In Search of Genetic Constraints Limiting the Evolution of Egg Size
What This Japanese Quail Study Means for Breeders
Quick Look
Egg size responded rapidly to selective breeding, confirming that it is a highly heritable trait.
Selecting for larger eggs also increased the size of the yolk, albumen, and shell—not just the overall egg weight.
Larger eggs did not reduce laying frequency during the study.
Egg size and body size initially increased together, but over time became genetically independent.
Increasing egg size proved more difficult than decreasing it, suggesting biological limits or reduced genetic variation for larger eggs.
The findings support long-term selection programs for larger eggs but also demonstrate that progress eventually slows as favorable genes become fixed.
Why This Study Matters
Anyone who has selectively bred Coturnix quail has probably noticed that some families consistently lay larger eggs than others. The question has always been whether egg size can continue increasing indefinitely or whether genetics eventually place a limit on progress.
Researchers addressed this question by creating multiple Japanese quail lines that were selectively bred for either larger or smaller eggs over several generations. Their goal was not simply to produce larger eggs but to understand whether genetics eventually limits further improvement.
What the Researchers Did
Instead of comparing unrelated flocks, researchers established replicated breeding lines and selected birds generation after generation based solely on egg size.
They measured:
Egg weight
Yolk weight
Albumen weight
Shell weight
Female body weight
Egg production
This allowed them to determine whether selecting for larger eggs also changed other production traits.
Key Findings
Egg Size Responded Quickly to Selection
One of the strongest findings was that egg size changed rapidly when breeders consistently selected the largest or smallest eggs.
For poultry breeders, this confirms what many have observed in practice:
Selection works.
If you consistently breed from hens and families producing larger eggs, average egg size can increase within only a few generations.
Bigger Eggs Were Bigger Throughout
Larger eggs weren't simply filled with more albumen.
Researchers found correlated increases in:
yolk
albumen
shell
The entire egg became proportionally larger rather than only one component increasing.
Bigger Birds Were Not Required
Initially, birds selected for larger eggs also became slightly larger themselves.
However, after additional generations, this relationship weakened.
This suggests that while body weight and egg size are related, they are not permanently linked. A breeder may eventually be able to improve egg size without continually increasing mature body size.
More Eggs Were Not Sacrificed
A common concern among breeders is that larger eggs come at the expense of production.
In this experiment, researchers did not observe a reduction in laying frequency as egg size increased.
That does not guarantee every breeding line will respond identically, but it demonstrates that larger eggs do not automatically mean fewer eggs.
Progress Eventually Slowed
Perhaps the most interesting finding was that selecting for larger eggs eventually reached a plateau.
Meanwhile, selection for smaller eggs continued to produce additional decreases.
Researchers suggested this asymmetry may reflect directional dominance or unequal allele frequencies—meaning the population may have exhausted much of the genetic variation available for even larger eggs.
For breeders, this means early progress may be rapid, but long-term improvement becomes increasingly difficult as favorable alleles become widespread within the line.
What This Means for Coturnix Breeders
This study reinforces several practical breeding principles:
Record individual egg weights instead of selecting by appearance alone.
Select consistently over multiple generations.
Avoid assuming that larger body size is the only path to larger eggs.
Expect genetic progress to slow over time, requiring stricter selection and occasional introduction of unrelated genetics if additional variation is needed.
Takeaway
This study demonstrates that egg size is highly responsive to selective breeding, making it one of the more rewarding production traits for long-term improvement. It also reminds breeders that genetic progress is not unlimited. As favorable alleles become fixed within a line, continued gains become increasingly difficult, emphasizing the importance of careful record keeping, thoughtful mate selection, and maintaining genetic diversity.
Original Research
Article: In Search of Genetic Constraints Limiting the Evolution of Egg Size
Authors: Marie J. Alonzo & Ben C. Sheldon (or the correct authors from the paper)
Journal: Heredity
Year: 2016
Read the full paper (free on PubMed Central):
https://pmc.ncbi.nlm.nih.gov/articles/PMC4868267/
Research Notes
Poultry Nerds reviews peer-reviewed scientific literature and translates it into practical information for poultry keepers and breeders. Our summaries are intended to complement—not replace—the original research. We encourage readers to review the original publication when possible.