Classification
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Candidates should be able to:
(a) define the terms classification, phylogeny and taxonomy;
(b) explain the relationship between classification and phylogeny;
(c) describe the classification of species into the taxonomic hierarchy of
domain, kingdom, phylum, class, order, family, genus and species;
(d) outline the characteristic features of the following five kingdoms:
Prokaryotae (Monera), Protoctista, Fungi, Plantae, Animalia;
(e) outline the binomial system of nomenclature and the use of scientific
(Latin) names for species;
(f) use a dichotomous key to identify a group of at least six plants, animals
or microorganisms;
(g) discuss the fact that classification systems were based originally on
observable features but more recent approaches draw on a wider range of
evidence to clarify relationships between organisms, including molecular
evidence (HSW1, 7a);
(h) compare and contrast the five kingdom and three domain classification
systems (HSW4, 7a, 7b).
Key Terms
• Classification: The process of placing
living things into groups- the detailed study
of the individuals in a species
• Phylogeny: the study of how closely
different species are related
• Taxonomy: the study of the principles
behind classification (the differences
between species). These differences can
be used to classify species.
Phylogeny
• The more closely
two species are
related, the closer
they appear on the
evolutionary tree
and the more
recently they shared
a common ancestor
The Current System of Classification
The current system uses
eight taxa:
• Domain
• Kingdom
• Phylum
• Class
• Order
• Family
• Genus
• Species
Five Kingdoms
• Traditionally, all living things have been
grouped into a number of kingdoms- this
used to be two- plants and animals, but
not all living things can fit into these two
categories, so it is now 5
• Prokaryotes, protoctists, fungi, plants,
animals
Prokaryotes
• No nucleus
• Loop of DNA that is not
arranged in linear proteins (not
associated with histone
proteins- Histones are the
major structural proteins of
chromosomes)
• No membrane bound
organelles
• Small ribosomes
• Carry out respiration in special
membrane systems called
mesosomes
• Have smaller cells than
eukaryotes
• May be free living, parasitic, or
cause disease
Protoctists
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All are eukaryotes
Mostly single celled
Have a variety of forms
Show plant or animal like features
Mostly free living
Have autotrophic (photosynthesis)
or heterotrophic (ingesting prey or
extracellular enzymes) nutrition
• It is sometimes referred to as the
dustbin group as they have
features that do not belong in any
other kingdom
Fungi
• Are eukaryotes
• Have a mycelium (a
network of strands called
hyphae)
• Have walls made of chitin
(not cellulose)
• Have a multinucleate
cytoplasm (more than
one nucleus)
• Are mostly free living and
saprophytic (cause decay
of organic matter)
Plants
• Eukaryotes
• Multicellular
• Have a cellulose cell
wall
• Produce multicellular
embryos from
fertilised eggs
• Have autotrophic
nutrition (carry out
photosynthesis)
Animals
• Are eukaryotes
• Are multicellular
• Have heterotrophic
nutrition (eat other
organisms, digest and
absorb them)
• Have fertilised eggs
that develop into a
ball of cells called a
blastula
• Can usually move
around
Classifying a Species
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A species means a group of organisms
that can breed together to produce
fertile offspring
Carl Linnaeus devised a system of
naming living organisms that uses two
names- called the binomial system
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Before this, scientists used to use
long, detailed descriptions or a
common name, however this did not
work well because:
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The same organisms had different
names in different parts of the country
Translation of languages may give
different names
The same common name may be
used for different species in other
parts of the world
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The Binomial System
• Latin is a universal language so species are given a
universal name
• All scientists around the world will use the same name
• Binomial means ‘two names’, the first name is for the
genus, the second is for the species e.g. Homo sapiensHomo is the genus, sapiens is the species
• The genus name always starts with a capital letter
• It is always used in a way that makes it stand out- if
typed, it is in italics, and if written it is underlined.
Identifying Organisms
• Before you collect a species sample, you must
check it will not harm the environment
• You would first have to carry out an EIAEnvironmental impact assessment
• Scientists need to know what species are
present, if they are rare, and if habitat
destruction would have a large impact
• For example: a pond that contains great crested
newts must not be harmed, as it is protected
under the wildlife and countryside act 1981- it is
illegal to catch, possess handle, cause death,
harm or disturb them
Using a Key
• A dichotomous key is a way of identifying
organisms you have found
• It is based on questions and answers- usually
yes or no
• The answer to each question leads to a different
question and finally the name of the specimen
• A good dichotomous key has one question fewer
than the number of species it can identify
You must make sure you know how to use a dichotomous key
to identify a group of at least six plants, animals or
microorganisms
Ancient Classification
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Early classification was based on
observable features, so was
limited to what you could see
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In the 17th century (1600s), they
had microscopes to help so they
could see cells
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Aristotle (384BC) based his
observation on appearance and
anatomy he said organisms were
either plant or animal, then he
further classified them into three
groups, those that: live and move
in water, live and move on land
and move through the air
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Unfortunately, this meant that he
grouped fish with turtles, birds with
insects and mammals with frogs.
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Classification is now more
accurate as more research has
been carried out.
Modern Classification
• Originally, one celled
organisms that were plant like
were classified with plants and
animal like ones classified with
animals, but some unicellular
organisms have features of
both animal and plant, also
there was the problem with
fungi having hyphae and not
carrying out photosynthesis.
• This is why we now have the 5
kingdom classification,
however there are now even
more ways of classifying
organisms…
Biochemistry
• Certain large biochemical molecules are
found in all living things, but they may not
be identical in all living things
• This can be used to show evolutionary
relationships and how closely related
species are to one another
Cytochrome C
• Cytochrome C is a protein used in respiration
• All living organisms except chemosynthetic
prokaryotes must respire, so must have
Cytochrome C
• Cytochrome C is not identical in all species- it is
a protein made from amino acids, and some of
the amino acid sequences differ between
species.
• If the amino acid sequence is the same, the two
species must be closely related, the more
differences there are, the less the species is
related
DNA
• All organisms use DNA or RNA
• Comparisons of DNA sequences provides
a way to classify species
• The more similar the sequence, the more
closely related the species
• We can use this to clarify or correct
evolutionary relationships that we are
unsure about
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In 1990, Carl Woese suggested a new
classification system based on RNA
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He divided the kingdom Prokaryote into
two: the bacteria (Eubacteria) and the
Archaeae (Archaebacteria)
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He said that Bacteria have: a different
cell membrane structure, different
flagella, different enzymes for building
RNA, no proteins bound to their genetic
material and different DNA replication
mechanisms
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He also said Archaea share features
with eukaryotes e.g. similar enzymes
for building RNA, similar DNA
replication mechanisms and production
of the same proteins that bind to DNA
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Woese suggested that the differences
between Archaeae and Eubacteria are
greater than the differences between
Eukaryotes and bacteria- this is now
widely accepted by biologists
Three Domains
Questions
[Total 4 marks]
Task
• Now complete the end of chapter test as
an open book activity