Summary
Selection favouring intermediate phenotypes, as seen in human birth weight.
In depth
Extremes are selected against, narrowing variation around the mean. Human birth weight is optimal at about 3 to 4 kg: very small and very large babies have higher mortality.
It maintains well-adapted phenotypes in stable environments.
It is the most common form in stable environments and keeps populations well adapted by removing extremes. Another example is clutch size in birds: too few eggs wastes potential, and too many leaves chicks underfed; Lack's studies of great tits suggest that the most common clutch is close to the optimum. Gall size in goldenrod flies is also held in check by wasps and birds.
Because it keeps the average the same, the effect is often not visible over time, though it reduces variance. It may maintain the status quo in living fossils such as horseshoe crabs and coelacanths, though these do evolve genetically.
Example: sickle cell trait is a case of heterozygote advantage and acts as a form of stabilising selection on the allele.
Frequently asked questions
What is Stabilising Selection?
Selection favouring intermediate phenotypes, as seen in human birth weight.
Which area of biology covers Stabilising Selection, and how advanced is it?
Stabilising Selection is one of the 53 topics in Evolution. It is rated Intermediate, so it suits learners who already know the core vocabulary. Related topics in this area include Natural Selection, Disruptive Selection, Evolutionary Trade-offs.
Can you give an example of Stabilising Selection?
Clutch size in birds tends toward an optimum.
What should I learn before Stabilising Selection?
Work through the Beginner topics in Evolution first, such as Evolution, Natural Selection, Charles Darwin, then return to Stabilising Selection. Intermediate topics assume the Beginner vocabulary.
What topics are related to Stabilising Selection?
The closest topics to Stabilising Selection are Natural Selection, Disruptive Selection, Evolutionary Trade-offs, Adaptation. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Stabilising Selection?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Stabilising Selection. To practise the whole of Evolution, use the Evolution quiz.
How is Stabilising Selection different from Natural Selection?
Stabilising Selection: Selection favouring intermediate phenotypes, as seen in human birth weight. Natural Selection: Individuals with traits better suited to their environment survive and reproduce more, passing those traits on. Read both side by side and note what each one explains.
How should I study Stabilising Selection step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Natural Selection and explain the difference aloud. Finally, take the Evolution quiz and come back to this page for any question you miss.