Directional selection causes trait distribution to:
Directional selection shifts population mean toward one extreme when environment changes, favoring novel phenotype such as larger beaks during drought or dark coloration during pollution. It increases frequency of alleles promoting favored extreme, decreasing opposite extreme, driving adaptation and potentially speciation. Peppered moth and Darwin's finches exemplify. This mode is common during colonization or climate change. Therefore it favors Shift mean, producing evolutionary trend. This concept integrates genetics, ecology and molecular evidence, frequently tested in NEET, GATE and CSIR-N
Ref: Hartl & Clark, Principles of Population Genetics, Drift and Effective Size.
