Variation Notes

Cambridge IGCSE Biology: Revision notes

Key facts

  • Variation is the differences between individuals of the same species.
  • Continuous variation gives a range of phenotypes with intermediates (height, mass); genes and environment both cause it.
  • Discontinuous variation gives distinct categories with no intermediates (blood groups); genes alone cause it.
  • A mutation is a random, permanent change in the base sequence of DNA that can create a new allele.
  • Ionising radiation and some chemicals increase the mutation rate.
  • Genetic variation comes from mutation, meiosis, random mating and random fertilisation.

What variation is

Variation is the differences between individuals of one species, and it is what natural selection works on.

Differences can be in physical features, body processes or behaviour. A population shows a range of phenotypes.

Variation explains why individuals are not identical, how organisms adapt to their environments, and gives the basis of evolution by natural selection.

  1. 1

    Individuals differ

    a range of phenotypes in the population

  2. 2

    Some suit the environment better

    they are better adapted

  3. 3

    They survive and reproduce

    passing on their alleles

  4. 4

    The population changes

    evolution by natural selection

Why variation matters for natural selection.
  • Variationdifferences between individuals of the same species
  • Phenotypeobservable characteristics

Which statement describes variation?

Continuous and discontinuous

Continuous variation gives a smooth range caused by genes and environment; discontinuous gives distinct categories caused by genes alone.

Continuous variation has many intermediate forms between two extremes, such as height, body mass and skin colour. Many genes and the environment (for example nutrition) both affect it.

Discontinuous variation has a few distinct phenotypes, such as ABO blood groups or round and wrinkled peas. It is caused by genes only, so the environment cannot change it.

Continuous

Phenotypes:
A range of phenotypes
Intermediates:
Yes, many
Cause:
Genes and environment
Examples:
Height, mass, skin colour
Graph:
Bell curve

Discontinuous

Phenotypes:
A few distinct categories
Intermediates:
No
Cause:
Genes only
Examples:
Blood groups, seed shape
Graph:
Bar chart
1401501601701801902000.20.40.60.811.2xyFrequency
Continuous variation gives a bell-shaped curve (schematic): most individuals are near the middle.

Which feature shows discontinuous variation?

Mutation

A mutation is a random change in the base sequence of DNA, and it can create a new allele.

A mutation is a random, permanent change in DNA. A gene mutation is a change in the base sequence of a gene, and even one base can give a new allele that codes for a different protein.

Most mutations are neutral or harmful, but a few are beneficial. The mutation rate rises with ionising radiation (X-rays, gamma rays, ultraviolet light) and some chemicals.

  1. 1

    Mutation

    random change in the base sequence of a gene

  2. 2

    New allele

    a different version of the gene exists

  3. 3

    Different protein

    the allele may code for a different amino acid sequence

  4. 4

    New phenotype

    variation increases in the population

From mutation to new variation

Which increases the rate of mutation?

Sources of genetic variation

Mutation makes new alleles; meiosis, random mating and random fertilisation shuffle them into new combinations.

Mutation creates new alleles. Meiosis makes every gamete different through crossing over and independent assortment of chromosome pairs. Random mating and random fertilisation bring alleles together in unpredictable combinations.

Without this variation a population could not adapt to change.

  1. 1

    Mutation

    creates new alleles

  2. 2

    Meiosis

    crossing over and independent assortment make each gamete different

  3. 3

    Random mating

    brings different parents together

  4. 4

    Random fertilisation

    any sperm can fertilise any egg, giving a new combination

How genetic variation is made and shuffled.

Mutation

  • Creates new alleles

Meiosis

  • Recombines alleles into unique gametes

Random mating

  • Brings different alleles together

Random fertilisation

  • Any sperm can fertilise any egg

Which source of variation creates entirely new alleles?

Try an exam question

Describe the differences between continuous and discontinuous variation. Give one example of each.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.