A Random Change In Allele Frequency Is Called

A random change in allele frequency is called genetic drift. This phenomenon, driven by chance events, plays a significant role in shaping the genetic makeup of populations over time. Unlike natural selection, which favors traits that enhance survival and reproduction, genetic drift is a non-selective process, meaning that the changes it causes are not necessarily beneficial or adaptive. Instead, it's a consequence of the inherent randomness in processes like reproduction, survival, and even the initial sampling of genes when a new population is established.

Understanding Allele Frequency

Before diving deeper into genetic drift, it’s crucial to understand the concept of allele frequency. An allele is a variant form of a gene. For example, a gene that determines flower color might have two alleles: one for red flowers and one for white flowers. Allele frequency refers to how common a particular allele is in a population. It's usually expressed as a proportion or percentage.

Imagine a population of butterflies where a gene controls wing color. There are two alleles for this gene: 'B' for blue wings and 'b' for brown wings. If, in a population of 500 butterflies, there are 800 'B' alleles and 200 'b' alleles, the allele frequencies would be:

• Frequency of 'B' allele = 800 / (800 + 200) = 0.8 or 80%
• Frequency of 'b' allele = 200 / (800 + 200) = 0.2 or 20%

These frequencies can change from one generation to the next. When these changes are due to random chance, it’s called genetic drift.

The Mechanics of Genetic Drift

Genetic drift operates primarily through two main mechanisms: bottleneck effect and founder effect, and through random variations in reproductive success. Let's examine each of these in detail.

1. Bottleneck Effect

The bottleneck effect occurs when a population undergoes a drastic reduction in size due to a random event, such as a natural disaster (e.g., earthquake, flood, fire) or human activities (e.g., overhunting, habitat destruction). This event doesn't selectively eliminate individuals based on their traits; it's a random cull. The surviving population carries only a subset of the original gene pool.

Key Characteristics of the Bottleneck Effect:

• Sudden Population Reduction: A significant and rapid decrease in the number of individuals in a population.
• Random Survival: Individuals survive irrespective of their genetic makeup. It’s a matter of being in the right place at the right time.
• Reduced Genetic Diversity: The surviving population has a smaller and less representative sample of the original gene pool. Some alleles may be lost entirely, while others become more common simply by chance.
• Long-Term Consequences: The reduced genetic diversity can make the population more vulnerable to future environmental changes or diseases because it lacks the genetic variation necessary to adapt.

Example:

Consider a population of seals living on an island. A tsunami strikes the island, killing the vast majority of the seals. The small group of survivors may not proportionally represent the genetic diversity of the original population. Some rare alleles might be lost completely, while other, previously uncommon alleles may become more frequent. As the seal population recovers, it will have a different genetic makeup than the original population due to this random sampling event.

2. Founder Effect

The founder effect occurs when a small group of individuals from a larger population colonizes a new area and establishes a new population. The founding individuals carry only a fraction of the original population's genetic diversity. Similar to the bottleneck effect, this can lead to a new population with allele frequencies that differ significantly from the parent population.

Key Characteristics of the Founder Effect:

• Establishment of a New Colony: A small group of individuals migrates and establishes a new, isolated population.
• Limited Genetic Diversity: The founding individuals carry only a subset of the genetic variation present in the original population.
• Non-Representative Allele Frequencies: The allele frequencies in the new population may not reflect those of the original population. Some alleles may be overrepresented, while others may be absent altogether.
• Increased Prevalence of Certain Traits: Rare genetic traits that were uncommon in the original population may become more prevalent in the new population simply because one or more of the founders happened to carry those alleles.

Example:

Imagine a flock of birds migrating from the mainland to a remote island. Only a few birds successfully make the journey and establish a new population. If, by chance, these founding birds carry a rare allele for a particular plumage color, that allele will be much more common in the island population than it was in the original mainland population.

3. Random Variations in Reproductive Success

Even in the absence of dramatic events like bottlenecks or founder effects, genetic drift can occur due to random variations in reproductive success. In any population, some individuals will, by chance, have more offspring than others. This doesn't necessarily mean they are genetically superior; it could simply be due to luck, such as finding a mate more easily or experiencing better environmental conditions during their reproductive years.

How Random Reproductive Success Drives Genetic Drift:

• Unequal Contribution to the Next Generation: Some individuals contribute more offspring to the next generation than others, purely by chance.
• Changes in Allele Frequencies: The alleles of individuals who have more offspring will become more common in the next generation, while the alleles of those who have fewer offspring will become less common.
• Cumulative Effect Over Time: Over many generations, these small, random fluctuations in allele frequencies can lead to significant changes in the genetic makeup of the population.

Example:

Consider a population of wildflowers in a meadow. Some flowers might, by chance, be pollinated more frequently by bees, leading to a greater number of seeds. Other flowers might be accidentally trampled by animals, reducing their reproductive success. These random events can cause certain alleles to become more or less common in the population over time, even if there is no selective advantage associated with those alleles.

The Consequences of Genetic Drift

Genetic drift has several important consequences for populations, including:

• Loss of Genetic Variation: Genetic drift tends to reduce genetic variation within populations. As some alleles become more common, others become rarer and may eventually be lost altogether. This loss of diversity can make populations less able to adapt to changing environmental conditions or resist diseases.
• Fixation of Alleles: Over time, genetic drift can lead to the fixation of alleles, meaning that one allele becomes the only allele present at a particular gene locus in the population. When an allele is fixed, there is no more genetic variation at that locus.
• Divergence Between Populations: Genetic drift can cause populations to diverge genetically from one another. If two populations are isolated and experience different random events, they will accumulate different genetic changes over time, leading to genetic differences between them.
• Increased Risk of Extinction: Small populations are particularly vulnerable to genetic drift. The smaller the population, the greater the impact of random events on allele frequencies. This can lead to a rapid loss of genetic diversity and an increased risk of extinction.

Factors Influencing Genetic Drift

The strength of genetic drift is influenced by several factors, most notably:

• Population Size: Genetic drift has a much stronger effect on small populations than on large populations. In small populations, random events can have a large impact on allele frequencies, while in large populations, the effects of random events are buffered by the large number of individuals.
• Starting Allele Frequencies: Alleles with a frequency near 50% have the highest probability of being affected greatly by random chance. Rare alleles may be lost, and common alleles may drift towards fixation, but those closer to 50% can swing either way.
• Migration: Gene flow, or migration of individuals from one population to another, can counter the effects of genetic drift by introducing new alleles and increasing genetic variation.
• Mutation Rate: While mutation is the ultimate source of new genetic variation, the rate at which new mutations arise is generally much slower than the rate at which genetic drift can cause alleles to be lost.

Genetic Drift vs. Natural Selection

It's essential to distinguish between genetic drift and natural selection, as they are both mechanisms of evolution but operate in different ways.

Natural selection favors traits that enhance survival and reproduction in a particular environment. It leads to adaptation, where populations become better suited to their surroundings. Genetic drift, on the other hand, is a non-adaptive process. The changes it causes are random and may not necessarily be beneficial. In some cases, genetic drift can even lead to the loss of beneficial alleles or the fixation of harmful alleles.

In reality, both genetic drift and natural selection often act simultaneously on populations. Natural selection may drive adaptation to a specific environment, while genetic drift introduces random fluctuations in allele frequencies. The relative importance of these two forces depends on the population size, the strength of selection, and other factors.

Examples of Genetic Drift in Action

Several real-world examples illustrate the effects of genetic drift:

• Pingelapese People of Micronesia: The island of Pingelap experienced a devastating typhoon in the 18th century that drastically reduced the population to just a few individuals. One of the survivors carried a recessive gene for achromatopsia, a condition that causes complete color blindness. As the population recovered, the frequency of this gene increased dramatically due to the bottleneck effect. Today, achromatopsia is much more common on Pingelap than in most other populations.
• Amish Communities: The Amish population in North America was founded by a small number of individuals who migrated from Europe. Due to the founder effect, certain rare genetic disorders, such as Ellis-van Creveld syndrome (a type of dwarfism), are more common in Amish communities than in the general population.
• Cheetahs: Cheetahs have experienced two major population bottlenecks in their history, leading to a severe loss of genetic diversity. As a result, cheetahs are highly susceptible to diseases and have a reduced ability to adapt to changing environments.

Mathematical Models of Genetic Drift

The effects of genetic drift can be modeled mathematically using concepts from probability and statistics. One common model is the Wright-Fisher model, which simulates the random sampling of alleles from one generation to the next. These models can help scientists understand how genetic drift operates and predict its effects on populations.

Implications for Conservation Biology

Genetic drift has important implications for conservation biology. Small, isolated populations of endangered species are particularly vulnerable to the effects of genetic drift, which can lead to a loss of genetic diversity and an increased risk of extinction. Conservation efforts often focus on maintaining or increasing the genetic diversity of these populations, for example, by translocating individuals from other populations to introduce new alleles.

The Ongoing Debate: Neutral Theory vs. Selectionism

The relative importance of genetic drift and natural selection in shaping genetic variation has been a subject of ongoing debate among evolutionary biologists. The neutral theory of molecular evolution, proposed by Motoo Kimura, argues that most genetic variation at the molecular level is selectively neutral, meaning that it has no effect on fitness. According to this theory, genetic drift is the primary force driving changes in allele frequencies at these neutral loci.

In contrast, selectionists argue that natural selection is a more important force in shaping genetic variation. They believe that many genetic variants, even those that appear to be neutral, may have subtle effects on fitness that are not easily detected. The debate between neutral theory and selectionism continues to this day, with researchers using a variety of approaches to investigate the relative importance of these two forces in evolution.

Conclusion

Genetic drift is a fundamental process in evolution, responsible for random changes in allele frequencies within populations. It operates through mechanisms such as the bottleneck effect, the founder effect, and random variations in reproductive success. While it is a non-adaptive process, genetic drift can have significant consequences for populations, including the loss of genetic diversity, the fixation of alleles, and divergence between populations. Understanding genetic drift is crucial for comprehending the complexities of evolutionary change and for developing effective strategies for conservation biology. While natural selection drives adaptation, genetic drift introduces an element of chance, reminding us that evolution is not always a predictable march towards perfection. It is a dynamic interplay of both deterministic and stochastic forces that shape the diversity of life on Earth.