The diversity of life on Earth is staggering. The science
of identifying, describing, naming, and classifying all of these
organisms is called taxonomy. Carolus
Linnaeus, an eighteenth-century Swedish botanist, is considered
the father of modern taxonomy. He carefully observed and compared
different species, grouping them according to the similarities and differences
he found. Taxonomists today still use his system of organization,
though they classify organisms based on their evolutionary relationships,
or phylogeny, rather than on simple physical characteristics.
The classification system used in taxonomy is hierarchical and contains
seven levels. The seven levels of taxonomic classification, from
broadest to most specific, are:
A good way to remember the sequence of taxonomic categories
is to use a mnemonic:
King Philip Came Over From German Shores
Each kingdom contains numerous phyla; each phylum
contains numerous classes; each class contains numerous orders;
etc. It is more accurate to draw the diagram of the taxonomic categories
in a tree structure, with each level of the hierarchy branching
into the next:
As one moves through the hierarchy from species to kingdom,
the common ancestor of all the species at a certain level dates
further back in evolutionary history than the common ancestor of
organisms in more specific levels. For example, the common ancestor
of humans and chimpanzees (which are both in the order Primates)
was alive more recently than the common ancestor of humans and dogs
(which are both in the class Mammalia). Much in the same way, members
of the same genus are more closely related than members of the same
family; members of the same family are more closely related than
members of the same order.
Each species is placed into the classification
system with a two-part name. The first half of the name is the species’
genus, while the second is the species’ own specific name. The genus name
is capitalized, and the species name is lowercase. Humans belong
to the genus Homo and the species sapiens, so
the name for humans is Homo sapiens.
The Five Kingdoms
Taxonomy splits all living things into five kingdoms:
Monera, Protista, Fungi, Plantae, and Animalia. For the SAT II Biology,
you should know the basic characteristics of the organisms that
belong in each of these kingdoms, and you should also be familiar
with the names and features of the major phyla within each kingdom.
Monerans are prokaryotic: they are single-celled
organisms that lack a nucleus and membrane-bound organelles. Of
the four kingdoms, monerans are the simplest, and they generally evolved
the earliest. Of all the kingdoms, only monerans are prokaryotic.
Monerans are characterized by a single circular
chromosome of DNA, a single cell membrane that controls the transport
of substances into and out of the cell, and a process of asexual reproduction
called binary fission that involves dividing into two identical
clones. Some monerans have a cell wall made of a sugar-protein complex
called peptidoglycan, which can be determined by Gram staining.
A Gram-positive moneran has a thick peptidoglycan cell wall, while
a Gram-negative moneran has a much thinner one. Monerans are broken
down into phyla according to their means of procuring food.
We cover the structure and function of monerans in more
detail in the section on microorganisms in the Organismal Biology
Bacteria are heterotrophic and can act as symbionts, parasites,
Phylum Cyanobacteria (blue-green algae)
Cyanobacteria are autotrophs that can perform photosynthesis.
Protists are eukaryotic. In general, protists are less
complex than the other eukaryotes and originated earlier in evolutionary
history. Most are unicellular, though some are organized in colonies
and some others are multicellular. The kingdom Protista can be separated
into three primary divisions: animal-like, plantlike, and funguslike.
The animal-like protists are heterotrophic and motile.
The most important protozoa for the SAT II Biology are the amoebas,
sporozoa, and ciliates:
The members of phylum Rhizopoda are amoebas,
known for their constantly changing body structure. Amoebas use
membrane extensions called pseudopods (“false feet”) to move and
to surround food particles, which they then engulf into their cytoplasm
via phagocytosis. Amoebas generally live in fresh water, but some
are found in soil or salt water. If an amoeba finds its way inside
a human through contaminated drinking water, it can cause severe
The phylum Apicomplexa consists of spore-forming parasitic
organisms, also known as sporozoa. The adult form lives
inside the cells of animals. The spores are transmitted to other
host animals, usually by a carrier animal. For example, a mosquito
bite transmits plasmodium, an apicomplexan that lives in red blood
cells and causes malaria.
All members of the phylum Ciliophora propel themselves
by waving many short, hairlike structures called cilia in a coordinated
fashion; cilia also help draw food particles into the oral groove.
Unlike other protozoa, ciliates have two nuclei: the smaller micronucleus
is involved in reproduction, while the macronucleus controls the
organism’s metabolic processes. A paramecium is the
classic example of a ciliate protozoan.
The plantlike protists include euglenoids and various
kinds of algae. They are all photo-synthetic autotrophs, transforming
light energy into food. Some are unicellular, but many are multicellular,
forming fibrous seaweed structures.
Euglenoids are classified with the plantlike
protists because many of them photosynthesize. But these unicellular
organisms have flagella that allow them to move.
Brown algae of phylum Phaeophyta are all multicellular
seaweeds, ranging in size from an inch to almost the length of a
football field (the large varieties are called kelp). Brown algae provide
both food and shelter to many animals in the coastal marine ecosystem.
Green algae of phylum Chlorophyta have the same photo-synthetic
pigments and the same cell wall structure as plants. In fact, they
are believed to be the ancestors of modern plants. Some are unicellular,
and some are multicellular; however, none have specialized tissues like
plants, and therefore they remain classified with the simpler organisms
in kingdom Protista.
The funguslike protists are called slime molds, which
belong to the phyla Myxomycota and Acrasiomycota. All slime molds
This phylum includes the plasmodial (acellular) slime
molds. A plasmodium consists of a single cell with multiple nuclei.
Plasmodial slime molds creep slowly along the decaying vegetation
they digest; when food or water is scarce, they produce small tough
spores that germinate when environmental conditions improve.
The cellular slime molds belong to. The mold
is really a large collection of individual amoebalike protists which
congregate into a “pseudo-plasmodium” or “slug” only when food is scarce.
In this cooperative form, they produce a single stalk that releases
Fungi are typically nonmotile and, like plants, have cell
walls. Unlike plants, fungi are heterotrophic and have cell walls
made of chitin rather than cellulose. Fungi secrete enzymes to digest
their food externally and then absorb the nutrients. They usually
live as decomposers, living off dead and decaying organisms, or
as parasites, growing on or in other living organisms. With the
exception of yeast, most fungi are multicellular. Structurally, multicellular
fungi are composed of filaments called hyphae; some have hyphae
that are segmented by divisions called septa, while others have
a continuous cytoplasm with many nuclei in each hyphae. Many fungi
exist as a tangle of hyphae, called a mycelium. Examples of fungi
are yeast and mushrooms.
Most fungi can also exist in the form of a spore, a microscopic
reproductive structure that is much more resistant to lack of food
or water. Unlike most plants and animals, which exist predominantly
in a diploid state, fungi spend most of their time in a haploid
state, with only a brief diploid phase during the reproductive cycle.
Some fungi grow in a mutually beneficial relationship
with a photosynthetic algae or plant. Lichen is an example of such
a partnership between a fungus and an algae. The benefits of the
merger are apparent: lichen can grow in a wider range of temperatures
than any individual plant or fungus, and lichen can often colonize
rocks that will not support any other multicellular life forms.
Plants are complex multicellular photosynthetic autotrophs,
with cellulose in their cell walls and a waxy cuticle covering their
aboveground parts. They are easily distinguishable from members
of all other kingdoms, with the possible exception of their simpler
ancestors in the Protista kingdom, the green algae. Over evolutionary
time, plants improved their ability to live on land by developing
a variety of important features. Plants can be divided into four
major groups, displaying a progressively greater degree of adaptation
to the terrestrial environment.
Bryophyta is the only phylum in the group of nonvascular
seedless plants. These mosses and worts are the most primitive true
plants. Because they lack a vascular system (vascular systems are
discussed in much more detail in the section on Structure and Function
of Plants, which is part of the Organismal Biology chapter), bryophytes
do not have a stem, leaves, or roots; they must distribute water
and nutrients throughout their bodies by absorption and diffusion.
As a result, they cannot grow beyond a small size and must keep their
bodies close to moist earth. Bryophytes reproduce by spores and
need water in order to bring about fertilization. Because the male
gamete is a flagellated sperm, reproduction requires water in which
the sperm can swim. Unlike all other plants, which have a diploid adult
stage, adult bryophytes are haploid, passing only briefly through
a diploid phase during the reproductive cycle.
Seedless Vascular Plants
There are three phyla of seedless vascular plants: Lycophyta
(club mosses), Sphenophyta (horsetails), and, most likely to appear
on the SAT II Biology, Pterophyta (ferns). Vascular plants have
a dual fluid transport system: xylem transports water and inorganic
minerals from the roots upward, and phloem transports sugars and
other organic nutrients up and down. This vascular system represents
a major evolutionary step in the adaptation to life on land. The
ability to transport water and nutrients across long distances allows
plants to grow much larger, sending specialized photosynthetic
structures (leaves) upward toward sunlight and specialized root
structures downward toward the water and minerals in the ground. Like
bryophytes, seedless vascular phyla reproduce by spores and have
flagellated sperm that require water in which to swim, limiting
these plants to relatively moist environments.
Flowerless Seed Plants—Gymnosperms
The evolution of seeds provided plants with another advantage
in their prolonged pilgrimage onto land. Unlike the spores of more
primitive plants, seeds are multicellular, containing both a complete
diploid embryo and a food supply. Having a food supply inside the seed
provides the newborn plant with a period of growth that is independent
of food resources in the environment. This independence allows seed
plants to grow in a greater variety of environments. Further freeing
seed plants, the male gametes of the seed plants take the form of
pollen, making reproduction independent of water.
The seed plants that evolved first, called gymnosperms
(“naked seeds”), do not produce flowers. Their seeds are exposed
directly to the air, without any capsule or fruit enclosing them.
The most important group of gymnosperms is phylum Coniferophyta;
these plants, commonly called conifers, produce cones that carry
seeds on their scales. Examples of gymnosperms are pines, firs,
cedars, and sequoias.
Flowering Seed Plants—Angiosperms
Flowering plants, called angiosperms (“covered seeds”),
are vascular seed plants with specialized reproductive structures,
which include both flowers and fruit. Instead of depending on currents
of wind or water for the dispersal of their gametes and seeds, plants
with flowers and fruit provide protection and attract animals that
then serve as the means of fertilization.
Flowering plants are divided into two classes, monocots
and dicots. Monocot seeds have a single cotyledon, while dicots
have two cotyledons in each seed. Monocots and dicots are covered
in more detail in the section on the Structure and Function of Plants.
Animals are eukaryotic, multicellular, and heterotrophic.
Animals also have specialized tissues to perform various functions.
Most animals are motile, at least during part of their life cycle,
reproduce sexually, and have nervous systems that allow them to
respond rapidly to changes in their environment.
Taxonomists use several observable features to classify
animals into groups according to their evolutionary relationships.
One of the most important of these features is body symmetry. In bilateral
symmetry, the left half of the organism is the mirror image
of the right half, but the top does not resemble the bottom, and
the front is dissimilar to the back. In radial symmetry,
the organism has a circular body plan, with similar structures arranged like
spokes on a wheel, such as a starfish. Most animals have three layers
of cells: the ectoderm, mesoderm, and endoderm. Almost all animals
have a hollow tube inside, which acts as a digestive tract; the
opening where food enters is called the mouth, and the opening where
digested material exists is called the anus.
Animals are the most diverse of the kingdoms. Any of their
various phyla may come up on the SAT II Biology, though the vertebrates
come up most often.
Phylum Porifera (Sponges)
Sponges are sessile (nonmoving), complex colonies of flagellated
unicellular protozoalike organisms. They do not exhibit any clear
symmetry, and they are the only animal phylum that does not possess
at least two distinct embryonic tissue layers. Their unique lack
of tissue organization has prompted taxonomists to classify sponges
as parazoa (“next to animals”). Nonetheless, some sponge cells are
specialized for reproductive or nutritional purposes, and this slight
organizational complexity gives them a toehold on the edge of the animal
kingdom. Although sponges do have a hollow space inside, they do
not have a digestive gut like other animals. Water flows into the
central space through the many pores in the sponge’s outer surface
and flows out through the large opening at the top of the sponge.
The flow of water brings food and oxygen and carries away waste
and carbon dioxide. All sponges secrete a skeleton that maintains
their shape (you might use these skeletal remains as “natural sponges”
Phylum Cnidaria includes all stinging marine organisms
that exhibit radial symmetry, such as jellyfish, hydras, sea anemones,
and coral. Cnidarians have a true digestive gut like other animals,
but one opening serves as both the mouth and anus. Additionally,
their body walls are made up of only two layers of cells: endoderm
Phylum Platyhelminthes (Flatworms)
Flatworms are bilaterally symmetric and are the most primitive
animals to possess all three embryonic tissue layers. Like cnidarians,
most flatworms have a digestive gut with only a single opening.
Flatworms are also the most primitive animals to exhibit discernable organs,
internal structures with at least two tissue layers and a specialized
function. There are three main kinds of flatworms: free-living carnivorous
planarians, parasitic flukes that feed off the blood of other animals,
and parasitic tapeworms that live inside the digestive tracts of
Phylum Nematoda (Roundworms)
Most nematodes, also called roundworms, are free-living;
however, some live as parasites in the digestive tracts of humans
and other animals. Soil-dwelling roundworms play an important ecological
role by helping to decompose and recycle organic debris. Roundworms
are bilaterally symmetric, have a complete gut tube with two openings,
and possess all three embryonic tissue layers with a cavity in between
the mesodermal and endodermal tissues. The roundworm species Caenorhabditis
elegans was the first animal to have its entire genome
Phylum Mollusca includes many familiar animals such as
snails, slugs, squid, octopuses, and shellfish such as clams and
oysters. Mollusks are bilaterally symmetric and have a complete
digestive tract and a circulatory system with a simple heart. They
move by means of a muscular structure called a foot, and they have
a rasping tongue called a radula and a mantle that secretes a hard
shell. Mollusks generally live in aquatic regions.
Phylum Annelida (Segmented Worms)
Annelida means “ringed” and refers to the repeated ringlike
segments that make up the bodies of annelids such as earthworms
and leeches. Annelids exhibit bilateral symmetry have a complete
digestive tract with two excretory organs called nephridia in
each segment and a closed circulatory system. Their nervous system
consists of a simple brain in front and a ventral (near the belly)
nerve cord connecting smaller clusters of nerve cells, or ganglia,
within each segment. Earthworms live freely within the soil, while
most leeches, on the other hand, are bloodsucking parasites. All
annelids must live in moist environments. Having not yet developed
more sophisticated respiratory systems, they exchange gases directly
with their surroundings.
Arthropoda is the most diverse and numerous animal phylum.
Insects, spiders, and crustaceans—which include lobsters, shrimp,
and crabs—constitute the major arthropod groups. The name Arthropoda
means “jointed feet”; arthropods have jointed appendages and, like
annelids, exhibit segmentation. Insects and crustaceans have three
body segments consisting of the head, thorax, and abdomen, while
arachnids only have two body segments. Arthropods are unique among
animals in having a hard exoskeleton made of chitin. The arthropod
nervous system resembles the annelid nervous system, with a simple
brain, a ventral nerve cord, and smaller ganglia within the various
body segments. However, many arthropods have very highly developed
sensory perception, including hearing organs, antennae, and compound
eyes. Arthropods have an open circulatory system, a full digestive
tract, and structures called Malphigian tubules to
The name Echinodermata means “spiny skin,” and this phylum
includes spiny marine animals such as starfish, sea urchins, and
sand dollars, all of which exhibit radial symmetry. Echinoderms
have several characteristic features, including an endoskeleton
that secretes a spiny skin and an unusual vascular system of water-filled
vessels that regulates the movement of their many tube feet and
also permits the exchange of carbon dioxide for oxygen. Echinoderms
have a very simple nervous system, with a ring of nerves around
their mouth and no brain. Some echinoderms filter food out of the
water, while others, like starfish, are carnivorous predators or
scavengers. Despite their primitive appearance, patterns in early embryonic
development strongly suggest that echinoderms are most closely related
to the chordates, the animal phylum that developed most recently
in evolutionary time.
Human beings belong to Chordata, the phylum that evolved
most recently in the animal kingdom. Chordates have three embryonic
tissues, a complete digestive tract, and well-developed circulatory,
respiratory, and nervous systems. Several features distinguish chordates
from all other animal phyla. The primary feature, for which chordates
are named, is the notochord, a tubular rod of tissue
that runs longitudinally down the back. Just above the notochord
runs a single, hollow nerve cord, the center of the nervous system.
Other animals, such as earthworms, also have nerve cords; however,
these run in ventral pairs along the belly and are not hollow. Two
other features, gill slits and tails, are present in all chordates
during embryonic development but disappear by adulthood in many
members of the phylum.
There are two groups of chordates, subphylum Urochordata
and subphylum Vertebrata. The former subphylum includes invertebrate
marine animals such as tunicates and lancelets, and almost never
appears on the SAT II Biology. Much more important for the test
are the vertebrates.
Subphylum Vertebrata contains those chordates that have
replaced the simple notochord with a segmented skeletal rod that
wraps around and protects the brain and nerve cord. The skeletal
segments, called vertebrae, are made of bone or cartilage, and the
entire series of segments is called the vertebral column. The portion
encasing the brain is called the skull. There are seven main classes
These fish are bottom-dwelling filter feeders without
jaws. They breathe through gills and lay eggs. Examples are lampreys
With a flexible endoskeleton made of cartilage, these
fish have well-developed jaws and fins, and they breathe through
gills. Their young hatch from eggs. Examples are sharks, eels, and
Bony fish mark an advance since they have much stronger
skeletons made of bone rather than cartilage. Bony fish are found
in both salt water and fresh water. They breathe through gills and
lay soft eggs. Almost every fish you can think of is a bony fish, from
goldfish to trout.
Amphibians such as frogs and salamanders embody the transition
from aquatic to terrestrial living. Born initially as fishlike tadpoles
living in the water, they undergo a metamorphosis and develop legs
and move onto land as adults. Most adult amphibians breathe through
lungs that develop during their metamorphosis, though some can breathe
through their skin. Their eggs lack shells, must be laid in water,
and receive little parental care.
With the development of the fluid-filled amniotic
sac, reptiles, including dinosaurs, were the first animals
to be able to hatch their eggs on land and make the full transition
to terrestrial life. Reptiles lay few eggs and provide some parental
care. Reptiles also have thick, scaly skin that resists water loss
and efficient lungs.
All classes of vertebrates that evolved before birds are cold-blooded (ectothermic).
The metabolism of these earlier classes is dependent on the environment.
When the temperature drops, their metabolism slows and speeds up
as the temperature rises. Birds and mammals, in contrast, are warm-blooded (endothermic).
They have developed structures such as feathers, hair, and fur to
help them maintain body temperature. The metabolism of birds and
mammals stays constant through far larger extremes of temperature,
making these two classes much more versatile.
Birds have specially evolved structures such as wings,
feathers, and light bones that allow for flight. In addition, birds
have four-chambered hearts and powerful lungs that can withstand
the extreme metabolic demands of flight. Birds lay hard eggs but
provide a great deal of care for their eggs and developing young.
Mammals have a number of unique features that
have allowed them to adapt successfully to many different environments.
They have the most highly developed nervous systems in the animal
kingdom, providing them with complex and adaptable behaviors. With
the exception of a few species such as the platypus, mammals do
not lay eggs like other vertebrates; instead, mammalian embryos
develop inside the mother and are not released until nearly or fully
developed and equipped for survival. Mammals are also unique in
having milk glands that provide nourishment for their infants. In
this way, the protection and feeding of their young is built directly
into mammalian bodies, dramatically increasing the ability of these
animals to raise surviving offspring in diverse environments. Examples
of mammals are whales, cows, mice, monkeys, and humans.