2.1 Biodiversity and Evolution
Classification – Page 2
Classification is the grouping together of organisms based on shared characteristics. The type
of classification most often employed groups organisms according to evolutionary
relationships (called natural or phylogenetic classification) i.e. all the organisms in a group
show a range of similarities because of a shared ancestry. Classification is hierarchical and
thus as the groups get smaller the members become more closely related.
Watch the ‘Brainpop’ video on ‘What’s in a name?’
Read 1. ‘Why we classify living things’ and answer the
a. What is a taxon?
b. Distinguish between the terms classification and
c. Which scientist developed the binomial naming system?
d. Describe the essential features of this system.
e. Why was a universal system of naming organisms
f. Give a general definition for a species.
 Read 2. ‘What features are used in classification?’
 Watch the pbs video Using DNA as evidence of evolution.
a. Natural classification is based on homology. Explain the
words in bold.
b. Complete the question looking at the amino acid sequence
similarity of haemoglobin in different organisms.
c. Distinguish between the terms divergent (adaptive radiation
is an example of this) and convergent evolution.
d. Complete the questions on the fish and convergent
Classification is essential to biology because there are too many different living things to sort
out and compare unless they are managed into manageable categories. The scheme of
classification has to be flexible, enabling newly discovered organisms to be added to the
scheme where they best fit. It should also be able to accommodate fossil organisms as they
are discovered, since Biologists believe that living and extinct species are related.
The process of classification involves:
 Giving every organism an agreed name
 Arrangement of organisms into groupings of apparently related organisms.
Taxonomists study the differences and similarities between organisms in order to place them
into different groups, called taxa (sing. taxon). They also study and discuss which features
should be taken into account. Organisms that share similar features are grouped together
whereas organisms that are different will be placed into different groups. The whole of the
living world is today organised into a hierarchy of ranked groups, which reflects evolutionary
closeness between organisms. The science of classification is called ‘taxonomy’.
The basic unit of biological classification is the species. A species is a group of organisms,
which have numerous features in common and are capable of interbreeding and producing
viable offspring.
The binomial system of naming
The system of naming organisms using two names is called binomial nomenclature. ‘Bi’
means two; ‘nomial’ meaning name and ‘nomenclature’ refers to a system used to name
The first name (a noun) in the binomial nomenclature system is always capitalized and it
refers to the genus; the second name (an adjective) always begins with a small letter and
refers to the species. Both are always written in italics when typed or underlined when
written by hand. Most words used in binomial nomenclature are Latin or Greek in origin.
Closely related organisms have the same generic name (belong to the same genus); only their
species name differs.
generic name
+ specific name
Water buttercup = Ranunculus aqauticus
Creeping buttercup = Ranunclulus repens
The system of naming organisms was consolidated and popularized by the Swedish
naturalist Carolus Linnaeus. In his book Systema Naturae (The Natural World, 1735) he
listed and explained the binomial system of nomenclature for species, which had been
brought to him from all over the world.
One clear advantage of the binomial nomenclature system is that scientists from all over the
world working in any language can share data about a species and be sure that they are
conversing about the same organism. Globally we speak a range of different languages and
we will have a multitude of different names for an organism. Even when we speak the
language we may have regional differences in the names we give to organisms.
What’s in a name? (reading for fun!!)
By Richard Conniff
In the 1750s, the Swedish botanist Carolus Linnaeus devised a system for naming species, and
zoologists have been fooling around with it ever since. There's a beetle named Agra vation and a
spider named Draculoides bramstokeri. There's a fish named after Frank Zappa, a crustacean
genus named for Godzilla, and a fly called Dicrotendipes thanatogratus after the Grateful Dead.
At least one entomologist named a genus of bugs after his mistress. A well-known American
entomologist, who was also a bigamist, named a couple of species for his two wives. Having
scientific colleagues name a new species after you can be an honor or an insult, however
unintended. The genus name Dyaria was coined by an amateur lepidopterist who thought he was
honoring a colleague named Dyar.
The potential for bizarre and jokey nomenclature is almost unlimited. A Smithsonian researcher
estimates that there are 30 million species on earth, almost all of them insects in need of names.
2. What features are used in classification?
The quickest way to classify organisms would be to do so according to obvious visual
similarities. For example birds and insects could be classified together because they have
wings. However, upon closer inspection it can be seen that classifying these together is
superficial as they are built from different tissues and have different origins in terms of their
development. Structures that have similar functions but differ in their basic structure are
called analogous structures. A classification based on analogous structures would be
referred to as an artificial classification.
Conversely a natural classification system is based on homologous structures, these are
structures that are thought to reflect evolutionary relationships. A classification based
upon evolutionary relationships is called phylogenetic.
Analogous structures
Resemble each other in function
Differ in their structure
Illustrate only superficial similarities
e.g. the wings of birds and insects
Homologous structures
Are similar in position and development
but not necessarily function
Are similar in basic structure
Similar due to common ancestry
e.g. the limbs of vertebrates, which
appear to be modifications of an ancestral
five-fingered pentadactyl limb
Today similarities and difference in the biochemistry of organisms, as well as structural
features have become important in taxonomy. The composition of nucleic acids and cell
proteins indicates a degree of relatedness between organisms, arguably more precisely than
structural features. Organisms, which have a closer evolutionary relationship, show fewer
differences in the composition of specific nucleic acids and cell proteins that they possess.
DNA and protein analysis can reduce the mistakes made due to convergent evolution (the
tendency of unrelated organisms to acquire similar structures).
DNA hybridization is the technique that involves the extraction and comparison of the DNA
sequences of two different species. The sequence of amino acids in a protein is determined
by DNA, therefore the more similar the sequence the more similar the protein will be between
Variations in protein molecules – looking at variations in the sequence of amino acids
The β chain of haemoglobin, which is built from 146 amino acids, shows variation in the sequence
of amino acids in different species that share this molecule. The longer it is since two different
species diverged from a common ancestor, the more likely it is that differences will have arisen.
Table 1 summarises the similarities and differences in haemoglobin between 8 species
The top number in each cell is the number of positions on the molecule where the two species
have identical amino acids. The lower number is the % of positions with identical amino acids.
Complete the table (look at the information on the next 2 pages) by filling in the blank cells. First,
count the number of positions where the two species have identical amino acids. Second,
calculate the % similarity using the formula below:
% Similarity = 100 X number of identical positions / 147
Compare your calculations with you peers. Why do you think that computers are almost always
used to construct these comparison tables?
The proteins of different species can also be compared using immunological techniques. The
principal relies on the antibodies of one species will respond to specific proteins in the blood
serum of another. Injecting human blood serum into rabbits forms antibodies against
human blood serum. When rabbit antibodies are mixed with human blood a precipitate is
formed. By comparing the amount of precipitate formed when the antihuman antibodies react
with a foreign blood serum hints at biological relatedness - the greater the reaction, the
closer the supposed relationship.
Man versus Man: 100%
Man versus Chimpanzee: 97%
Man versus Baboon: 50% Man
versus the dog: 0%
Convergent Evolution
Classification :
 
Grouping organisms based on their evolutionary relationships
  Taxonomy – branch of Biology concerned with naming and
classifying life forms
 
Looks at:
 
physical features
  biochemical features (DNA ‘genetic fingerprinting’ and
enzyme studies)
 
Is dynamic and changes
 
with expanding knowledge about organisms
  with differences of opinion about whether morphology or
genetics are more central for the basis of classification
  Binomial Nomenclature - Two part name (Genus & species names in
italics or underlines if written) ex: Panthera tigris
  Hierarchical Classification –ranks groups in ascending order from
large to small groups - Seven Taxonomic Categories (Kingdom, Phylum, Class,
Order Family, Genus, Species)
  Phylogenetic - study of the evolutionary relationship between
organisms -usually uses a phylogenetic tree diagram with the oldest species at
the base and more recent ones on the branches
 
Ex: simple phylogenetic tree (shows evolutionary relationship)
  Ex: cladogram (similar but based on common traits) (often both
terms are used interchangeably)
Mammals Turtles Lizards and Crocodiles Birds Snakes
Phylogenetic Tree
Closely related species:
recognised by:
their similar morphology (eg: the homology of the pentadactyl limb in the four
classes of terrestrial vertebrates)
Biochemical methods
  Looks at the proportion of genes or proteins shared between
species to estimate relatedness (uses a process called gel electrophoresis that
shows bands on a gel which can be looked at to see if they have the same
  Biochemical methods can reduce the mistakes made in classification
due to convergent evolution.
Homologous V. Analogous:
  Homology - traits inherited by two different organisms from a
common ancestor
  Analogy - similarity due to convergent evolution, not common
ancestry Homologous:
Ex: pentadactyl limbs
Analogous- organisms evolve a similar characteristic independently of one another.
This often occurs because both lineages face similar environmental challenges and
selective pressures.
Ex: Wings in birds and insects Fins in sharks and dolphins
Simple observation tells us that these limbs are probably not homologous because
they have such different structure
Sharks Dolphins
skeleton made of cartilage
use gills to get oxygen from the water in which they swim
don't nurse their young
don't have hair
skeleton made of bone
go to the surface and breathe atmospheric air in through their
do nurse their young
do have hair — they are born with hair around their "noses"