Artificial Selection and Egg Size Evolution: Why Larger Eggs May Reach a Genetic Plateau
Quick Look
Researchers artificially selected Japanese quail for larger and smaller eggs over multiple generations.
Egg size responded rapidly to selection in both directions during the first generation.
Selection for larger eggs plateaued after the initial response.
Selection for smaller eggs continued over subsequent generations.
The asymmetrical response suggests that increasing egg size may be genetically constrained.
Researchers proposed directional dominance or unequal allele frequencies as possible explanations.
The study reinforces that egg size is a highly heritable, polygenic trait.
The findings help explain why selective breeding may not always produce continuous increases in egg size.
Why This Research Matters
Egg size is one of the most important reproductive traits in poultry. Larger eggs generally produce larger chicks and are often desirable in both commercial and hobby breeding programs. Many breeders assume that continually selecting the largest eggs each generation will continue producing progressively larger eggs.
This study tested that assumption experimentally.
By applying artificial selection in both directions, researchers examined whether egg size could continue increasing indefinitely or whether biological and genetic limits eventually restrict further improvement.
Study Summary
Objective
To determine how egg size responds to artificial selection and to investigate whether genetic constraints limit the evolution of larger eggs.
Study Design
Researchers established experimental Japanese quail populations and applied directional selection for both increased and decreased egg size over multiple generations.
Birds producing the largest eggs were selected to create one breeding line, while birds producing the smallest eggs established another. A control population was maintained without directional selection for comparison.
The researchers measured changes in egg size across generations while estimating genetic responses to selection.
Major Findings
Selection produced an immediate response in both directions during the first generation.
However, the response differed substantially afterward.
The line selected for larger eggs reached a plateau after the first generation, despite continued selection.
In contrast, the line selected for smaller eggs continued responding across multiple generations, producing progressively smaller eggs.
This asymmetrical response demonstrated that increasing egg size may become genetically constrained even though sufficient genetic variation remains for continued reduction in egg size.
The researchers concluded that the observed plateau was unlikely to result from a lack of selection pressure. Instead, they suggested that genetic architecture—including directional dominance or unequal allele frequencies—may limit continued increases in egg size.
The study also confirmed that egg size behaves as a quantitative trait controlled by numerous genes rather than a single major gene.
Practical Takeaways for Breeders
This research demonstrates that successful selection for larger eggs may not continue indefinitely.
Even when breeders consistently select birds producing the largest eggs, genetic improvement may eventually slow or stop if favorable allele combinations become fixed within the breeding population.
The study also illustrates an important principle of quantitative genetics: traits controlled by many genes often respond differently depending on the direction of selection.
For breeders, this means that a lack of continued improvement does not necessarily indicate poor selection practices. Instead, the breeding population itself may have reached the limits of available genetic variation for that particular trait.
Limitations of the Study
The research was conducted using experimental Japanese quail populations maintained under controlled conditions. Responses to selection may differ among unrelated breeding populations with different genetic backgrounds.
Although the study demonstrated asymmetrical responses to selection, it did not identify the specific genes responsible for the observed plateau.
The proposed explanations—including directional dominance and unequal allele frequencies—remain hypotheses supported by the observed selection response rather than direct molecular evidence.
Finally, the study focused exclusively on egg size. Other correlated traits, including fertility, hatchability, chick quality, and long-term fitness, were beyond the scope of the experiment.
Original Research
Pick JL, Hutter P, & Tschirren B.
Artificial selection reveals the energetic expense of producing larger eggs.
Heredity. 2016;116:542–549.
DOI: 10.1038/hdy.2016.16
Original publication:
https://www.nature.com/articles/hdy201616
Open-access version:
https://pmc.ncbi.nlm.nih.gov/articles/PMC4868267/
How to Cite This Review
Poultry Nerds. (2026). Artificial Selection and Egg Size Evolution: Why Larger Eggs May Reach a Genetic Plateau (Research Review). Poultry Nerds Research Library. Retrieved July 13, 2026, from https://www.poultrynerdspodcast.com/research-library/artificial-selection-egg-size-japanese-quail
Research Review
This article summarizes a peer-reviewed scientific publication and explains its practical application for poultry keepers, breeders, and educators. It is an original review written by Poultry Nerds and is not a reproduction of the original manuscript. Readers are encouraged to review the complete publication using the original publisher links provided above.