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Difficulty: HardModern Evolutionary Theory and Genetic Basis of Evolution

In modern evolutionary theory (Neo-Darwinism), natural selection operates on genetic variation within a population in distinct ways depending on selective pressures. Match each mode of natural selection on the left with its corresponding effect on phenotypic distribution and allele frequencies on the right.

  • Stabilizing selectionSelects against phenotypic extremes, favoring average traits and narrowing genetic variance.
  • Directional selectionShifts allele frequencies toward one phenotypic extreme in response to environmental change.
  • Disruptive selectionFavors extreme phenotypes over intermediate forms, resulting in a bimodal trait distribution.
  • Balancing selectionPreserves multiple alleles in a gene pool over generations due to heterozygote advantage.

Answer

Stabilizing selection matches selecting against extremes to narrow genetic variance; Directional selection matches shifting allele frequencies toward one phenotypic extreme; Disruptive selection matches favoring extreme phenotypes over intermediate forms; Balancing selection matches preserving multiple alleles in a gene pool due to heterozygote advantage.
Stabilizing selection lowers variance by favoring intermediate phenotypes; directional selection pushes trait means toward one extreme; disruptive selection favors both phenotypic extremes causing a bimodal distribution; balancing selection retains genetic variation through mechanisms like heterozygote superiority.

Step-by-Step Solution

1
Analyze how stabilizing selection alters phenotypic variance in population genetics.
Establish that stabilizing selection eliminates extreme phenotypes and maintains the average phenotype, thereby narrowing variation around the mean.
In unchanging environments, intermediate phenotypes yield optimal fitness.
2
Analyze the population genetic outcome of directional selection.
Establish that persistent selection for a specific extreme phenotype shifts the allele frequencies in the direction of that adaptive trait.
Environmental shifts create selective pressure favoring one phenotypic tail.
3
Analyze the impact of disruptive selection on population structure.
Establish that selection against the intermediate form favors both phenotypic extremes, splitting the trait distribution into two distinct peaks.
Heterogeneous environments or varied resource partitioning can favor extreme adaptations.
4
Analyze how balancing selection maintains gene pool diversity.
Establish that mechanisms like overdominance (heterozygote advantage) actively retain alternative alleles rather than driving any single allele to fixation.
Heterozygotes possessing higher selective value preserve both alleles in equilibrium.

Key Concept

Modes of Natural Selection in Population Genetics
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Topics

Modern Evolutionary Theory and Genetic Basis of Evolution

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