Natural selection is a key mechanism of evolution. It acts on the heritable traits of organisms, leading to differential survival and reproduction. Depending on which phenotypes are favored, natural selection can take different forms, leading to different patterns of change in the genetic makeup and trait distribution within a population.

Directional Selection Effect

Statement A says: Directional selection changes the average value of a trait.

Directional selection occurs when individuals with phenotypes at one extreme of the range are favored. This means the average phenotype in the population shifts over time towards that favored extreme. For example, if larger individuals survive better than smaller ones, the average size of the population will increase over generations. Therefore, statement A is true.

Stabilizing Selection Effect

Statement B says: Stabilizing selection increases variation in a trait.

Stabilizing selection occurs when individuals with intermediate phenotypes are favored, and those with extreme phenotypes (both high and low) are selected against. This type of selection reduces the frequency of extreme variations and increases the frequency of the average phenotype. This results in a decrease in the overall variation of the trait within the population. For example, in human babies, intermediate birth weight is favored over very low or very high birth weight. Therefore, statement B is false.

Disruptive Selection Effect

Statement C says: Disruptive selection reduces variation in a trait.

Disruptive selection (also known as diversifying selection) occurs when individuals with phenotypes at both extremes of the range are favored over those with intermediate phenotypes. This type of selection tends to increase the overall variation in the trait within the population, or maintain high variation, potentially leading to the divergence of the population into distinct groups. For example, in a population of finches, birds with very small beaks might be good at eating small seeds, while birds with very large beaks might be good at eating large seeds. Birds with intermediate beaks are not efficient at eating either size seed and are selected against. Therefore, statement C is false.

Balancing Selection Effect

Statement D says: Balancing selection maintains variation in a trait.

Balancing selection refers to any form of natural selection that actively maintains multiple alleles or phenotypes within a population. This prevents favored alleles from becoming fixed and lost alleles from being eliminated. Examples include heterozygote advantage (where the heterozygote genotype has a higher fitness than either homozygote, like in the case of sickle-cell trait in malaria-prone areas) or frequency-dependent selection. This process maintains genetic diversity and thus trait variation within the population. Therefore, statement D is true.

Summary of Selection Effects

Let's summarize the effects of these selection types:

Incorrect Natural Selection Statements

Based on our analysis:

• Statement A is true.
• Statement B is false.
• Statement C is false.
• Statement D is true.

The question asks for the combinations of statements that are NOT true about natural selection. The statements that are not true are B and C.

Therefore, the correct option represents the combination B and C.