Classification

1. Classification • Each person might divide these shells into different categories • Scientists often group and name, or classify, organisms using certain guidelines • This makes it easier to discuss the types and characteristics of living things

2. Classification

3. Finding Order in Diversity • For more than 3.5 billion years, life on Earth has been constantly changing • Natural selection and other processes have led to a staggering diversity of organisms • A tropical rain forest, for example, may support thousands of species per acre • Recall that a species is a population of organisms that share similar characteristics and can breed with one another and produce fertile offspring • Biologists have identified and named about 1.5 million species so far • They estimate that anywhere between 2 and 100 million additional species have yet to be discovered

4. Why Classify? • To study this great diversity of organisms, biologists must give each organism a name • Biologists must also attempt to organize living things into groups that have biological meaning • To study the diversity of life, biologists use a classification system to name organisms and group them in a logical manner

5. Why Classify? • In the discipline known as taxonomy, scientists classify organisms and assign each organism a universally accepted name • By using a scientific name, biologists can be certain that everyone is discussing the same organism • When taxonomists classify organisms, they organize them into groups that have biological significance • When you hear the word “bird,” for example, you immediately form a mental picture of the organism being discussed—a flying animal that has feathers • But science often requires smaller categories as well as larger, more general categories • In a good system of classification, organisms placed into a particular group are more similar to each other than they are to organisms in other groups

6. Why Classify? • You use classification systems also, for example, when you refer to “teachers” or “mechanics,” or more specifically, “biology teachers” or “auto mechanics” • Such a process, like scientific classification, uses accepted names and common criteria to group things

7. Assigning Scientific Names • By the eighteenth century, European scientists recognized that referring to organisms by common names was confusing • Common names vary among languages and even among regions within a single country • For example, a cougar can also be called a puma, a panther, or a mountain lion • Furthermore, different species sometimes share a single common name • In the United Kingdom, the word buzzard refers to a hawk, whereas in many parts of the United States, buzzard refers to a vulture • To eliminate such confusion, scientists agreed to use a single name for each species • Because eighteenth-century scientists understood Latin and Greek, they used those languages for scientific names • This practice is still followed today in naming newly discovered species

8. Early Efforts at Naming Organisms  • The first attempts at standard scientific names often described the physical characteristics of a species in great detail • As a result, these names could be twenty words long! • For example, the English translation of the scientific name of a particular tree might be “Oak with deeply divided leaves that have no hairs on their undersides and no teeth around their edges” • This system of naming had another major drawback • It was difficult to standardize the names of organisms because different scientists described different characteristics

9. Binomial Nomenclature  • A major step was taken by Carolus Linnaeus, a Swedish botanist who lived during the eighteenth century • He developed a two-word naming system called binomial nomenclature • This system is still in use today • In binomial nomenclature, each species is assigned a two-part scientific name • The scientific name is always written in italics • The first word is capitalized, and the second word is lowercased

10. Binomial Nomenclature  • For example, the grizzly bear is called Ursus arctos • The first part of the scientific name—in this case, Ursus—is the genus to which the organism belongs • A genus (plural: genera) is a group of closely related species • The genus Ursus contains five other kinds of bears, including Ursus maritimus, the polar bear

11. Binomial Nomenclature  • The second part of a scientific name—in this case, arctos or maritimus—is unique to each species within the genus • Often, this part of the name is a Latinized description of some important trait of the organism or an indication of where the organism lives • The Latin word maritimus, referring to the sea, comes from the fact that polar bears often live on pack ice that floats in the sea

12. Linnaeus's System of Classification • Linnaeus's classification system is hierarchical; that is, it consists of levels • Linnaeus's hierarchical system of classification includes seven levels • They are—from smallest to largest—species, genus, family, order, class, phylum, and kingdom • In taxonomic nomenclature, or naming system, each of those levels is called a taxon (plural: taxa), or taxonomic category

13. CLASSIFICATION • Taxonomy: is the science of grouping organisms according to their presumed natural relationship • Common names add cause confusion to the classification system • System used today is binomial nomenclature (two names) • Developed by Linnaeus • Placed structurally similar organisms into a group called a species • Similar species into a larger group called a genus • Similar genera into a family • Similar families were placed into an order • Similar orders in a class • Similar classes into phylum • Phylum into kingdom • Rather than use all seven categories in naming organisms, Linnaeus chose to use the genus and specie names

14. Linnaeus's System of Classification • The two smallest categories, genus and species, were discussed in the example of the bears • The giant panda, resembles the grizzly bear and the polar bear • However, it differs enough from them and other species in the genus Ursus that it is placed in its own genus, Ailuropoda

15. Linnaeus's System of Classification • The grizzly bear, Ursus arctos, and the polar bear, Ursus maritimus, are classified as different species in the same genus, Ursus • The giant panda is placed in a separate genus

16. Linnaeus's System of Classification

17. Linnaeus's System of Classification • Genera that share many characteristics, such as Ursus and Ailuropoda, are grouped in a larger category, the family—in this case, Ursidae • These bears, together with six other families of animals, such as dogs (Canidae) and cats (Felidae), are grouped together in the order Carnivora • An order is a broad taxonomic category composed of similar families • The next larger category, the class, is composed of similar orders • For example, order Carnivora is placed in the class Mammalia, which includes animals that are warm-blooded, have body hair, and produce milk for their young

18. Linnaeus's System of Classification • Several different classes make up a phylum (plural: phyla) • A phylum includes many different organisms that nevertheless share important characteristics • The class Mammalia is grouped with birds (class Aves), reptiles (class Reptilia), amphibians (class Amphibia), and all classes of fishes into the phylum Chordata • All these organisms share important features of their body plan and internal functions • Finally, all animals are placed in the kingdom Animalia • Thekingdom is the largest and most inclusive of Linnaeus's taxonomic categories • Linnaeus named two kingdoms, Animalia and Plantae

19. CLASSIFICATION

20. Linnaeus's System of Classification • Linnaeus’s hierarchical system of classification uses seven taxonomic categories • This illustration shows how a grizzly bear, Ursus arctos, is grouped within each taxonomic category • Only some representative species are illustrated for each category above the species

21. Linnaeus's System of Classification

22. CLASSIFICATION

23. Modern Evolutionary Classification • In a sense, organisms determine who belongs to their species by choosing with whom they will mate! • Taxonomic groups above the level of species are “invented” by researchers who decide how to distinguish between one genus, family, or phylum, and another • Linnaeus and other taxonomists have always tried to group organisms according to biologically important characteristics • Like any taxonomic system, however, Linnaeus's system had limitations and problems

24. Which Similarities Are Most Important? • Linnaeus grouped species into larger taxa, such as genus and family, mainly according to visible similarities and differences • But which similarities and differences are most important? • If you lived in Linneaus's time, for example, how would you have classified dolphins? • Would you have called them fishes because they live in water and have finlike limbs? • Or would you call them mammals because they breathe air and feed their young with milk? • How about the animals shown in the figure? • Adult barnacles and limpets live attached to rocks and have similarly shaped shells with holes in the center • Crabs, on the other hand, have body shapes unlike those of barnacles or limpets • Based on these features, would you place limpets and barnacles together, and crabs in a different group?

25. Which Similarities Are Most Important? • Classifying species based on easily observed adult traits can pose problems • Observe the crab (top left), barnacles (bottom left), and limpet (right) • Which seems most alike?

26. Which Similarities Are Most Important?

27. Evolutionary Classification • Darwin's ideas about descent with modification have given rise to the study of phylogeny, or evolutionary relationships among organisms • Biologists now group organisms into categories that represent lines of evolutionary descent, or phylogeny, not just physical similarities • The strategy of grouping organisms together based on their evolutionary history is called evolutionary classification

28. Evolutionary Classification • Species within a genus are more closely related to each another than to species in another genus • According to evolutionary classification, that is because all members of a genus share a recent common ancestor • Similarly, all genera in a family share a common ancestor • This ancestor is further in the past than the ancestor of any genus in the family but more recent than the ancestor of the entire order • The higher the level of the taxon, the farther back in time is the common ancestor of all the organisms in the taxon

29. Evolutionary Classification • Organisms that appear very similar may not share a recent common ancestor • Natural selection, operating on species in similar ecological environments, has often caused convergent evolution • For example, superficial similarities once led barnacles and limpets to be grouped together, as shown on the left of the figure

30. Evolutionary ClassificationTraditional Classification and Cladogram • Early systems of classification grouped organisms together based on visible similarities • That approach might result in classifying limpets and barnacles together (left)

31. Evolutionary ClassificationTraditional Classification and Cladogram

32. Evolutionary Classification • However, barnacles and limpets are different in important ways • For example, their free-swimming larvae, or immature forms, are unlike one another • Certain adult characteristics are different too • Adult barnacles have jointed limbs and a body divided into segments • Barnacles periodically shed, or molt, their external skeleton • These characteristics make barnacles more similar to crabs than to limpets • Limpets, in turn, have an internal anatomy that is closer to that of snails, which are mollusks • And like mollusks, limpets do not shed their shells • Because of such characteristics, taxonomists infer that barnacles are more closely related to crabs than to mollusks • In other words, barnacles and crabs share an evolutionary ancestor that is more recent than the ancestor that barnacles share with limpets • Thus, both barnacles and crabs are classified as crustaceans, and limpets are mollusks

33. Classification Using Cladograms • To refine the process of evolutionary classification, many biologists now prefer a method called cladistic analysis • Cladistic analysis identifies and considers only those characteristics of organisms that are evolutionary innovations—new characteristics that arise as lineages evolve over time • Characteristics that appear in recent parts of a lineage but not in its older members are called derived characters

34. Classification Using Cladograms • Derived characters can be used to construct a cladogram, a diagram that shows the evolutionary relationships among a group of organisms • You can see an example of a cladogram on the right-hand side of the figure • Notice how derived characters, such as “free-swimming larva” and “segmentation,” appear at certain locations along the branches of the cladogram • These locations are the points at which these characteristics first arose • You can see that crabs and barnacles share some derived characters that barnacles and limpets do not • One such shared derived character is a segmented body • Another is a molted external skeleton • Thus, this cladogram groups crabs and barnacles together as crustaceans and separates them from limpets, which are classified as a type of mollusk

35. Classification Using CladogramsTraditional Classification and Cladogram • Biologists now group organisms into categories that represent lines of evolutionary descent, or phylogeny, not just physical similarities • Crabs and barnacles are now grouped together (right) because they share several characteristics that indicate that they are more closely related to each other than either is to limpets • These characteristics include segmented bodies, jointed appendages, and an external skeleton that is shed during growth

36. Classification Using CladogramsTraditional Classification and Cladogram

37. Classification Using Cladograms • Cladograms are useful tools that help scientists understand how one lineage branched from another in the course of evolution • Just as a family tree shows the relationships among different lineages within a family, a cladogram represents a type of evolutionary tree, showing evolutionary relationships among a group of organisms

38. CLASSIFICATION • Inferring Phylogeny • Infer the probable evolutionary relationships among species that have been classified • Sometimes a Phylogenetic Tree is used

39. PHYLOGENETIC TREE

40. CLASSIFICATION • Binomial name of a species is called its scientific name • Describes the organism or the range of the organism, or honors another scientist or friend • Classification: • Phylum used in animal classification • Division used in plant classification • Classification of species: • Subspecies (races): morphological different and are often geographically separated • Varieties: morphologically different and are often not geographically separated • Some produced by humans (apples, peaches and nectarines) • Strain: biochemically dissimilar group within a species • Usually used in reference to microorganisms

41. CLASSIFICATION • Evidence Used in Classification • Comparative morphology • Embryology • Homologous structures show evolutionary relationships between organisms (bones in the forelimb of a lizard are embryologically similar to those in a cat) • Chromosomes • Karyotypes: compare numbers and shapes • Biochemistry • Sequence of bases in DNA • Amino acid sequence in proteins • Physiology • Function of systems • Phylogeny • Evolutionary relationships • Biosystematics • Using reproductive compatibility to infer evolutionary relationships

42. Similarities in DNA and RNA • All of the classification methods discussed so far are based primarily on physical similarities and differences • But even organisms with very different anatomies have common traits • For example, all organisms use DNA and RNA to pass on information and to control growth and development • Hidden in the genetic code of all organisms are remarkably similar genes • Because DNA and RNA are so similar across all forms of life, these molecules provide an excellent way of comparing organisms at their most basic level—their genes

43. Similarities in DNA and RNA • The genes of many organisms show important similarities at the molecular level • Similarities in DNA can be used to help determine classification and evolutionary relationships • Now that scientists can sequence, or “read,” the information coded in DNA, they can compare the DNA of different organisms to trace the history of genes over millions of years

44. Similar Genes  • Even the genes of diverse organisms such as humans and yeasts show many surprising similarities • For example, humans have a gene that codes for myosin, a protein found in our muscles • Researchers have found a gene in yeast that codes for a myosin protein • As it turns out, myosin in yeast helps enable internal cell parts to move • Myosin is just one example of similarities at the molecular level—an indicator that humans and yeasts share a common ancestry

45. DNA Evidence  • DNA evidence can also help show the evolutionary relationships of species and how species have changed • The more similar the DNA sequences of two species, the more recently they shared a common ancestor, and the more closely they are related in evolutionary terms • And the more two species have diverged from one another, or changed in comparison to one another during evolution, the less similar their DNA will be

46. DNA Evidence • Consider the case of the American vulture and the African vulture, which resemble each other • Both birds have traditionally been classified together as “vultures” • One group of birds inhabits Africa and Asia, and the other, the Americas • But American vultures have a peculiar behavior: When they get overheated, they urinate on their legs, and evaporative cooling removes some body heat • The only other birds known to behave this way are storks, which look quite different from vultures and have always been put in a separate family • Does this similarity in behavior indicate a close evolutionary relationship?

47. DNA Evidence • Scientists analyzed the DNA of these three birds • The analysis showed that the DNA sequences of the American vulture and the stork were more similar than those of the American vulture and the African vulture • This similarity in DNA sequences indicates that the American vulture and the stork share a more recent common ancestor than do the American vulture and the African vulture • Therefore, the American vulture is more closely related to storks than to other vultures

48. Molecular Clocks • Comparisons of DNA can also be used to mark the passage of evolutionary time • A model known as a molecular clockuses DNA comparisons to estimate the length of time that two species have been evolving independently • To understand molecular clocks, think about a pendulum clock • It marks time with a periodically swinging pendulum • A molecular clock also relies on a repeating process to mark time—mutation

49. Molecular Clocks • Simple mutations occur all the time, causing slight changes in the structure of DNA, as shown in the figure • Some mutations have a major positive or negative effect on an organism's phenotype • These mutations are under powerful pressure from natural selection • Other mutations have no effects on phenotype • These neutral mutations accumulate in the DNA of different species at about the same rate • A comparison of such DNA sequences in two species can reveal how dissimilar the genes are • The degree of dissimilarity is, in turn, an indication of how long ago the two species shared a common ancestor

50. Molecular Clocks • By comparing the DNA sequences of two or more species, biologists estimate how long the species have been separated • What evidence indicates the species C is more closely related to species B than to species A?