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.
Completed
1.

Watch the ‘Brainpop’ video on ‘What’s in a name?’
2.

Read 1. ‘Why we classify living things’ and answer the
following:
a. What is a taxon?
b. Distinguish between the terms classification and
taxonomy.
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
adopted?
f. Give a general definition for a species.
3.
 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
evolution.
1. WHY WE CLASSIFY LIVING THINGS.
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.
TAXONOMY
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
things.
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
(noun)
(adjective)
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.
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).
At least 1.7 million species of living organisms have been discovered, and the list grows longer every year
(especially of insects in the tropical rain forest). How are they to be classified?
Ideally, classification should be based on homology; that is, shared characteristics that have been
inherited from a common ancestor. The more recently two species have shared a common ancestor,
the more homologies they share, and
the more similar these homologies are.
Until recent decades, the study of homologies was limited to
anatomical structures (comparative anatomy e.g. pentadactyl limb)
pattern of embryonic development.
With advances in molecular biology studies of DNA, proteins and immunological responses can be used to
give evidence for classifying organisms. Biochemical methods have helped to prevent mistakes in
classification, which have been made due to morphological convergence.
What is convergence?
Protein Sequencing
Sequencing the amino acids within shared protein and looking for differences can provide evidence for how
closely organisms are related to a common ancestor.
Two examples of proteins that have been extensively studied include haemoglobin and cytochrome c (a
protein that forms part of the electron transport chain in mitochondria that electrons are passed down
during aerobic respiration.
Why might cytochrome C prove to be amore useful protein to compare than haemoglobin?
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?
Comparing Proteins Using Immunological Techniques
Explain this technique:
This example below is given in your textbook when comparing human proteins.
DNA base sequences
Over time DNA changes due to errors in copying (mutations). If the error occurs in a sex cell it could be
passed onto the next generation. Often these mutations make no difference to the observable
characteristics of a species but they can cause the species to very slowly change due to the action of
natural selection.
The DNA changes gradually over thousands of years and we can use computers to compare the base
sequences of DNA in different organisms. The more differences we find the more it is suggested that the
organisms are not so closely related and the longer it is since they had a common ancestor.
By studying the sequence of bases we can see where single differences in the code have occurred and
how they have been passed on through evolution.
Comparison of DNA base sequences is used to elucidate relationships between organisms.
Traditional taxonomic methods such as comparative anatomy (bones, teeth etc.) placed humans on a
separate evolutionary branch and grouped all the other apes together in one family (Pongidae).
However, evidence from studying DNA shows that humans are more closely related to chimpanzees and
gorillas than to orangutans. And that chimpanzees (and Bonobo) are closer in terms of an evolutionary
ancestor than gorillas.
DNA Hybridisation
DNA sequencing is expensive and time consuming and only small length of DNA can be studied and
compared in detail. Another method called DNA hybridisation compares whole DNA molecules from
different species.
The double helix structure of DNA is held together by hydrogen bonds that form between the
complimentary base pairs.
In DNA hybridisation a hybrid DNA molecule is made
from one strand of DNA from one species joined to
another DNA strand from another species. Where the
bases line up to form pairs, hydrogen bonds will form.
So the more the strands match, the more bonds will
have formed.
The general rule is that the closer two species are the
greater the number of hydrogen bonds formed, so the
more strongly the 2 strands are held together.
If we heat single species DNA it will need to be heated
to about 70-80 degrees Celsius before enough energy is
provided to break the bonds.
In hybrid DNA less energy is needed to separate the strands because there are fewer hydrogen bonds.
(Around 1 degree lower for every 1% difference in bases)
The table below shows data obtained from a study of the DNA of humans, chimpanzees, gorillas and
orangutans.
1. Which species DNA is most similar to human DNA?
2. If human DNA split at 94 degrees what temperature would you expect the human/gorilla hybrid stand to
split at?
Species from which hybrid was produced
Difference between the temperature at which human
DNA strands separated and hybrid DNA strands
separated
Human: Chimpanzee
1.6
Human: Gorilla
2.3
Human: Orangutan
3.6
DNA genetic fingerprinting (DNA profiling)
This technique can also be used to look for evolutionary relationships between organisms.
Outline of the technique:
DNA is cut using a restriction enzyme.
(Restriction enzymes cut at specific recognition sites)
This will cut DNA into different length fragments.
(Fragment lengths may vary as different organisms may have evolved different VNTR {variable number
tandem repeats}
The fragments will be different sizes and the fragments can be separated according to their size using a
technique called gel electrophoresis.
Banding patterns then can then be compared. The more banding patterns that are shared the closer the
organisms probably are in terms of their common evolutionary ancestor.
PHYLOGENY
Phylogenists study how closely different species are related and build evolutionary trees to show those
relations. The more closely two species are related, the closer they appear together on the evolutionary
tree.
Who is the thrush more closely related to? Explain your answer
Summary:
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
Ways:
  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
proteins/genes)
  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
blowholes
do nurse their young
do have hair — they are born with hair around their "noses"