03 April 2014

# 131 Variation continuous and discontinous

Variation is all the differences which exist between members of the same species. It is caused by a combination of genetic and environmental factors. There are two kinds of variation: continuous and discontinuous.

Continuous variation 

- shows a complete range of the characteristic within a population.

- caused both by both gens (often a number of different genes) and environment:
  • Plants: availability of/competition for: nutrients, light, water;
                  exposure to disease…
  • Animals: availability of food/balanced diet
                     exposure to disease (or the availability of health serviced for                                                                                                           humans).

Discontinuous variation 

- seen where there are obvious, distinct categories for a feature.
- no intermediates between categories, the feature cannot usually change during life.
- caused by a single gen/a small number of genes, with no environmental influence. 

Try this

Seventy seeds were collected from a cross between two plants of the same species. The seeds were sown at the same time and, after 3 weeks, the heights of the plants which grew were measured and found to fall in to two groups, A and B, as shown in figure below.

a) Calculate the percentage of seeds which germinated. Show your working. 
                                                                                                    [2 marks]
b) i) Name the type of variation shown within each group               [1 mark]
    ii) State three factors which might have caused this variation.    [3 marks]


a) 56/70x100=80%

b) i) continuous variation.

    ii) Any 3 factors from: genes, temperature, disease, seed size, light, O2, CO2, H2O, minerals, mutation, trampling by animals. 

Video: What is meant by genetic difference?