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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).

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.

Hardy-Weinberg Principle

Hardy-Weinberg Principle

Diploid organisms have two alleles of each gene, one from each parent, in their somatic cells. Therefore, each individual contributes two alleles to the gene pool of the population. The gene pool of a population is the sum of every allele of all genes within that population and has some degree of variation. Genetic variation is typically expressed as a relative frequency, which is the percentage of the total population that has a given allele, genotype or phenotype.

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.

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...

Limits to Natural Selection

Limits to Natural Selection

Organisms that are well-adapted to their environment are more likely to survive and reproduce. However, natural selection does not lead to perfectly adapted organisms. Several factors constrain natural selection.

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Updated: Sep 13, 2025

Development of an Individual-Tree Basal Area Increment Model using a Linear Mixed-Effects Approach

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Published on: July 3, 2020

Bounded environmental stochasticity generates secondary Allee thresholds.

Sebastian J Schreiber¹

¹Department of Evolution and Ecology and Center for Population Biology, University of California, Davis, CA, 95616, USA.

Journal of Theoretical Biology

|August 2, 2025

Summary

This summary is machine-generated.

Populations with Allee effects can persist despite bounded environmental fluctuations, not just inevitable extinction. Realistic models reveal two thresholds for population persistence and stochastic extinction risk.

Keywords:

Allee effects Environmental stochasticity Extinction Mate limitation,Persistence Population dynamics Predation

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Area of Science:

Ecology
Population Dynamics
Mathematical Biology

Background:

The Allee effect describes a critical population density below which extinction is probable.
Classical models often assume normal environmental variation, predicting inevitable extinction.
Realistic environmental fluctuations are bounded and not always normally distributed.

Purpose of the Study:

To analyze population dynamics under Allee effects with bounded environmental fluctuations.
To investigate the impact of realistic environmental variation on population persistence.
To develop more accurate models for conservation and management.

Main Methods:

Utilized piecewise deterministic Markov models (PDMPs).
Incorporated finite-state Markov chains for environmental dynamics.
Analyzed population persistence through threshold densities and stochastic bistability.

Main Results:

Identified two threshold densities: one for deterministic extinction, one for deterministic persistence.
Demonstrated stochastic bistability at intermediate densities, with probabilities of extinction or persistence.
Found persistence impossible when carrying capacity in one environment falls below the Allee threshold in another.

Conclusions:

Bounded environmental fluctuations significantly alter population persistence predictions compared to classical models.
Realistic environmental modeling is crucial for accurate conservation and management strategies.
Non-monotonic effects of environment on growth rates can lead to persistence challenges.