Classification Each person might divide these shells into different categories Scientists often group and name, or classify, organisms using certain guidelines

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

Classification

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 1.5 million species so farspecies so far

• They estimate that anywhere between 2 and 100 million additional species have yet to be discovered

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

Why Classify?• In the discipline known as taxonomytaxonomy, 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

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

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 To eliminate such confusion, scientists agreed to use a

single name for each speciessingle 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 This practice is still followed today in naming newly discovered

speciesspecies

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

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 nomenclaturebinomial nomenclature

• This system is still in use today• In binomial nomenclature, each species is assigned each species is assigned

a two-part scientific namea two-part scientific name• The scientific name is always written in italicsitalics• The first word is capitalized, and the The first word is capitalized, and the second word is second word is

lowercasedlowercased


• 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 genusgenus (plural: genera) is a group of closely related species

• The genus Ursus contains five other kinds of bears, including Ursus maritimus, the polar bear


• 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 oror 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

Linnaeus's System of Classification

• Linnaeus's classification system is hierarchical;hierarchical; that is, it consists of levels

• Linnaeus's hierarchical system of classification includes seven levels

• They are—from smallest to largest—species, They are—from smallest to largest—species, genus, family, order, class, phylum, and genus, family, order, class, phylum, and kingdomkingdom

• In taxonomic nomenclature, or naming system, each of those levels is called a taxon (plural: taxa), or taxonomic category

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


• 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


• 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



• 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


• 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• The kingdom is the largest and most inclusive of

Linnaeus's taxonomic categories• Linnaeus named two kingdoms, Animalia and Plantae

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


CLASSIFICATION

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

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?


• 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?


Evolutionary Classification

• Darwin's ideas about descent with modification have given rise to the study of phylogenyphylogeny, 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 classificationevolutionary 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 The higher the level of the taxon, the farther back in time is the common ancestor of all the organisms in time is the common ancestor of all the organisms in the taxonthe taxon


• 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

Evolutionary Classification Traditional 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)

Evolutionary Classification Traditional Classification and Cladogram

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 taxonomists infer that barnacles are

more closely related to crabs than to mollusksmore 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 Thus, both barnacles and crabs are classified as crustaceans, and

limpets are molluskslimpets are mollusks

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


• Derived characters can be used to construct a cladogram 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 aroseThese 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 Thus, this cladogram groups crabs and barnacles together as crustaceans and separates them from limpets, which are classified as crustaceans and separates them from limpets, which are classified as a type of molluska type of mollusk

Classification Using Cladograms Traditional 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

Classification Using Cladograms Traditional Classification and Cladogram


• 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

CLASSIFICATION

• Inferring Phylogeny– Infer the probable evolutionary relationships

among species that have been classified– Sometimes a Phylogenetic Tree is used

PHYLOGENETIC TREE

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

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

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

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

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

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

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?

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 Therefore, the American vulture is more closely related to storks than to other vulturesrelated to storks than to other vultures

Molecular Clocks

• Comparisons of DNA can also be used to mark the passage of evolutionary time

• A model known as a molecular clock uses 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 A molecular clock also relies on a repeating process

to mark time—mutationto mark time—mutation

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 These mutations are under powerful pressure from natural

selectionselection• 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 A comparison of such DNA sequences in two species can reveal how dissimilar the genes arereveal how dissimilar the genes are

• The degree of dissimilarity is, in turn, an indication of how long The degree of dissimilarity is, in turn, an indication of how long ago the two species shared a common ancestorago the two species shared a common ancestor

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?

Molecular Clocks

Molecular Clocks• The use of molecular clocks is not simple, however, because there

is not just one molecular clock in a genome• Instead, there are many, each of which “ticks” at a different

rate– This is because some genes accumulate mutations faster than

others– These different clocks allow researchers to time different kinds of

evolutionary events• Think of a conventional clock• If you want to time a brief event, you pay attention to the second

hand• To time an event that lasts longer, you use the minute hand or the

hour hand• In the same way, researchers would use a different molecular In the same way, researchers would use a different molecular

clock to compare modern bird species than they would to clock to compare modern bird species than they would to estimate the age of the common ancestor of yeasts and estimate the age of the common ancestor of yeasts and humanshumans

Kingdoms and Domains • As in all areas of science, systems of classification adapt to new

discoveries• Ideas and models change as new information arises• Some explanations have been discarded altogether, whereas

others, such as Darwin's theory of evolution by natural selection, have been upheld and refined through years of research

• So, it should not be surprising that early attempts at drawing life's universal tree were based on some misguided assumptions

• Some of the earliest trees of life were dominated by humans– These models represented vertebrates as the most important and

abundant animals• They also implied that “higher” animals evolved from “lower”

animals that were identical to modern forms• Biologists now know these notions are incorrectBiologists now know these notions are incorrect

The Tree of Life Evolves

• The scientific view of life was simpler in Linnaeus's time

• The only known differences among living things were the fundamental traits that separated animals from plants

• Animals were mobile organisms that used food for energy

• Plants were green, photosynthetic organisms that used energy from the sun

Five Kingdoms • As biologists learned more about the natural world, they

realized that Linnaeus's two kingdoms, Animalia and Plantae, did not adequately represent the full diversity of life

• First, microorganisms such as protists and bacteria were recognized as being significantly different from plants and animals

• Scientists soon agreed that microorganisms merited their own kingdom, which was named Protista

• Then, the mushrooms, yeasts, and molds were separated from the plants and placed in their own kingdom, Fungi

• Later still, scientists realized that bacteria lack the nuclei, mitochondria, and chloroplasts found in other forms of life– Therefore, they were placed in another new kingdom, Monera

• This process produced five kingdoms—Monera, Protista, This process produced five kingdoms—Monera, Protista, Fungi, Plantae, and Animalia Fungi, Plantae, and Animalia

CLASSIFICATION

• Five Kingdom System– Monera

• Prokaryotic organisms• Bacteria and blue-green algae

– Protista• Eukaryotic organisms that lack specialized tissue systems• Unicellular or multicellular• Algae and protozoa

– Fungi• Heterotrophic unicellular and multicellular eukaryotic organisms

– Plantae• Eukaryotic, multicellular, autotrophic organisms with tissues

– Animalia• Eukaryotic, multicellular, heterotrophic organisms with tissues

CLASSIFICATION

CLASSIFICATION

CLASSIFICATION

CLASSIFICATION

Six Kingdoms

• In recent years, as evidence about microorganisms continued to accumulate, biologists came to recognize that the Monera were composed of two distinct groups

• Some biologists consider the differences between these two groups to be as great as those between animals and plants

• As a result, the Monera have been separated into two kingdoms, Eubacteria Eubacteria and ArchaebacteriaArchaebacteria, bringing the total number of bringing the total number of kingdoms to sixkingdoms to six

Six Kingdoms

• The six-kingdom system of The six-kingdom system of classification includes the kingdoms classification includes the kingdoms Eubacteria, Archaebacteria, Protista, Eubacteria, Archaebacteria, Protista, Fungi, Plantae, and AnimaliaFungi, Plantae, and Animalia

• This system of classification is shown in the bottom row of the table

Six Kingdoms

• This diagram shows some of the ways organisms have been classified into kingdoms over the years

• The six-kingdom system includes the following kingdoms: Eubacteria, Eubacteria, Archaebacteria, Archaebacteria, Protista, Fungi, Plantae, Protista, Fungi, Plantae, and Animaliaand Animalia

Six Kingdoms

The Three-Domain System

• Some of the most recent evolutionary trees have been produced using comparative studies of a small subunit of ribosomal RNA that occurs in all living things

• Using a molecular clock model, scientists have grouped modern organisms according to how long they have been evolving independently


• Molecular analyses have given rise to a new taxonomic category that is now recognized by many scientists

• The domaindomain is a more inclusive category than any other—larger than a kingdom

• The three domains are:– Eukarya:Eukarya: which is composed of protists, fungi, plants, and

animals– Bacteria:Bacteria: which corresponds to the kingdom Eubacteria– Archaea:Archaea: which corresponds to the kingdom Archaebacteria

• As scientists continue to accumulate new information about organisms in the domains Bacteria and Archaea, these domains may be subdivided into additional kingdoms


• Clearly, modern classification is a rapidly changing science, and we must pick a convention to classify life's diversity for the purposes of this Text

• In this Text, we recognize the three domains and also refer frequently to the six kingdoms

• The relationship between the three domains and the six kingdoms is shown in the table

• It also summarizes the key characteristics of each kingdom

• You can see that some groups share one or more traits with other groups


• Organisms are grouped in three domains

• There is a simple relationship between the three domains and the six kingdoms

• This table summarizes key evidence used in classifying organisms into these major taxonomic groups


Domain Bacteria

• The members of the domain Bacteriadomain Bacteria are unicellular and prokaryotic

• Their cells have thick, rigid cell walls that surround a cell membrane

• The cell walls contain a substance known as peptidoglycan

• The domain Bacteria corresponds to the kingdom kingdom EubacteriaEubacteria

• These bacteria are ecologically diverse, ranging from free-living soil organisms to deadly parasites

• Some photosynthesize, while others do not• Some need oxygen to survive, while others are killed

by oxygen

Domain Archaea

• Also unicellular and prokaryotic, members of the domain Archaeadomain Archaea live in some of the most extreme environments you can imagine—volcanic hot springs, brine pools, and black organic mud totally devoid of oxygen

• Indeed, many of these bacteria can survive only in the absence of oxygen

• Their cell walls lack peptidoglycanTheir cell walls lack peptidoglycan, and their their cell membranes contain unusual lipids that cell membranes contain unusual lipids that are not found in any other organismare not found in any other organism

• The domain Archaea corresponds to the kingdom Archaebacteriakingdom Archaebacteria.

Domain Eukarya

• The domain Eukaryadomain Eukarya consists of all organisms that have a nucleus

• It is organized into the four remaining kingdoms of the six-kingdom system:– ProtistaProtista– FungiFungi– PlantaePlantae– AnimaliaAnimalia

• Organisms in these kingdoms are diverse and varied

Domain Eukarya

• The domains Bacteria and Archaea include the same organisms that are in the kingdoms Eubacteria and Archaebacteria

• The domain Eukarya includes the protists, fungi, plants, and animals

• Biologists continue to investigate how these three large groups originated

• Which domain includes organisms from more than one kingdom?

Domain Eukarya

Protista

• The kingdom Protistakingdom Protista is composed of eukaryotic organisms that cannot be classified as animals, plants, or fungi

• Of the six kingdoms, Protista is the least satisfying classification, because its members display the greatest variety

• Most protists are unicellular organisms, but some, such as the multicellular algae, are not

• Some protists are photosynthetic, while others are heterotrophic

• Some share characteristics with plants, others with fungi, and still others with animals

Fungi

• Members of the kingdom FungiFungi are heterotrophs

• Most feed on dead or decaying organic matter

• Unlike other heterotrophs, these fungi secrete digestive enzymes into their food source

• They then absorb the smaller food molecules into their bodies

• The most recognizable fungi, including mushrooms, are multicellular

• Some fungi, such as yeasts, are unicellular

Plantae

• Members of the kingdom Plantae are multicellular organisms that are photosynthetic autotrophs– In other words, they carry out photosynthesis

• Plants are nonmotile—they cannot move from place to place

• They also have cell walls that contain cellulose• The plant kingdom includes cone-bearing and flowering

plants as well as mosses and ferns• Although older classification systems regard multicellular

algae as plants, in this book we group algae with the protists

Animalia

• Members of the kingdom Animaliakingdom Animalia are multicellular and heterotrophic

• The cells of animals do not have cell walls• Most animals can move about, at least for some

part of their life cycle• As you will see in later chapters, there is

incredible diversity within the animal kingdom, and many species of animals exist in nearly every part of the planet

Documents

Classification Each person might divide these shells into different categories Scientists often group and name, or classify, organisms using certain guidelines