Page 208 - (DK) Ocean - The Definitive Visual Guide

206 INTRODUCTION TO OCEAN LIFE

Classification

BY CLASSIFYING ORGANISMS AND FITTING them into a universally accepted
framework, scientists have created a massive reference system that accommodates WHAT IS
all forms of life. Over 2 million organisms have been described, of which about 16 A SPECIES?
percent live in the oceans. The marine proportion is likely to increase because many A species is the basic unit of
classification. One commonly
new species continue to be discovered annually, particularly in the deep ocean. accepted definition of a species
is a group of living organisms
LINNAEAN HIERARCHY Principles of Classification that have so many features in
Linnaeus used a hierarchy of ranked categories of Classification helps us make sense of the natural common that they can interbreed
and exchange genes in natural
increasing exclusiveness. Today’s expanded system conditions. This definition cannot
includes many ranks, from domain down to species. world by grouping organisms on the basis of features be applied to fossil species. It
Below is an example of a series of ranked categories, that they share. It gives scientists a clear and accurate also does not work for Bacteria
illustrating those that classify the common dolphin. understanding of the diversity of life, and because and Archaea and there can be no
everyone uses the same system, the knowledge is
DOMAIN Eucarya one universal definition. Other
Includes all Eukaryotes—organisms that accessible on a worldwide basis. The hierarchical factors such as geographical
have complex cells with distinct nuclei. Only system devised by the Swedish scientist Carolus isolation and DNA (see below)
bacteria and archaea fall outside this domain. Linnaeus (see panel, left) in the 18th century still are also important.
forms the basis of today’s classification. Each species
KINGDOM Animalia
Includes all animals—multicellular eukaryotes is identified with a unique two-part scientific name
that need to eat food for energy. All animals (made up of the genus and species names), then categorized in a series of ever-larger
are mobile for at least part of their lives.
groupings. However, as our knowledge increases, it is often necessary to revise the
PHYLUM Chordata groups. Sometimes, this leads to subdivision of categories, for example phylum
Includes all chordates—animals possessing Arthropoda has been split into four subphyla. Many new species are discovered each
a notochord. In most cases, the notochord year but describing and publishing them is laborious and time-consuming.
is replaced before birth by the backbone.
CLASS Mammalia
Includes all mammals—air-breathing
chordates that feed their young on milk. The Evidence
The jaw is made up of a single bone.
In the past, scientists could identify and classify organisms
ORDER Cetartiodactyla only by studying anatomy, by looking at form, function,
Includes all cetaceans (whales and dolphins)—
marine mammals that have a tail with and embryological development (animals only), and by
boneless, horizontal flukes for propulsion. examining the fossil record. Recently, scientists have also
been able to investigate organisms by looking at their
FAMILY Delphinidae proteins and their DNA. DNA is a
Includes all dolphins (a subgroup of toothed DETAILED ANATOMY
cetaceans) with beaks and 50–100 vertebrae. By making a detailed complex molecule whose sequential
The skull lacks a crest. anatomical examination structure is unique to each organism.
of material in museum The relatedness of organisms can
GENUS Delphinus collections, scientists
Includes a few colorful, oceanic dolphins can distinguish between be determined by comparing these
with 40–50 teeth on each side of the jaw. similar organisms and DNA molecules for shared features.
These dolphins form large social groups. This molecular evidence has led to
classify them according
to shared characters. many revisions of classification.
SPECIES Delphinus delphis
Specifies a single type of dolphin with a
V-shaped black cape under the dorsal fin and
criss-cross hour-glass patterning on its sides. ANIMALS WITH A SKULL
A skull is a derived character that unites
all the organisms below. The skull is said
to have evolved in their common ancestor.
Cladistics
By the 1950s, although most people used the same system of JAWED VERTEBRATES
Animals beyond this point form a clade of
classification, the criteria they used for placing organisms in organisms with jaws, again assumed to have
categories were often neither measurable nor repeatable. been inherited from a common ancestor.
The idea emerged to analyze many characters using an
automatic, computer-like process, not only to classify
BONY VERTEBRATES
organisms, but also to trace their evolution.
All animals beyond this point form a clade
This process became known as cladistics, possessing an inherited bony skeleton,
and it is a widely used technique today. not shared by sharks, lampreys, or hagfish.
LAMPREY
A cladistic analysis examines a wide selection
of characters shared by a study group of FISH CLADOGRAM HAGFISH RAY-FINNED FISH
This simplified
organisms. It finds the most likely pattern of
OCEAN LIFE involving the least number of steps (evolutionary used to classify fish. CARTILAGINOUS FISH without the limblike lobes of
Below is a clade of fish with fins
cladogram indicates
made up of radiating bones only,
evolutionary changes that link the organisms,
just three of the steps
lobe-fins, or limbs of tetrapods.
Clades include all the
branching points). It then arranges the organisms
descendants of a common
in a tree diagram (cladogram) that reflects their
ancestor, so some new groups, such
relationships. A cladogram is made up of nested
as “lobe-finned fish and tetrapods”
groups called clades. A clade encompasses all
result, since all tetrapods (land
vertebrates) descend from lobe-fins.
the descendants of the group’s common ancestor.
LOBE-FINNED FISH
AND TETRAPODS RAY-FINNED FISH