The Mating Dance of Genes: How Being Rare Can Make You Irresistible
Discover how evolution sometimes favors the unusual over the common in nature's complex mating game
The Evolutionary Puzzle
Imagine a grand party where everyone is dressed in the same style. Suddenly, someone walks in with a completely unique, captivating outfit. All eyes turn to them. This everyday social phenomenon has a profound parallel in the natural world, where evolution sometimes favors the unusual over the common.
For decades, evolutionary biologists have debated a powerful idea known as frequency-dependent mating—the theory that an individual's reproductive success can depend not just on its own traits, but on how common or rare those traits are in the population. This article explores the compelling evidence for this phenomenon, answering a challenge once posed by scientist D. J. Merrell and revealing how the very act of being different can become a powerful evolutionary advantage.
Genetic Diversity
Frequency-dependent selection maintains genetic variation that would otherwise be lost through natural selection.
Evolutionary Cycles
Creates predictable cycles where different traits gain and lose advantages over time.
The Rules of Attraction: What is Frequency-Dependent Selection?
Negative Frequency-Dependence
Rare traits have an advantage over common ones, creating balancing selection that maintains genetic diversity 5 .
- Preserves multiple morphs in populations
- Creates evolutionary stability
- Examples: Color morphs in lizards, mating strategies in insects
Positive Frequency-Dependence
Common traits have an advantage over rare ones, leading to fixation of single traits.
- Reduces genetic variation
- Can lead to evolutionary dead ends
- Examples: Warning coloration in toxic species
Why It Matters: The Power of Being Rare
Genetic Diversity
From an evolutionary perspective, negative frequency-dependent selection is remarkably powerful. It can:
- Maintain genetic diversity that would otherwise be lost 3
- Explain puzzling polymorphisms in natural populations
- Drive evolutionary cycles through constant frequency changes
- Promote speciation when combined with assortative mating 6
This mechanism has been documented across the tree of life, from the vivid color patterns of side-blotched lizards to the floral morphs of plants and even the mating strategies of insects and mites 1 4 .
Nature's Living Laboratories: Evidence from the Field
The Adder's Colored Skin: A 37-Year Revelation
One of the most compelling demonstrations of frequency-dependent selection comes from a remarkable 37-year study of adder populations on a Swedish island 3 .
Researchers meticulously documented two distinct color morphs—zigzag and melanistic—within this undisturbed population.
The Lizard's Rock-Paper-Scissors Game
In the Pyrenean mountains of Europe, the common lizard presents another fascinating case study. Male lizards occur in three color morphs—orange, yellow, and white—that engage in complex evolutionary dynamics resembling a game of rock-paper-scissors 4 .
Documented Examples in Nature
| Species | Trait | Type of Frequency Dependence | Observed Cycle |
|---|---|---|---|
| European Adder (Vipera berus) | Color pattern (zigzag vs. melanistic) | Negative | Maintained over 37 years |
| Common Lizard (Zootoca vivipara) | Male color (orange, yellow, white) | Negative (RPS dynamics) | 3-8 years |
| Bulb Mite (Rhizoglyphus robini) | Male mating tactics (fighter vs. scrambler) | None detected in recent study | N/A |
| North American Side-blotched Lizard (Uta stansburiana) | Throat color & mating strategy | Negative (RPS dynamics) | 2-3 years |
Lizard Morph Frequency Cycles (Simulated Data)
A Closer Look: The Bulb Mite Experiment
While many studies support frequency-dependent mating, a recent carefully designed experiment on bulb mites surprisingly found no evidence for this phenomenon, providing an important counterpoint to the narrative and highlighting the complexity of evolutionary processes.
Fighter Males
Thickened third pairs of legs used in combat
Scrambler Males
Avoid confrontation, alternative mating strategy
Sterile Male Technique
Used to precisely measure reproductive success
Experimental Design
| Experimental Group | Male Morph Ratio (Fighter:Scrambler) | Sterilization Protocol | Key Measurements |
|---|---|---|---|
| Fertile Control | 50:50 | None | Baseline reproduction |
| Sterile Control | 100% sterile | 40 kRad γ-radiation | Verify sterilization |
| Experimental 1 | 25:75 | Mixed fertile/sterile | Progeny sired by each morph |
| Experimental 2 | 50:50 | Mixed fertile/sterile | Progeny sired by each morph |
| Experimental 3 | 75:25 | Mixed fertile/sterile | Progeny sired by each morph |
Progeny Sired by Fighter vs Scrambler Mites
Mating Success Across Frequencies
Surprising Results: Challenging Assumptions
Contrary to expectations, the results revealed no significant advantage to being rare 1 . Fighter males consistently sired more progeny than scramblers across all frequency treatments, and this advantage wasn't enhanced when they were the rarer morph.
The Scientist's Toolkit: How We Study Mating Games
Understanding frequency-dependent selection requires sophisticated methods that can track individuals, measure reproductive success, and manipulate frequencies.
Long-term Field Studies
Projects like the 37-year adder study 3 provide invaluable data on how selection operates across generations in natural populations.
Sterile Male Techniques
Used in the bulb mite experiment 1 , this approach allows precise measurement of paternity by rendering males sterile but behaviorally normal.
Genetic Markers
Modern molecular tools enable researchers to track specific alleles and determine paternity without disturbing natural behaviors.
Population Manipulations
Experimental introductions or removals of specific morphs can test frequency-dependent effects directly.
Strengths
- Direct observation of natural behaviors
- High ecological validity
- Long-term evolutionary insights
- Comprehensive data collection
Limitations
- Time and resource intensive
- Difficult to control variables
- Limited to observable species
- Ethical considerations
Conclusion: The Ever-Changing Dance of Evolution
The evidence for frequency-dependent mating presents a compelling picture of evolution as a dynamic, ever-changing process where success is always contextual. From the color morphs of adders and lizards to the surprising case of bulb mites where the phenomenon appears absent, we see that evolution rarely follows simple rules.
Key Insight
The answer to D. J. Merrell's challenge is nuanced: yes, frequency-dependent mating is a real and powerful evolutionary force documented across numerous species, but it is not the universal explanation for all maintained polymorphisms.
Nature's dance is more complex—sometimes favoring the rare, sometimes rewarding consistency, but always maintaining a diversity that ensures life's resilience in the face of change.
Future Research Directions
Genomic Approaches
Identifying specific genes involved in frequency-dependent selection
Cross-Species Comparisons
Examining patterns across diverse taxonomic groups
Cognitive Mechanisms
Understanding how animals perceive and respond to rarity