Glossary

5.1 Types of natural selection: directional, stabilizing, and disruptive

2 min read•july 25, 2024

Natural selection shapes populations in diverse ways. Directional, stabilizing, and each affect trait distributions differently, influencing mean values and variation within populations over time.

Understanding these selection types helps explain observed patterns in nature. By analyzing trait distributions and environmental pressures, scientists can identify and quantify selection's effects on populations, revealing evolution in action.

Types of Natural Selection

Types of natural selection

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  • favors one extreme of a phenotypic trait shifting population mean towards one end of distribution changing allele frequencies over time (beak size in Galápagos finches during drought)

  • favors intermediate phenotypes reducing variation around mean maintaining current population average (human birth weight)

  • Disruptive selection favors extreme phenotypes at both ends of distribution increasing variation within population leading to bimodal distribution of traits (shell color in land snails)

Effects on phenotype distribution

  • Directional selection gradually shifts entire distribution towards one extreme reducing variation on non-favored side potentially resulting in loss of alleles for non-favored phenotype (antibiotic resistance in bacteria)

  • Stabilizing selection narrows overall distribution of phenotypes reducing variation at both extremes increasing frequency of individuals with intermediate traits (number of offspring in mammals)

  • Disruptive selection widens overall distribution of phenotypes creating two distinct peaks potentially leading to speciation if mating becomes assortative (beak size in African seedcrackers)

Traits influenced by selection

  • Directional selection: antibiotic resistance in bacteria, beak size in Galápagos finches during drought, industrial melanism in peppered moths

  • Stabilizing selection: human birth weight, number of offspring in mammals, enzyme efficiency at body temperature

  • Disruptive selection: shell color in land snails, beak size in African seedcrackers, coloration in guppies

Analysis of selection in data

  • Examine distribution of phenotypes in population looking for shifts in mean and assessing changes in variation

  • Compare data from different time points or generations observing trends in trait frequencies over time and identifying emerging patterns in distribution

  • Consider environmental factors and selective pressures analyzing how trait relates to survival and reproduction evaluating potential advantages of different phenotypes

  • Use statistical methods to quantify changes:

    1. Calculate mean, variance, and skewness of distribution
    2. Perform tests for normality and modality

  • Apply mathematical models of selection using breeder's equation R=h2SR = h^2S and calculating selection differential and selection intensity

  • Interpret results in context of evolutionary theory considering alternative explanations for observed patterns and evaluating strength and consistency of selection over time

Key Terms to Review (13)

Adaptive Evolution: Adaptive evolution is the process through which a population becomes better suited to its environment through genetic changes that enhance survival and reproduction. This phenomenon is often driven by natural selection, where advantageous traits increase in frequency over generations, resulting in organisms that are more fit for their specific ecological niches. The connection between adaptive evolution and genetic mutations, types of natural selection, and environmental changes illustrates how species respond dynamically to various pressures over time.
Allele frequency: Allele frequency refers to how often a specific allele appears in a population compared to all alleles for that gene. It plays a crucial role in understanding genetic variation, evolutionary processes, and the genetic structure of populations, influencing how traits are inherited and how populations adapt over time.
Charles Darwin: Charles Darwin was a 19th-century naturalist and biologist best known for his theory of evolution through natural selection, which fundamentally changed our understanding of life on Earth. His work provided crucial insights into the processes of evolution and laid the foundation for modern evolutionary biology, influencing various scientific fields and changing perceptions of species adaptation and diversification.
Darwin's finches: Darwin's finches are a group of about 15 species of small birds found on the Galápagos Islands, known for their diverse beak shapes and sizes adapted to different food sources. These finches played a crucial role in Charles Darwin's formulation of the theory of evolution by natural selection, illustrating how species can adapt and diverge based on environmental pressures.
Directional selection: Directional selection is a mode of natural selection where one extreme phenotype is favored over others, leading to a shift in the population's traits over time. This process drives evolutionary change by promoting the prevalence of beneficial traits that enhance survival and reproduction in a specific environment. Understanding this concept is crucial for grasping how populations adapt to changing conditions, interact with their environments, and deviate from genetic equilibrium.
Disruptive Selection: Disruptive selection is a type of natural selection that favors individuals at both extremes of a trait distribution while selecting against the average or intermediate phenotypes. This process can lead to increased genetic variance and may promote speciation, especially when the extremes have distinct advantages in their environment. Over time, disruptive selection can contribute to the emergence of new species by enhancing ecological divergence and altering allele frequencies in populations.
Fitness landscape: A fitness landscape is a metaphorical representation of the relationship between genotypes (or phenotypes) and reproductive success, often visualized as a multi-dimensional space where peaks represent high fitness and valleys represent low fitness. This concept helps in understanding how natural selection influences evolutionary processes, adaptation, and the dynamics of evolutionary arms races.
Gene flow: Gene flow refers to the transfer of genetic material between populations through processes such as migration, interbreeding, or the movement of gametes. This exchange of genetic information is crucial for maintaining genetic diversity within populations and can impact evolutionary trajectories by introducing new alleles or modifying allele frequencies over time.
Genetic drift: Genetic drift is the random fluctuation in allele frequencies within a population due to chance events, leading to changes in genetic variation over time. This process can significantly impact small populations where random events can lead to large changes in allele frequencies, affecting evolution and the overall genetic diversity of populations.
Gregor Mendel: Gregor Mendel was a 19th-century Austrian scientist known as the father of genetics due to his pioneering work on inheritance patterns in pea plants. His experiments laid the groundwork for understanding how traits are passed from one generation to the next, connecting heredity with evolution and helping explain population changes over time. Mendel's laws of inheritance reveal how variation within populations can lead to evolutionary changes, making his work critical in linking genetics to natural selection.
Stabilizing Selection: Stabilizing selection is a type of natural selection that favors the average or intermediate phenotypes in a population, reducing variation and maintaining the status quo. This process occurs when extreme traits are selected against, leading to a decrease in the frequency of those traits over time. It highlights how certain traits can be advantageous for survival and reproduction, making them more common within a stable environment.
Survival of the fittest: Survival of the fittest is a phrase that describes the process of natural selection, where individuals better adapted to their environment tend to survive and reproduce more successfully than those less suited. This concept is key to understanding evolution and species change, illustrating how advantageous traits become more common in a population over generations.
The modern synthesis: The modern synthesis is a comprehensive theory of evolution that integrates principles from genetics with Darwinian natural selection. It emphasizes the role of genetic variation and mutation in the process of evolution, explaining how populations adapt over time to their environments. This synthesis unites diverse biological fields, including paleontology, systematics, and population genetics, providing a cohesive framework for understanding the mechanisms of evolutionary change.
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