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Classification

Taxonomymethod of classifying organisms

As the science of biology developed in the 18th & 19th centuries, a major goal was to name and

classify Earth's diverse species. You might think this task would be

fairly complete by now—but in fact that is not the case. New

discoveries, methods, technology, and molecular evidence have

revitalized the field, making it one of today's most exciting areas of

biological research.

What Is Taxonomy?A branch of biology that involves the identification, naming, and

classification of species.

� Assigning scientific names to species is an important part of studying the history of life.

� Although common names such as monkey, fruit fly, and pine generally work well in everyday language, common names can also cause confusion. Each of the names above actually refers to many different species. And sometimes the same common name can even refer to very different organisms.

� For example, a tortoise might be called a gopher in Florida, but in Kansas, "gopher" might refer to either a ground squirrel or a pocket gopher.

� Often, a common name doesn't accurately reflect the organism. Consider the names catfish, crayfish, and silverfish—the first is a fish, the second is a small freshwater lobster, and the third is an insect.

� Then there's the fact that organisms have different common names in different languages. Yet biologists must be able to communicate with one another about the species they study, no matter what language they speak.

Scientific Names

� To reduce this confusion in discussing organisms, one goal of taxonomy is to assign a universal scientific name to each known species.

� Taxonomists also attempt to organize the diversity of life by classifying species into larger groups of related species. This section describes some of their methods.

Astrophytum myriostigma

This poison frog's

scientific name,

Dendrobates azureus,

means “blue tree-walker”. Binomial Nomenclature

� Carolus Linnaeus (1700’s)

� System for naming things using Genus and species (two names)

� Used by all countries in all languages to avoid confusion among scientists.• Ex. Homo sapiens, Canus lupus, Felis domesticus

• Genus comes 1st and is Upper case , Species comes 2nd is lower case

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The Linnaeus binomial system also groups species into broader categories.

Closely related species are grouped into the same genus.

• For example the leopard (Panthera pardus) , and the African lion (Panthera leo) both belong to the genus Panthera. Taxonomists group similar genera in the same family. Next, similar families are placed into orders, orders into classes, and classes into phyla(singular, phylum). Phyla are grouped into kingdoms.

• Classifying a species by kingdom, phylum, and so on, is like placing students in a large school system. First a student might be identified by school, then by specific grade, and finally as a unique individual by name.

Classification and Evolution

� Darwin viewed Linnaeus's system of classification in the context of evolution. Ever since, biologists have strived to have classification represent the evolutionary relationships among species.

� A diagram that reflects such hypotheses of evolutionary relationships has a branching pattern called a phylogenetic tree . (The diagram's name comes from the word phylogeny, meaning "evolutionary history.")

� In a phylogenetic tree, each branch point represents a common ancestor of the species above that point. In this diagram, the branches are labeled to reinforce how taxonomy reflects the branching pattern of evolution.

Phylogenetic Tree 1, 2

How do we classify?

Based on Morphology- branch of biology dealing with the study of form &

structure of organisms

How do we classify?

� Structural (anatomy and physiology)

� Biochemical (enzymes, proteins, DNA)

� Cytological (cell structure)

� Embryological (development)

� Behavioral (patterns of actions)

� Fossil (common ancestor)

� Evolutionary relations

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Homologous structures are one of the best

clues to assess how closely organisms are

related.

� Homologous structures—like a

whale's flipper and a bat's wing—may look different and function

very differently in different species.

However, they will have basic underlying similarities if they both

evolved from a single structure in

a common ancestor.

� The greater the number of homologous structures 2

species have, the more closely the

species are thought to be related.

Convergent EvolutionConvergent Evolution

process in which unrelated species from similar environments

have adaptations that seem very similar.

� There are, however, pitfalls in the search for homologous characteristics. Not all similar structures are inherited from a common ancestor. Similar adaptations that result from convergent evolution are called analogous structures.

� For example, the wings of insects and those of birds are analogous, not homologous, flight equipment—they evolved independently. And, they are built from entirely different structures. There is no evidence that insects and birds shared a common winged ancestor.

Homology & Analogy

Molecular Data as a Taxonomic Tool

� The relatedness of species can be measured by comparing their genes and gene products (proteins). The more the sequences match up, the more closely the species are probably related. Researchers are now sequencing the genomes of species at a rapid rate. The data—readily available on the Internet—have sparked a boom in the study of evolutionary history. Such molecular data are independent of the structural data that have traditionally been used for classification.

� Such molecular comparisons provide a new way to test hypotheses about evolutionary history. The strongest support for any such hypothesis is when molecular data agree with evidence from other sources, such as anatomy (body structure). For instance, fossil data have indicated that whales are closely related to the group of mammals that includes hippos, cows, deer, and pigs. Molecular data have backed up this hypothesis.

� Taxonomy began a new era in the 1960s. One advance that led the way occurred when the molecular methods just described became available for comparing species' DNA. At the same time, computer technology provided greater power to analyze information. These innovations coincided with a new ways of building phylogenetic trees as hypotheses about evolutionary relationships.

� A key feature of a phylogenetic tree is the pattern of branches. A particular branching in the cat family could be represented as the diagram on the right. The tree can be expanded to include additional species, as in the diagram on the left. The "deeper" branch point represents the evolutionary split from a common ancestor of the wolf and cat groups. This phylogenetic tree represents the hypothesis that the wolf and cat groups diverged earlier in their evolution than did leopards and house cats.

� Leopards and house cats compose a branch of two species that share a common ancestor (right). A larger branch that also includes wolves (left) has a common ancestor that would have lived longer ago than the ancestor of leopards and house cats.

Cladistics

� Cladistics - is a branch of biology that determines the evolutionary relationships between organisms based on derived similarities

� Cladistics is a particular method of hypothesizing relationships among organisms. Like other methods, it has its own set of assumptions, procedures, and limitations. Cladistics is now accepted as the best method available for phylogenetic analysis, for it provides an explicit and testable hypothesis of organismal relationships.

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Reexamining Traditional Classification� In most cases, cladistic analysis has supported traditional classification. However, basing phylogenetic trees strictly on derived characters produces

some surprises. � For instance, biologists have traditionally placed birds and reptiles (lizards,

snakes, and crocodiles) in separate classes of vertebrates. Cladistic analysis now suggests that crocodiles are more closely related to birds than they are to

lizards and snakes.

� This cladogram shows birds and crocodiles in one clade, and lizards and snakes in another. Now look at the larger clade that includes the ancestor that

crocodiles share with lizards and snakes. In this scheme, the reptile group must also include birds. This classification seems to go against common

sense and tradition. For many people, the unique flight adaptations of birds justify separating them from the crocodiles and other reptiles.

� Keeping birds in a separate group from reptiles may be more comfortable, but it probably does not accurately reflect evolutionary relationships.

Cladogram: Major Groups of

Plants

Section 2 How Biologists

Classify OrganismsChapter 14

Comparing Classification

Schemes

� Phylogenetic trees and classifications

represent hypotheses about evolutionary

history. Like all hypotheses, they are revised to

correspond with the discovery of new

evidence. In some cases they are even

rejected. Cladistic analysis of molecular data is

changing scientists' understanding of certain

evolutionary relationships. The changes even

affect classification at the kingdom level.

Two- and Three-Kingdom Schemes� Biologists have traditionally considered the kingdom to be the broadest taxonomic

category. In high school, your grandparents learned that there are only two kingdoms of life—plants and animals.

� The two-kingdom system had a long tradition in taxonomy. Linnaeus divided all known forms of life between the plant and animal kingdoms. The two-kingdom system prevailed for over 200 years, but it had its problems. Where do prokaryotes fit in such a system? Can they be considered members of the plant kingdom? And what about fungi?

Two- and Three-

Kingdom Schemes

� Later on your grandparents learned a three-kingdom model. This model placed protists such as protozoans into their own kingdom. But this model also failed to fit new evidence about the biology and evolutionary history of certain forms of life.

A Five-Kingdom Scheme

In 1969, American ecologist Robert H. Whittaker

proposed a five-kingdom system.

� It places prokaryotes such as bacteria in the kingdom Monera.

� Organisms of the other 4 kingdoms all consist of eukaryotic cells.

� The kingdoms for plants, fungi, and animals consist of multicellular eukaryotes. Among eukaryotes, these kingdoms differ in structure, development, and modes of nutrition.

� Plants make their own food by photosynthesis.

� Fungi live by decomposing the remains of other organisms and absorbing small organic molecules.

� Most animals live by ingesting food and digesting it within their bodies.

MONERA

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5In the five-kingdom system, the kingdom

Protista contains all eukaryotes that do not fit

the definitions of plant, fungus, or animal.

� Most protists are unicellular. Amoebas and

other so-called protozoa are examples.

� But protists also include certain large, multicellular organisms (green Ulva)

that are thought to be close relatives of unicellular protists.

Three Domains� In the last decade, molecular data and

cladistics have led to a reevaluation of the five-kingdom system. A three-domain system is one alternative to the five-kingdom system.

� A domain is a taxonomic category above the kingdom level.

� Three-domains of Life:

- Archaea – no nuclear membrane (Live in very harsh conditions)

- Bacteria (Monera) – no nuclear membrane

- Eukarya – Nuclear membrane� What is most important to understand here is

that classifying Earth's diverse species of life is a work in progress. As more is learned about organisms and their evolution, classification schemes will continue to be revised. It is quite possible that if you continue your study of biology in the coming years, you may learn about new and different taxonomic systems.

Kingdom and Domain

Characteristics

Section 1 Introduction to

Kingdoms and DomainsChapter 19

Dichotomous Key

� Tool used by biologists to identify an unknown

organism

� Series of paired statements of anatomical description that leads to an identification.

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What bird am I ?

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� Bird W is Geospiza

� Bird X is Platyspiza

� Bird Y is Certhidea

� Bird Z is Camarhynchus

Key for Vertebrate Identification

� 1. a) animal has a spine…………………..go to 2 b) animal has no spine………..…invertebrate

2. a) animal has no gills and fins……..…. go to 3 b) animal has gills and fins…………….. Fish

3. a) animal has no scales…………..........go to 4b) animal has scales………………..….reptile

4. a) animal has feathers …………………..birdb) animal has no feathers ……………..go to 5

5. a) animal has hair…………………….mammalb) animal has no hair………………..amphibian

What am I ???

1. Give two reasons why common names of organisms can lead to confusion.

2. Why are analogous structures not useful for classifying species in an evolutionary context?

3. What does a branch point in a cladogram represent?

4. How does the three-domain model of classification differ from the five-kingdom model?

Concept Check

Online Review

� Taxonomy Quiz 1

� Phylogenetic Tree Quiz 1, 2

� Homology and Analogy

� Convergent Evolution

� Vertebrate Flight

� Cladistics Quiz 1, 2, 3