|
GCSE level biology:
Classification in biology:
Part 5.
Modern developments in classification systems of living things - three
domain system - DNA analysis, phylogenetics and the three domain system of classification
[Author
©
Dr WP Brown PhD:
Doc Brown's biology exam revision notes suitable for students of UK
IGCSE & GCSE level biology courses & ~ US grades 9-10 biology [page updated
Feb 20th 2026 *]
[Key
points and learning objectives for this page, after the main body of
notes]
Index
of notes on classification of organisms in biology
[email doc
b: comment? query?]
*
[privacy-policies-disclaimer] *
]SEARCH doc b's
website]
(5) Modern developments in classification systems of living things - three
domain system
Since the days of the 18th century Carl `Linnaeus,
there have been two very significant developments in the science of
living organisms.
(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
tissue.
You look for DNA similarities or
differences 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
common ancestor.
(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
evolution.
(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 later 1970s onwards scientists like
Carl Woese proposed a
three domain system.
This was proposed from
genetic 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 ..
(1)
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 can't see the latter
without a microscope!). They are usually multi-cellular organisms.
(2)
Archaea
(types of prokaryotes): Archaea may be described as primitive
bacteria and often found living in extreme environmental conditions
e.g.
around very hot volcanic hydrothermal
vents on the seabed - often associated with areas of
tectonic activity in the earth's crust,
hot volcanic springs on the surface,
salt lakes - such a high concentration of
dissolved salts that few organisms can survive in,
and in very acidic soils or anaerobic
environments like marshes and animal guts.
These are environments 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 to justify the split into two domains.
The ribosomes of archaeans is similar in
size and structure to archaeans, yet the DNA/RNA is closer in
structure to eukaryotic cells.
Further justification for treating archaea
as a separate domain comes from the fact that 2/3rds of the
genes in them did not match genes in other organisms.
In a sense archaea were 'discovered'
because of their genetic uniqueness, and would have remained a
mystery without the advent of modern techniques of genetic
analysis - genome sequencing!
(3)
Bacteria
('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, Chlamydia, Helicobacter, Listeria, Staphylococcus etc.
Although they often look similar to Archaea, there are significant
biochemical differences between these domains i.e. there are
significant differences in the DNA and RNA sequences of archaea
and 'true bacteria'.
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,
in a similar way to the traditional classification system.
Strictly speaking, under this system, organisms are classified into three domains and
six kingdoms.
As already described, the domains are Archaea, Bacteria, and Eukarya.
However, the six kingdoms can be considered
as:
Archaebacteria (ancient bacteria), Eubacteria (true
bacteria), Protista, Fungi, Plantae, and Animalia.
|
Classification in biology:
An
evolutionary tree of life based on modern genetics
An
evolutionary tree of life based on modern genetics - three domains
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.
Combining traditional and modern genetic evidence
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 main
branches are based on the
Carl Woese theory of a three domain system.
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.
|
Key points -
Summary of ideas
Based on
the syllabus-specifications for students taking the AQA, Edexcel and OCR
GCSE level biology examinations (~US grades 9-10).
Modern Developments in Classification: The Three-Domain
System
Introduction
The classification of living organisms has evolved
significantly with advancements in technology, particularly in DNA
phylogenetics.
Traditional classification relied on observable
characteristics, but modern methods use genetic analysis to determine
evolutionary relationships more accurately.
One of the most significant
developments is the Three-Domain System, proposed by Carl
Woese in 1990, which divides life into Eukarya, Archaea, and
Bacteria.
The Three-Domain System
Woese's system is based on differences in
ribosomal
RNA (rRNA) sequences, which provide insights into evolutionary
relationships. The three domains are:
-
Eukarya:
-
Includes protists, fungi, plants, and
animals.
-
Cells have a nucleus and
membrane-bound organelles.
-
DNA is contained within the nucleus and organized
into chromosomes.
-
Archaea:
-
Consists of primitive bacteria,
often found in extreme environments (e.g., hot
springs, deep-sea vents).
-
Cell walls lack peptidoglycan,
distinguishing them from true bacteria.
-
Unique biochemical pathways and
membrane lipids.
-
Bacteria:
-
Includes true bacteria, such as
cyanobacteria.
-
Cell walls contain peptidoglycan.
-
No nucleus; DNA is in a single circular chromosome.
DNA Phylogenetics and Classification
-
DNA sequencing allows scientists to
compare genetic material across species.
-
Organisms with similar DNA sequences
are more closely related.
-
Evolutionary trees (phylogenetic trees)
illustrate relationships based on genetic data.
Impact on Classification
-
Traditional classification relied on morphology
(physical traits).
-
DNA analysis has redefined relationships,
leading to reclassification of some species.
-
The Three-Domain System provides a more accurate
representation of evolutionary history.
-
So, note that ...
-
Differences between Eukarya, Archaea, and
Bacteria.
-
How DNA sequencing has influenced
classification.
-
The role of rRNA analysis in defining
domains.
Summary of learning objectives and key words or phrases
Know about the modern organism classification system
proposed by Carl Woese, that is the three domain system of classification
into eukarya (eukaryotes), bacteria and archaea (prokaryotes), an example of modern developments in
classification systems of living things organisms.
Appreciate the classification using an evolutionary tree of
life diagram based on modern genetics three domain system of eukarya,
archaea and bacteria from a common ancestor
WHAT NEXT?
TOP OF PAGE
Index
of notes on classification of organisms in biology
INDEX of all my BIOLOGY NOTES
HOME PAGE of Doc Brown's Science
website
UK KS3 Science Quizzes for
KS3 science students aged ~11-14, ~US grades 6, 7 and 8
Biology * Chemistry
* Physics UK
GCSE level students aged ~14-16, ~US grades 9-10
Advanced Level Chemistry
for pre-university age ~16-18 ~US grades 11-12, K12 Honors
Find your GCSE/IGCSE
science course for more help links to all science revision notes
email doc
brown - comments - query?
Doc Brown's school biology revision notes: GCSE biology, IGCSE
biology, O level biology, ~US biology science grade 8, grade 9 and
grade 10 school science courses or equivalent for ~14-16 year old students of
biology IGCSE AQA GCSE Biology Edexcel GCSE Biology IGCSE OCR Gateway Science
Biology OCR 21st Century Science Biology Some of these biology revision notes
might be suitable for UK KS3 Science-Biology courses for ages 12-14 (~US biology
science grades 6, grade 7 and
grade 8),
Based on the syllabus-specifications
for students taking the IGCSE/GCSE level biology examinations summary
revision notes and key points on modern classification of organisms
based on genetics DNA analysis for students studying the AQA
igcse/gcse biology notes on modern classification of organisms based on genetics
DNA analysis, Edexcel gcse
biology notes on modern classification of organisms based on
genetics DNA analysis, OCR 21st century GCSE
biology notes on modern classification of organisms based on
genetics DNA analysis, OCR gateway
GCSE biology notes on modern classification of organisms based on
genetics DNA analysis, WJEC gcse biology notes on modern classification of
organisms based on genetics DNA analysis, CCEA
gcse biology notes on modern classification of organisms based on
genetics DNA analysis for students taking & studying CIE Cambridge igcse
biology, or any other GCSE or IGCSE level biology exams notes on
modern classification of organisms based on genetics DNA analysis, useful for US grade 9-10
students taking biology courses,
Explaining the importance of classification of organisms based on 3
domain system
in GCSE level biology, What you need to know about classification of
organisms based on 3 domain system for
GCSE level
biology,
Explaining the use of classification of organisms based on 3 domain
system knowledge in GCSE level biology, Examples of classification of organisms
based on 3 domain system explained
when studying GCSE level biology, What is
the significance of classification of organisms based on 3 domain system in GCSE level biology, describing and
explaining the theory of classification of organisms based on 3 domain
system when studying GCSE level biology, revision
notes for classification of organisms based on 3 domain system in exams,
online help for understanding classification of organisms based on 3 domain
system for GCSE biology, exam revision
notes for classification of organisms based on 3 domain system, what do I need to learn about
classification of organisms based on 3 domain system? revision summary for
classification of organisms based on 3 domain system in GCSE biology exams, learning notes for
classification of organisms based on 3 domain system, help to pass the
classification of organisms based on 3 domain system
topic in an exam question, how to prepare for questions on
classification of organisms based on 3 domain system in a GCSE
biology examination? Explaining the importance of phylogenetics &
classification of organisms
in GCSE level biology, What you need to know about phylogenetics &
classification of organisms for
GCSE level
biology,
Explaining the use of phylogenetics & classification of organisms knowledge in GCSE level biology, Examples of
phylogenetics & classification of organisms explained
when studying GCSE level biology, What is
the significance of phylogenetics & classification of organisms in GCSE level biology, describing and
explaining the theory of phylogenetics & classification of organisms when studying GCSE level biology, revision
notes for phylogenetics & classification of organisms in exams, online
help for understanding phylogenetics & classification of organisms for GCSE
biology, exam revision
notes for phylogenetics & classification of organisms, what do I need to learn about
phylogenetics & classification of organisms? revision summary for phylogenetics
& classification of organisms in GCSE biology exams, learning notes for
phylogenetics & classification of organisms, help to pass the phylogenetics &
classification of organisms
topic in an exam question, how to prepare for questions on phylogenetics
& classification of
organisms in a GCSE
biology examination?
SITEMAP Website content © Dr
Phil Brown 2000+. All copyrights reserved on Doc Brown's biology revision notes, images,
quizzes, worksheets etc. Copying of website material is NOT
permitted. Exam revision summaries and references to science course specifications
are unofficial.
|
