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Levetin−McMahon: Plants and Society, Fifth Edition II. Introduction to Plant Life: Botanical Principles 8. Plant Systematics and Evolution © The McGraw−Hill Companies, 2008 C H A PT E R OU T L I N E Early History of Classification 124 Carolus Linnaeus 124 How Plants are Named 126 Common Names 126 A CLOSER LOOK 8.1 The Language of Flowers 128 Scientific Names 129 Taxonomic Hierarchy 130 Higher Taxa 130 What Is a Species? 131 A CLOSER LOOK 8.2 Saving Species Through Systematics 133 The Influence of Darwin’s Theory of Evolution 134 The Voyage of the HMS Beagle 134 Natural Selection 136 Phylocode 137 Chapter Summary 137 Review Questions 137 Further Reading 138 K EY C O N CE P T S 1. 2. 3. 4. Scientific names are two-word names called binomials that are internationally recognized by the scientific community. Carolus Linnaeus, an eighteenth-century Swedish botanist, started the binomial system and is therefore known as the Father of Taxonomy. With the publication in 1859 of On the Origin of Species, Charles Darwin proposed that species are not static entities but are works in progress that evolve in response to environmental pressures. Natural selection favors the survival and reproduction of those individuals in a species that possess traits that better adapt them to a particular environment. C H A P T E R 8 Plant Systematics and Evolution Fossils, such as this 160 million year old arucarian pine cone from Argentina, were part of the evidence Charles Darwin used to formulate his theory of evolution of species by means of natural selection. 123 Levetin−McMahon: Plants and Society, Fifth Edition 124 UNIT II II. Introduction to Plant Life: Botanical Principles 8. Plant Systematics and Evolution © The McGraw−Hill Companies, 2008 Introduction to Plant Life: Botanical Principles P lant systematics is the branch of botany that is concerned with the naming, identification, evolution, and classification (arrangement into groups with common characteristics) of plants. In a strict sense, plant taxonomy is the science of naming and classifying plants; however, in this book the terms taxonomy and systematics are used interchangeably. The simplest form of classification is a system based on need and use; early humans undoubtedly classified plants into edible, poisonous, medicinal, and hallucinogenic categories. EARLY HISTORY OF CLASSIFICATION The earliest known formal classification was proposed by the Greek naturalist Theophrastus (370–285 B.C.), who was a student of Aristotle. In his botanical writings (Enquiries into Plants and The Causes of Plants), he described and classified approximately 500 species of plants into herbs, undershrubs, shrubs, and trees. Because his influence extended through the Middle Ages, he is regarded as the Father of Botany. Two Roman naturalists who also had long-lasting impacts on plant taxonomy were Pliny the Elder (A.D. 23–79) and Dioscorides (first century A.D.). Both described medicinal plants in their writings, and Dioscorides’s Materia Medica remained the standard medical reference for 1,500 years. From this period through the Middle Ages, little new botanical knowledge was added. Blind adherence to the Greek and Roman classics prevailed, using manuscripts painstakingly copied by hand in monasteries throughout Europe. The revival of botany after its stagnation in the Middle Ages began early in the Renaissance with the renewed interest in science and other fields of study. The invention of the printing press in the middle of the fifteenth century allowed botanical works to be more easily produced than ever before. These richly illustrated books, known as herbals, dealt largely with medicinal plants and their identification, collection, and preparation. The renewal of interest in taxonomy can be traced to the work of several herbalists; in fact, this period of botanical history from the fifteenth through the seventeenth centuries is known as the Age of Herbals. Another factor in the revival of taxonomy was the global exploration by the Europeans during this period, which led to the discoveries of thousands of new plant species. In less than 100 years more plants were introduced to Europe than in the previous 2,000 years. Carolus Linnaeus (fig. 8.1) was born in May 1707, in southern Sweden, the son of a clergyman. He became interested in botany at a very young age through the influence of his father, who was an avid gardener and amateur botanist. It was expected that Linnaeus would also become a clergyman, but in school he did not do well in theological subjects. He did, however, excel in the natural sciences and entered the University of Lund in 1727 to pursue studies in natural science and medicine. (At this time medical schools were the centers of botanical study because physicians were expected to know the plant sources of medicines in use.) After one year he transferred to the University of Uppsala, the most prestigious university in Sweden. It was here that he published his first botanical papers, which laid the foundations for his later works in classification and plant sexuality. In 1732, he undertook a solo expedition to Lapland to catalog the natural history of this relatively unknown area. He later published Flora Lapponica, a detailed description of the plants of this area. Linnaeus received his medical degree in 1735 from the University of Harderwijk in the Netherlands. Soon he came under the patronage of George Clifford, a director of the Dutch East India Company and one of the wealthiest men in Europe. He served as Clifford’s personal physician and as curator of his magnificent gardens, which housed specimens from around the world. The 3 years he spent in the Netherlands Carolus Linnaeus By the beginning of the eighteenth century, it was common to name plants using a polynomial (see fig. 8.4), which included a single word name for the plant (today called the genus name), followed by a lengthy list of descriptive terms, all in Latin. This system had flaws. It was not standardized; different polynomials existed for the same plant; and it was cumbersome to remember some of the longer polynomials, which could be a paragraph in length. This was the state of taxonomy during the time of Linnaeus. Figure 8.1 Statue of Carolus Linnaeus (1707–1778) holding flowers of Indian blanket (Gaillardia pulchella) at the Linnaeus Teaching Garden, Tulsa, OK. Levetin−McMahon: Plants and Society, Fifth Edition II. Introduction to Plant Life: Botanical Principles 8. Plant Systematics and Evolution CHAPTER 8 Figure 8.2 Frontispiece of Systema Naturae, one of the writings of Linnaeus, in which he expounded on his ideas of classification. were the most productive period in his life. During that time, he completed several books and papers including Systema Naturae, Fundamenta Botanica, and Genera Plantarum, which expanded on his ideas of classification (fig. 8.2). © The McGraw−Hill Companies, 2008 Plant Systematics and Evolution 125 He returned to Sweden in 1738 and soon married Sara Elisabeth Moraea. After setting up a medical practice in Stockholm, he was appointed physician to the Swedish Admiralty, specializing in the treatment of venereal diseases. In 1741, he returned to the University of Uppsala as professor of medicine and botany, a position he retained until retirement in 1775. Linnaeus was a popular teacher who attracted students from all over Europe. Many of his students became famous professors in their own right; others traveled to distant lands collecting unknown specimens for Linnaeus to classify. After suffering several strokes, he died in January 1778. One of Linnaeus’s achievements was his sexual system of plant classification, which did much to popularize the study of botany. This system was based on the number, arrangement, and length of stamens and thus divided flowering plants into 24 classes. Using this system, it was possible for anyone to identify and name unknown plants. At the time, his language was risqué because he compared floral parts to human sexuality, with stamens referred to as husbands and pistils∗ as wives; for example, “husband and wife have the same bed” meant stamens and pistils in the same flower (fig. 8.3). ∗ Note that the older term of pistil, rather than carpel, has been used in this key. Vegetable Kingdom Key of the Sexual System Marriages of plants Florescence Public marriages Flowers visible to every one In one bed Husband and wife have the same bed All the flowers hermaphrodite: stamens and pistils in the same flower Without affinity Husbands not related to each other Stamens not joined together in any part With equality All the males of equal rank Stamens have no determinate proportion of length 4. Four males 1. One male 2. Two males 5. Five males 3. Three males 6. Six males 7. Seven males 8. Eight males 9. Nine males 10. Ten males 11. Twelve males 12. Twenty males 13. Many males With subordination Some males above others Two stamens are always lower than the others 14. Two powers 15. Four powers (a) With affinity Husbands related to each other Stamens cohere with each other, or with the pistil 16. One brotherhood 18. Many brotherhoods 17. Two brotherhoods 19. Confederate males 20. Feminine males In two beds Husband and wife have separate beds Male flowers and female flowers in the same species 21. One house 22. Two houses Clandestine marriages Flowers scarce visible to the naked eye 24. Clandestine marriages (b) 23. Polygamies Figure 8.3 (a) Linnaeus’s sexual system related floral parts to human sexuality. (b) Hibiscus in the Mallow Family (Malvaceae) keys out to feminine males because the stamens are attached to the style. Levetin−McMahon: Plants and Society, Fifth Edition 126 UNIT II II. Introduction to Plant Life: Botanical Principles 8. Plant Systematics and Evolution © The McGraw−Hill Companies, 2008 Introduction to Plant Life: Botanical Principles Dr. Johann Siegesbeck, a contemporary of Linnaeus and director of the botanical garden in St. Petersburg, was shocked at the analogies and said such loathsome harlotry as several males to one female would not be permitted in the vegetable kingdom by the Creator . . . . Who would have thought that bluebells, lilies, and onions could be up to such immorality? Despite the opposition of some, the Linnaean sexual method was easy to understand and simple for even the amateur botanist to use. This method, however, was an artificial system grouping together clearly unrelated plants (in his system, cherries and cacti were grouped together); by the early nineteenth century it was abandoned in favor of systems that reflected natural relationships among plants. Linnaeus’s greatest accomplishment was his adoption and popularization of a binomial system of nomenclature. When he described new plants, he conformed to the current practice of using a polynomial. For convenience, however, he began to add in the margin a single descriptive adjective that would identify unequivocally a particular species (fig. 8.4). He called this adjective the trivial name. This combination later developed into the two-word scientific name, or binomial, described in the next section. Linnaeus used this system consistently in Species Plantarum, published in 1753. This work contains descriptions and names of 5,900 plants, all the plants known to Linnaeus. The binomial system simplified scientific names and was soon in wide use. In 1867, a group of botanists at the International Botanical Congress in Paris established rules governing plant nomenclature and classification. They established Species Plantarum as the starting point for scientific names. Although the rules (formalized in the International Code of Botanical Nomenclature) have been modified over the years, the 1753 date is still valid, and many names first proposed by Linnaeus are still in use today. Linnaeus’s contributions were not limited to botany since the binomial system is used for all known organisms. He is credited with naming approximately 12,000 plants and animals; for all his contributions to the field of taxonomy, he is known as the Father of Taxonomy. HOW PLANTS ARE NAMED Names are useful because they impart some information about a plant; it may be related to flower color, leaf shape, flavor, medicinal value, season of blooming, or location. Names are necessary for communication; “if you know not the name, knowledge of things is wasted.” This discussion begins with a look at common names, or what plants are called locally, and follows with an examination of internationally recognized scientific names. Figure 8.4 A photograph from Species Plantarum illustrates the beginning of the binomial system. Note the trivial names in the margin next to the polynomial description for each species. The trivial name was later designated as the species epithet, which, together with the generic, forms the binomial. Common Names A close look at common names often reveals a keen sense of observation, a fanciful imagination, or even a sense of humor: trout lily, milkweed, Dutchman’s pipe, Texas bluebonnet, ragged sailor, and old maid’s nightcap (table 8.1). Sometimes the names even convey feelings or emotions (see A Closer Look 8.1—The Language of Flowers). Names have evolved over centuries but are sometimes only used in a limited geographical area. Even short distances away, other common names may be used for the same plant. Consider, for example, the many names for the tree that many people call osage orange (Maclura pomifera) (fig. 8.5a): bodeck, bodoch, bois d’arc, bow-wood, osage apple tree, hedge, hedge apple, hedge osage, hedge-plant osage, horse apple, mock orange, orange-like maclura, osage apple, and wild orange. Levetin−McMahon: Plants and Society, Fifth Edition II. Introduction to Plant Life: Botanical Principles 8. Plant Systematics and Evolution CHAPTER 8 © The McGraw−Hill Companies, 2008 Plant Systematics and Evolution 127 Table 8.1 Some Common Names and Their Meanings Names in Commemoration Douglas-fir David Douglas, plant collector (1798–1834) Camellia Georg Josef Kamel, pharmacist (1661–1706) Gerber daisy Traugott Gerber, German explorer (?–1743) Freesia Friedrich H. T. Freese, German physician (?–1876) Names That Describe Physical Qualities Dusty miller White woolly leaves Dutchman’s breeches Shape of flower Goldenrod Shape and color of inflorescence Indian pipe Shape of flower with stem Cattail Inflorescence of carpellate flowers Lady’s slipper Shape of this orchid’s flower Milkweed Milky juice when plant is cut Skunk cabbage Fetid odor of inflorescence Cheeses Fruit resembles a round head of cheese Smoke tree Plumelike pedicels Shagbark hickory Shedding bark Redbud Color of flower buds Quaking aspen Rustling leaves Bluebell Color and shape of flower Crape myrtle Wavy edges of petals (a) Scientific Names That Have Become Common Names Hydrangea Abelia Vanilla Narcissus Coreopsis Gladiolus Names That Indicate Use Daisy fleabane Gets rid of fleas Boneset A tonic from this plant can heal bones Feverwort Medicinal property to reduce fever Kentucky coffee tree Seeds roasted for coffee substitute Belladonna Juice used to beautify by producing pallid skin and dilated, mysterious eyes Names That Indicate Origin, Location, or Season Pacific yew Grows along northern Pacific coast Spring beauty One of the first flowers of spring Marshmallow Found in wet, marshy habitat Daylily Flowers last only a day Four-o’clock Flowers open in late afternoon Japanese honeysuckle Country of origin is Japan (b) Figure 8.5 Mock orange is a common name shared by (a) the tree Maclura pomifera and (b) the shrub Philadelphus lewisii—two entirely different species of plants. Levetin−McMahon: Plants and Society, Fifth Edition II. Introduction to Plant Life: Botanical Principles 8. Plant Systematics and Evolution © The McGraw−Hill Companies, 2008 A CLOSER LOOK 8.1 The Language of Flowers Through traditions, some flowers became symbolic of certain emotions and feelings. This was sometimes even reflected in their common names; two straightforward examples are forget-me-not flowers, which conveyed the sentiment “remember me,” and bachelor’s button, which indicated the single status of the wearer. This symbolism reached its peak during the Victorian era, when almost every flower and plant had a special meaning. In Victorian times, it was possible to construct a bouquet of flowers that imparted a whole message (box fig. 8.1). A Victorian suitor might send a bouquet of jonquils, white roses, and ferns to his intended, which indicated that he desired a return of affection, he was worthy of her love, and he was fascinated by her. This “language” became so popular that dictionaries were printed to interpret floral meanings. One of the most popular dictionaries was the Language of Flowers (1884) by Kate Greenaway, a well-known illustrator of children’s books. Following is a small sampling of some common flowers and plants and what they symbolized: Amaryllis: pride Apple blossom: preference Bachelor’s buttons: celibacy Bluebell: constancy Buttercup: ingratitude Yellow chrysanthemum: slighted love Daffodil: regard Daisy: innocence Dogwood: durability Elm: dignity Goldenrod: caution Holly: foresight Honeysuckle: generous and devoted affection Ivy: fidelity Lavender: distrust Lichen: dejection Lily of the valley: return of happiness Live oak: liberty Magnolia: love of nature Marigold: grief Mock orange: counterfeit Oak leaves: bravery Palm: victory Pansy: thoughts Spring crocus: youthful gladness 128 Box Figure 8.1 Flowers convey a message all their own. Dwarf sunflower: adoration Tall sunflower: haughtiness Yellow tulip: hopeless love Blue violet: faithfulness Wild grape: charity Zinnia: thought of absent friends Even today, several plants have well-known symbolic meanings. Red roses convey passionate love; a four-leaf clover means luck; orange blossoms symbolize weddings; and an olive branch indicates peace. Floral colors can also communicate feelings, with red indicating passion; blue, security; yellow, cheer; white, sympathy; and orange, friendship. With some thought, it is possible to find the right flower and color to express the exact message. Levetin−McMahon: Plants and Society, Fifth Edition II. Introduction to Plant Life: Botanical Principles 8. Plant Systematics and Evolution © The McGraw−Hill Companies, 2008 CHAPTER 8 On the other hand, different plants may share the same common name. Although mock orange is one of the common names for osage orange, the name mock orange is usually associated with a completely unrelated group of flowering shrubs (Philadelphus spp.) (fig. 8.5b). These examples point out the difficulties with common names; one plant may be known by several different names, and the same name may apply to several different plants. The need to have one universally accepted name is fulfilled with scientific names. Scientific Names Each kind of organism is known as a species, and similar species form a group called a genus (pl., genera). Each species has a scientific name in Latin that consists of two elements; the first is the genus and the second is the specific epithet. Such a name is a binomial, literally two names, and is always italicized or underlined; for example, Maclura pomifera is the scientific name for osage orange. A rough analogy of the binomial concept can be seen in a list of names in a telephone directory, where the surname “Smith” (listed first) represents the genus and the first names (John, Frank, and Mary) define particular species within the genus. In the binomial, the first name is a noun and is capitalized; the second, written in lower case, is usually an adjective. After the first mention of a binomial, the genus name can be abbreviated to its first letter, as in M. pomifera, but the specific epithet can never be used alone. The genus name, however, can be used alone, especially when referring to several species within a genus; for example, Philadelphus refers to over 50 species of mock orange. The specific epithet can be replaced by an abbreviation for species, “sp.” (or “spp.” plural), when the name of the species is unknown or unnecessary for the discussion. In the previous example, Philadelphus sp. refers to one species of mock orange whereas Philadelphus spp. refers to more than one species. Scientific names may be just as descriptive as common names, and translation of the Latin (or latinized Greek) is informative (table 8.2). Sometimes either the genus name or the specific epithet is commemorative, derived from the name of a botanist or other scientist. Some specific epithets are frequently used with more than one genus, and knowledge of their meanings will provide some insight into scientific names encountered later in this text (table 8.3). A complete scientific name also includes the name or names of the author or authors (often abbreviated) who first described the species or placed it in a particular genus. For example, the complete scientific name for corn is Zea mays L.; the “L” indicates that Linnaeus named this species. On the other hand, the complete name for osage orange is Maclura pomifera (Raf.) Schneid. This author citation indicates that Rafinesque-Schmaltz first described the species, giving it the specific epithet pomifera, but Schneider later put it in the genus Maclura. In this text, the author citations are omitted for simplicity. Plant Systematics and Evolution 129 Table 8.2 Genus Names and Their Meanings Names in Commemoration Begonia Michel Begon, patron of botany (1638–1710) Forsythia William Forsyth, gardener at Kensington Palace (1737–1804) Bougainvillea Louis Antoine de Bougainville, explorer and scientist (1729–1811) Fuchsia Leonhard Fuchs, German physician and herbalist (1501–1566) Zinnia Johann Gottfried Zinn (1727–1759) Wisteria Caspar Wistar, American professor of plant anatomy (1761–1818) Names That Describe Physical Qualities Myriophyllum Finely divided leaves Chlorophytum Green plant Lunaria Moon, refers to appearance of pods Helianthus Sunflower Zebrina Zebra, refers to striped leaves Trillium Floral parts in threes Tetrastigma Four-lobed stigma Ribes Acid tasting; refers to fruit Polygonum Many knees; refers to jointed stems Zanthoxylum Yellow wood Sagittaria Arrow; refers to arrowhead leaves Names from Aboriginal or Classic Origins Avena Oats (Latin) Triticum Wheat (Latin) Allium Garlic (Greek) Catalpa Catalpa (North American Indian) Vitis Grape vine (Latin) Ulmus Elm (Latin) Pinus Pine (Latin) Solidago Make whole or strengthen Angelica Angelic medicinal properties Cimcifuga Repel bugs Saponaria Soap; refers to soap that can be made from the plant Pulmonaria Lung; used to treat infections of the lung Potentilla Powerful; refers to its potent medicinal properties Names That Indicate Use Names That Indicate Location Elodea Grows in marshes Petrocoptis Break rock; refers to habit of growing in rock crevices Levetin−McMahon: Plants and Society, Fifth Edition 130 UNIT II II. Introduction to Plant Life: Botanical Principles 8. Plant Systematics and Evolution © The McGraw−Hill Companies, 2008 Introduction to Plant Life: Botanical Principles Table 8.3 Common Scientific Epithets and Their Meanings A scientific name is unique, referring to only one species and universally accepted among scientists. It is the key to unlocking the door to the accumulated knowledge about a plant. Imagine the confusion if only common names were used and a reference was made to mock orange. Would this reference allude to Maclura pomifera or to a species of Philadelphus? acidosus, -a, -um Sour aestivus, -a, -um Developing in summer albus, -a, -um White alpinus, -a, -um Alpine annus, -a, -um Annual arabicus, -a, -um Of Arabia arboreus, -a, -um Treelike arvensis, -a, -um Of the field biennis, -a, -um Biennial campester, -tris, -tre Of the pasture canadensis, -is, -e From Canada carolinianus, -a, -um From the Carolinas chinensis, -is, -e From China coccineus, -a, -um Scarlet deliciosus, -a, -um Delicious dentatus, -a, -um Having teeth Higher Taxa domesticus, -a, -um Domesticated edulis, -is, -e Edible esculentus, -a, -um Tasty europaeus, -a, -um From Europe fetidus, -a, -um Bad smelling floridus, -a, -um Flowery foliatus, -a, -um Leafy hirsutus, -a, -um Hairy japonicus, -a, -um From Japan lacteus, -a, -um Milky white littoralis, -is, -e Growing by the shore luteus, -a, -um Yellow mellitus, -a, -um Honey-sweet niger, -ra, -um Black occidentalis, -is, -e Western odoratus, -a, -um Fragrant officinalis, -is, -e Used medicinally robustus, -a, -um Hardy ruber, -ra, -rum Red saccharinus, -a, -um Sugary sativus, -a, -um Cultivated silvaticus, -a, -um Of the woods sinensis, -is, -e Chinese speciosus, -a, -um Showy tinctorius, -a, -um Used for dyeing utilis, -is, -e Useful vernalis, -is, -e Spring flowering virginianus, -a, -um From Virginia Species that have many characteristics in common are grouped into a genus, one of the oldest concepts in taxonomy (fig. 8.6). In almost every society, the concept of genus has developed in colloquial language; in English the words oak, maple, pine, lily, and rose represent distinct genera. These intuitive groupings reflect natural relationships based on shared vegetative and reproductive characteristics. Many of the scientific names of genera are directly taken from the ancient Greek and Roman common names for these genera (Quercus, old Latin word for oak). The next higher category, or taxon (pl., taxa), above the rank of genus is the family. Families are composed of related genera that again (as in a genus) share combinations of morphological traits. In the angiosperms, floral and fruit features are often used to characterize a family. Ideally, the family represents a natural group with a common evolutionary lineage; some families may be very small while others are very large, but still cohesive, groups. A few common angiosperm families that have special economic importance are listed in Table 8.4. According to the International Code of Botanical Nomenclature, each family is assigned one name, which is always capitalized and ends in the suffix -aceae. The old established names of several well-known families present exceptions to this rule. Both the traditional and standardized names are used for these families (table 8.5). The taxa above the rank of family and their appropriate endings are presented in Table 8.6. The higher the taxonomic category, the more inclusive the grouping (fig. 8.7). Families are grouped into orders, orders into classes, classes into divisions (phyla)*, and divisions into kingdoms. A domain is above the kingdom level and is the vulgaris, -is, -e Common TAXONOMIC HIERARCHY In addition to genus and species, other taxonomic categories exist to conveniently group related organisms. As pointed out, Linnaeus used an artificial system; however, today scientists use a phylogenetic system to group plants. In a phylogenetic system, information is gathered from morphology, anatomy, cell structure, biochemistry, genetics, and the fossil record to determine evolutionary relationships and, therefore, natural groupings among plants. *Either division or phylum (sing.; phyla, plural) may be used to indicate the taxonomic rank that is composed of a group of related classes. Traditionally, division has been the term preferred by botanists and will be used throughout this textbook. Levetin−McMahon: Plants and Society, Fifth Edition II. Introduction to Plant Life: Botanical Principles 8. Plant Systematics and Evolution CHAPTER 8 (a) Quercus phellos (willow oak) © The McGraw−Hill Companies, 2008 Plant Systematics and Evolution 131 (b) Quercus rubra (red oak) Figure 8.6 A genus is a group of species that share many characteristics in common. Although willow oak (Quercus phellos) and red oak (Quercus rubra) are clearly distinct species, they are both recognizable as belonging to the oak (Quercus) genus by the presence of acorns. most inclusive taxonomic category. The complete classification of a familiar species is also illustrated in Table 8.6. In addition to the categories already described, biologists also recognize intermediate categories with the “sub” for any rank; for example, divisions may be divided into subdivisions, and species may be divided into subspecies (varieties and forms are also categories below the rank of species). Although the International Code of Botanical Nomenclature has rules that govern the assignment of names and define the taxonomic hierarchy, it does not set forth any particular classification system. As a result, there are several organizational schemes that have supporters. These systems differ in the numbers of classes, divisions, kingdoms, and even domains and how they are related to one another. Presently most biologists use a three-domain, six-kingdom system, which will be described fully in Chapter 9. There is general agreement about the use of a three-domain, six-kingdom system, but biologists still debate the definition of a species. What Is a Species? As indicated previously, each kind of organism is known as a species. Although this intuitive definition, based on morphological similarities, works fairly well in many circumstances, it is limited; scientists have given much thought to the biological basis of a species. Many accept the biological species concept first proposed by Ernst Mayr in 1942, which defines a species as “a group of interbreeding populations reproductively isolated from any other such group of populations.” This definition presents problems when defining plant species. Many closely related plant species that are distinct morphologically are, in fact, able to interbreed; this is true for many species of oaks and sycamores. By contrast, a single plant species may have diploid and polyploid (more than the diploid number of chromosomes) individuals that may be reproductively isolated from each other. It is estimated that as many as 40% of flowering plants may be polyploids, with the evening primrose group a thoroughly studied example; an even higher percentage of polyploid species occurs in ferns. Because of these limitations, alternatives to the biological species concept have been suggested. The ecological species concept recognizes a species through its role in the biological community as defined by the set of unique adaptations within a particular species to its environment. The availablity of molecular sequence data for nucleic acids and proteins had led to the development of the genealogical species concept. Proponents utilize the distinct genetic history of organisms to differentiate species. Despite the lack of an all-inclusive botanical definition, the concept of “species” facilitates the naming, describing, and classifying of plants in a uniform manner. An inventory of the world’s species is the first step in preserving biodiversity, as discussed in A Closer Look 8.2—Saving Species through Systematics. Levetin−McMahon: Plants and Society, Fifth Edition 132 UNIT II II. Introduction to Plant Life: Botanical Principles 8. Plant Systematics and Evolution © The McGraw−Hill Companies, 2008 Introduction to Plant Life: Botanical Principles Table 8.4 Economically Important Angiosperm Families Scientific Family Name Common Family Name Economic Importance Aceraceae Maple Lumber (ash, maple), maple sugar Apiaceae Carrot Edibles (carrot, celery), herbs (dill), poisonous (poison hemlock) Arecaceae Palm Edibles (coconut), fiber oils and waxes, furniture (rattan) Asteraceae Sunflower Edibles (lettuce), oils (sunflower oil), ornamentals (daisy) Brassicaceae Mustard Edibles (cabbage, broccoli) Cactaceae Cactus Ornamentals, psychoactive plants (peyote) Cannabaceae Hemp Psychoactive (marijuana), fiber plants Cucurbitaceae Gourd Edibles (melons, squashes) Euphorbiaceae Spurge Rubber, medicinals (castor oil), edibles (cassava), ornamentals (poinsettia) Fabaceae Bean Edibles (beans, peas), oil, dyes, forage, ornamentals Fagaceae Beech Lumber (oak), dyes (tannins), ornamentals Iridaceae Iris Ornamentals Juglandaceae Walnut Lumber, edibles (walnut, pecan) Lamiaceae Mint Aromatic herbs (sage, basil) Lauraceae Laurel Aromatic oils (bay leaves), lumber Liliaceae Lily Ornamentals, poisonous plants Magnoliaceae Magnolia Ornamentals, lumber Malvaceae Mallow Fiber (cotton), seed oil, edibles (okra), ornamentals Musaceae Banana Edibles (bananas), fibers Myrticaceae Myrtle Timber, medicinals (eucalyptus), spices (cloves) Oleaceae Olive Lumber (ash), edible oil and fruits (olive) Orchidaceae Orchid Ornamentals, spice (vanilla) Papaveraceae Poppy Medicinal and psychoactive plants (opium poppy) Piperaceae Pepper Black pepper, houseplants Poaceae Grass Cereals, forage, ornamentals Ranunculaceae Buttercup Ornamentals, medicinal and poisonous plants Rosaceae Rose Fruits (apple, cherry), ornamentals (roses) Rubiaceae Coffee Beverage (coffee), medicinals (quinine) Rutaceae Citrus Edible fruits (orange, lemon) Salicaceae Willow Ornamentals, furniture (wicker), medicines (aspirin) Solanaceae Nightshade Edible (tomato, potato), psychoactive, poisonous (tobacco, mandrake) Theaceae Tea Beverage (tea) Vitaceae Grape Fruits (grapes), wine Levetin−McMahon: Plants and Society, Fifth Edition II. Introduction to Plant Life: Botanical Principles 8. Plant Systematics and Evolution © The McGraw−Hill Companies, 2008 A CLOSER LOOK 8.2 Saving Species through Systematics Earth is blessed with a tremendous variety of living organisms. About 1.4 million living microbes, fungi, plants, and animals have been identified by systematics. The number of yet undescribed species is much greater, with estimates ranging between 10 million and 100 million. Biodiversity is an inventory of the number and variety of organisms that inhabit Earth. We are currently in the midst of a biodiversity crisis; the variety of living species is declining owing to an accelerated extinction rate. Human activities are responsible for this terrible loss. Over 6.6 billion people at present inhabit Earth, and this number is expected to increase to over 9 billion in the next 50 years. Population pressures cause natural areas to be cleared for agriculture or expanding urbanization. More people also results in more pollution that fouls the land, sea, and air. All of these human-induced changes translate into a death toll upon the world’s biodiversity. Consider the tropical rain forests of the world. These forests are some of the most biologically diverse areas on the planet, home to approximately 70% of the world’s species. Unfortunately, these forests have been subjected to massive destruction. Scientists have calculated that species loss in the tropical rain forests is currently 1% to 5% per decade and will increase to 2% to 8% by the year 2015. This rate translates to an average loss of 9,000 species per year, or a heartbreaking 225,000 extinctions between 1990 and 2015. Why should we care about biodiversity? Biodiversity is the basis for the necessary essentials to human existence: food, fiber, fuel, and shelter. Of the estimated more than 250,000 species of angiosperms, nearly 20,000 have been used at one time or another as food for humans. Advances in agriculture are dependent upon the interaction between systematics and biodiversity. Since the 1960s, world crop yields have increased two- to four-fold. Part of this increase is due to the creation of improved crop varieties through breeding programs and more recently through genetic engineering. Locating and identifying relatives of crop species have been of critical importance to agricultural research in breeding for desirable characteristics. With the advent of genetic engineering, nearly any plant species is a potential source of genes for transfer to agricultural crops. Ironically, the conversion of native ecosystems to agricultural lands in an attempt to accommodate the food demands of an exponentially growing human population may eliminate the very organisms on which agriculture depends for its future. Fertile soil, obviously essential for the vitality of agricultural crops, is also a by-product of biodiversity because it is formed through the interactions of a number of soil organisms: fungi, earthworms, bacteria, plant roots, and burrowing mammals. Species loss could result in soils unable to support vegetation. Not all of the world’s supply of food comes from cultivated sources. There is still a substantial harvest of wild plants and animals. Commercial fishing is in essence the hunting of wild fish populations. Blueberries and maple syrup are just two examples of foods gleaned from nature in the United States. Wood and wood pulp are other products harvested from biodiversity resources. Biodiversity in itself is a major economic force, as evidenced by the increasing popularity of ecotourism. Sport fishing, hunting, and bird-watching are other examples of economically profitable activities that depend on the preservation of biodiversity. Lastly, nearly half of the medicinals now in use originated from a wild plant, and it has been estimated that between 35,000 and 70,000 species of plants are used directly as medicines worldwide. Knowledge of systematics has many practical applications. There has been a movement to reduce our dependence upon chemical pesticides and instead rely more heavily on biological controls to manage nuisance organisms. Biological control methods depend upon proper identification of a pest, knowledge of its life cycle, and correct identification of its predators and susceptibility to disease. Misidentification can be costly. Mealybugs are noxious pests that can cause massive damage to crops. A species of mealybug was identified as the culprit in the devastation of coffee plantations in Kenya. Biological control methods were employed using the natural enemies of the identified species of mealybug but were ineffective. Further investigation revealed that the mealybug had been misidentified. Once the correct species was assigned, natural pests of the mealybug were brought in from its native habitat in Uganda, and the mealybug infestation was soon brought under control. Knowledge of systematics can be used to predict economic uses of little known but related species. Researchers identified anticancer compounds from Kenyan populations of Maytenus buchananii. There was a problem, however; the species was rare in this locality. Knowledge of systematics suggested that closely related species would probably possess the same chemical compounds. This proved to be the case when a population of the same genus but different species was collected from India. Clearly, the preservation of biodiversity should be of utmost importance to everyone. Systematic research is fundamental to learning about the characteristics and dimensions of biodiversity. Systematics is necessary to identify localities of high species diversity or rare species. Baseline data must be collected to ascertain which species are declining in numbers or those whose range is becoming limited. Knowledge of systematics will determine if exotic pests are moving into new areas and threatening native species. Without scientific identification and mapping, valuable habitats and the species found there will be lost. In fact, Systematics Agenda 2000 is an ongoing global initiative by the scientific community to discover, describe, and classify the world’s species in an effort to understand and conserve biodiversity. 133 Levetin−McMahon: Plants and Society, Fifth Edition 134 UNIT II II. Introduction to Plant Life: Botanical Principles 8. Plant Systematics and Evolution © The McGraw−Hill Companies, 2008 Introduction to Plant Life: Botanical Principles Kingdom: Plantae Plant kingdom Figure 8.7 Major ranks in the taxonomic hierarchy. Note that the higher the ranking, the broader the defining characteristics and the more inclusive the group. Table 8.5 Traditional and Standardized Names for Some Common Families Division: Magnoliophyta Class: Liliopsida Flowering plants Family Name Traditional Name Standardized Name Sunflower Compositae Asteraceae Mustard Cruciferae Brassicaceae Grass Gramineae Poaceae Mint Labiatae Lamiaceae Pea Leguminosae Fabaceae Palm Palmae Arecaceae Carrot Umbelliferae Apiaceae Table 8.6 The Taxonomic Hierarchy and Standard Endings Monocots Rank Standard Ending Example Division (Phylum) -phyta Magnoliophyta Class Order Family Genus Order: Liliales Species Family: Liliaceae Genus: Lilium Lily family Lilies -opsida Liliopsida -ales Liliales -aceae Liliaceae Lilium Lilium superbum L. THE INFLUENCE OF DARWIN’S THEORY OF EVOLUTION The theory of evolution by means of natural selection was to irrevocably change the way biologists view species. Instead of unchanging organisms and generations created all alike, it was realized that species are dynamic and variable, continually evolving through the mechanism of natural selection in which adaptions are refined to a changing environment. The Voyage of the HMS Beagle Species: Lilium superbum L. Turk’s Cap Lily Charles Robert Darwin (fig. 8.8) was born in England in 1809 to a family of distinguished naturalists and physicians. His grandfather was Erasmus Darwin, a well-known poet and physician, and his father, Robert Darwin, was a successful country doctor. At 15 years of age, Charles was sent to the University of Edinburgh Medical School to study medicine. Not finding it to his liking, he transferred after Levetin−McMahon: Plants and Society, Fifth Edition II. Introduction to Plant Life: Botanical Principles 8. Plant Systematics and Evolution CHAPTER 8 Figure 8.8 Charles Darwin (1809–1882) published The Origin of Species in 1859. British Isles North America Galápagos Islands 135 Plant Systematics and Evolution 2 years to Cambridge University to study theology. While at Cambridge, he spent much of his free time with the students and professors of natural history. This association later proved invaluable. In 1831, at the age of 22, Darwin graduated from Cambridge with a degree in theology. Shortly thereafter he was recommended as ship naturalist by John Henslow, one of the natural history professors at Cambridge. The ship in question was the HMS Beagle, commissioned by King William IV to undertake a voyage around the world for the purpose of charting coastlines, particularly that of South America, for the British navy. The voyage of the Beagle began on December 27, 1831, and was to last 5 years (fig. 8.9). During his time on the Beagle, Darwin collected thousands of plants and other specimens from South America, the Galápagos Islands (off the coast of Ecuador), Australia, and New Zealand. He studied geological formations and noted fossil forms of extinct species. He found that some fossils of extinct species bore a striking resemblance to extant species, as though the former had given rise to the latter. Darwin spent some time studying the species found on the Galápagos Islands. He noted that animals and plants found in the Galápagos were obviously similar to species found in South America, but there were distinct differences. These observations led Darwin to question the fixity of species concept. According to this concept, widely held at the time of Darwin, species were acts of Divine Creation, unchanging over time. When the Beagle returned to England in 1836, Darwin married his cousin Emma Wedgwood (of the famous Europe Azores Atlantic Ocean Asia Canary Islands Cape Verde Islands Pacific Ocean Africa Bahia South America Rio de Janeiro Tahiti © The McGraw−Hill Companies, 2008 Valparaíso Montevideo Falkland Islands Cocos Islands Cape of Good Hope Australia Sydney King George Sound Tasmania New Zealand Cape Horn Figure 8.9 The 5-year voyage of the HMS Beagle. Darwin’s observations on the geology and distributions of plants and animals in South America and the Galápagos Islands were the groundwork for the development of the theory of evolution by means of natural selection. Levetin−McMahon: Plants and Society, Fifth Edition 136 UNIT II II. Introduction to Plant Life: Botanical Principles 8. Plant Systematics and Evolution © The McGraw−Hill Companies, 2008 Introduction to Plant Life: Botanical Principles Wedgwood china family) and settled, at age 27, in the English countryside. He continued his work in natural history, conducting experiments, writing papers, and corresponding with other naturalists. Among his works was a four-volume treatise on the classification and natural history of barnacles. In 1842, he began putting his thoughts together on what was to become his theory of evolution by natural selection. Darwin continued to expand and fine-tune his thoughts over the next 16 years. In June of 1858, he received a manuscript from Alfred Russel Wallace (1823–1913), a young British naturalist working in Malaysia. Wallace’s work was entitled On the Tendency of Varieties to Depart Indefinitely from the Original Type; Wallace had independently arrived at the concept of natural selection. Wallace and Darwin jointly presented their ideas on July 1, 1858, at a meeting of the Linnean Society in London. During the next few months, Darwin completed writing what was to become one of the most influential texts of all time. With the publication on November 24, 1859 of On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life by Charles Darwin, biological thought was changed forever. Natural Selection There are four underlying premises to Darwin’s theory of evolution by natural selection: 1. Variation: Members within a species exhibit individual differences, and these differences are heritable. 2. Overproduction: Natural populations increase geometrically, producing more offspring than will survive. 3. Competition: Individuals compete for limited resources, what Darwin called “a struggle for existence.” 4. Survival to reproduce: Only those individuals that are better suited to the environment survive and reproduce (survival of the fittest), passing on to a proportion of their offspring the advantageous characteristics. Offspring that inherit the advantageous traits are selected for survival and many will live to reproductive age passing on the desirable attributes. Those that do not inherit these traits are not likely to survive or reproduce. Gradually, the species evolves, or changes, as more and more individuals carry these traits. Darwin gave this example: If the number of individuals of a species with plumed seeds could be increased by greater powers of dissemination within its own area (that is, if the checks to increase fell chiefly on the seeds), those seeds which were provided with ever so little more down, would in the long run be most disseminated; hence a greater number of seeds thus formed would germinate, and would tend to produce plants inheriting the slightly better-adapted down. Concept Quiz Darwin identified four conditions that are necessary if evolution is to occur: genetic variation, overproduction of offspring, competition for limited resources, and reproduction of the fittest. Imagine a plant population that reproduces entirely by asexual methods, such as spreading by underground stems. Although there are many individual plants in the population, they are essentially a single plant genetically; that is, they are clones. Can natural selection act on a population of clones? Is this population capable of evolving? Explain. In addition to natural selection, humans have long used artificial selection, natural selection as practiced by humans (see Chapter 11), to shape the characteristics of crop plants to suit the needs of humanity. The most serious flaw in Darwin’s Theory of Evolution was the mechanism of heredity. Darwin had not worked out the source of variation in species, nor did he understand the means by which traits are passed down from generation to generation. It would take an Austrian monk, Gregor Mendel (see Chapter 7), working in relative obscurity with pea plants, to come up with the answers to Darwin’s questions about inheritance. A well-known example of natural selection is the case of heavy-metal tolerance in bent grass, Agrostis tenuis. Certain populations of bent grass were found growing near the tailings, or soil heaps, excavated from lead mines in Wales despite the fact that mine soils had high concentrations of lead and other heavy metals (copper, zinc, and nickel). When mine plants were transplanted into uncontaminated pasture soil, all survived but were small and slow growing. A nearby population of bent grass from uncontaminated pasture soil exhibited no such tolerance when transplanted into mine soil; in fact, most (57 out of 60) of the pasture plants died in the lead-contaminated soil. The survival of the three pasture plants in mine soil is significant; undoubtedly these three possessed an advantageous trait, the ability to tolerate heavy-metal soil. A trait that promotes the survival and reproductive success of an organism in a particular environment is an adaptation. The mine plants had descended from bent grass plants that possessed the adaptation that conferred tolerance to the mine soil; over time (less than 100 years in this case) populations of Agrostis tolerant to heavy metal evolved from those few tolerant individuals. Although Darwin’s theory of natural selection is the foundation of modern evolutionary concepts, biologists today are still learning about the forces that shape evolution. Levetin−McMahon: Plants and Society, Fifth Edition II. Introduction to Plant Life: Botanical Principles 8. Plant Systematics and Evolution CHAPTER 8 Concept Quiz Natural selection favors the survivorship of those individuals in a population that possess characteristics crucial for survival. You observe that trees in a part of a forest in which deer are plentiful have higher branches than the trees in a fenced-off part of the forest. Explain the different selective forces at work in these two different environments. PHYLOCODE The Linnaean system of nomenclature and the hierarchy of classification that has been presented in this chapter were created more than 250 years ago, before Charles Darwin and Alfred R. Wallace had proposed their evolutionary theory by means of natural selection. Linnaean nomenclature is an artificial system based upon the appearance of organisms that often does not reflect their evolutionary relationships, or phylogeny. Currently, there is a movement to reject this preevolutionary taxonomy and replace it with a new system of nomenclature, called PhyloCode, that is truly phylogenetic. PhyloCode is based upon the work of the twentiethcentury German entomologist Willi Hennig, who proposed that only shared derived characteristics should be used to define a group of related organisms. He further proposed that each group constructed should be monophyletic, or composed of only those organisms that can trace their descent from a common ancestor. These natural groupings are known as clades. First introduced in 1983, the PhyloCode abandons the Linnaean ranks of the taxonomic hierarchy. In this system, as new information that may change a group’s ranking accumulates, names are not changed, as they would be with the Linnaean system, which associates different suffixes with different ranks. Instead of ranks, clades are the only groups recognized. Opponents fear that a complete abandonment of all ranks will result in a loss of comparative information and encourage a proliferation of names that, without any context, will serve only to confuse the nomenclature. Released in 1991, the APG (American Phylogeny Group) system compared the sequence data of select genes to classify the flowering plants. The highest formal rank in this classification system is the order; higher categories are only identified as clades. As more data accumulated, APG II, an update of the classification, became available in 2003. In this system, angiosperms are recognized as a clade, sharing several distinct characteristics, such as ovules enclosed in a carpel and double fertilization. Within the angiosperms, all monocots appear to belong to a distinct clade, but molecular data © The McGraw−Hill Companies, 2008 Plant Systematics and Evolution 137 indicates that the traditional dicots represent several evolutionary lineages. Most of the dicots do comprise a clade and are now called the eudicots or true dicots. Approximately 75% of all angiosperm species are now classified as eudicots. Traditional dicots excluded from the clade eudicot are called the paleodicots (literally old dicots) by some authorities and include several ancient lineages in the evolution of angiosperms. CHAPTER SUMMARY 1. Plant systematics has its origins in the classical works of Theophrastus of ancient Greece, who is generally regarded as the Father of Botany. The study of plants, as did many other intellectual endeavors, went into a decline during the Dark Ages of Europe but was later revived owing to renewed interest in herbalism during the fifteenth to seventeenth centuries. 2. Linnaeus, a Swedish botanist of the eighteenth century, is credited with the creation of the binomial, or scientific name. Although common names are often informative and readily accessible, scientific names have the advantage of being recognized the world over and unique to a single species. 3. The taxonomic hierarchy includes the major ranks: domain, kingdom, division (phylum), class, order, family, genus, and species. 4. Biologists have wrestled with the concept of the species; the biological concept describes a species as a group of interbreeding populations, reproductively isolated from other populations. 5. Charles Darwin and his theory of evolution by natural selection irrevocably changed the way biologists viewed species. Natural selection favors those individuals that possess traits that better enable them to survive in the environment. These individuals survive to reproduce, and many of their offspring will tend to have these adaptations and pass them on to future generations. In this way, populations change over time. The four underlying conditions of Darwin’s theory of evolution by natural selection are variation, overproduction of offspring, competition, and survival to reproduce. REVIEW QUESTIONS 1. List the common names of some of the wildflowers in your area. Determine the type of information each name imparts. 2. Using a plant dictionary (see Further Reading) look up the scientific names and their meanings for common houseplants and landscape plants in your area. 3. Briefly describe the concept of evolution by natural selection. Levetin−McMahon: Plants and Society, Fifth Edition 138 UNIT II II. Introduction to Plant Life: Botanical Principles 8. Plant Systematics and Evolution © The McGraw−Hill Companies, 2008 Introduction to Plant Life: Botanical Principles 4. Why are only inherited traits important in the evolutionary process? Laufer, Geraldine Adamich. 1996. Tussie-Mussies. The Herb Companion April/May: 48–53. 5. How do mutations (Chapter 7) lead to the evolution of new species? 6. What was the lasting contribution of Linnaeus? How was the binomial system an improvement over polynomials? 7. In what ways can systematics preserve biodiversity? Litt, Amy. 2006. Origins of Floral Diversity. Natural History 115(5): 34 –40. FURTHER READING Blunt, Wilfrid. 1971. The Compleat Naturalist: A Life of Linnaeus. The Viking Press, New York, NY. Briggs, David, and S. Max Walters. 1984. Plant Variation and Evolution, 2nd Edition. Cambridge University Press, Cambridge, MA. Coombes, Allen J. 1994. Dictionary of Plant Names: Botanical Names and Their Common Equivalents. Timber Press, Beaverton, OR. Conniff, Richard. 2006/2007. Happy Birthday Linnaeus. Natural History 115(10): 42–47. Darwin, Charles (author) and Edward O Wilson (editor) 2006. From So Simple A Beginning: The Four Great Books of Charles Darwin. W. W. Norton & Company, New York. Friedman, William E. 2006. Sex Among the Flowers. Natural History 115(9): 48–53. Gilbert, Bil. 1984. The Obscure Fame of Carl Linnaeus. Audubon 86:102–115. Greenaway, Kate. 1992. Language of Flowers. Dover Press, New York, NY. Irvine, William. 1983. Apes, Angels, and Victorians: The Story of Darwin, Huxley, and Evolution. University Press of America, New York, NY. Judd, Walter S., Christopher S. Campbell, Elizabeth A. Kellogg, Peter F. Stevens, and Michael J. Donoghue. 2002. Plant Systematics, A Phylogenetic Approach, 2nd Edition. Sinauer Associates, Sunderland, MA. Kohn, David. 2005. The Miraculous Season: The Historical Darwin. Natural History 114(9): 38–40. Laufer, Geraldine Adamich. 1993. The Language of Flowers. Workman Publishing, New York, NY. Mayr, Ernst. 2000. Darwin’s Influence on Modern Thought. Scientific American 283(1): 78–83. Miller, Douglass R., and Amy Y. Rossman. 1995. Systematics, Biodiversity, and Agriculture. BioScience 45(10): 680– 686. Pennisi, Elizabeth. 2001. Linnaeus’s Last Stand? Science 291: 2304–2307. Piementel, David, Christa Wilson, Christine McCullum, Rachel Huang, Paulette Dwen, Jessica Flack, Quynh Tran, Tamara Saltman, and Barabara Cluff. 1995. Economic and Environmental Benefits of Biodiversity. BioScience 47(11): 747–757. Quammen, David. 2004. Darwin’s Big Idea. National Geographic 206(5): 2–35. Quammen, David. 2007. The Name Giver. National Geographic 211(6): 72–87. Raby, Peter. 2001. Alfred Russel Wallace: A Life. Princeton University Press, Princeton, NJ. Savage, Jay M. 1995. Systematics and the Biodiversity Crisis. BioScience 45(10): 673–679. Schiebinger, Londa. 1996. The Loves of Plants. Scientific American 274(2): 110–115. Sulloway, Frank J. 2005. The Evolution of Charles Darwin. Smithsonian 36(9): 58–69. Withgott, Jay. 2000. Is It “So Long, Linnaeus?” BioScience 50(8): 646–651. ONLINE LEARNING CENTER Visit www.mhhe.com/levetin5e for online quizzing, web links to chapter-related material, and more!