Topic 18: Variation and selection

No two organisms are exactly alike, even within the same species. This topic explores variation — the differences between individuals — and how it arises from genes and the environment. You will then see how variation drives evolution through natural selection, the process that adapts populations to their environment over time, and how humans harness the same idea on purpose through selective breeding.

Continuous and discontinuous variation

Variation is the differences between individuals of the same species. It can be caused by genes, by the environment, or by a combination of both. Variation falls into two types based on how the feature is distributed.

• Continuous variation gives a range of values between two extremes, such as height or mass. It is usually controlled by many genes and is influenced by the environment. Plotted on a graph it gives a smooth curve.
• Discontinuous variation gives a limited number of distinct categories with no in-between values, such as ABO blood group or tongue rolling. It is usually controlled by one or a few genes and is not much affected by the environment. It is shown as a bar chart.

Identical twins raised apart are a classic way to investigate how much the environment, rather than genes, affects a feature.

Mutation and adaptive features

Mutation is a genetic change. It is the way new alleles are formed and is therefore the original source of all variation. Most mutations are neutral or harmful, but occasionally one is beneficial. The rate of mutation is increased by ionising radiation (such as X-rays and gamma rays) and by some chemicals called mutagens.

An adaptive feature is an inherited feature that helps an organism to survive and reproduce in its environment. The fitness of an organism is the probability of it surviving and reproducing in its environment. For example, the thick fur and small ears of an Arctic fox reduce heat loss, while the deep roots of a desert plant reach water far underground.

• Adaptive features are inherited, not learned during the organism’s life.
• Beneficial mutations can spread through a population over many generations by natural selection.

Natural selection

Natural selection is the mechanism of evolution. Because organisms produce more offspring than the environment can support, there is competition (a struggle for survival). Individuals show variation, much of it inherited. Those with features best suited to the environment are more likely to survive, reproduce and pass on their alleles. Over many generations the helpful alleles become more common in the population.

A clear example is antibiotic resistance in bacteria. A random mutation makes a few bacteria resistant; when an antibiotic is used it kills the non-resistant bacteria, leaving the resistant ones to reproduce. Over time the whole population becomes resistant. This is why doctors warn against overusing antibiotics.

• Variation exists in a population — the result of mutation and sexual reproduction.
• Selection pressure (such as predators, disease or competition) favours certain alleles.
• Better-adapted individuals survive and reproduce, so advantageous alleles increase in frequency.

Selective breeding

Selective breeding (artificial selection) is when humans choose which organisms reproduce. People select individuals with the desired features, breed them together, then select the best of the offspring and repeat this over many generations. It is used to improve crops and livestock, for example breeding cows for high milk yield, wheat for disease resistance or dogs for particular traits.

The key difference from natural selection is who does the selecting: in natural selection the environment selects, while in selective breeding humans select. A drawback of selective breeding is reduced variation, which can leave a population vulnerable to disease and may cause inherited health problems.

• Choose parents with the desired feature.
• Breed them together and select the best offspring.
• Repeat over many generations until the feature is well established.