What are the alternative names used for genetic hitchhiking?

Besides genetic hitchhiking, the phenomenon is also referred to as genetic draft or the hitchhiking effect.

What initiates a selective sweep, according to the provided text?

A selective sweep is initiated when a newly arisen mutation is advantageous, leading it to increase in frequency within a population.

What types of alleles can be affected by genetic hitchhiking?

Neutral or even slightly disadvantageous alleles that are physically close to a beneficial mutation undergoing a selective sweep can hitchhike along with it, meaning their frequencies change as a consequence.

How does genetic hitchhiking differ from background selection?

Genetic hitchhiking involves neutral or slightly deleterious alleles changing frequency due to linkage with advantageous mutations undergoing a selective sweep. Background selection, conversely, refers to the effects on a neutral locus caused by its linkage disequilibrium with newly appearing deleterious mutations.

How are genetic hitchhiking and background selection classified in terms of evolutionary forces?

Both genetic hitchhiking and background selection are categorized as stochastic, or random, evolutionary forces, similar in nature to genetic drift.

Who coined the term 'hitchhiking' in the context of genetics, and when?

The term hitchhiking was first introduced in 1974 by researchers John Maynard Smith and John Haigh.

Which researchers contributed to the study of genetic hitchhiking after its initial description?

Following the coining of the term, John H. Gillespie and other scientists further investigated the phenomenon of genetic hitchhiking.

What is the direct consequence for alleles when a polymorphism is in linkage disequilibrium with a locus undergoing a selective sweep?

When a polymorphism is linked to a locus undergoing a selective sweep, the allele associated with the advantageous adaptation will increase in frequency, potentially becoming fixed in the population. Simultaneously, the allele linked to the non-advantageous version will decrease in frequency, possibly leading to its extinction.

What is the overall impact of genetic hitchhiking on the genetic diversity within a population?

Genetic hitchhiking generally leads to a reduction in the overall amount of genetic variation present within a population.

Can the allele that 'hitchhikes' have its own selective value?

Yes, a hitchhiker allele can be neutral, advantageous, or even deleterious in its own right. In contexts like cancer biology, such alleles are sometimes referred to as passenger mutations.

What role does genetic recombination play in the process of genetic hitchhiking?

Genetic recombination can interrupt genetic hitchhiking. It provides an opportunity to break the linkage between the selected allele and nearby hitchhiking alleles, potentially halting the process before the hitchhiking allele becomes fixed or extinct.

How does the physical distance between a hitchhiking polymorphism and the selected gene affect recombination?

The closer a hitchhiking polymorphism is located to the gene under selection, the less opportunity there is for genetic recombination to occur between them. This proximity strengthens the hitchhiking effect over a larger genomic region.

How can the patterns resulting from genetic hitchhiking be used in evolutionary studies?

The reduction in genetic variation observed near a selective sweep, a consequence of hitchhiking and limited recombination, provides a useful pattern for researchers to detect genes that have been under very recent selection using population genetic data.

In what fundamental way are genetic drift and genetic draft similar?

Both genetic drift and genetic draft are considered stochastic, or random, evolutionary processes. This means they operate by chance and are not directly correlated with the selective advantage or disadvantage of the specific allele in question.

How is genetic drift defined in population genetics?

Genetic drift is defined as the random fluctuation of allele frequencies in a population from one generation to the next, resulting from random sampling of individuals during reproduction.

How is genetic draft distinguished from genetic drift?

Genetic draft is distinguished from genetic drift by its cause: changes in allele frequency are driven by the random associations an allele has with other alleles that are undergoing selection, rather than solely by random sampling.

What mathematical expression describes the variance in allele frequency due to genetic drift in idealized populations?

The variance in allele frequency across replicated idealized populations of size N, after one generation, due to genetic drift is given by the formula pq / (2N), where p and q represent the frequencies of the two alleles.

How does population size influence the impact of genetic drift?

The formula for genetic drift variance, pq / (2N), demonstrates that the effect of drift is inversely proportional to the population size (N). Larger populations experience less drift, while smaller populations experience more pronounced effects.

What is the role of effective population size in the context of genetic draft?

Genetic draft operates with an effective population size that may differ significantly from the actual number of individuals. This effective size can be influenced by factors such as the rate of recombination and the characteristics of beneficial mutations, rather than just the census population size.

How do the frequency changes caused by genetic draft differ in predictability from those caused by genetic drift?

Changes in allele frequency due to genetic drift are independent from one generation to the next. In contrast, changes due to genetic draft are autocorrelated; if an allele's frequency increases due to draft, it is more likely to increase further in subsequent generations.

Does genetic draft produce the same allele frequency spectrum as genetic drift?

No, genetic draft generates a different allele frequency spectrum compared to that produced by genetic drift.

Why is the Y chromosome particularly susceptible to genetic hitchhiking?

The Y chromosome does not undergo genetic recombination. This lack of recombination means that any advantageous mutation on the Y chromosome can cause a selective sweep that affects a larger region, making linked neutral or deleterious mutations more prone to hitchhiking.

What evolutionary hypothesis does genetic hitchhiking offer regarding the Y chromosome's gene content?

Genetic hitchhiking on the non-recombining Y chromosome is proposed as a potential explanation for why there are relatively few functional genes found on the Y chromosome, as deleterious mutations may accumulate more easily.

How is genetic hitchhiking relevant to the evolution of higher mutation rates?

Hitchhiking is considered necessary for the evolution of increased mutation rates to be favored by natural selection acting on evolvability. A gene that increases mutation rate (a mutator) can hitchhike to fixation if it facilitates the rise of advantageous mutations nearby.

Describe the process by which a mutator gene might spread through genetic hitchhiking.

A mutator gene increases the mutation rate in its vicinity. If a nearby allele mutates into a beneficial form, that beneficial allele will spread via selection. The linked mutator gene is then 'dragged' along with the beneficial allele due to hitchhiking, provided recombination does not separate them.

What is the impact of recombination on the evolution of mutator genes?

Recombination can disrupt the linkage between a mutator gene and the advantageous mutations it helps create. This disruption makes the spread and fixation of mutator genes less likely, particularly in species with high recombination rates.

In which type of species is the evolution of mutators generally expected to be more common, and why?

The evolution of mutators is generally expected to occur more readily in asexual species. This is because the absence of recombination prevents the mutator gene from being easily separated from the advantageous mutations it helps generate, facilitating its spread.

How does genetic hitchhiking pose a challenge to the neutral theory of molecular evolution?

Genetic hitchhiking challenges the neutral theory by providing a mechanism that can explain the fixation of alleles due to selection, even if those alleles are not directly under selection. This contrasts with the neutral theory's premise that most fixation events are driven by genetic drift acting on neutral mutations.

What observation related to the McDonald-Kreitman test can be explained by genetic hitchhiking?

Genetic hitchhiking offers an explanation for why genome-wide applications of the McDonald-Kreitman test often suggest that a significant proportion of mutations become fixed due to selection, rather than solely through neutral processes.

What is a polymorphism in the context of population genetics and genetic hitchhiking?

A polymorphism refers to the existence of two or more common forms of a gene or DNA sequence within a population. In genetic hitchhiking, these are variations that may be neutral or slightly deleterious but are physically linked to a gene under strong positive selection.

What does it mean for genetic variations to be in linkage disequilibrium?

Linkage disequilibrium means that certain alleles or genetic variations are inherited together more often than would be expected by random chance, typically because they are located close to each other on the same chromosome.

Can you define a selective sweep in evolutionary terms?

A selective sweep is a process where a beneficial mutation rapidly increases in frequency within a population due to natural selection. This rapid increase leads to a reduction in genetic variation at linked sites, as the beneficial allele 'sweeps' through the population.

What is the definition of allele frequency?

Allele frequency refers to the relative proportion of a specific allele (a variant of a gene) within a population's gene pool.

What characterizes a stochastic process in evolution?

A stochastic process in evolution is one that involves randomness or chance. The outcome is not predetermined and can vary unpredictably between different populations or over time.

What is the concept of effective population size in genetics?

Effective population size is a theoretical measure representing the size of an idealized population that would experience the same rate of genetic drift as the actual population being studied. It often differs from the actual census size due to various biological factors.

What does an allele frequency spectrum represent?

An allele frequency spectrum illustrates the distribution of frequencies of all alleles present within a population or a sample. Different evolutionary forces, like drift and selection, shape this spectrum in characteristic ways.

What is a mutator gene?

A mutator gene is a gene that increases the rate at which mutations occur in other genes within an organism or population.

What does the term 'evolvability' refer to in biology?

Evolvability is the capacity of a biological system or lineage to generate evolutionary novelty and adapt over time. It relates to the potential for future adaptation and diversification.

What is the central tenet of the neutral theory of molecular evolution?

The neutral theory of molecular evolution posits that most evolutionary changes at the molecular level are caused by the random fixation of neutral mutations through genetic drift, rather than by positive natural selection acting on advantageous mutations.

What is the purpose of the McDonald-Kreitman test in evolutionary genetics?

The McDonald-Kreitman test compares patterns of genetic variation within a species (polymorphisms) to patterns of divergence between species (fixed differences) at specific genetic loci. It is used to detect evidence of positive selection.

How does linkage disequilibrium enable genetic hitchhiking?

Linkage disequilibrium ensures that alleles located near a gene undergoing a selective sweep are inherited together. This physical linkage allows the frequency of these nearby alleles to change passively as the selected allele increases in frequency.

What is the relationship between recombination rates and the extent of genetic hitchhiking?

Higher recombination rates break down linkage disequilibrium more effectively, limiting the distance over which genetic hitchhiking can occur. Conversely, low recombination rates allow hitchhiking to influence larger segments of the genome.

Why does genetic hitchhiking result in a decrease in genetic variation?

Genetic hitchhiking reduces genetic variation because the rapid spread of a beneficial allele carries linked alleles along with it. This process diminishes the diversity of alleles at loci near the selected site, creating a pattern of reduced variation.

How does the variance generated by genetic draft compare to that generated by genetic drift in terms of predictability?

While both genetic drift and draft introduce variance in allele frequencies, drift's effects are independent between generations. Genetic draft's effects are autocorrelated, meaning an increase in frequency due to draft is likely to continue in subsequent generations.

What is a key application of understanding genetic hitchhiking patterns in population genetics?

By recognizing the patterns of reduced genetic variation caused by hitchhiking around selective sweeps, researchers can identify specific genes that have been under recent positive selection.

What is the proposed consequence of genetic hitchhiking for deleterious mutations on the Y chromosome?

Due to the absence of recombination on the Y chromosome, genetic hitchhiking may facilitate the fixation of deleterious mutations, as they can be carried along with any advantageous mutations that spread on the chromosome.

Why are mutator genes more likely to evolve in asexual populations according to the text?

In asexual populations, the lack of recombination prevents the separation of a mutator gene from beneficial mutations it helps create. This linkage allows the mutator to spread along with the benefits it confers, making its evolution more probable than in sexual populations where recombination can disrupt this association.

How does genetic hitchhiking challenge the neutral theory of molecular evolution?

Genetic hitchhiking challenges the neutral theory by providing a mechanism where alleles can increase in frequency due to selection, even if they are not directly under selection themselves. This suggests that selection may play a larger role in molecular evolution than the neutral theory initially proposed.

What is the interplay between linkage disequilibrium, recombination, and genetic hitchhiking?

Genetic hitchhiking relies on linkage disequilibrium to keep alleles physically linked on a chromosome. Recombination acts as a force that breaks down this linkage, thereby limiting the extent and impact of genetic hitchhiking. The closer the alleles, the less likely recombination is to occur, and the stronger the hitchhiking effect.

An allele is a variant form of a gene, representing a specific version of a genetic sequence that can lead to different traits or characteristics.

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Defining Genetic Hitchhiking

The Phenomenon

Genetic hitchhiking, also referred to as genetic draft or the hitchhiking effect, describes a phenomenon where an allele's frequency changes not due to direct natural selection acting upon it, but because it is physically located near another gene that is undergoing a selective sweep. This occurs on the same DNA chain, where linkage disequilibrium facilitates the co-inheritance of alleles.

Mechanism of Association

When a beneficial mutation arises and increases in frequency through a selective sweep, any other polymorphisms (alleles) in close proximity on the same chromosome, which are in linkage disequilibrium, tend to alter their allele frequencies as well. Neutral or even slightly detrimental alleles can 'hitchhike' along with the advantageous allele, influencing their own frequency dynamics within the population.

Terminology

The term "hitchhiking" was formally introduced by John Maynard Smith and John Haigh in 1974. It is distinct from genetic drift, although both are considered stochastic (random) evolutionary forces. In contrast to hitchhiking, background selection refers to the effects on a neutral locus due to linkage disequilibrium with newly appearing deleterious mutations.

The Mechanics of Draft

Selective Sweeps

A selective sweep occurs when a new, advantageous mutation arises and rapidly increases in frequency within a population. This process is driven by positive natural selection, favoring the allele that confers a survival or reproductive advantage. As this beneficial allele sweeps through the population, it carries along linked alleles.

Linkage Disequilibrium

Linkage disequilibrium (LD) is the non-random association of alleles at different loci. When two loci are physically close on a chromosome, recombination is less likely to occur between them. Consequently, alleles at these linked loci tend to be inherited together. During a selective sweep, the strong directional selection on one locus can create or exacerbate LD with nearby loci, causing alleles at those loci to change frequency alongside the selected allele.

Evolutionary Consequences

Reduction in Genetic Variation

A primary outcome of genetic hitchhiking is a reduction in genetic variation within the region surrounding the selected locus. As the advantageous allele sweeps to high frequency or fixation, linked neutral or deleterious alleles are dragged along, decreasing the diversity of alleles present in the population in that genomic region. This pattern of reduced variation is a key signature used to detect past selective sweeps.

Fixation and Extinction

The allele that is linked to the adaptation will increase in frequency, potentially becoming fixed (reaching 100% frequency) in the population. Conversely, alleles at linked loci that are not advantageous, or are even slightly deleterious, will decrease in frequency. In some cases, these linked alleles may be driven to extinction, effectively being removed from the population's gene pool due to their association with the sweeping beneficial allele.

Draft versus Drift

Stochastic Forces

Both genetic drift and genetic draft are considered random evolutionary processes, meaning they act stochastically and are not directly correlated with the selection acting on the specific allele in question. Genetic drift refers to random fluctuations in allele frequencies from one generation to the next due to sampling error, particularly pronounced in small populations.

In a population of size N, the variance in allele frequency change due to genetic drift in one generation is approximately pq / (2N), where 'p' and 'q' are the allele frequencies. This highlights the strong dependence of drift on population size.

Genetic draft exhibits similar behavior but its effective population size may not correlate directly with the actual number of individuals. Instead, the effective size can be influenced by factors such as the recombination rate and the frequency and strength of beneficial mutations. While drift's impact is purely random sampling, draft's impact is tied to the random association with alleles undergoing selection.

Correlated vs. Uncorrelated Changes

A key distinction lies in the correlation of allele frequency changes across generations. Changes due to genetic drift are independent from one generation to the next. In contrast, if an allele frequency increases due to genetic draft, it suggests a higher likelihood of further increase in subsequent generations because it is linked to a beneficial allele that is still spreading.

Furthermore, genetic draft generates a distinct allele frequency spectrum compared to genetic drift, reflecting the influence of directional selection on linked neutral variation.

Applied Genetics: Hitchhiking in Action

Sex Chromosomes

The Y chromosome, which does not undergo genetic recombination, is particularly susceptible to the accumulation of deleterious mutations via hitchhiking. This phenomenon has been proposed as a significant factor contributing to the reduced number of functional genes observed on the Y chromosome compared to other chromosomes.

Mutator Evolution

Genetic hitchhiking plays a crucial role in the evolution of higher mutation rates. A hypothetical 'mutator' gene (M) that increases the mutation rate in its vicinity can be favored if it facilitates the creation of advantageous mutations (A*) in nearby alleles (A). The mutator M can then hitchhike to fixation along with the beneficial A* allele, provided recombination does not separate them. This process is more prevalent in asexual species where recombination is absent or limited.

Neutral Theory

The neutral theory of molecular evolution posits that most mutations are neutral or deleterious. Genetic hitchhiking presents a challenge to this theory by explaining how selection-driven processes can influence the frequencies of neutral alleles, potentially leading to an overestimation of adaptive evolution when analyzing genome-wide patterns. Tests like the McDonald-Kreitman test can reveal signatures consistent with hitchhiking, indicating selection's influence beyond directly beneficial mutations.

Historical Context

Origins of the Concept

The concept of genetic hitchhiking was formally articulated in 1974 by evolutionary biologists John Maynard Smith and Roger Haigh. Their seminal work laid the foundation for understanding how selection acting on one part of the genome could indirectly influence the evolutionary trajectory of linked, neutral, or even deleterious alleles. Subsequent research by scientists like John H. Gillespie further elucidated the mathematical properties and implications of this evolutionary mechanism.

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Genetic Hitchhiking: The Unseen Journey of Alleles

💡 Dive in with Flashcard Learning! 💡

Defining Genetic Hitchhiking ℹ️

🚶 The Phenomenon

🔗 Mechanism of Association

💡 Terminology

The Mechanics of Draft ⚙️

⚡ Selective Sweeps

⛓️ Linkage Disequilibrium

Evolutionary Consequences 📊

📉 Reduction in Genetic Variation

⬆️ Fixation and Extinction

Draft versus Drift ⚖️

🎲 Stochastic Forces

🎯 Correlated vs. Uncorrelated Changes

Applied Genetics: Hitchhiking in Action 🔬

🚹🚺 Sex Chromosomes

📈 Mutator Evolution

📜 Neutral Theory

Historical Context ⏳

📜 Origins of the Concept

Teacher's Corner 🧑‍🏫

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Dive in with Flashcard Learning!

Defining Genetic Hitchhiking

The Phenomenon

Mechanism of Association

Terminology

The Mechanics of Draft

Selective Sweeps

Linkage Disequilibrium

Evolutionary Consequences

Reduction in Genetic Variation

Fixation and Extinction

Draft versus Drift

Stochastic Forces

Correlated vs. Uncorrelated Changes

Applied Genetics: Hitchhiking in Action

Sex Chromosomes

Mutator Evolution

Neutral Theory

Historical Context

Origins of the Concept

Teacher's Corner

True or False?

Test Your Knowledge!

Gamer's Corner

Discover other topics to study!

References

Feedback & Support

Disclaimer

Important Notice