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:

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.

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 chapter.

Bacteria are heterotrophic and can act as symbionts, parasites, or decomposers.

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 dysentery.

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 are heterotrophs.

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 spores.

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.

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.

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 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.

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” in bathing).

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 and ectoderm.

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 other animals.

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 sequence determined.

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.

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 eliminate waste.

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 of vertebrates.

These fish are bottom-dwelling filter feeders without jaws. They breathe through gills and lay eggs. Examples are lampreys and hagfish.

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 rays.

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.