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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.

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.

What is Natural Selection?

What is Natural Selection?

Natural selection is an evolutionary process in which individuals with survival-promoting traits reproduce at higher rates. These favorable traits become more common within a population or species. Naturally selected traits initially arise via random genetic mutations. In order for selection to occur, there must be variation within a population, the trait controlling the variation must be heritable, and there must be an evolutionary advantage for variation in the trait.

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...

Complementation Tests

Complementation Tests

A complementation test is a simple cross to identify whether the two mutations are located on the same gene or different genes. It was first performed by Edward Lewis in the 1940s while working on fruit flies. He developed the test to identify the location and arrangement of different mutations on chromosomes.
Organisms heterozygous for different mutations are crossed pairwise in all combinations. If present on different genes, the mutations can complement each other by providing the missing...

Position-effect Variegation

Position-effect Variegation

In 1928, a German botanist Emil Heitz observed the moss nuclei with a DNA binding dye. He observed that while some chromatin regions decondense and spread out in the interphase nucleus, others do not. He termed them euchromatin and heterochromatin, respectively. He proposed that the heterochromatin regions reflect a functionally inactive state of the genome. It was later confirmed that heterochromatin is transcriptionally repressed, and euchromatin is transcriptionally active chromatin.

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Related Experiment Video

Updated: Nov 16, 2025

Probing the Limits of Egg Recognition Using Egg Rejection Experiments Along Phenotypic Gradients

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Published on: August 22, 2018

An interplay between compositional constraint and natural selection dictates the codon usage pattern among select

Indrani Sarkar¹, Saurabh Singh Rathore², Ram Pratap Singh³

¹National Avian Forensic Laboratory, Sálim Ali Centre for Ornithology and Natural History, Anaikatty, Coimbatore, 641108, Tamil Nadu, India.

|February 26, 2021

Summary

This summary is machine-generated.

Genomic analysis of Galliformes birds reveals selection pressure dominates mutation, with AT-biased genomes and frequent use of cytosine in optimal codons. Key proteins for immunity and fundamental biological processes were identified.

Keywords:

Codon usage Enrichment analysis Evolution Galliformes Translational efficiency

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Published on: September 11, 2017

Area of Science:

Genomics
Bioinformatics
Avian Biology

Background:

Galliformes represent the first avian order domesticated by humans.
Whole genome sequences are now available for several Galliformes species, including chicken, turkey, and quail.
Understanding Galliform genomic strategies requires bioinformatic analysis of codon usage, evolution, and functional enrichment.

Purpose of the Study:

To investigate the genomic features of chicken, helmeted guinea fowl, turkey, and Japanese quail.
To analyze codon usage patterns, evolutionary dynamics, and functional enrichment across these Galliform genomes.
To elucidate the interplay between mutational and selection pressures in Galliform genome evolution.

Main Methods:

Comparative genomic analysis of four Galliformes species.
Bioinformatic analysis of codon usage bias, including optimal codons and dinucleotide frequencies (TpA, CpG).
Assessment of mutational pressures using neutrality plots and selection indices; functional enrichment analysis of protein-coding genes.

Main Results:

Galliform genomes exhibit AT bias, with highly expressed genes showing higher GC content.
Selection pressure was found to dominate over mutational pressure in these species.
Specific dinucleotides (TpA, CpG) were underrepresented in protein-coding genes; functional enrichment highlighted proteins involved in immunity, signal transduction, and the central dogma.

Conclusions:

Galliform genomes are shaped by strong selection pressures, influencing codon usage and gene expression.
The identified genes are crucial for fundamental biological processes, particularly immunity and cellular maintenance in domesticated birds.
Comparative genomics provides insights into the evolutionary adaptations of Galliformes associated with domestication.