Continuous and Discontinuous Variation

Variation refers to the differences in characteristics between individuals within a single species. There are two types of variation that we can observe:

  • Continuous variation
  • Discontinuous variation

Continuous Variation

Continuous variation refers to a characteristic that can have many different values and take on any value within that range. An example of this is human height. There is no fixed point at which someone becomes short or tall. Instead, height can range from very short to very tall and everything in between. It is continuous because a person’s height can fall within a wide range of values (e.g. 1.7m, 1.71m, 1.72m, 1.725…).

Continuous variation is also known as quantitative variation.

Some other examples of continuous variation are:

  • Weight
  • Heart rate
  • Rate of photosynthesis

You can record data with a continuous range of values and plot it as a histogram. If you draw a line of best fit, the graph will look similar to the graph below. The graph shows a smooth bell-shaped curve of normal distribution.

A graph depicting a normal distribution curve overlaying red bars representing frequency of a given characteristic. The horizontal axis is labelled "Characteristic" and the vertical axis is labelled "Frequency". The area under the curve is shaded with cross-hatch patterns.

The more data you collect and the more categories you use, the closer your results will be to the bell curve and a normal distribution

With continuous variation, the characteristics:

  • are usually quantitative, with each category being continuous with the next
  • do not have distinct categories that individuals in a species can be placed into
  • are determined by a large number of genes (which is known as polygenic inheritance)
  • are significantly affected by the environment

Discontinuous Variation

Discontinuous variation refers to the variation in a characteristic within a population that has a limited number of possible values, or categories. Individuals can only belong to one category. These characteristics are usually controlled by alleles of one gene or a small number of genes, and are not strongly influenced by the environment.

An example of discontinuous variation is blood type in humans. There are four categories (A, B, AB or O), and humans can only belong to one of these categories.

Some other examples of discontinuous variation:

  • Eye colour
  • Gender

When plotting discontinuous variation on a graph, the data is called discrete or categorical data. For example, with human blood types:

A bar chart with red bars representing the frequency of different blood types. Blood types 'AB' and 'B' have shorter bars, while 'A' and 'O' have taller bars, indicating a higher frequency.

Features of discontinuous variation graphs:

  • Usually qualitative
  • Well defined categories
  • Controlled by few genes
  • Unaffected by the environment

You’ve used 9 of your 10 free revision notes for the month

Sign up to get unlimited access to revision notes, quizzes, audio lessons and more

Sign up