Classification and evolution: (f) the different types of variation

The idea

Variation is the differences between individuals in a population. It can be classified in several ways. Genetic variation arises from mutation, meiosis (crossing over and independent assortment) and random fertilisation, and it is heritable. Environmental variation arises from differences in conditions such as diet, temperature or exposure and is not heritable. Many traits show both genetic and environmental contributions. Variation can also be described as continuous, where a characteristic varies smoothly across a range and is usually polygenic, or discontinuous, where individuals fall into distinct categories and is often controlled by one or a few genes. Interspecific variation is between species; intraspecific variation is within a species.

What earns the marks

• Genetic variation is caused by mutation, crossing over and independent assortment during meiosis, and random fertilisation, and it is heritable.
• Environmental variation results from differences in external conditions and is not passed on to offspring.
• Most characteristics result from an interaction of genetic and environmental factors rather than one alone.
• Continuous variation shows a smooth range of values, is often polygenic, and is usually represented by a normal distribution.
• Discontinuous variation places individuals into distinct categories and is often controlled by one or a few genes.
• Intraspecific variation occurs within a species, whereas interspecific variation occurs between species.

Common mistakes and how to put them right

• Treating environmental variation as heritable.

  Put it right: Only genetic differences are passed to offspring.

• Assuming all variation is either purely genetic or purely environmental.

  Put it right: Many traits are influenced by both.

• Confusing continuous with discontinuous variation.

  Put it right: Continuous variation forms a smooth range while discontinuous variation forms distinct categories.

• Confusing intraspecific with interspecific variation.

  Put it right: Intraspecific is within a species and interspecific is between species.

Exam tips

• Identify the source of the variation (genetic, environmental or both) before choosing an option.
• Decide whether the characteristic forms a smooth range or distinct categories to classify it as continuous or discontinuous.
• Check whether the question refers to variation within one species or between species.