Summary
Selection favouring one extreme phenotype, shifting the population mean in one direction.
In depth
The mean of a trait moves toward one extreme, as in antibiotic resistance, body size in warming climates and beak size in finches. It reduces variation if continued.
Graphically, the bell curve shifts.
It occurs when the environment changes or a population enters a new one, and the response depends on heritability: R equals h squared times S. Examples include increasing body size in the evolution of horses, rising tolerance to heavy metals in plants growing near mines, and rapid evolution of industrial melanism. In the fossil record, it appears as a trend over time.
Continued directional selection eventually meets limits from trade-offs, loss of variation and developmental constraints. Hunting and fishing can cause such selection, as when heavy fishing led to earlier maturity and smaller body size in cod.
Frequently asked questions
What is Directional Selection?
Selection favouring one extreme phenotype, shifting the population mean in one direction.
Which area of biology covers Directional Selection, and how advanced is it?
Directional 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 Experimental Evolution, Antibiotic Resistance Evolution, Evolutionary Trade-offs.
Why does Directional Selection matter?
Understanding it lets us predict responses of pests and pathogens to control measures.
Can you give an example of Directional Selection?
Elephants in poaching areas are increasingly born without tusks because tusked animals are killed.
What should I learn before Directional Selection?
Work through the Beginner topics in Evolution first, such as Evolution, Natural Selection, Charles Darwin, then return to Directional Selection. Intermediate topics assume the Beginner vocabulary.
What topics are related to Directional Selection?
The closest topics to Directional Selection are Experimental Evolution, Antibiotic Resistance Evolution, Evolutionary Trade-offs, Macroevolution. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Directional Selection?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Directional Selection. To practise the whole of Evolution, use the Evolution quiz.
How is Directional Selection different from Experimental Evolution?
Directional Selection: Selection favouring one extreme phenotype, shifting the population mean in one direction. Experimental Evolution: Long-term laboratory studies, like Lenski's E. coli, that watch evolution happen in real time. Read both side by side and note what each one explains.
How should I study Directional 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 Experimental Evolution and explain the difference aloud. Finally, take the Evolution quiz and come back to this page for any question you miss.