Allelic Frequency Changes: Videos & Practice Problems

Natural selection, genetic drift, mutation, migration, and non-random mating are key mechanisms influencing allele frequencies in populations. Natural selection favors advantageous traits, while genetic drift causes random changes, especially in small populations. Mutations introduce new alleles, and migration enhances genetic diversity through gene flow. Non-random mating, including assortative mating and inbreeding, affects genetic variation and can lead to reproductive isolation and speciation. Understanding these concepts is crucial for grasping population genetics and evolutionary processes.

Natural Selection

Hardy-Weinberg equilibrium is a theoretical model that describes a population where allele frequencies remain constant over generations, based on five key assumptions. However, real-world populations often deviate from this model due to various factors, particularly natural selection. Natural selection is the process by which certain traits become more common in a population because they confer a survival or reproductive advantage. This occurs due to the struggle for survival, where not all organisms can survive and reproduce, leading to the prevalence of advantageous alleles.

There are several types of natural selection. Directional selection occurs when an allele frequency shifts towards fixation (where one allele becomes the only variant in the population) or loss (where an allele disappears entirely). For example, if a population initially has multiple alleles for a trait, directional selection can lead to one allele becoming fixed while others are lost. In contrast, positive selection favors beneficial mutations, increasing their frequency in the population, while purifying selection removes harmful mutations, thereby enhancing the overall fitness of the population.

Balancing selection maintains multiple alleles in a population because they provide distinct advantages, resulting in an equilibrium where these alleles coexist. An example of this is stabilizing selection, which favors intermediate phenotypes over extremes, leading to a reduction in variation for that trait.

Fitness is a crucial concept in understanding natural selection, defined as the contribution of an individual's genetic makeup to the next generation. There are two types of fitness: absolute fitness, which measures the total number of offspring produced by an individual, and relative fitness, which compares the reproductive success of one individual to another. For instance, if a brown fish produces two offspring and a blue fish produces ten, the relative fitness of the brown fish is lower compared to the blue fish.

Natural selection is an ongoing process observable in real-time, highlighting the limitations of the Hardy-Weinberg model as a representation of actual populations. Instead, it serves as a baseline for understanding genetic variation and evolutionary dynamics within populations.

New Alleles and Migration

New alleles are continuously generated within populations, significantly influencing allelic frequencies. A primary mechanism for the creation of these new alleles is mutation, which occurs consistently. Mutations can not only introduce entirely new alleles but can also transform existing alleles into different forms. To quantify this process, scientists calculate the mutation rate, which reflects how frequently mutations arise in a population. This rate can be determined using various formulas, allowing researchers to understand how changes in one allele, such as allele p, can affect the frequency of another allele, like allele q, through mutation.

While some educational resources may delve deeply into the calculations surrounding mutation rates, it is essential to grasp the fundamental concept that alleles are perpetually being created, primarily through mutations. Another critical factor in genetic variation is migration, also referred to as gene flow. This process involves the movement of individuals between different populations or subpopulations. When an individual from one population migrates to another and interbreeds, it results in genetic admixture, which is the blending of genetic material from multiple populations.

For example, if an individual from a black circle population migrates to a red circle population and mates, the offspring will exhibit genetic traits from both populations, contributing to genetic diversity. This genetic variation is crucial as it often violates the assumptions of Hardy-Weinberg equilibrium, which describes a stable, non-evolving population. The ongoing processes of mutation and migration are vital for maintaining genetic diversity within populations, ensuring that they adapt and evolve over time.

Genetic Drift

Genetic drift is a key evolutionary mechanism that describes how allele frequencies in a population can change over time due to random sampling effects, particularly in finite populations. Unlike infinite populations, where genetic drift is negligible, finite populations experience fluctuations in allele frequencies simply by chance. This randomness arises because not all gametes produced contribute to the next generation, leading to a subset of alleles being passed on.

When individuals reproduce, they create gametes that contain a random assortment of alleles. For instance, if a person has a diverse set of alleles, only a fraction of these will be represented in their offspring due to the random selection process. This phenomenon is known as sampling error, where the limited number of offspring can lead to significant deviations from the expected allele frequencies found in the parental generation. For example, if a population starts with equal numbers of two alleles, say red and blue, random gamete selection might result in a generation with a higher frequency of blue alleles, illustrating genetic drift.

Genetic drift can lead to two significant outcomes: fixation and loss of alleles. Fixation occurs when all individuals in a population become homozygous for a particular allele, meaning that allele is the only one present. Conversely, loss happens when an allele completely disappears from the population. These changes can drastically reduce genetic variation, which is crucial for a population's adaptability and survival.

Two primary mechanisms contribute to genetic drift: the Founder Effect and the Bottleneck Effect. The Founder Effect occurs when a small group from a larger population establishes a new population, carrying only a limited number of alleles. This can lead to reduced genetic diversity in the new population compared to the original. For example, if a few birds from a large flock migrate to a new area, they may not represent the full genetic diversity of the original group.

The Bottleneck Effect, on the other hand, refers to a sharp reduction in population size due to environmental events, such as natural disasters or habitat loss. This drastic decrease can lead to a significant loss of genetic variation, as only a small number of individuals survive to reproduce. For instance, polar bears facing habitat destruction may experience a bottleneck, resulting in a population with limited genetic diversity, which can hinder their ability to adapt to changing conditions.

In summary, genetic drift is a crucial concept in understanding how allele frequencies can change in finite populations due to random sampling. It highlights the importance of genetic variation for the survival and adaptability of species, emphasizing that both the Founder Effect and Bottleneck Effect can significantly impact a population's genetic landscape.

Non-Random Mating

Non-random mating is a significant concept in population genetics that deviates from the assumptions of the Hardy-Weinberg equilibrium, which requires random mating. In reality, organisms, including humans, often engage in non-random mating based on phenotypes, which are influenced by different alleles. This phenomenon is known as assortative mating, which can be categorized into two types: positive assortative mating, where individuals select mates with similar phenotypes, and negative assortative mating, where they choose mates with dissimilar phenotypes.

An intriguing example of negative assortative mating in humans involves a study where males wore T-shirts for several days to collect their body odor. Women then rated these odors for attractiveness, revealing a preference for the scent of men with different major histocompatibility complex (MHC) genotypes. This suggests that individuals can subconsciously select mates based on genetic compatibility, which is crucial for immune system diversity.

Another form of non-random mating is isolation by distance, where populations separated by geographical barriers, such as mountains or rivers, do not interbreed. Over time, this separation can lead to genetic variations and ultimately speciation, the process by which new species arise due to reproductive isolation. Reproductive isolation can be classified into two categories: prezygotic and postzygotic mechanisms. Prezygotic mechanisms prevent mating or fertilization, such as geographical isolation or differences in mating behaviors. In contrast, postzygotic mechanisms occur after fertilization, leading to offspring that are inviable or sterile, such as mules, which are hybrids of horses and donkeys.

Inbreeding, another form of non-random mating, occurs when individuals mate with close relatives. This can lead to inbreeding depression, where the likelihood of homozygosity for harmful recessive alleles increases, resulting in a higher incidence of genetic diseases. The inbreeding coefficient (F) quantifies the probability that two alleles in an individual are identical by descent from a common ancestor. For example, the inbreeding coefficient for a father-daughter pairing is 25%, indicating a significant risk for recessive genetic disorders.

In summary, non-random mating, whether through assortative mating, isolation by distance, or inbreeding, plays a crucial role in shaping genetic diversity and evolutionary processes. Understanding these concepts helps clarify why the assumptions of Hardy-Weinberg may not always apply to real-world populations.

Which of the following terms describes a change in allelic frequency due to random disappearance of genes in a small population?

• A. Natural Selection
• B. Allele creation through mutation
• C. Gene Flow
• D. Genetic Drift
• E. Non-random mating

Answer: (D)

A group of finches live on a small, isolate island. One day, a few finches travel to a distant island and start a new population of finches. This type of change in a population is called what?

• A. Speciation
• B. Founder effect
• C. Bottleneck effect
• D. Genetic drift

Answer: (B)

Which of the following is an example of natural selection?

• A. A neutral mutation is carried from generation to generation
• B. A rabbit migrated to a new location and brought new alleles to the endogenous rabbit population
• C. A mutation causes a finch to develop a stronger beak, which makes it more likely to grow, survive, and reproduce.
• D. One allele becomes fixed in a population due to random genetic drift over time.

Answer: (C)

Here’s what students ask on this topic:

What is the Hardy-Weinberg principle and its assumptions?

The Hardy-Weinberg principle states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of evolutionary influences. The five key assumptions for a population to be in Hardy-Weinberg equilibrium are:

• No selection: No differential survival or reproduction of individuals.
• No mutation: No new alleles are introduced into the population.
• Large population size: The population is infinitely large to prevent genetic drift.
• No migration: No gene flow between populations.
• Random mating: Individuals pair by chance, not according to their genotypes or phenotypes.

These conditions are rarely met in real populations, making the principle a useful null model for studying evolutionary processes.

How does natural selection affect allele frequencies in a population?

Natural selection affects allele frequencies by favoring alleles that enhance survival and reproduction. This process increases the frequency of advantageous alleles and decreases the frequency of harmful ones. There are different types of selection:

• Directional selection: Favors one extreme phenotype, leading to allele fixation or loss.
• Stabilizing selection: Favors intermediate phenotypes, reducing variation.
• Disruptive selection: Favors extreme phenotypes at both ends, increasing variation.

Overall, natural selection drives populations toward greater adaptation to their environments.

What is genetic drift and how does it influence allele frequencies?

Genetic drift is the random fluctuation of allele frequencies in a population, especially significant in small populations. It occurs due to random sampling of alleles during gamete formation and reproduction. Over time, genetic drift can lead to the fixation or loss of alleles, reducing genetic variation. Two key phenomena associated with genetic drift are:

• Founder effect: A new population is established by a small number of individuals, carrying only a fraction of the original population's genetic diversity.
• Bottleneck effect: A significant reduction in population size due to environmental events, leading to a loss of genetic diversity.

Genetic drift can have profound effects on the genetic structure of populations, independent of natural selection.

How do mutations contribute to changes in allele frequencies?

Mutations are changes in the DNA sequence that create new alleles, contributing to genetic diversity. They can occur spontaneously or due to environmental factors. Mutations can be:

• Beneficial: Increase an organism's fitness, potentially becoming more common through positive selection.
• Neutral: Have no effect on fitness and may persist in the population by chance.
• Harmful: Decrease fitness and are often removed by purifying selection.

Although mutation rates are generally low, their cumulative effect over generations can significantly alter allele frequencies and drive evolutionary change.

What is non-random mating and how does it affect genetic variation?

Non-random mating occurs when individuals select mates based on specific traits, rather than randomly. This can affect genetic variation in several ways:

• Assortative mating: Individuals mate with others that have similar phenotypes, increasing homozygosity and potentially leading to inbreeding.
• Disassortative mating: Individuals mate with those that have different phenotypes, increasing heterozygosity and genetic diversity.
• Inbreeding: Mating between relatives, increasing the likelihood of homozygosity for harmful recessive alleles, leading to inbreeding depression.

Non-random mating can lead to changes in allele frequencies and influence the genetic structure of populations, potentially leading to reproductive isolation and speciation.