What is variation?
Variation is the difference in characteristics between individuals of the same species. No two individuals, apart from identical twins, are exactly alike. Variation is the raw material for natural selection and is grouped into two main types — continuous and discontinuous.
Continuous variation
Continuous variation shows a full range of values between two extremes, with no clear-cut categories. Examples include human height, body mass and hand span. These traits are controlled by many genes (polygenic) and are strongly influenced by the environment. When plotted, they usually form a smooth, bell-shaped (normal) distribution curve.
Example
The height of Form 5 students ranges continuously from short to tall, with most students near the middle and few at the extremes — a typical bell-shaped curve. Diet and exercise (environment) affect the final height alongside genes.
Discontinuous variation
Discontinuous variation falls into distinct, separate categories with no in-between values. Examples include ABO blood group, ability to roll the tongue, and attached or free earlobes. These traits are usually controlled by one or a few genes and are little affected by the environment. A bar chart, not a curve, is used to display them, because the data belongs to separate groups rather than a smooth range.
Factors causing variation
Variation is produced by two sets of factors. Genetic factors arise mainly during meiosis — crossing over and independent assortment shuffle alleles — together with random fertilisation and mutation, which creates entirely new alleles. Environmental factors such as diet, climate, sunlight and disease act on top of the genes, especially for continuous traits, so the same genotype can give slightly different phenotypes in different surroundings.
Key idea
Continuous variation: many genes + environment, a range of values, bell curve. Discontinuous variation: few genes, distinct categories, little environmental effect, bar chart.
Remember
Genetic factors come from meiosis (crossing over, independent assortment), random fertilisation and mutation. Environmental factors include diet, climate and disease.