Classification and Evolution

4.3 Classification and Evolution

Biological Classification

Binomial system – a system that uses the genus name and the species name to avoid confusion when naming organism

Classification – the process of placing living things into groups

Reasons for classification:

  • For convenience
  • To make the study of living things easier
  • For easier identification
  • To show the relationships between species

Modern Classification Hierarchy

As you descend the taxonomic ranks from Domain à Species it becomes harder to distinguish and separate closely related organisms from each other and to place them accurately.

Reasons for the binomial naming system:

  • The same organism may have a completely different common name in different parts of a country
  • Different common names are used in different countries
  • Translation of languages and dialects may give different names
  • The same common name may be used for a different species in a different part of the world

Using observable features for classification

Species – a group of organisms that can freely interbreed to produce fertile offspring

This definition does not work for organisms that reproduce asexually and is very hard to apply to organisms known only from fossil records and the like.

Phylogenetic definition of species – a group of individual organisms that are very similar in appearance, anatomy, physiology, biochemistry and genetics

Early classification systems by Linnaeus and Aristotle were based solely on appearance and features which limited the classification to observable features only.

The Five Kingdoms

Evidence used in Classification

  • Biological Molecules

Some biological molecules, such as those for DNA replication and respiration are essential for life and therefore all living things have a variant that can be compared to show how closely related they are. If we assume that the earliest living common ancestors to living things had the same version of these molecules then any changes are a direct result of evolution.

  1. Cytochrome C

The protein cytochrome C is essential in respiration but is not identical in all species due to evolution. The sequences of amino acids in the protein can help draw conclusions about how closely related they are. If the sequences are the same then the two species must be closely related and if they are different they are not so closely related. The more differences found between the sequences, the less closely related the two species.

  1. RNA Polymerase

RNA Polymerase is also used as an indicator of evolution because of its essential role in protein synthesis


  • DNA
  1. Genome Sequencing

Advances in genome sequencing have meant that the entire base sequence of an organism’s DNA can be determined. The DNA sequence of one organism can then be compared to the DNA sequence of another organism. This will show you how closely related they are to each other.

  1. Comparing amino acid sequences

Proteins are made of amino acids. The sequence of amino acids in a protein is coded for by a base sequence in the DNA. Related organisms have similar DNA sequences and so similar amino acid sequences in their proteins.

     3. Immunological Comparisons

Similar proteins will bind to the same antibodies. So, if antibodies to a human version of a protein were added to isolated samples from other species, then any protein similar to the human version will be recognised and bind to the antibody.

Artificial Classification – classification for convenience, e.g. in plant identification books, sorting by flower colour

Natural Classification – Biological classification involving a detailed study of the individuals in a species, it uses many characteristics, reflects evolutionary relationships and may change with advancing knowledge

Phylogeny– the study of the evolutionary relationships between organisms

Divergent evolution is where another species has evolved from the original common ancestor and the two species get progressively less similar.

Convergent evolution is where two species, who may share the same environment and therefore the same factors that affect survival, evolve similar characteristics.

Natural Selection

Natural Selection – the term used to explain how features of the environment apply a selective force on the reproduction of individual in a population

Charles Darwin did not invent the theory of evolution but he proposed natural selection as a mechanism towards the theory. It was controversial at the time as it countered the popular religious beliefs.

Darwin developed his ideas from the expedition he sailed with the HMS Beagle around the Galapagos Islands. Wallace was another naturalist who came to the same conclusion as Darwin.

  1. Offspring generally appear similar to their parents
  2. No two individuals are identical
  3. Organisms have the ability to produce large numbers of offspring
  4. Populations in nature tend to remain fairly stable in size
  5. There is a struggle to survive
  6. Better adapted individuals survive and pass on their characteristics
  7. Over time a number of changes may give rise to a new species

Fossil Evidence

In the past the world was inhabited by species that were different from those present today.

Old species have dies out and new species have arisen.

The new species that have appeared are often similar to the older ones found in the same place.

One of the most complete fossil records is that of the horse.



Variation – the presence of variety – the differences between individuals

Intraspecific variation – variation between members of the same species

Interspecific variation – the differences between species

Continuous variation – variation where there are two extremes and a full range of values in between

Discontinuous variation – variation where there are distinct categories and nothing in between

Causes of Variation

  1. Inherited or genetic variation

This includes the combination of alleles that is inherited from our parents which is completely unique to us (unless there is an identical twin).

  1. Environmental variation

Many characteristics are brought out by environmental changes. For example, an overfed pet can become obese and a person’s skin tone may change due to exposure to the sun.

  1. Combined effects

Humans have become taller as the result of a better overall diet but however well your diet, you are unlikely to grow as tall as other people if your family is short.

Not all genes are active at any one time e.g. puberty is a time when many different genes are activating.

Changes in the environment can also directly affect which genes are active.


  1. Sample a population – this has to be random
  1. Mean – to show variation between samples

3. Standard Deviation – to show the spread of values about the mean

4. Spearman’s Rank Correlation Coefficient – to consider the relationship of the data

5. Student’s t-test – used to compare two means


Adaptation – a characteristic that enhances survival in the habitat

Anatomical adaptations – structural features

Behavioural adaptations – the ways that behaviour is modified for survival

Physiological adaptations – affect the way that processes work (also called biochemical)

Marram Grass (example)

Natural Selection

  1. Mutation creates an alternative version of a gene (alleles)
  2. This creates genetic variation between the individuals of a species (intraspecific variation)
  3. When resources are scarce, the environment will select those variations (characteristics) that give an advantage. There is a selection pressure.
  4. Individuals with an advantageous characteristic will survive and reproduce
  5. Therefore they pass on their advantageous characteristics (inheritance)
  6. The next generation will have a higher proportion of individuals with the successful characteristics. Over time, the group of organisms becomes well adapted to their environment.

Pesticide Resistance in insects

An insecticide applies a very strong selection pressure. If the individual insect is susceptible then it will die, but if it has resistance it will survive and reproduce, spreading the resistance through the entire population.

e.g. Mosquitos have developed and enzyme that can break down the pyrethroids used to treat mosquito nets.

e.g. Insect populations have become resistant to the insecticide DDT which binds to a receptor on the plasma membrane of certain cells in insects. This is due to mutations in the genes coding for cell surface receptors.

When insects become resistant it leads to pesticides accumulating in the food chain. When predators eat the insect they may get a large dose of the insecticide. This is why DDT is banned in many areas.


The use of antibiotics is a strong selection pressure on bacteria. MRSA is a very resistant bacteria that has cropped up because of over prescription of antibiotics. Medical researchers are struggling to develop new and effective drugs as the bacterial populations rapidly become resistant to them.