on the basis of their female parts. He applied
his system in a rigid fashion, resulting in classifications that
are surprising to modern botanists. For example, he placed
oaks, beeches, hazelnuts, and cattails into the same category;
modern botanists recognize cattails as very different from the
other three (see angiosperms, evolution of).
The central feature of the Linnaean system is that it is
a nested hierarchy. The specific epithet (the second of the
Linnaean system
two words in the name) referred to the species, and the genus
name (the first word) was given to a set of closely related species.
As the system was modified by later scientists, closely
related genera were clustered into families; closely related
families into orders; closely related orders into classes; closely
related classes into phyla; phyla into the major kingdoms, of
which there were originally only two: plants and animals.
Linnaeus inspired the scientific world because he brought, or
revealed, order within the diversity of organisms on the Earth
which was just then being thoroughly explored.
The Linnaean system seemed to correspond closely to the
concept of the Great Chain of Being (see scala naturae). All
things, from rocks to plants to animals to humans to angels to
God, were believed to be connected like a tapestry of fibers.
Scientists were not surprised to find intermediate entities—for
example, a sponge as intermediate between animal and rock, or
beings halfway between humans and apes—because it was all
part of the created tapestry. Linnaeus had no thought of evolutionary
origins of organisms; this is revealed by the fact that he
applied his system to rocks as well as to organisms. Linnaeus
tried to make the system as natural as possible by using characteristics
that were a realistic reflection of the Chain of Being.
While Linnaeus gave no thought to evolution, the fact
that the natural world could be so readily classified into nested
categories struck Charles Darwin as being a major piece of
evidence favoring evolution (see origin of species [book]).
Despite naturalists’ belief in the Scala Naturae, there actually
were not many intermediates between modern entities.
Sponges are not really half-rock, half-animal. Dogs, wolves,
coyotes, and foxes form a group of similar canines; cats, lions,
tigers, leopards, and cheetahs form a group of similar felines;
and there are no cat-dogs. Organisms, as Linnaeus classified
them, display a pattern like branches of an evolutionary tree.
Today, biologists attempt to classify organisms using
the Linnaean system but try to make the system as natural
as possible by defining genera, families, orders, classes, and
phyla on the basis of evolutionary divergence (see cladistics).
The species themselves are recognized when possible
on the basis of reproductive isolation (see speciation). When
a group of species share many features with one another, scientists
conclude that they evolved from a common ancestor
more recently than species that share few features.
The Latin (or Greek) generic and specific names may
describe the characteristics that make the genus or species different
from other species or genera; they may describe visible
features of the organism, or the location in which it lives; they
may be the ancient Greek or Latin names for the organisms;
or they may be a tribute to a scientist who studied the organisms.
For example, dogs are Canis familiaris—genus Canis,
species familiaris. Canis is a Latin word for dog. Other members
of the genus Canis include Canis lupus, the wolf (lupus is
Latin for wolf), and Canis latrans, the coyote (latrans comes
from the Latin for barking).
This system of groupings also explains the higher levels
of the Linnaean system (see figure on page 248):
• Genera that evolved from a common ancestor most recently,
and are therefore most similar to one another, are