Transient polymorphism describes a temporary genetic state within a population where two alleles for a specific gene exist, but one allele is actively and gradually replacing the other. This dynamic shift is driven by strong environmental selective pressure, which leads to directional selection, ultimately causing the elimination of one of the alleles over time.
Understanding Transient Polymorphism
In essence, transient polymorphism represents a snapshot during an ongoing evolutionary change. It's a phase where a gene pool is polymorphic (has multiple alleles), but this polymorphism is not stable; it's in transition. The process is characterized by a significant shift in allele frequencies, where the advantageous allele increases in prevalence while the disadvantageous one diminishes.
Key Characteristics
- Dynamic State: Unlike balanced polymorphism, where allele frequencies are relatively stable due to heterozygote advantage or frequency-dependent selection, transient polymorphism is a temporary phase.
- Strong Selective Pressure: A powerful environmental factor or change acts as the driving force, favoring one allele over the other.
- Directional Selection: This type of natural selection continuously favors individuals at one extreme of the phenotypic range over others, leading to a shift in the average trait value of the population.
- Allele Replacement: The hallmark of transient polymorphism is the eventual replacement of one allele by another, rather than their co-existence indefinitely.
Mechanisms Behind the Shift
The replacement of one allele by another in transient polymorphism is a direct consequence of natural selection responding to environmental changes. Here's how it generally unfolds:
- Environmental Change: A significant shift in the environment occurs, such as a new predator, a change in climate, or the introduction of a new disease or pollutant.
- Differential Fitness: This environmental change makes one existing allele (and the trait it confers) suddenly more advantageous for survival and reproduction than another. Individuals possessing the favored allele have higher fitness.
- Directional Selection: Organisms with the advantageous allele are more likely to survive, reproduce, and pass on that allele to their offspring. Conversely, those with the less advantageous allele are less likely to survive and reproduce.
- Allele Frequency Shift: Over generations, the frequency of the advantageous allele steadily increases in the gene pool, while the frequency of the disadvantageous allele decreases.
- Eventual Fixation/Loss: If the selective pressure remains consistent and strong, the advantageous allele may eventually become fixed (reach 100% frequency) in the population, and the disadvantageous allele may be completely lost.
For a deeper dive into how natural selection drives these changes, explore resources on evolution by natural selection.
Examples in Nature
Several real-world phenomena illustrate transient polymorphism:
- Industrial Melanism in Peppered Moths ( Biston betularia ):
- Context: Before the Industrial Revolution, light-colored peppered moths were camouflaged against lichen-covered trees, making the dark (melanic) form rare.
- Environmental Change: Industrial pollution darkened tree trunks by killing lichens and depositing soot.
- Shift: The dark allele became advantageous as dark moths were now camouflaged, while light moths became conspicuous to predators. This led to a rapid increase in the frequency of the dark allele and a decrease in the light allele.
- Current State: With cleaner air in many regions, tree trunks are lightening again, and the light allele is making a comeback, demonstrating the transient nature of this polymorphism driven by changing selective pressures.
- Antibiotic Resistance in Bacteria:
- Context: A bacterial population initially consists mostly of antibiotic-susceptible individuals.
- Environmental Change: Exposure to antibiotics creates a strong selective pressure.
- Shift: Any bacteria with pre-existing resistance alleles (even if rare) survive the antibiotic treatment, while susceptible bacteria are eliminated. These resistant bacteria then reproduce, rapidly increasing the frequency of resistance alleles in the population.
- Outcome: The population transitions from being predominantly susceptible to being largely resistant, representing a transient polymorphism where the resistance allele replaces the susceptibility allele. Learn more about antibiotic resistance from the World Health Organization.
Distinguishing from Other Polymorphisms
It's helpful to understand how transient polymorphism differs from other forms of genetic variation:
| Feature | Transient Polymorphism | Balanced Polymorphism |
|---|---|---|
| Allele Frequencies | Actively changing; one allele replacing another | Relatively stable; maintained over generations |
| Selective Pressure | Directional selection; one allele highly favored | Stabilizing selection; often heterozygote advantage (e.g., sickle cell trait in malaria-prone areas) |
| Outcome | Eventual fixation of one allele or loss of another | Stable coexistence of multiple alleles |
| Nature of Polymorphism | Temporary, a phase during evolutionary change | Stable, long-term feature of the population |
Transient polymorphism is a powerful indicator of ongoing evolutionary processes, revealing how populations adapt to their changing environments in real time.
