5.5 Variation

Continuous and Discontinuous Variation

• Variation: differences in characteristics among individuals of the same species.

Continuous variation	Discontinuous variation
Differences between individuals in the same population are not distinct or obvious	Differences between individuals in the same population are distinct and very obvious
Quantitative: can be measured	Qualitative: grouped into distinct categories
Represented by a histogram forming a normal distribution curve	Represented by a discrete bar chart
Determined by genes and may be influenced by environmental factors	Determined only by genetic factors; inherited and not influenced by the environment
Examples: height, body mass, skin colour	Examples: ability to roll the tongue, fingerprint type, earlobe type, blood group

Activity 5.8: Height and Body Mass

Aim: Investigate variation in height and body mass among pupils.

Material: Graph paper
Apparatus: Measuring tape, weighing machine

1. Measure every pupil’s height.
2. Record the number of pupils in 5 cm height intervals.
3. Plot a histogram of number of pupils against height interval.
4. Repeat Steps 1–3 for body mass using 5 kg intervals.
5. Identify the most frequent ranges, describe each histogram’s shape and determine whether the differences are distinct.

Expected interpretation: values cover a range with intermediate values and form a normal-distribution pattern; height and body mass show continuous variation.

Activity 5.9: Ability to Roll the Tongue

Aim: Investigate variation in the ability to roll the tongue among pupils.

Material: Graph paper

1. Record whether each pupil can or cannot roll the tongue.
2. Tabulate the number in each trait category.
3. Plot a bar chart of number of pupils against tongue-rolling trait.
4. Describe the discrete bars and determine whether the two traits differ distinctly.

Expected interpretation: two separate categories with no intermediate value show discontinuous variation.

Factors Causing Variation

Genetic Factors

• Crossing over between homologous chromosomes during meiosis exchanges genetic material and produces new genetic combinations.
• Independent assortment of chromosomes produces gametes with different genetic information.
• Random fertilisation between male and female gametes produces zygotes with different genotype combinations.
• Chromosomal and gene mutations produce new phenotypes by changing genetic information.

Environmental Factors

• pH, sunlight, temperature, water, climate and nutrition may change an organism’s characteristics.
• Examples:
  • frequent sunlight exposure produces darker skin;
  • overeating increases body mass;
  • Hydrangea sp. flowers are blue in acidic soil and pink in alkaline or neutral soil.

Importance of Variation

• Enables organisms to adapt to environmental changes; individuals with different characteristics may survive and maintain their species when conditions change drastically.
• Makes it easier to distinguish and identify individuals of the same species.
• Enables natural selection: organisms with suitable characteristics survive while those with less suitable characteristics become extinct; evolution occurs through natural selection.
• Supports camouflage, protecting organisms from predators.
• Adds value to animals and plants: genetic engineering and selective breeding produce desired new genetic characteristics and increase the quality and quantity of agricultural yield.