Chapter 18 – Classification
18-1 Finding Order in Diversity
• To study the diversity of organisms, biologists must give each
organism a name and 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.
• Taxonomy – scientists classify organisms and assign each organism a
universally accepted name.
• By using a scientific name, biologists can be sure that everyone is
discussing the same organism.
Example: this animal is commonly referred to as a mountain lion, a
puma, a cougar, or a panther. These are all common names for the
same animal. The scientific name for this animal is Felis concolor
Assigning Scientific Names
• Early names tended to be very long, in order to fully describe the
animal. A Swedish Botanist named Carolus Linnaeus, who lived
during the eighteenth century, developed a two-word naming system
called binomial nomenclature. This system is still used 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.
• Grizzly bear is called Ursus arctos
• The first word of the scientific name, in this case, Ursus, is the genus
to which the animal belongs.
• Genus – a group of closely related species.
• The second part of the scientific name, in this case, arctos, is unique
to each species within the genus. It usually describes some important
trait of the organism or an indication of where the organism lives.
• Ursus maritimus is the scientific name of a polar bear. Maritimus
refers to the sea and 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 consists of levels. It includes seven
levels. They are- from smallest to largest – species, genus, family,
order, class, phylum, and kingdom.
• Each of these levels is called a taxon.
• Family – group of genera that share many characteristics
bears (Ursidae), dogs (Canidae), cats (Felidae)
• Order – a broad taxonomic category composed of similar families
• Class – composed of similar orders (ex - order Carnivora is placed in
the class Mammalia, which includes animals that are warm-blooded,
have body hair and produce milk for their young)
• Phylum – several different classes make up phylum. A phylum
includes many different organisms that 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.
• Kingdom – the largest and most inclusive of Lennaeus’s taxonomic
categories.
• Linnaeus names two kingdoms, Animalia and Plantae. All animals are
placed in the kingdom Animalia.
Seven Taxonomic Categories
18-2 Modern Evolutionary Classification
Which Similarities Are Most Important?
• Linnaeus grouped species into larger taxa, such as genus and family,
mainly according to visible similarities and differences. Which
similarities and differences are important? For example, would you
classify dolphins as a fish because it lives in the water and has fins or
as a mammal because it breathes air and feed their young their milk?
Evolutionary Classification
• Phylogeny- evolutionary relationships among organisms
• Biologists now group organisms into categories that represent lines of
evolutionary descent, or phylogeny, not just physical similarities.
• Evolutionary classification – grouping organisms together based on
their evolutionary history
• All members of the same genus share a recent common ancestor. All
genera in a family share a common ancestor, further in the past.
• Organisms that appear very similar may not share a recent common
ancestor. For example, barnacles and limpets were once grouped
together because the look similar.
Barnacle:
Limpet:
However, barnacles and limpets are different in important ways. Their
free-swimming larvae(immature forms) are unlike one another. Adult
barnacles have jointed limbs and a body divided into segments.
Barnacles periodically shed their external skeleton. Limpets have an
internal anatomy that is closer to snails.
Classification Using Cladograms
• Many biologists prefer cladistics analysis, which identifies and
considers only those characteristics of organisms that are
evolutionary innovations – new characteristics that arise as lineages
evolve over time.
• Derived characters – characteristics that appear in recent parts of a
lineage but not in its older members
• Cladogram – a diagram that shows the evolutionary relationships
among a group of organisms
• Cladograms help scientists understand how one lineage branched
from another in the course of evolution.
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 “read” the information coded in DNA, they
can compare the DNA of different organisms to trace the history of
genes over millions of years.
• Even genes of different organisms like humans and yeasts have many
similarities. Both have a gene that code for myosin, which is a protein
found in our muscles. This shows that humans and yeasts share a
common ancestry.
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.
The more two species have changed from one another during
evolution, the less similar their DNA will be.
Molecular Clocks
• Molecular clock – a model that uses DNA comparisons to estimate
the length of time that two species have been evolving
independently. A molecular clock relies on mutations.
• A comparison of mutations in DNA in two species can sow how
dissimilar the genes are. The degree of dissimilarity shows how long
ago the two species shared a common ancestor.
18-3 Kingdoms and Domains
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. Biologist now know that this is incorrect.
The Tree of Life Evolves
• 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. Microorganisms, such as the protist
and bacterium, are significantly different from plants and animals.
Scientists agreed that microorganisms needed their own kingdom,
which was name Protista.
• Mushrooms, yeasts, and molds were also placed in their own
kingdom called Fungi.
• Bacteria do not have a nuclei, mitochondria, and chloroplasts, so they
are placed in a kingdom called Monera.
• Scientists then realized that the Monera are made up of two distinct
groups. So, Monera have been separated into two groups: Eubacteria
and Archaebacteria.
• There are a total of six kingdoms: Eubacteria, Archaebacteria,
Protista, Fungi, Plantae, and Animalia
The Three-Domain System
• Scientists have come up with a new taxonomic category that is bigger
than a kingdom call the Domain. The Domain is more inclusive than
any other category.
The three domains are:
• Eukarya, made up of protists, fungi, plants, and animals
• Bacteria, which corresponds to the kingdom Eubacteria
• Archaea, which corresponds to the kingdom Archaebacteria
Domain Bacteria
• Members of the domain Bacteria are unicellular and prokaryotic.
Their cells have thick, rigid cell walls that surround a cell membrane.
• These bacteria range from free-living soil organisms to deadly
parasites.
Domain Archaea
• Members of the domain Archaea are unicellular and prokaryotic.
They live in extreme environments, like volcanic hot springs, brine
pools, and black organic mud with no oxygen.
Domain Eukarya
• Members of the domain Eukarya consist of all organisms that have a
nucleus. It consists of Protista, Fungi, Plantae,and Animalia
• Protista is composed of eukaryotic organisms that can’t be classified
as animals, plants, or fungi. They show great variety. Most are
unicellular but some are not. Some are photosynthetic and some are
not.
Fungi
• Fungi are heterotrophs. Most feed on dead or decaying organic
matter. Most are multicellular, like mushrooms. Some are unicellular,
like yeast.
Plantae
• Members of the domain Plantae are multicellular, photosynthetic
autotrophs.
Animalia
• Members of the kingdom Animalia are multicellular and
heterotrophic. Their cells do not have cell walls.