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Types of Selection

Types of Selection

Natural selection influences the frequencies of particular alleles and phenotypes within populations in several different ways. Primarily, natural selection can be directional, stabilizing, or disruptive. Directional selection favors one extreme trait and shifts the population towards that phenotype while selecting against individuals displaying alternate traits. Stabilizing selection favors an intermediate trait with a narrow range of variation. Deviation from the optimal phenotype towards an...

Frequency-dependent Selection

Frequency-dependent Selection

When the fitness of a trait is influenced by how common it is (i.e., its frequency) relative to different traits within a population, this is referred to as frequency-dependent selection. Frequency-dependent selection may occur between species or within a single species. This type of selection can either be positive—with more common phenotypes having higher fitness—or negative, with rarer phenotypes conferring increased fitness.

Genetic Drift

Genetic Drift

Natural selection—probably the most well-known evolutionary mechanism—increases the prevalence of traits that enhance survival and reproduction. However, evolution does not merely propagate favorable traits, nor does it always benefit populations.

Mutation, Gene Flow, and Genetic Drift

Mutation, Gene Flow, and Genetic Drift

In a population that is not at Hardy-Weinberg equilibrium, the frequency of alleles changes over time. Therefore, any deviations from the five conditions of Hardy-Weinberg equilibrium can alter the genetic variation of a given population. Conditions that change the genetic variability of a population include mutations, natural selection, non-random mating, gene flow, and genetic drift (small population size).

Law of Segregation

Law of Segregation

When crossing pea plants, Mendel noticed that one of the parental traits would sometimes disappear in the first generation of offspring, called the F1 generation, and could reappear in the next generation (F2). He concluded that one of the traits must be dominant over the other, thereby causing masking of one trait in the F1 generation. When he crossed the F1 plants, he found that 75% of the offspring in the F2 generation had the dominant phenotype, while 25% had the recessive phenotype.

Genetics of Speciation

Genetics of Speciation

Speciation is the evolutionary process resulting in the formation of new, distinct species—groups of reproductively isolated populations.

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Related Experiment Video

Updated: Jul 30, 2025

Following the Dynamics of Structural Variants in Experimentally Evolved Populations

Following the Dynamics of Structural Variants in Experimentally Evolved Populations

Published on: February 3, 2023

The divergence of mean phenotypes under persistent directional selection.

Archana Devi¹, Gil Speyer², Michael Lynch¹

¹Biodesign Center for Mechanisms of Evolution, Arizona State University, Tempe, AZ 85287, USA.

|May 18, 2023

Summary

Organismal traits evolve due to selection and genetic drift. Differences in mutation effects across sites influence trait evolution, impacting how phenotypes scale with population size.

Keywords:

evolutionary divergence mutation bias phenotypic divergence phenotypic scaling random genetic drift selective interference

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Area of Science:

Evolutionary biology
Population genetics
Molecular evolution

Background:

Organismal traits, especially cellular ones, are often under directional selection across lineages.
Phenotypic gradients arise from varying strengths of random genetic drift, which differs significantly across the Tree of Life.
Previous theories assumed uniform mutation effects at all genomic sites.

Purpose of the Study:

To extend evolutionary theory to account for varying mutation effects on traits.
To investigate how differing mutation effects influence selective interference and trait evolution.
To develop a more biologically realistic model of trait evolution under selection and drift.

Main Methods:

Developed semi-analytic expressions for selective interference.
Extended single-effects models to scenarios with varied mutation effects.
Analyzed linkage effects in models with differing mutation impacts.

Main Results:

Clarified conditions where mutations with different selective effects interfere with fixation.
Demonstrated that variance in mutation effects modifies expected scaling relationships between phenotypes and population sizes.
Showed how differing mutation effects can alter the dynamics of trait evolution.

Conclusions:

The biological realism of varying mutation effects is crucial for understanding trait evolution.
Differences in mutation effects significantly impact the interplay between selection, drift, and linkage.
This work provides a more nuanced understanding of how population size influences the evolution of organismal traits.