CLASSIFICATION of living organisms
domain, kingdom, phylum, class, order, family,
modern developments - Carl Woese,
the work of Carl Linnaeus, evolutionary tree of life
how organisms are named
Doc Brown's Biology Revision Notes
Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent
Structure - eukaryotes and prokaryotes
traditional method of classifying living organisms
In biology, a classification system is a way of
organising living organisms into groups.
Early classification systems used observable
features to place organisms in groups - sometimes referred to as an
artificial classification system - although now regarded as
outdated and not suitable in the light of more advanced knowledge,
such systems have their use in e.g. identifying animals using a
As we understanding of evolutionary relationships
developed there was a need for a more advanced classification system,
sometimes referred to as natural classification system.
Natural classification systems look at an
organism's common ancestors and common structural features to help
out organisms into suitable groupings.
e.g. bats and humans have many different
features, the pentadactyl hands of them means they are both grouped
The traditional 'natural' system, still in wide use today,
is based on the classification work of Carl Linnaeus working in the 18th
His system grouped living things according to
their particular observable physical characteristics and
structural features seen with an optical microscope.
This gives rise to the 'Linnaean system' in
which living organisms are first divided up into five kingdoms.
You are basically looking at similarities
and differences between species and the data is expressed in the
idea of a five kingdom system.
The Five Kingdoms of life and their
characteristics CAN be defined as ...
animalia - all animals animals are multicellular, do
not have cell walls, do not have chlorophyll, feed heterotrophically (heterotrophs
can't make their own food) e.g. fish, insects, mammals, reptiles etc.
plantae - all plants - are multicellular, have cell
walls, have chlorophyll, feed autotroprically (autotrophs can make their own
food from photosynthesis) e.g. grasses, flowers, trees etc.
fungi - multicellular, have cell walls,
do not have chlorophyll, feed saprophytically (saprophytes feed off
dead organisms and decaying material) e.g. mushrooms, toadstools,
protoctista (protists) - unicellular (single
celled eukaryotes), have a nucleus, protoctista include algae.
prokaryotes - unicellular (single
celled), have no nucleus e.g. bacteria and archaea.
This 'five kingdom' system is still in
use, BUT it is getting out of date.
With reference to the above diagram of the Linnaean
the five kingdoms of life are sub-divided into
Species groups of organisms that have
many features in common (Homo sapien - you and me!)
Reminder: A species is defined
as a group of similar organisms that can reproduce to give
fertile offspring e.g. cats, humans.
Genus contains several species with
similar characteristics (us - Homo)
Family comprising of several genera
(us - hominidae)
Order comprising of several families
(us - primate)
Class comprising of several orders
(us - mammalian)
Phylum comprising of several classes
(us - chordata - vertebrates)
(Kingdom - animal)
Even for the
blue pathway of
'Dr Phil Brown', the
diagram ignores many subspecies e.g. vertebrates are part of the phylum
Chordata and the vertebrates themselves are sub-divided into five
subphyla: Vertebrata - fish, amphibians, reptiles, birds, and mammals.
Extra note on 'traditional' classification
(which you may not need for your GCSE biology exam)
- Scientists do not classify
viruses in any of the five kingdoms and regard them as non-living.
- Viruses, which are smaller than bacteria,
cannot reproduce themselves, have protein coat containing a few genes,
they invade cells and make them reproduce the invading virus.
- The main characteristics of
the phylum Chordata as animals with a supporting rod running the
length of the body, an example of this being the backbone in vertebrates.
- Vertebrates are divided into five classes, groups of
amphibians, birds, fish, mammals and reptiles
- Scientists place
vertebrates into the five groups based on:
- a) Oxygen absorption methods lungs, gills and skin
- b) Reproduction internal or external fertilisation,
oviparous (lay eggs) or viviparous (give birth to live young)
- c) Thermoregulation homeotherms ('warm blooded' -
kept at a constant temperature) and poikilotherms ('cold blooded' - body
temperature varies with external temperature).
- There can be
problems associated with assigning vertebrates to a specific group based on
their anatomy and reproduction methods - why many vertebrates are
difficult to classify.
- e.g. the duck-billed platypus has a bill like a duck, tail
like a beaver, its homeothermic, lays eggs but suckles its young. Not an
easy one to classify! but its closer to a mammal than any of the other four
- Accurate classification
may also be complicated by:
- a) variation within a species
- b) hybridisation in ducks produces
viable new species
- c) ring species - a group of related
populations that live near each other, neighbouring populations may
interbreed but those well separated geographically may not. Sorting out
which are genuinely different species is not easy.
- The definition of a
species as organisms that produce fertile offspring may have limitations:
- Some organisms do not always reproduce sexually and
some hybrids are fertile.
- Some organisms can reproduce asexually but are still
classed as the same species.
- Many closely related species can interbreed producing
viable offspring and technically classed as a different species.
developments in the classification system of living things
Since the days of the 18th century Carl `Linnaeus,
there have been two very significant developments in the science of
(i) Now that we know the structure of DNA and
RNA we have a much greater knowledge of the biochemistry of life
- new discoveries are being made all the time.
You can now compare DNA sequences
for particular genes or the whole genome for different organisms
- and all you need is a small sample of cells or a piece of
You look for similarities between
organisms e.g. do they have the same number of genes, do they
have a similar number of variants for a gene.
The more similar the DNA sequences
of two or more species, the more closely they are genetically
related and therefore be more likely to be more correctly
classified in the same group.
If two or more organisms share the same
number of genes and genetic variants, so they have a similar,
but not identical genomes, its likely these organisms have a
(Quote: "94% of the DNA base sequences is
the same for chimpanzees and humans". You can therefore deduce
we have a common ancestor, and not that long ago in terms of the
millions of years of 'geological time'.)
The study of the history of evolutionary
relationships at the molecular level is called molecular
phylogenetics - looking at DNA sequences of the genome.
In biology, phylogenetics is defined as
the study of the evolutionary history and relationships among
individuals or groups of organisms to determine the course of
(ii) Developments in microscopy, using
more advanced techniques, enable us to
see and understand the most fundamental structures of cells of
living organisms e.g. cells, sub-cellular structures e.g. organelles
like ribosomes and mitochondria.
This is allowing biological scientists to propose new models of classification - which in time
will change too!
The new discoveries are helping to clarify the
relationships between organisms.
In the 1990s the scientist Carl Woese proposed a
three domain system.
This was proposed from the evidence of e.g.
RNA sequence analysis which showed that some species thought to be
closely related, where in fact, quite distinct from each other, and
not as closely related as was thought.
The three domain system is based on the following
divisions of life forms ..
Eukarya (eukaryota, types of eukaryotes): This our
most familiar domain which includes all the life you see around you!
e.g. animals, fungi, plants and protists (but you see the latter
without a microscope).
('true bacteria', types of prokaryotes): These are bacteria, some of
whose names we are quite familiar with as examples of infections!
e.g. E. coli, Cholera, Helicobacter, Listeria, Staphylococcus etc.
Although they often look similar to Archaea, there are significant
biochemical differences between these domains - there are
significant differences in the DNA and RNA sequences of archaea
and 'true bacteria'.
(types of prokaryotes): Archaea may be described as primitive
bacteria and often found living in extreme environmental conditions
e.g. hot volcanic springs or salt lakes (high concentration of
dissolved salts) where few other life forms can survive. Therefore,
many archaea are examples of extremophiles.
Again, although archaea
often look similar to 'true' bacteria, there are significant
biochemical differences between these two domains.
As you can see from the diagram, the three domain
system is added to the top of the traditional Linnaean classification
system, and each domain is the subdivided into kingdoms, phylum, class, order, family, genus and species categories.
Strictly speaking, under this system, organisms are classified into three domains and
As already described, the domains are Archaea, Bacteria, and Eukarya.
However, the six kingdoms can be considered
Archaebacteria (ancient bacteria), Eubacteria (true
bacteria), Protista, Fungi, Plantae, and Animalia.
Evolutionary Trees -
evolutionary tree of life
Evolutionary trees are a way of representing the
relationship between species and the pathway they may have evolved.
Evolution and DNA and the evolutionary tree
Scientists can use DNA sequences to estimate
how long ago different species separated from each other.
This is worked out from how frequently
mutations have occurred giving rise to variants.
By knowing the number of different genetic
variants between two species, you can work out how long ago that
particular speciation occurred i.e. how long ago did the new species
appear in the timeline of evolution.
By combining traditional classification data and
the new evidence from DNA sequencing, you can join species together to
form an evolutionary tree - see examples below.
In an evolutionary tree you connect the species
together by lines that come from their most recent ancestor indicating
their evolutionary relationship.
The more closely two species are related, the
smaller the number of steps between them on the evolutionary tree.
The diagram above illustrates the idea of distant
and recent common ancestors (a to o represent species).
e.g. the evolutionary path for species
h is a ==> b ==> d => h
species d would be a recent ancestor,
species a would be a
more distant ancestor.
Such a diagram shows how closely, or otherwise,
how species might be related.
e.g. the characteristics of species h and i
would be closely related to each other, and those of j and k would
be similar too.
BUT, there would be a greater difference
between the species pairs h/i and j/k because they have different
previous ancestors of d and e respectively, despite the earlier
common ancestor b.
Scientists are using all sorts of data these days
to try and 'formulate' the evolutionary tree of ALL life.
The diagram above is principally based on (i) structural details and (ii)
DNA and RNA sequence analysis of
currently existing organisms.
For extinct species, scientists have to
rely on fossil evidence, but with modern instrumental
techniques, amazing details can be obtained on the structure of long
extinct organisms - even those of a 'soft flesh' bodied nature.
The naming of organisms
The adoption by biologists of a system of strictly
binomial nomenclature is due to Swedish botanist and physician
Carl von Linnι, more commonly known by his Latinized name Carl Linnaeus
In this context, the word binomial means
consisting of 'two parts' - usually two Latin words, the first has a capital
letter (upper case), the second word has a small letter (lower
The name of the organism is written in
italics (I may have forgotten this sometimes!)
By using Latin, every name can be
unambiguously recognised around the world, no matter what the
native language of any scientist.
The use of a universal name avoids
confusion in scientific communication.
(I'm afraid its not quite the same in
With millions of species to name, the world of
science needs a VERY systematic way of naming life forms.
Although the classification system of Carl
Linnaeus is being 'updated' on the basis of modern biological research,
his proposed system of naming living organisms is still the basis of
The name is based on the two 'lowest' sections
of the classification systems described above:
The first part gives you the
genus of the species.
This gives you information on the
In the case of 'us', our genus is
The second part tells you the
In the case of 'us', our species is
Therefore, as an organism, OUR two-part
name is Homo sapiens.
- So, be able to explain why binomial
classification is needed to identify, study and conserve species, and can be
used to target conservation efforts.
- The binomial name of species consists of a two part
Latin name (handy for use any country with its own language!).
- The Latin name cannot be confused linguistically with
'local' or country names.
- Study and identification produces a common data base
of information on species-organisms with a universal name.
- From the database, species at threat can be
identified and preservation strategies put in place.
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