The Taxonomic hierarch y The categories at the different levels may include many a few, or only one taxon. For example, there is only one liy n the decades following Linnaeus, taxonomists began to ing genus of the family Hominidae, but several living gen- group organisms into larger, more inclusive categories. era of Fagaceae. To someone familiar with classification or Genera with similar properties were grouped into a cluster with access to the appropriate reference books, each taxon called a family, and similar families were placed into the implies both a set of characteristics and a group of organ- ame order(figure 32. 4). Orders with common properties isms belonging to the taxon. For example, a honeybee has were placed into the same class, and classes with similar the species(level 1) name Apis mellifera. Its genus name characteristics into the same phylum(plural, phyla). For (level 2)Apis is a member of the family Apidae(level 3). All historical reasons, phyla may also be called divisions among members of this family are bees, some solitary, others liv plants, fungi, and algae. Finally, the phyla were assigned to ing in hives as 4. mellifera does. Knowledge of its order one of several great groups, the kingdoms. Biologists cur- (level 4), Hymenoptera, tells you that A. mellifera is likel rently recognize six kingdoms: two kinds of bacteria(Ar- able to sting and may live in colonies. Its class (level 5)In chaebacteria and Eubacteria), a largely unicellular group of ecta indicates that A. mellifera has three major body seg- eukaryotes(Protista), and three multicellular groups ments, with wings and three pairs of legs attached to the (Fungi, Plantae, and Animalia). In order to remember the middle segment. Its phylum(level 6), Arthropoda, tells us seven categories of the taxonomic hierarchy in their prop that the honeybee has a hard cuticle of chitin and jointed order, it may prove useful to memorize a phrase such as appendages. Its kingdom (level 7), Animalia, tells us that A kindly pay cash or furnish good security"(kingdom-phy- mellifera is a multicellular heterotroph whose cells lack cell lum-class-order-family-genus-species) wall In addition, an eighth level of classification, called do- mains, is sometimes used. Biologists recognize three do- mains, which will be discussed later in this chapter. The are ped into genera, genera into families, scientific names of the taxonomic units higher than the classes. and classes into Phyla are the basic units within kingdoms; such a genus level are capitalized but not printed distinctively system is hierarchical. talicized. or underlined nN astern gray squirrel FIGURE 32. 4 ciurus carolinensis The hierarchical system Kingdom I in classifying organism. The organism is first recognized as a eukaryote within this domain it is an Claes animal (kingdom: Animalia) ,CA正色的三当当33出 Orde Among the different phyla of ,的 色 (phylum: Chordata, Genus subphylum: Vertebrata). The organism's fur characterizes it ta Sciuridae has four front toes and five back toes. it Mammalia (family: Sciuridae). Within this family, it is a tree squirrel Vernet ala (genus: Sciurus), with gray fur and white-tipped hairs on the Anura carolinensis, the eastern gray 652 Part IX Viruses and Simple organism
The Taxonomic Hierarchy In the decades following Linnaeus, taxonomists began to group organisms into larger, more inclusive categories. Genera with similar properties were grouped into a cluster called a family, and similar families were placed into the same order (figure 32.4). Orders with common properties were placed into the same class, and classes with similar characteristics into the same phylum (plural, phyla). For historical reasons, phyla may also be called divisions among plants, fungi, and algae. Finally, the phyla were assigned to one of several great groups, the kingdoms. Biologists currently recognize six kingdoms: two kinds of bacteria (Archaebacteria and Eubacteria), a largely unicellular group of eukaryotes (Protista), and three multicellular groups (Fungi, Plantae, and Animalia). In order to remember the seven categories of the taxonomic hierarchy in their proper order, it may prove useful to memorize a phrase such as “kindly pay cash or furnish good security” (kingdom–phylum–class–order–family–genus–species). In addition, an eighth level of classification, called domains, is sometimes used. Biologists recognize three domains, which will be discussed later in this chapter. The scientific names of the taxonomic units higher than the genus level are capitalized but not printed distinctively, italicized, or underlined. The categories at the different levels may include many, a few, or only one taxon. For example, there is only one living genus of the family Hominidae, but several living genera of Fagaceae. To someone familiar with classification or with access to the appropriate reference books, each taxon implies both a set of characteristics and a group of organisms belonging to the taxon. For example, a honeybee has the species (level 1) name Apis mellifera. Its genus name (level 2) Apis is a member of the family Apidae (level 3). All members of this family are bees, some solitary, others living in hives as A. mellifera does. Knowledge of its order (level 4), Hymenoptera, tells you that A. mellifera is likely able to sting and may live in colonies. Its class (level 5) Insecta indicates that A. mellifera has three major body segments, with wings and three pairs of legs attached to the middle segment. Its phylum (level 6), Arthropoda, tells us that the honeybee has a hard cuticle of chitin and jointed appendages. Its kingdom (level 7), Animalia, tells us that A. mellifera is a multicellular heterotroph whose cells lack cell walls. Species are grouped into genera, genera into families, families into orders, orders into classes, and classes into phyla. Phyla are the basic units within kingdoms; such a system is hierarchical. 652 Part IX Viruses and Simple Organisms Eastern gray squirrel Sciurus carolinensis FIGURE 32.4 The hierarchical system used in classifying an organism. The organism is first recognized as a eukaryote (domain: Eukarya). Second, within this domain, it is an animal (kingdom: Animalia). Among the different phyla of animals, it is a vertebrate (phylum: Chordata, subphylum: Vertebrata). The organism’s fur characterizes it as a mammal (class: Mammalia). Within this class, it is distinguished by its gnawing teeth (order: Rodentia). Next, because it has four front toes and five back toes, it is a squirrel (family: Sciuridae). Within this family, it is a tree squirrel (genus: Sciurus), with gray fur and white-tipped hairs on the tail (species: Sciurus carolinensis, the eastern gray squirrel)
What Is a Species: In the previous section we discussed how species are named and grouped, but how do biologists decide when one or- ganism is distinct enough from another to be called its own species? In chapter 22, we reviewed the nature of species and saw there are no absolute criteria for the definition of this category. Looking different, for example, is not a use- ful criterion: different individuals that belong to the same (a) ecle dogs)may look very unlike one an- FIGURE 32.5 other, as different as a Chihuahua and a St Bernard. These The biological species very different-appearing individuals are fully capable of hy concept. Horses(a)and one ano donkeys(b) are not the The biological species concept(figure 32.5)essentially same species, be ecause the says that two organisms that cannot interbreed and produce ev produce fertile offspring are different species. This definition of when they interbreed, species can be useful in describing sexually reproducing species that regularly outcross-interbreed with individu als other than themselves. However, in many groups of or- ganisms, including bacteria, fungi, and many plants and an- (c) imals, asexual reproduction--reproduction without sex--predominates. Among them, hybridization cannot be How Many Species Are There? used as a criterion for species recognition Scientists have described and named a total of 1.5 million Defining species species, but doubtless many more actually exist. Some ps of organisms, such as flowering plants, vertebrate Despite such difficulties, biologists generally agree on the animals, and butterflies, are relatively well known with an organisms they classify as species based on the similarity of estimated 90% of the total number of species that actually morphological features and ecology. As a practical defini- exist in these groups having already been described. Many tion, we can say that species are groups of organisms that other groups, however, are very poorly known. It is gener remain relatively constant in their characteristics, can be ally accepted that only about 5% of all species have been distinguished from other species, and do not normally ecos gnized for bacteria, nematodes(roundworms), fungi terbreed with other species in nature and mites(a group of organisms related to spiders) By taking representative samples of organisms from dif- Evolutionary Species Concept ferent environments, such as the upper branches of tropical trees or the deep ocean, scientists have estimated the total This simple definition of species leaves many problems un numbers of species that may actually exist to be about 10 Ived. How, for instance, are we to compare living species million, about 15% of them marine organisms with seemingly similar ones now extinct? Much of the dis agreement among alternative species concepts relates to Most Species Live in the Tropics unique species name, and when do we assign them o solving this problem. When do we assign fossil specimens Most species, perhaps 6 or 7 million, are tropical. Presently species living today? If we trace the lineage of two sister only 400,000 species have been named in tropical Asia, species backwards through time, how far must we go before Africa, and Latin America combined, well the two species converge on their common ancestor? It is species that occur in the tropics. This is an incredible gap often very hard to know where to draw a sharp line be n our knowledge concerning biological diversity in a world tween two closely related species. that depends on biodiversity for its sustainabilit To address this problem, biologists have added an evo- These estimates apply to the number of eukaryotic or- lutionary time dimension to the biological species concept. numk ns only. There is no functional way of estimating the A current definition of an evolutionary species is a single numbers of species of prokaryotic organisms, although it is lineage of populations that maintains its distinctive identity from clear that only a very small fraction of all species have been otber sucb lineages. Unlike the biological species concept, the discovered and characterized so far evolutionary species concept applies to both asexual and sexually reproducing forms. Abrupt changes in diagnostic Species are groups of organisms that differ from one features mark the boundaries of different species in evolu another in recognizable ways and generally do not interbreed with one another in nature tionary time. Chapter 32 How We Classify Organisms 653
What Is a Species? In the previous section we discussed how species are named and grouped, but how do biologists decide when one organism is distinct enough from another to be called its own species? In chapter 22, we reviewed the nature of species and saw there are no absolute criteria for the definition of this category. Looking different, for example, is not a useful criterion: different individuals that belong to the same species (for example, dogs) may look very unlike one another, as different as a Chihuahua and a St. Bernard. These very different-appearing individuals are fully capable of hybridizing with one another. The biological species concept (figure 32.5) essentially says that two organisms that cannot interbreed and produce fertile offspring are different species. This definition of a species can be useful in describing sexually reproducing species that regularly outcross—interbreed with individuals other than themselves. However, in many groups of organisms, including bacteria, fungi, and many plants and animals, asexual reproduction—reproduction without sex—predominates. Among them, hybridization cannot be used as a criterion for species recognition. Defining Species Despite such difficulties, biologists generally agree on the organisms they classify as species based on the similarity of morphological features and ecology. As a practical definition, we can say that species are groups of organisms that remain relatively constant in their characteristics, can be distinguished from other species, and do not normally interbreed with other species in nature. Evolutionary Species Concept This simple definition of species leaves many problems unsolved. How, for instance, are we to compare living species with seemingly similar ones now extinct? Much of the disagreement among alternative species concepts relates to solving this problem. When do we assign fossil specimens a unique species name, and when do we assign them to species living today? If we trace the lineage of two sister species backwards through time, how far must we go before the two species converge on their common ancestor? It is often very hard to know where to draw a sharp line between two closely related species. To address this problem, biologists have added an evolutionary time dimension to the biological species concept. A current definition of an evolutionary species is a single lineage of populations that maintains its distinctive identity from other such lineages. Unlike the biological species concept, the evolutionary species concept applies to both asexual and sexually reproducing forms. Abrupt changes in diagnostic features mark the boundaries of different species in evolutionary time. How Many Species Are There? Scientists have described and named a total of 1.5 million species, but doubtless many more actually exist. Some groups of organisms, such as flowering plants, vertebrate animals, and butterflies, are relatively well known with an estimated 90% of the total number of species that actually exist in these groups having already been described. Many other groups, however, are very poorly known. It is generally accepted that only about 5% of all species have been recognized for bacteria, nematodes (roundworms), fungi, and mites (a group of organisms related to spiders). By taking representative samples of organisms from different environments, such as the upper branches of tropical trees or the deep ocean, scientists have estimated the total numbers of species that may actually exist to be about 10 million, about 15% of them marine organisms. Most Species Live in the Tropics Most species, perhaps 6 or 7 million, are tropical. Presently only 400,000 species have been named in tropical Asia, Africa, and Latin America combined, well under 10% of all species that occur in the tropics. This is an incredible gap in our knowledge concerning biological diversity in a world that depends on biodiversity for its sustainability. These estimates apply to the number of eukaryotic organisms only. There is no functional way of estimating the numbers of species of prokaryotic organisms, although it is clear that only a very small fraction of all species have been discovered and characterized so far. Species are groups of organisms that differ from one another in recognizable ways and generally do not interbreed with one another in nature. Chapter 32 How We Classify Organisms 653 (a) (b) (c) FIGURE 32.5 The biological species concept. Horses (a) and donkeys (b) are not the same species, because the offspring they produce when they interbreed, mules (c), are sterile
32.2 Scientists construct phylogenies to understand the evolutionary relationships among organisms. Evolutionary Classifications Monophyletic group After naming and classifying some 1.5 million organisms what have biologists learned? One very important advan tage of being able to classify particular species of plants, an- imals, and other organisms is that individuals of species that are useful to humans as sources of food and medicine an be identified. For example, if you cannot tell the fungus Penicillium from Aspergillus, you have little chance of pro- ducing the antibiotic penicillin. In a thousand ways, just having names for organisms is of immense importance in our modern world Taxonomy also enables us to glimpse the evolutionary Paraphyletic group istory of life on earth. The more similar two taxa are, the more closely related they are likely to be. By lookin at the differences and similarities between organisms, bi ologists can construct an evolutionary tree, or phy- geny, inferring which organisms evolved from which other ones. in what order. and when. The reconstruction and study of phylogenies is called systematics. Within a phylogeny, a grouping can be either monophyletic, para- phyletic, or polyphyletic. A monophyletic group in-(b) cludes the most recent common ancestor of the group and all of its descendants. A paraphyletic group includes Polyphyletic group le most recent common ancestor of the group but not all of its descendants. And, a polyphyletic group does not include the most recent common ancestor of all the members of the group. Monophyletic groups are com monly assigned names, but systematists will not assign a taxonomic classification to a polyphyletic group. Para- phyletic groups may be considered taxa by some scien lutionary relaton ey do not accurately represent the evo- (figure 32.6) FIGURE 32.6 (a)A monophyletic group consists of the most recent common Cladistics ancestor and all of its descendants. All taxonomists accept monophyletic groups in their classifications and in the above A simple and objective way to construct a phylogenetic example would give the name"Apes"to the orangutans, gorillas, tree is to focus on key characters that a group of organ- chimpanzees, and humans.()A paraphyletic group consists of the isms share because they have inherited them from a com- most recent common ancestor and some of its descendants mon ancestor. A clade is a group of organisms related by Taxonomists differ in their acceptance of paraphyletic groups. For xample, some taxonomists arbitrarily group orangutans, gorillas, descent, and this approach to constructing a phylogeny is and chimpanzees into the paraphyletic family Pongidae, separate called cladistics. Cladistics infers phylogeny (that is, from humans. Other taxonomists do not use the family pongidae builds family trees) according to similarities derived from their classifications because gorillas and chimpanzees are more a common ancestor, so-called derived characters. a de- closely related to humans than to orangutans. () A polyphyletic rived character that is unique to a particular clade is group does not contain the most recent common ancestor of the ometimes called a synapomorphy. The key to the ap- group, and taxonomists do not assign taxa to polyphyletic groups proach is being g able to identify morphological, physio For example, sharks and whales could be classified in the same logical,or behavioral traits that differ among the organ- group because they have similar shapes, anatomical features, and isms being studied and can be attributed to a common habitats. However, their similarities reflect convergent evolution, ancestor. By examining the distribution of these traits not common ancestry among the organisms, it is possible to construct a clado- 654 Part IX Viruses and Simple organism
Evolutionary Classifications After naming and classifying some 1.5 million organisms, what have biologists learned? One very important advantage of being able to classify particular species of plants, animals, and other organisms is that individuals of species that are useful to humans as sources of food and medicine can be identified. For example, if you cannot tell the fungus Penicillium from Aspergillus, you have little chance of producing the antibiotic penicillin. In a thousand ways, just having names for organisms is of immense importance in our modern world. Taxonomy also enables us to glimpse the evolutionary history of life on earth. The more similar two taxa are, the more closely related they are likely to be. By looking at the differences and similarities between organisms, biologists can construct an evolutionary tree, or phylogeny, inferring which organisms evolved from which other ones, in what order, and when. The reconstruction and study of phylogenies is called systematics. Within a phylogeny, a grouping can be either monophyletic, paraphyletic, or polyphyletic. A monophyletic group includes the most recent common ancestor of the group and all of its descendants. A paraphyletic group includes the most recent common ancestor of the group but not all of its descendants. And, a polyphyletic group does not include the most recent common ancestor of all the members of the group. Monophyletic groups are commonly assigned names, but systematists will not assign a taxonomic classification to a polyphyletic group. Paraphyletic groups may be considered taxa by some scientists, although they do not accurately represent the evolutionary relationships among the members of the group (figure 32.6). Cladistics A simple and objective way to construct a phylogenetic tree is to focus on key characters that a group of organisms share because they have inherited them from a common ancestor. A clade is a group of organisms related by descent, and this approach to constructing a phylogeny is called cladistics. Cladistics infers phylogeny (that is, builds family trees) according to similarities derived from a common ancestor, so-called derived characters. A derived character that is unique to a particular clade is sometimes called a synapomorphy. The key to the approach is being able to identify morphological, physiological, or behavioral traits that differ among the organisms being studied and can be attributed to a common ancestor. By examining the distribution of these traits among the organisms, it is possible to construct a clado- 654 Part IX Viruses and Simple Organisms 32.2 Scientists construct phylogenies to understand the evolutionary relationships among organisms. Ray Shark Whale Cow Orangutan Gorilla Chimpanzee Human Monophyletic group Ray Shark Whale Cow Orangutan Gorilla Chimpanzee Human Paraphyletic group Ray Shark Whale Cow Orangutan Gorilla Chimpanzee Human Polyphyletic group (a) (b) (c) FIGURE 32.6 (a) A monophyletic group consists of the most recent common ancestor and all of its descendants. All taxonomists accept monophyletic groups in their classifications and in the above example would give the name “Apes” to the orangutans, gorillas, chimpanzees, and humans. (b) A paraphyletic group consists of the most recent common ancestor and some of its descendants. Taxonomists differ in their acceptance of paraphyletic groups. For example, some taxonomists arbitrarily group orangutans, gorillas, and chimpanzees into the paraphyletic family Pongidae, separate from humans. Other taxonomists do not use the family Pongidae in their classifications because gorillas and chimpanzees are more closely related to humans than to orangutans. (c) A polyphyletic group does not contain the most recent common ancestor of the group, and taxonomists do not assign taxa to polyphyletic groups. For example, sharks and whales could be classified in the same group because they have similar shapes, anatomical features, and habitats. However, their similarities reflect convergent evolution, not common ancestry
Traits. Jaws Lungs Amniotic No tail Bipedal FIGURE 32.7 a cladogram. Morphological data for a group of seven vertebrates is tabulated.A“1” Lamprey 0 indicates the presence of a trait, or derived character,anda“o” indicates the absence of Shark 0 0 0 0 the trait. a tree, or cladogram, diagrams the proposed evolutionary relationships among 0 0 0 based on the derived characters. The derived characters Lizard 0 tween the cladogram branch points are 0 0 shared by all organisms above the branch point and are not present in any below it. The 1 outgroup, in this case the lamprey, does not possess any of the derived characters Human 1 1 Lamprey Shark Salamander Lizard Gorilla Human Amniotic embrane gram(figure 32.7), a branching diagram that represents Cladistics is a relatively new approach in biology and has ogen become popular among students of evolution. This is be- In traditional phylogenies, proposed ancestors will cause it does a very good job of portraying the order in often be indicated at the nodes between branches, and the which a series of evolutionary events have occurred. The lengths of branches correspond to evolutionary time, with great strength of a cladogram is that it can be completely extinct groups having shorter branches. In contrast, clado- objective. In fact, most cladistic analyses involve man grams are not true family trees in that they do not identify characters, and computers are required to make the com- ancestors, and the branch lengths do not reflect evolution ary time(see figure 32. 6). Instead, they convey compara Sometime it is necessary to"weight"characters, or take tive information about relative relationships. Organisms into account the variation in the "strength"of a character that are closer together on a cladogram simply share such as the size or location of a fin or the effectiveness of more recent common ancestor than those that are farther a lung. To reduce a systematist's bias even more, many apart. Because the analysis is comparative, it is necessary to analyses will be run through the computer with the traits have something to anchor the comparison to, some solid weighted differently each time. Under this procedure, ound against which the comparisons can be made. To several different cladograms will be constructed, the goal rather different organism(but not too different)to serve as o ing to choose the one that is the most parsimonious, achieve this, each cladogram must contain an outgroup, amplest and thus most likely. Reflecting the impor a baseline for comparisons among the other organisms tance of evolutionary processes to all fields of biolog ing evaluated, the ingroup. For example, in figure 32.7, most taxonomy today includes at least some element of the lamprey is the outgroup to the clade of animals that cladistic analysis Chapter 32 How We Classify Organisms 655
gram (figure 32.7), a branching diagram that represents the phylogeny. In traditional phylogenies, proposed ancestors will often be indicated at the nodes between branches, and the lengths of branches correspond to evolutionary time, with extinct groups having shorter branches. In contrast, cladograms are not true family trees in that they do not identify ancestors, and the branch lengths do not reflect evolutionary time (see figure 32.6). Instead, they convey comparative information about relative relationships. Organisms that are closer together on a cladogram simply share a more recent common ancestor than those that are farther apart. Because the analysis is comparative, it is necessary to have something to anchor the comparison to, some solid ground against which the comparisons can be made. To achieve this, each cladogram must contain an outgroup, a rather different organism (but not too different) to serve as a baseline for comparisons among the other organisms being evaluated, the ingroup. For example, in figure 32.7, the lamprey is the outgroup to the clade of animals that have jaws. Cladistics is a relatively new approach in biology and has become popular among students of evolution. This is because it does a very good job of portraying the order in which a series of evolutionary events have occurred. The great strength of a cladogram is that it can be completely objective. In fact, most cladistic analyses involve many characters, and computers are required to make the comparisons. Sometime it is necessary to “weight” characters, or take into account the variation in the “strength” of a character, such as the size or location of a fin or the effectiveness of a lung. To reduce a systematist’s bias even more, many analyses will be run through the computer with the traits weighted differently each time. Under this procedure, several different cladograms will be constructed, the goal being to choose the one that is the most parsimonious, or simplest and thus most likely. Reflecting the importance of evolutionary processes to all fields of biology, most taxonomy today includes at least some element of cladistic analysis. Chapter 32 How We Classify Organisms 655 Lamprey Tiger Gorilla Human Jaws Lungs Amniotic membrane Hair No tail Bipedal Shark Salamander Lizard Traits: Organism Jaws Lungs Amniotic membrane Hair No tail Bipedal Lamprey Shark Salamander Lizard Tiger Gorilla Human 00 0 0 0 0 10 0 0 0 0 11 0 0 0 0 11 1 0 0 0 11 1 1 0 0 11 1 1 1 0 11 1 1 1 1 FIGURE 32.7 A cladogram. Morphological data for a group of seven vertebrates is tabulated. A “1” indicates the presence of a trait, or derived character, and a “0” indicates the absence of the trait. A tree, or cladogram, diagrams the proposed evolutionary relationships among the organisms based on the presence of derived characters. The derived characters between the cladogram branch points are shared by all organisms above the branch point and are not present in any below it. The outgroup, in this case the lamprey, does not possess any of the derived characters
Class Reptilia Mammalia Reptilia Class Aves Archosaurs Crocodilians Birds Lizards and Lizards and Mammals Turtles Crocodilians Birds Dinosaur (a) Traditional phylogeny and taxonomic classification FIGURE 32.8 Traditional and cladistic interpretations of vertebrate classification. Traditional and cladistic taxonomic analyses of the same set of traditional analysis, key characteristics such as feathers and hollow bones are weighted more heavily than others, placing the birds in their own group and the reptiles in a paraphyletic group. (b) Cladistic analysis gives equal weight to these and many other characters and places irds in the same grouping with crocodiles, reflecting the close evolutionary relationship between the two. Also, in the traditional phylogeny, the branch leading to the dinosaurs is shorter because the length corresponds to evolutionary time. In cladograms, branch lengths do not correspond to evolutionary time. Traditional Taxonomy cestry but ignores the immense evolutionary impact of a Weighting characters lies at the core of traditional taxon- derived character such as feathers omy. In this approach, taxa are assigned based on a vast Overall, classifications based on traditional taxonomy amount of information about the morphology and biology are information-rich, while classifications based on clado- of the organism gathered over a long period of time. Tradi- grams need not be. Traditional taxonomy is often used tional taxonomists consider both the common descent and when a great deal of information is available to guide char- amount of adaptive evolutionary change when grouping or- little information is available about how the character af- anisms. The large amount of information used by tradi tional taxonomists permits a knowledgeable weighting of fects the life of the organism. DNA sequence comparisons, characters according to their biological significance. In tra for example, lend themselves well to cladistics--you have a ditional taxonomy, the full observational power and judg- great many derived characters(DNA sequence differences) ment of the biologist is brought to bear--and also any bi- but little or no idea of what impact the sequence diffe ses he or she may have. For example, in classifying the ences have on the organ terrestrial vertebrates, traditional taxonomists place birds in their own class(Aves), giving great weight to the characters A phylogeny may be represented as a cladogram based that made powered flight possible, such as feathers. How on the order in which groups evolved. Traditional ever,cladists(figure 32.8) lumps birds in among the rep- taxonomists weight characters according to assumed tiles with crocodiles. This accurately reflects their true an- mportance 656 Part IX Viruses and Simple organism
Traditional Taxonomy Weighting characters lies at the core of traditional taxonomy. In this approach, taxa are assigned based on a vast amount of information about the morphology and biology of the organism gathered over a long period of time. Traditional taxonomists consider both the common descent and amount of adaptive evolutionary change when grouping organisms. The large amount of information used by traditional taxonomists permits a knowledgeable weighting of characters according to their biological significance. In traditional taxonomy, the full observational power and judgment of the biologist is brought to bear—and also any biases he or she may have. For example, in classifying the terrestrial vertebrates, traditional taxonomists place birds in their own class (Aves), giving great weight to the characters that made powered flight possible, such as feathers. However, cladists (figure 32.8) lumps birds in among the reptiles with crocodiles. This accurately reflects their true ancestry but ignores the immense evolutionary impact of a derived character such as feathers. Overall, classifications based on traditional taxonomy are information-rich, while classifications based on cladograms need not be. Traditional taxonomy is often used when a great deal of information is available to guide character weighting, while cladistics is a good approach when little information is available about how the character affects the life of the organism. DNA sequence comparisons, for example, lend themselves well to cladistics—you have a great many derived characters (DNA sequence differences) but little or no idea of what impact the sequence differences have on the organism. A phylogeny may be represented as a cladogram based on the order in which groups evolved. Traditional taxonomists weight characters according to assumed importance. 656 Part IX Viruses and Simple Organisms Mammals Mammals Turtles Turtles Crocodilians Crocodilians Birds Birds Dinosaurs Dinosaurs Lizards and snakes Lizards and snakes Early reptiles Class Mammalia Class Reptilia Class Aves Mammalia Reptilia Archosaurs (a) Traditional phylogeny and taxonomic classification (b) Cladogram and cladistic classification FIGURE 32.8 Traditional and cladistic interpretations of vertebrate classification. Traditional and cladistic taxonomic analyses of the same set of data often produce different results: in these two classifications of vertebrates, notice particularly the placement of the birds. (a) In the traditional analysis, key characteristics such as feathers and hollow bones are weighted more heavily than others, placing the birds in their own group and the reptiles in a paraphyletic group. (b) Cladistic analysis gives equal weight to these and many other characters and places birds in the same grouping with crocodiles, reflecting the close evolutionary relationship between the two. Also, in the traditional phylogeny, the branch leading to the dinosaurs is shorter because the length corresponds to evolutionary time. In cladograms, branch lengths do not correspond to evolutionary time