Stabilising Selection

Introduction

• Stabilising selection is a type of natural selection where the average phenotype within a population is favoured.
• Extreme phenotypes at both ends of the phenotypic range are selected against.
• It results in a reduction in the genetic variation for a particular trait.

Example 1: Moth Camouflage

• Consider an environment where there are trees of intermediate darkness and moderate amounts of light lichen.

Moth Camouflage Scenario

• In this scenario, moths with extreme colouration at either end of the spectrum are at a disadvantage. a. Very Dark Moths: Moths with extremely dark colouration would be less successfully camouflaged against the moderately dark trees. b. Very Light Moths: Moths with extremely light colouration would stand out against the moderate amount of light lichen.

Intermediate Melanin Moths

• Moths with intermediate melanism, having a colouration that matches the average environment, are favoured by natural selection.
• They are more effectively camouflaged and have a higher chance of survival.

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Stabilising Selection

Example 2: Bird Clutch Size

• Another example of stabilising selection can be observed in bird clutch size.

Clutch Size Scenario

• Birds that lay too many eggs may face the risk of not being able to provide enough food for all offspring, leading to starvation.
• On the other hand, birds that lay too few eggs may have a decreased chance of successfully passing on their genes.

Intermediate Clutch Size

• Natural selection favours birds that lay an intermediate number of eggs in their clutches.
• This clutch size optimises the chances of offspring survival and gene transmission.

Result of Stabilising Selection

• Stabilising selection tends to maintain the status quo for a particular trait within a population.
• Over generations, it reduces the genetic diversity associated with that trait.