1. Members of a species show genetic variation. New alleles arise due to mutation.

2. Individuals compete for resources such as food, or to survive disease or predation.

3. The better adapted individuals survive to breed and pass on the alleles/genes for the features that made them better adapted to the next generation.

4. This leads to an increase in the numbers of individuals displaying the beneficial characteristics in subsequent generations.

5. Over time (thousands of generations) the species changes (it has evolved)

The stages of evolution

This process is calledNATURAL SELECTIONNATURAL SELECTION

Human head louse approachingHumans develop chemical poisons to kill off head lice.

Use your knowledge of natural selection to explain how resistant varieties of head lice can quickly evolve

Remember:

• Think through the sequence of events

• Try to use the correct terms/phrases in your explanation

Natural selection has led to the evolution of antibiotic resistant strains of bacteria

Use your knowledge of natural selection to explain how antibiotic resistant strains can evolve

• remember to include the key phrases linked to natural selection

Describe what is meant by speciation and what is needed for it to take place

(B grade)

Explain the role of natural selection and isolation in bringing about speciation

(A grade)

Speciation requires isolation

Natural selection is the mechanism that causes species to change (evolve).

New species develop (speciation) when part of the population of an existing species is isolated (often by a geographical barrier – mountains, oceans) from the rest of the population.

If nature selects different features in the isolated population it can change so much, that when the populations do mix again, they no longer breed together. We now have two species, instead of just the one we started with.

Isolation is needed for the development of new species

• Use a series of diagram to show how isolation can cause a new species to evolve from an existing species

• Annotate your work to show what is happening at each stage of the diagram

• Highlight key words/phrases

Extension:

It becomes much wetter and a new stream is established through a grassy field separated a large slug population into two sub populations one on each side of the stream. Explain how a thousand years later, there could be two species of slugs present in the field

Hint: You will have to think of a change that produces a

different selection pressure in the field on one side of the stream