[生物] Classifying Life

The diversity of life on Earth is staggering. The science ofidentifying, describing, naming, and classifying all of these organismsis called taxonomy.Carolus Linnaeus, aneighteenth-century Swedish botanist, is considered the father of moderntaxonomy. 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 theyclassify organisms based on their evolutionary relationships, or phylogeny,rather than on simple physical characteristics. The classificationsystem used in taxonomy is hierarchical and contains seven levels. Theseven levels of taxonomic classification, from broadest to mostspecific, are:

KingdomPhylumClassOrderFamilyGenusSpecies

    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; eachphylum contains numerous classes; each class contains numerous orders;etc. It is more accurate to draw the diagram of the taxonomiccategories in a tree structure, with each level of the hierarchybranching into the next:

    As one moves through the hierarchy fromspecies to kingdom, the common ancestor of all the species at a certainlevel dates further back in evolutionary history than the commonancestor of organisms in more specific levels. For example, the commonancestor of humans and chimpanzees (which are both in the orderPrimates) was alive more recently than the common ancestor of humansand 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 thesame family; members of the same family are more closely related thanmembers of the same order.

    Each species is placed into theclassification system with a two-part name. The first half of the nameis the species’ genus, while the second is the species’ own specificname. 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 fivekingdoms: Monera, Protista, Fungi, Plantae, and Animalia. For the SATII Biology, you should know the basic characteristics of the organismsthat belong in each of these kingdoms, and you should also be familiarwith the names and features of the major phyla within each kingdom.

    Kingdom Monera
    Monerans are prokaryotic: they aresingle-celled organisms that lack a nucleus and membrane-boundorganelles. Of the four kingdoms, monerans are the simplest, and theygenerally evolved the earliest. Of all the kingdoms, only monerans areprokaryotic.

    Monerans are characterized by a singlecircular chromosome of DNA, a single cell membrane that controls thetransport of substances into and out of the cell, and a process ofasexual reproduction called binary fission that involves dividing intotwo identical clones. Some monerans have a cell wall made of asugar-protein complex called peptidoglycan, which can be determined byGram staining. A Gram-positive moneran has a thick peptidoglycan cellwall, while a Gram-negative moneran has a much thinner one. Moneransare broken down into phyla according to their means of procuring food.

    We cover the structure and function ofmonerans in more detail in the section on microorganisms in theOrganismal Biology chapter.

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

    Phylum Cyanobacteria (blue-green algae)
    Cyanobacteria are autotrophs that can perform photosynthesis.

    Kingdom Protista
    Protists are eukaryotic. In general,protists are less complex than the other eukaryotes and originatedearlier in evolutionary history. Most are unicellular, though some areorganized in colonies and some others are multicellular. The kingdomProtista can be separated into three primary divisions: animal-like,plantlike, and funguslike.

    The animal-like protists are heterotrophicand motile. The most important protozoa for the SAT II Biology are theamoebas, sporozoa, and ciliates:

    Phylum Rhizopoda
    The members of phylum Rhizopoda are amoebas,known for their constantly changing body structure. Amoebas usemembrane extensions called pseudopods (“false feet”) to move and tosurround food particles, which they then engulf into their cytoplasmvia phagocytosis. Amoebas generally live in fresh water, but some arefound in soil or salt water. If an amoeba finds its way inside a humanthrough contaminated drinking water, it can cause severe dysentery.

    Phylum Apicomplexa
    The phylum Apicomplexa consists of spore-forming parasitic organisms, also known as sporozoa.The adult form lives inside the cells of animals. The spores aretransmitted to other host animals, usually by a carrier animal. Forexample, a mosquito bite transmits plasmodium, an apicomplexan thatlives in red blood cells and causes malaria.

    Phylum Ciliophora
    All members of the phylum Ciliophora propelthemselves by waving many short, hairlike structures called cilia in acoordinated fashion; cilia also help draw food particles into the oralgroove. Unlike other protozoa, ciliates have two nuclei: the smallermicronucleus is involved in reproduction, while the macronucleuscontrols the organism’s metabolic processes. A paramecium is the classic example of a ciliate protozoan.

    The plantlike protists include euglenoidsand various kinds of algae. They are all photo-synthetic autotrophs,transforming light energy into food. Some are unicellular, but many aremulticellular, forming fibrous seaweed structures.

    Phylum Euglenophyta
    Euglenoids are classified with the plantlikeprotists because many of them photosynthesize. But these unicellularorganisms have flagella that allow them to move.

    Phylum Phaeophyta
    Brown algae of phylum Phaeophyta are allmulticellular seaweeds, ranging in size from an inch to almost thelength of a football field (the large varieties are called kelp). Brownalgae provide both food and shelter to many animals in the coastalmarine ecosystem.

    Phylum Chlorophyta
    Green algae of phylum Chlorophyta have thesame photo-synthetic pigments and the same cell wall structure asplants. In fact, they are believed to be the ancestors of modernplants. Some are unicellular, and some are multicellular; however, nonehave specialized tissues like plants, and therefore they remainclassified with the simpler organisms in kingdom Protista.
    The funguslike protists are called slimemolds, which belong to the phyla Myxomycota and Acrasiomycota. Allslime molds are heterotrophs.

    Phylum Myxomycota
    This phylum includes the plasmodial(acellular) slime molds. A plasmodium consists of a single cell withmultiple nuclei. Plasmodial slime molds creep slowly along the decayingvegetation they digest; when food or water is scarce, they producesmall tough spores that germinate when environmental conditions improve.

    Phylum Acrasiomycota
    The cellular slime molds belong to. The moldis really a large collection of individual amoebalike protists whichcongregate into a “pseudo-plasmodium” or “slug” only when food isscarce. In this cooperative form, they produce a single stalk thatreleases spores.

    Kingdom Fungi
    Fungi are typically nonmotile and, likeplants, have cell walls. Unlike plants, fungi are heterotrophic andhave cell walls made of chitin rather than cellulose. Fungi secreteenzymes to digest their food externally and then absorb the nutrients.They usually live as decomposers, living off dead and decayingorganisms, 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 calledhyphae; some have hyphae that are segmented by divisions called septa,while others have a continuous cytoplasm with many nuclei in eachhyphae. 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 aspore, a microscopic reproductive structure that is much more resistantto lack of food or water. Unlike most plants and animals, which existpredominantly in a diploid state, fungi spend most of their time in ahaploid state, with only a brief diploid phase during the reproductivecycle.

    Some fungi grow in a mutually beneficialrelationship with a photosynthetic algae or plant. Lichen is an exampleof such a partnership between a fungus and an algae. The benefits ofthe merger are apparent: lichen can grow in a wider range oftemperatures than any individual plant or fungus, and lichen can oftencolonize rocks that will not support any other multicellular life forms.

    Kingdom Plantae
    Plants are complex multicellularphotosynthetic autotrophs, with cellulose in their cell walls and awaxy cuticle covering their aboveground parts. They are easilydistinguishable from members of all other kingdoms, with the possibleexception of their simpler ancestors in the Protista kingdom, the greenalgae. Over evolutionary time, plants improved their ability to live onland by developing a variety of important features. Plants can bedivided into four major groups, displaying a progressively greaterdegree of adaptation to the terrestrial environment.

    Nonvascular Plants—Bryophytes
    Bryophyta is the only phylum in the group ofnonvascular seedless plants. These mosses and worts are the mostprimitive true plants. Because they lack a vascular system (vascularsystems are discussed in much more detail in the section on Structureand Function of Plants, which is part of the Organismal Biologychapter), bryophytes do not have a stem, leaves, or roots; they mustdistribute water and nutrients throughout their bodies by absorptionand diffusion. As a result, they cannot grow beyond a small size andmust keep their bodies close to moist earth. Bryophytes reproduce byspores and need water in order to bring about fertilization. Becausethe male gamete is a flagellated sperm, reproduction requires water inwhich the sperm can swim. Unlike all other plants, which have a diploidadult stage, adult bryophytes are haploid, passing only briefly througha diploid phase during the reproductive cycle.

    Seedless Vascular Plants
    There are three phyla of seedless vascularplants: Lycophyta (club mosses), Sphenophyta (horsetails), and, mostlikely to appear on the SAT II Biology, Pterophyta (ferns). Vascularplants have a dual fluid transport system: xylem transports water andinorganic minerals from the roots upward, and phloem transports sugarsand other organic nutrients up and down. This vascular systemrepresents a major evolutionary step in the adaptation to life on land.The ability to transport water and nutrients across long distancesallows plants to grow much larger, sending specialized photosyntheticstructures (leaves) upward toward sunlight and specialized rootstructures downward toward the water and minerals in the ground. Likebryophytes, seedless vascular phyla reproduce by spores and haveflagellated sperm that require water in which to swim, limiting theseplants to relatively moist environments.

    Flowerless Seed Plants—Gymnosperms
    The evolution of seeds provided plants withanother advantage in their prolonged pilgrimage onto land. Unlike thespores of more primitive plants, seeds are multicellular, containingboth a complete diploid embryo and a food supply. Having a food supplyinside the seed provides the newborn plant with a period of growth thatis independent of food resources in the environment. This independenceallows seed plants to grow in a greater variety of environments.Further freeing seed plants, the male gametes of the seed plants takethe form of pollen, making reproduction independent of water.

    The seed plants that evolved first, calledgymnosperms (“naked seeds”), do not produce flowers. Their seeds areexposed directly to the air, without any capsule or fruit enclosingthem. The most important group of gymnosperms is phylum Coniferophyta;these plants, commonly called conifers, produce cones that carry seedson their scales. Examples of gymnosperms are pines, firs, cedars, andsequoias.

    Flowering Seed Plants—Angiosperms
    Flowering plants, called angiosperms(“covered seeds”), are vascular seed plants with specializedreproductive structures, which include both flowers and fruit. Insteadof depending on currents of wind or water for the dispersal of theirgametes and seeds, plants with flowers and fruit provide protection andattract animals that then serve as the means of fertilization.

    Flowering plants are divided into twoclasses, monocots and dicots. Monocot seeds have a single cotyledon,while dicots have two cotyledons in each seed. Monocots and dicots arecovered in more detail in the section on the Structure and Function ofPlants.

    Kingdom Animalia
    Animals are eukaryotic, multicellular, andheterotrophic. Animals also have specialized tissues to perform variousfunctions. Most animals are motile, at least during part of their lifecycle, reproduce sexually, and have nervous systems that allow them torespond rapidly to changes in their environment.

    Taxonomists use several observable featuresto classify animals into groups according to their evolutionaryrelationships. One of the most important of these features is bodysymmetry. In bilateral symmetry, the left half of the organismis the mirror image of the right half, but the top does not resemblethe bottom, and the front is dissimilar to the back. In radial symmetry,the organism has a circular body plan, with similar structures arrangedlike spokes on a wheel, such as a starfish. Most animals have threelayers of cells: the ectoderm, mesoderm, and endoderm. Almost allanimals have a hollow tube inside, which acts as a digestive tract; theopening where food enters is called the mouth, and the opening wheredigested material exists is called the anus.

    Animals are the most diverse of thekingdoms. 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), complexcolonies of flagellated unicellular protozoalike organisms. They do notexhibit any clear symmetry, and they are the only animal phylum thatdoes not possess at least two distinct embryonic tissue layers. Theirunique lack of tissue organization has prompted taxonomists to classifysponges as parazoa (“next to animals”). Nonetheless, some sponge cellsare specialized for reproductive or nutritional purposes, and thisslight organizational complexity gives them a toehold on the edge ofthe animal kingdom. Although sponges do have a hollow space inside,they do not have a digestive gut like other animals. Water flows intothe central space through the many pores in the sponge’s outer surfaceand flows out through the large opening at the top of the sponge. Theflow of water brings food and oxygen and carries away waste and carbondioxide. All sponges secrete a skeleton that maintains their shape (youmight use these skeletal remains as “natural sponges” in bathing).

    Phylum Cnidaria
    Phylum Cnidaria includes all stinging marineorganisms that exhibit radial symmetry, such as jellyfish, hydras, seaanemones, and coral. Cnidarians have a true digestive gut like otheranimals, 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.

    Phylum Platyhelminthes (Flatworms)
    Flatworms are bilaterally symmetric and arethe most primitive animals to possess all three embryonic tissuelayers. Like cnidarians, most flatworms have a digestive gut with onlya single opening. Flatworms are also the most primitive animals toexhibit discernable organs, internal structures with at least twotissue layers and a specialized function. There are three main kinds offlatworms: free-living carnivorous planarians, parasitic flukes thatfeed off the blood of other animals, and parasitic tapeworms that liveinside the digestive tracts of other animals.

    Phylum Nematoda (Roundworms)
    Most nematodes, also called roundworms, arefree-living; however, some live as parasites in the digestive tracts ofhumans and other animals. Soil-dwelling roundworms play an importantecological role by helping to decompose and recycle organic debris.Roundworms are bilaterally symmetric, have a complete gut tube with twoopenings, and possess all three embryonic tissue layers with a cavityin between the mesodermal and endodermal tissues. The roundworm speciesCaenorhabditis elegans was the first animal to have its entire genome sequence determined.

    Phylum Mollusca
    Phylum Mollusca includes many familiaranimals such as snails, slugs, squid, octopuses, and shellfish such asclams and oysters. Mollusks are bilaterally symmetric and have acomplete digestive tract and a circulatory system with a simple heart.They move by means of a muscular structure called a foot, and they havea rasping tongue called a radula and a mantle that secretes a hardshell. Mollusks generally live in aquatic regions.

    Phylum Annelida (Segmented Worms)
    Annelida means “ringed” and refers to therepeated ringlike segments that make up the bodies of annelids such asearthworms and leeches. Annelids exhibit bilateral symmetry have acomplete digestive tract with two excretory organs called nephridia ineach segment and a closed circulatory system. Their nervous systemconsists 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 mostleeches, on the other hand, are bloodsucking parasites. All annelidsmust live in moist environments. Having not yet developed moresophisticated respiratory systems, they exchange gases directly withtheir surroundings.

    Phylum Arthropoda
    Arthropoda is the most diverse and numerousanimal phylum. Insects, spiders, and crustaceans—which includelobsters, shrimp, and crabs—constitute the major arthropod groups. Thename Arthropoda means “jointed feet”; arthropods have jointedappendages and, like annelids, exhibit segmentation. Insects andcrustaceans have three body segments consisting of the head, thorax,and abdomen, while arachnids only have two body segments. Arthropodsare unique among animals in having a hard exoskeleton made of chitin.The arthropod nervous system resembles the annelid nervous system, witha simple brain, a ventral nerve cord, and smaller ganglia within thevarious body segments. However, many arthropods have very highlydeveloped sensory perception, including hearing organs, antennae, andcompound eyes. Arthropods have an open circulatory system, a fulldigestive tract, and structures called Malphigian tubules to eliminate waste.

    Phylum Echinodermata
    The name Echinodermata means “spiny skin,”and this phylum includes spiny marine animals such as starfish, seaurchins, and sand dollars, all of which exhibit radial symmetry.Echinoderms have several characteristic features, including anendoskeleton that secretes a spiny skin and an unusual vascular systemof water-filled vessels that regulates the movement of their many tube feetand also permits the exchange of carbon dioxide for oxygen. Echinodermshave a very simple nervous system, with a ring of nerves around theirmouth 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 embryonicdevelopment strongly suggest that echinoderms are most closely relatedto the chordates, the animal phylum that developed most recently inevolutionary time.

    Phylum Chordata
    Human beings belong to Chordata, the phylumthat evolved most recently in the animal kingdom. Chordates have threeembryonic tissues, a complete digestive tract, and well-developedcirculatory, respiratory, and nervous systems. Several featuresdistinguish chordates from all other animal phyla. The primary feature,for which chordates are named, is the notochord, a tubular rodof tissue that runs longitudinally down the back. Just above thenotochord runs a single, hollow nerve cord, the center of the nervoussystem. 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 chordatesduring embryonic development but disappear by adulthood in many membersof the phylum.
There are two groups of chordates, subphylumUrochordata and subphylum Vertebrata. The former subphylum includesinvertebrate marine animals such as tunicates and lancelets, and almostnever appears on the SAT II Biology. Much more important for the testare the vertebrates.

    Subphylum Vertebrata contains thosechordates that have replaced the simple notochord with a segmentedskeletal 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. Theportion encasing the brain is called the skull. There are seven mainclasses of vertebrates.

    Jawless fish:
These fish are bottom-dwelling filter feederswithout jaws. They breathe through gills and lay eggs. Examples arelampreys and hagfish.

    Cartilaginous fish:
With a flexible endoskeleton made of cartilage,these fish have well-developed jaws and fins, and they breathe throughgills. Their young hatch from eggs. Examples are sharks, eels, and rays.

    Bony fish:
Bony fish mark an advance since they have muchstronger skeletons made of bone rather than cartilage. Bony fish arefound in both salt water and fresh water. They breathe through gillsand lay soft eggs. Almost every fish you can think of is a bony fish,from goldfish to trout.

    Amphibians:
Amphibians such as frogs and salamanders embodythe transition from aquatic to terrestrial living. Born initially asfishlike tadpoles living in the water, they undergo a metamorphosis anddevelop legs and move onto land as adults. Most adult amphibiansbreathe through lungs that develop during their metamorphosis, thoughsome can breathe through their skin. Their eggs lack shells, must belaid in water, and receive little parental care.

    Reptiles:
With the development of the fluid-filled amniotic sac,reptiles, including dinosaurs, were the first animals to be able tohatch their eggs on land and make the full transition to terrestriallife. Reptiles lay few eggs and provide some parental care. Reptilesalso 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 theenvironment. When the temperature drops, their metabolism slows andspeeds up as the temperature rises. Birds and mammals, in contrast, arewarm-blooded (endothermic). They have developed structures suchas feathers, hair, and fur to help them maintain body temperature. Themetabolism of birds and mammals stays constant through far largerextremes of temperature, making these two classes much more versatile.

    Birds:
Birds have specially evolved structures such aswings, feathers, and light bones that allow for flight. In addition,birds have four-chambered hearts and powerful lungs that can withstandthe extreme metabolic demands of flight. Birds lay hard eggs butprovide a great deal of care for their eggs and developing young.

    Mammals:
Mammals have a number of unique features thathave allowed them to adapt successfully to many different environments.They have the most highly developed nervous systems in the animalkingdom, providing them with complex and adaptable behaviors. With theexception of a few species such as the platypus, mammals do not layeggs like other vertebrates; instead, mammalian embryos develop insidethe mother and are not released until nearly or fully developed andequipped for survival. Mammals are also unique in having milk glandsthat provide nourishment for their infants. In this way, the protectionand feeding of their young is built directly into mammalian bodies,dramatically increasing the ability of these animals to raise survivingoffspring in diverse environments. Examples of mammals are whales,cows, mice, monkeys, and humans.