5.4.1 Define evolution. (1)

Define means to give the precise meaning of a word, phrase or physical quantity.

Syllabus definition: Evolution is the cumulative change in the heritable characteristics of a population.

Darwinian Evolution is not simply based on natural selection but was in fact a composition of at least five different 'sub theories':

1. Evolution: that all life is and has been perpetually changing. This contrasts strongly with notions that all forms of life are constant an unchanging.

2. Common descent: that all living things share a common ancestor if the traced back far enough.

3. Gradualism: that evolutionary change takes places slowly and gradually. This contrasts with saltation in which changes are sudden and extreme.

4. Multiplication of species: the diversity of life is a consequence of speciation. Populations adapting to locations and becoming reproductively isolated from other populations.

5. Natural selection: a two stage process in which there is:

5.4.2 Outline the evidence for evolution provided by the fossil record, selective breeding of domesticated animals and homologous structures.(2)

Outline means to give a brief account of summary.

The theory of evolution requires evidence to show that organisms change overtime, production of new species of organism and that the mechanism is natural selection..

Types of evidence:

Fossil record: A fossil is the ancient preserved remains of an organism. The fossil can be dated from the the age of the rock formation. Sequences of fossil can show the gradual change of an organism over geological time. Continuous fossil records are rare with most containing large time gaps until subsequent discoveries are made.

Evolution and the fossil record This is an excellent site with links to all aspects of this section of the syllabus including the fossil record.

Homologous structures: All of life is connected through evolutionary history and consequently those organisms more closely connected might reasonably be expected to share common structures or homologous. Group of organisms closely related share a common form or derived trait which has been inherited from the common ancestor.

This classic example of homologous structures is the pentadactyl limb of the vertebrate.

a) Humerus

b) Radius

c) Ulna

In each example the bones are modified and adapted to the locomotion of the animal.


In homologous structures it is normal to find that parts of the structure will be modified, enlarged or reduced (vestigial).

Divergence: The pentadactyl limb structure shows adaptation and modification from a common limb (ancestor) structure.

Convergence: Two organisms with different ancestors have a limb structure that fulfills the same function but has evolved form different origins. Examples Wing of a bird and the wing of an insect.


Selective breeding: man has selectively breed animals and plants for thousands of years. If an animal posses a characteristic that is considered useful or valuable then this animal is selected for breading. The hope then is that this characteristic will be present in the next generation and at a higher frequency than before. In subsequent generations it may even then be possible to select from an even more advantageous characteristic.

A Thought experiment: Selection of high milk yields in cows.


The observations of artificial selection (selective breeding) suggested that natural populations would:

  • show phenotypic variation

  • be subject to natural selection pressures

  • there would be selection of those individuals possessing the advantageous characteristic.

However selective breeding can work in another manner in which breeders selectively cull the weaker members of the population. Therefore selective breeding provides a model for nature suggesting that there could be selection two modes of selection.

  • Selection for

  • Selection against

Reading reference: Artificial selection

5.4.3State that populations tend to produce more offspring than the environment can support.(1)

State means to give a specific name, value or other brief answer without explanation or calculation.

Population growth produces more offspring than the carrying capacity of an environment can support.



5.4.4 Explain that the consequence of the potential overproduction of offspring is a struggle for survival.(3)

Explain means to give a detailed account of the causes, reasons or mechanisms.


Population modeling




5.4.5 State that the members of a species show variation.(1)

State means to give a specific name, value or other brief answer without explanation or calculation.

This type of variation is called discontinuous.

There are distinct classes of individual

e.g. Blood groups of a human population

Discontinuous variation usually indicates the condition is controlled by one to two genes.




This type of variation is called continuous variation with no distinct classes but a complete range of the characteristic

e.g. Height of a trees in a forest

Continuous variation like this normally indicates a polygenic condition or multiple alleles




5.4.6 Explain how sexual reproduction promotes variation in a species.(3)

Explain means to give a detailed account of causes, reasons or mechanisms.

Asexual and sexual populations both experience mutation which increases the variation within the members of a population. However sexually reproducing populations also experience significant additional sources of variation.

The sources of genetic variation in a populations :



5.4.7 Explain how natural selection leads to evolution.(3)

Explain means to give a detailed account of causes, reasons or mechanisms.

Darwin C. (1859) The Origin of Species

''........can we doubt (remembering that many more individuals are born than can possible survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and procreating their kind? On the other hand, we may feel sure that any variation in the least injurious would be rigidly destroyed. This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection'

or in a most concise definition from contemporary Biologist Richard Dawkins

"the non-random survival of random variants."

Natural selection is a process (not a thing) which requires:

When a population evolves there is a cumulative change in the heritable characteristics of the population.

Natural selection can act on a population without speciation occurring.

In effect the genetic profile of the population is adapting to changes in local conditions.

Every phase in the process of evolution is affected by variation and by selection.



Natural selection models

Newbyte.com Frogs

Evolution Lab


Biology corner - Peppered moth



5.4.8 Explain two examples of evolution in response to environmental change; one must be antibiotic resistance in

Example 1: Staphylococcus aureus

This bacteria is associated with a variety of conditions including skin and lung infections. As an example of evolution it can be shown that the population of S. aureus has diverged into two forms.


The bacteria is in two forms;




How MRSA evolved:

Currently this organisms is the subject of much concern amongst Health professionals particularly in the UK and USA. . This concern stems from the evolution of a new antibiotic resistant form of the species (eMRSA-16 or MRSA252). Figures from the USA indicate that some 51% of infections are contracted in hospital itself and 31% are contracted from within the community.


Example 2 New Zealand Kaka

European Black Cap evolution (?) is given here

Evolutionary Biology research

Frontiers in Evolutionary Biology a publication by the National Science Foundation March 2005 (pdf 299kb).

Some reading for those with time. This document puts into perspective the kind of questions and tasks on which evolutionary biologists are working





Click4Biology:5.4 Evolution


5.4.1 Definition.

5.4.2 Evidence of evolution.

5.4.3 Population size and evolution.

5.4.4 Population size and survival.

5.4.5 Variation in a species.

5.4.6 Sexual reproduction and variation

5.4.7 Natural selection

5.4.8 Examples of evolution.

My main reference for this section was:

Mayr, E (2004) What Makes Biology Unique? Cambridge University Press: Cambridge