Genetic Variation and Evolution


This topic straddles material that falls into two important areas of your A-level syllabus: genetics and evolution. The link between these topics may not be clear to you, and so this module will guide you through some of the complex ways in which these topics are related. You will start by discovering the ideas behind the Modern Synthesis, before investigating concepts such as genetic drift, adaptive landscapes, phenotypic plasticity and heritability.

The activities below include several reading resources, including research reviews and some evolutionary websites that contain lots of useful information and case studies. You will need to read these resources as you work through a number of tasks. Questions for consideration and discussion are also included.

If you are interested in this topic, there are many more open-access sources of information available to you. However, when using these sources, be aware that:

• Some websites may not support the widely-accepted scientific view of evolution by natural selection, favouring a more creationist approach. This resource will not consider the somewhat meaningless debate between advocates of evolution and creationism, as there is no point is comparing an objective approach to explaining life on earth (science) with a subjective approach to explaining life on earth (faith). This site will focus on widely-accepted scientific theories that are based on hypothesis testing and evidence. If you stick to using scientific articles and science-department websites you should be okay.

• The literature for this topic involves a lot of advanced mathematics. Please do not be put off by this! When you progress to university, these equations and models will become a lot clearer. This topic will just give you a “taster” of some of the key concepts involved in evolutionary genetics.

Natural Selection and Genetics

Natural selection is one of the main processes by which organisms evolve across time (the other is genetic drift; see activity 2). The theory of natural selection was first put forward by Charles Darwin and Alfred Russel Wallace (1858). Natural selection is the gradual, non-random process by which biological traits become either more or less common in a population as a function of differential reproduction of their bearers. It will occur if three conditions are met:

1. Heredity: Individuals resemble their parents: they are in general more like their parents than are other members of the population.

2. Variation: Individuals vary in their characteristics

3. Fitness differences: There is a relationship between some of the characteristics of an individual and its ability to survive and reproduce (its fitness).

Although these prerequisites are still widely accepted today, for a long time after Darwin’s theory was proposed, a theory of inheritance was lacking. The first step in resolving this issue was taken in 1900 when Hugo de Vries and Carl Correns rediscovered the work of Gregor Mendel (the “father of modern genetics”) whose work on plant hybridisation identified a number of laws of inheritance. For the next 40 years, there was then a long struggle to incorporate genetics into evolutionary theory, culminating in what is generally known as the Modern Synthesis (ca 1940-50). The theory of natural selection could now be stated in terms of genes: what actually changes during biological evolution is the relative frequency of genes. Natural selection is thus the differential survival of alternative alleles.

You can read more about the modern synthesis and natural selection in general on the website of Christs College (the college Darwin attended in Cambridge – link below). You may think that a historical perspective is not useful, but understanding the process by which theories change and develop is an important skill in modern science.

Before you progress to the next section, make sure that you understand the ways in which variation arises in the genome. A good video to show this is found on this page.

Video Resource

Resource activities

Genetic Drift

Did you know there are actually two processes by which organisms evolve at the genetic level? You'll be familiar with natural selection but this resource discusses the other process: genetic drift.


Continuous Genetic Variation

You will be familiar with traits whose variation is apparently controlled by one or just a small number of genes. This activity looks at the ways we can analyse traits controlled by multiple genes.


Gene-Environment Interactions

This activity looks at how the environment itself can influence how genes affect a trait.



Heritability is the extent to which genetic individual differences contribute to individual differences in phenotype. This activity explores the importance of heritability to understanding genetic variation.  


Activity questions

  • How has the heritability of egg number in chickens changed since their domestication?
  • What is the heritability of finger number in your village/town/city?
  • What is the heritability of having ear-rings in your village/town/city?
  • When you all arrive at university, what happens to the heritability of skin colour amongst you and your fellow students?

Reflective questions

To answer and record these questions you will need to have an account and be logged in.

Task 1

What are the key arguments, concepts, points contained within it?

Task 2

What are you struggling to understand?

What could you do to improve your understanding of these concepts/terminology etc.?

Task 3

What further questions has this resource raised for you?

What else are you keen to discover about this topic and how could you go about learning more?

Can you make any links between this topic and your prior knowledge or school studies?

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Further reading