12.4

Evolution by Natural Selection

Overview of Natural Selection

Natural selection is a fundamental mechanism of evolution, responsible for the adaptation of populations over time. It operates by increasing the frequency of alleles that confer reproductive advantages, leading to adaptive evolution.

• Definition: Natural selection is the process by which organisms with traits better suited to their environment tend to survive and reproduce more successfully.

• Chance and Sorting: Evolution by natural selection involves both random genetic variation and non-random sorting of these variations based on fitness.

• Fitness: The ability of an organism to survive and reproduce in its environment.

• Example: Darwin’s finches in the Galápagos Islands, where beak shape and size evolved in response to available food sources.

Modes of Selection

Types of Selection

Selection can act on populations in different ways, shaping the distribution of phenotypes. The three primary modes are directional, disruptive, and stabilizing selection.

• Directional Selection: Favors individuals at one extreme of the phenotypic range, shifting the population’s trait distribution in one direction.

• Disruptive Selection: Favors individuals at both extremes of the phenotypic range, often resulting in a bimodal distribution.

• Stabilizing Selection: Favors intermediate phenotypes, reducing variation and maintaining the status quo for a particular trait.

• Example: Mouse fur coloration in different environments, where selection favors coloration that provides camouflage.

Mode of Selection	Effect on Population	Example
Directional	Shifts trait distribution toward one extreme	Beak size in finches during drought
Disruptive	Favors both extremes, may lead to speciation	Mouse fur color in patchy environments
Stabilizing	Reduces variation, favors intermediate traits	Human birth weight

Balancing Selection

Maintaining Genetic Variation

Balancing selection preserves genetic diversity in a population by favoring multiple alleles. This can occur through heterozygote advantage or frequency-dependent selection.

• Heterozygote Advantage: Heterozygous individuals have higher fitness than either homozygote. Example: Sickle-cell allele in humans provides resistance to malaria.

• Frequency-Dependent Selection: The fitness of a phenotype depends on its frequency relative to other phenotypes. Example: Scale-eating fish with left- or right-mouthed morphs.

Type	Mechanism	Example
Heterozygote Advantage	Maintains two or more alleles	Sickle-cell allele in malaria regions
Frequency-Dependent	Fitness varies with phenotype frequency	Scale-eating fish mouth orientation

Sexual Selection

Mechanisms and Outcomes

Sexual selection is a form of natural selection that favors traits increasing mating success. It can be intersexual (mate choice) or intrasexual (competition among the same sex).

• Intersexual Selection: Individuals of one sex choose mates based on certain traits, often leading to sexual dimorphism.

• Intrasexual Selection: Competition among individuals of the same sex for access to mates.

• Example: Peacock tail feathers are selected by female choice for their size and color.

Adaptation and Compromise

Limits and Trade-offs

Adaptations are not perfect; they often involve compromises due to competing selective pressures. Evolution works with existing traits, not by creating new ones from scratch.

• Trade-offs: Adaptations may improve one function while reducing another. Example: Seals have flippers for swimming but are less efficient on land.

• Historical Constraints: Evolution modifies existing structures rather than inventing new ones.

Chance, Environment, and Evolution

Role of Random Events

Chance events and environmental changes can affect evolutionary outcomes. Not all traits are perfectly adapted, and evolution does not produce perfect organisms.

• Genetic Drift: Random changes in allele frequencies can influence evolution, especially in small populations.

• Environmental Change: Rapid changes can make previously adaptive traits less beneficial.

Key Terms and Concepts

• Natural Selection

• Fitness

• Directional, Disruptive, Stabilizing Selection

• Balancing Selection

• Heterozygote Advantage

• Frequency-Dependent Selection

• Sexual Selection

• Adaptation

• Genetic Drift

Equations and Formulas

• Hardy-Weinberg Equation:

$p^2 + 2pq + q^2 = 1$

• Relative Fitness (w):

$w = \frac{\text{Number of offspring of genotype}}{\text{Number of offspring of reference genotype}}$

Additional info: Some examples and definitions have been expanded for clarity and completeness. The notes cover content relevant to Chapter 23: The Evolution of Populations, including mechanisms and outcomes of natural selection, modes of selection, and adaptation.