Directional Selection

Introduction

• Directional selection is a type of natural selection that favours the survival and reproduction of individuals with traits at one extreme of the phenotypic range.
• It leads to a shift in the population's characteristics towards that extreme.

Example: Industrial Melanism

• Industrial melanism is a classic example of directional selection.
• It was observed in peppered moths during the industrial revolution in England.

Industrial Melanism in Peppered Moths

• Before industrialisation, peppered moths had a variety of colours, with both light and dark individuals.
• With the onset of industrial pollution, lichens on trees died, and tree bark became darker due to soot deposits.

Selection for Dark Moths

• Dark moths had a selective advantage because they were better camouflaged against the darkened bark.
• Predators, such as birds, were less likely to spot and eat them.

Survival and Reproduction

• Dark moths had a higher chance of surviving to reproductive age.
• They could pass on their genetic traits, which coded for more melanin, to their offspring.

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Directional Selection

Result of Directional Selection

• Over time, the frequency of the DNA sequences coding for more melanin increased in the moth population.
• This resulted in a higher proportion of dark moths compared to lighter ones.