Directional selection

In population genetics, directional selection is a mode of natural selection in which individuals with a trait (for example, beak size) at one extreme of a phenotypic distribution have better fitness than individuals with intermediate or opposite extreme phenotypes. Over time, the allele frequencies, and consequently the population mean for the trait, shift consistently in the direction of the extreme phenotype with greater fitness. An example is the evolution of antibiotic resistance in bacteria – the introduction of a strong selective pressure (the antibiotic) selects resistant strains of bacteria, thereby shifting allele frequencies toward phenotypes with strong resistance to the antibiotic.

This type of selection plays an important role in the emergence of complex and diversifying traits and is also a primary force in speciation. Natural phenomena that might promote strong directional selection include:

• Sudden environmental changes (biotic or abiotic) favouring one phenotype over a previously dominant phenotype;
• Colonization of a new habitat with novel selection pressures (as was the case with Darwin’s finches migrating to the Galápagos Islands two million years ago);
• The genetic context offers sufficient heritable variation and involves relatively minor interactions or correlations among genes (pleiotropy or epistasis) and trade-offs (antagonistic pleiotropy). Such interactions would not necessarily preclude directional selection, but would make it more complicated.