ABSTRACT
Stabilizing selection works against the phenotypes distant from the mean on either side. The result is a reduction in the phenotypic variance so that the population after selection is distributed more closely around the mean than before selection. In a normally-distributed trait, the resulting distribution is described statistically as leptokurtic. In the 1900s, the population geneticist J.B.S. Haldane believed that natural selection can detect single-allele substitutions. He based this belief on the observation of industrial melanism, where an allele specifying one phenotype was substituted by another, specifying a protected phenotype. Similar single-allele substitutions are detectable in many cases of insecticide resistance in insects. Many evolutionists found it difficult to explain how behavioral traits that seem to be deleterious for the individual performing them, could evolve by natural selection. The evolution of another kind of trait is also difficult to explain by natural selection – the extreme development of ornaments.
