7.1 The neutral theory of molecular evolution
2 min read•july 25, 2024
Neutral theory challenges the idea that natural selection drives all evolutionary changes. It proposes that most molecular changes are selectively neutral, with as the primary force. This concept revolutionized our understanding of molecular evolution.
Evidence for neutral theory includes constant rates of across species and high levels of polymorphism in natural populations. It contrasts with the selectionist view, which emphasizes natural selection as the main driver of molecular evolution.
Neutral Theory Foundations
Principles of neutral theory
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- proposed in 1968 challenged prevailing view of natural selection driving all evolutionary changes
- Most molecular changes selectively neutral mutations have no effect on fitness neither advantageous nor deleterious
- Genetic drift primary force driving molecular evolution random changes in allele frequencies over time
- balances new mutations and loss of existing variants
- hypothesis posits relatively constant rate of molecular evolution over time
- influences fixation rate smaller populations experience faster genetic drift
Genetic drift in neutral theory
- Random sampling of alleles between generations leads to changes in allele frequencies
- Fixation of neutral mutations process variant becomes only allele in population
- Loss of genetic variation drift reduces diversity over time
- Founder effects and population bottlenecks enhance impact of drift on molecular variation (island colonization)
- Stronger drift effect in smaller populations (endangered species)
- Interaction with mutation rate balances loss of variation due to drift
Evidence and Comparisons
Evidence for neutral theory
- Constant rate of amino acid substitutions across species (hemoglobin)
- High levels of polymorphism in natural populations (fruit flies)
- Correlation between evolutionary rate and generation time (mice vs elephants)
- occur more frequently than nonsynonymous ones
- Molecular evolution studies in various organisms (Drosophila, humans)
- DNA sequencing advances provide comprehensive data to test theory
Neutral theory vs selectionist view
- Neutral theory:
- Most molecular changes due to genetic drift
- Selection plays minor role in molecular evolution
- Emphasizes importance of random processes
- Predicts higher levels of polymorphism
- Relatively constant rate of evolution
- Selectionist view:
- Natural selection primary driver of molecular evolution
- Adaptive changes common at molecular level
- Emphasizes role of environmental pressures
- Predicts lower levels of polymorphism due to selective sweeps
- Variable rate of evolution depending on selective pressures
- compromise incorporates slightly deleterious mutations
- Implications for speciation and adaptation:
- Neutral theory emphasizes non-adaptive processes
- Selectionist view focuses on adaptive divergence
Key Terms to Review (22)
Allele frequency: Allele frequency refers to how often a specific allele appears in a population compared to all alleles for that gene. It plays a crucial role in understanding genetic variation, evolutionary processes, and the genetic structure of populations, influencing how traits are inherited and how populations adapt over time.
Amino acid substitutions: Amino acid substitutions refer to the changes in the sequence of amino acids in a protein that occur when one amino acid is replaced by another in the polypeptide chain. These substitutions can arise from mutations in the DNA sequence that codes for the protein and may have various effects on the protein's structure and function. Understanding these substitutions is crucial for studying molecular evolution, as they can influence evolutionary processes and the adaptation of organisms.
Effective Population Size: Effective population size refers to the number of individuals in a population who contribute offspring to the next generation, which can differ from the actual population size due to various factors like unequal sex ratios, variance in reproductive success, and genetic drift. Understanding effective population size is crucial for assessing gene flow, maintaining genetic diversity, and measuring genetic variation within populations, especially in the context of evolutionary processes and conservation efforts.
Evolutionary stasis: Evolutionary stasis refers to a period in which a species remains relatively unchanged over long periods of geological time, showing little to no evolutionary change in its morphology or genetic makeup. This phenomenon can occur when the environment is stable and selective pressures do not favor significant adaptations, resulting in a plateau of evolutionary development.
Fixation Probability: Fixation probability refers to the likelihood that a new mutation will eventually become fixed in a population, meaning that it becomes the only allele present at a particular genetic locus. This concept is crucial for understanding how neutral mutations spread through populations over time, particularly in the context of genetic drift and the neutral theory of molecular evolution, which posits that most evolutionary changes at the molecular level are caused by random drift rather than natural selection.
Founder effect: The founder effect occurs when a small group of individuals breaks away from a larger population to establish a new population, leading to reduced genetic diversity. This phenomenon is significant in understanding how isolated populations can experience rapid evolutionary changes, particularly in the context of genetic drift, adaptation, and speciation.
Genetic diversity: Genetic diversity refers to the variety of genes within a given population, influencing its ability to adapt to environmental changes and contribute to the overall health of species. This variation is crucial for processes like evolution and natural selection, allowing populations to survive in changing environments and resist diseases.
Genetic drift: Genetic drift is the random fluctuation in allele frequencies within a population due to chance events, leading to changes in genetic variation over time. This process can significantly impact small populations where random events can lead to large changes in allele frequencies, affecting evolution and the overall genetic diversity of populations.
Kimura's Two-Parameter Model: Kimura's Two-Parameter Model is a mathematical framework used to understand the rates of nucleotide substitutions in DNA sequences, emphasizing that most mutations are neutral and not subject to natural selection. This model introduces two parameters: the transition rate and the transversion rate, which help explain how these mutations accumulate over time in populations, aligning with the principles of the neutral theory of molecular evolution.
Molecular Clock: A molecular clock is a technique used in evolutionary biology to estimate the time of divergence between species based on the rate of genetic mutations. This method relies on the assumption that mutations accumulate at a relatively constant rate over time, allowing scientists to gauge how long ago two species shared a common ancestor. Molecular clocks provide insights into evolutionary timelines, aiding in understanding biogeographic patterns, and supporting concepts in molecular evolution and phylogenetics.
Motoo Kimura: Motoo Kimura was a prominent Japanese biologist best known for his formulation of the neutral theory of molecular evolution. His work emphasized that much of the genetic variation seen in populations is due to random genetic drift of neutral mutations, rather than being driven by natural selection. This theory has significant implications for understanding how evolutionary processes shape genetic diversity over time.
Mutation-drift equilibrium: Mutation-drift equilibrium refers to the balance between the introduction of new genetic variations through mutations and the random changes in allele frequencies due to genetic drift. This concept highlights how, over time, the effects of mutation can counteract the effects of drift in a population, maintaining genetic diversity. It plays a crucial role in understanding how neutral mutations can accumulate and lead to evolutionary change without being influenced by natural selection.
Nearly neutral theory: Nearly neutral theory is an extension of the neutral theory of molecular evolution, suggesting that many mutations that are not strictly neutral are close to neutral in their effects on fitness. This theory posits that slightly deleterious or advantageous mutations can accumulate in populations over time, influencing molecular variation and evolution. By incorporating the effects of both nearly neutral and neutral mutations, this theory helps explain patterns of genetic diversity observed in various species.
Neutral Mutation: A neutral mutation is a change in the DNA sequence that does not affect the organism's fitness, meaning it neither benefits nor harms the organism. This type of mutation can occur due to various reasons such as replication errors or environmental factors and is significant in understanding genetic variation and molecular evolution, as it provides insight into how mutations accumulate over time without impacting survival or reproduction.
Nonsynonymous mutations: Nonsynonymous mutations are genetic alterations that result in a change in the amino acid sequence of a protein. These mutations can have significant effects on protein function and contribute to evolutionary processes, as they often lead to phenotypic variations that can be subject to natural selection. Understanding nonsynonymous mutations is essential in studying molecular evolution and the mechanisms through which genetic diversity arises within populations.
Phylogenetic tree: A phylogenetic tree is a diagram that represents the evolutionary relationships among various biological species based on their genetic, morphological, or behavioral traits. It illustrates the concept of descent with modification, showing how different species diverged from common ancestors over time and can help in understanding patterns of evolution and speciation.
Population bottleneck: A population bottleneck is an event that drastically reduces the size of a population, resulting in a loss of genetic diversity. This reduction often occurs due to environmental events, such as natural disasters or human activities, and can have lasting effects on the genetic makeup of the surviving population. The consequences of a bottleneck can influence evolutionary processes and impact how species adapt to changing environments.
Punctuated equilibrium: Punctuated equilibrium is an evolutionary theory suggesting that species experience long periods of stability, or stasis, interrupted by brief episodes of rapid change during speciation. This concept highlights how evolutionary processes can be irregular, contrasting with gradualism, which posits that evolution occurs at a slow and constant rate over time. It connects to the fossil record by illustrating how transitional forms can emerge suddenly rather than gradually, influencing our understanding of macroevolutionary patterns.
Sequence alignment: Sequence alignment is a method used to arrange the sequences of DNA, RNA, or protein to identify regions of similarity that may indicate functional, structural, or evolutionary relationships between the sequences. This technique is crucial for understanding molecular evolution and can reveal patterns of genetic variation that help in comparative studies of genomes across different species.
Silent mutations: Silent mutations are changes in the DNA sequence that do not result in a change to the protein sequence they encode. These mutations occur when a nucleotide is replaced by another nucleotide, but because of the redundancy in the genetic code, the amino acid remains the same. This phenomenon is significant in understanding the neutral theory of molecular evolution, which suggests that many genetic variations are selectively neutral and can accumulate over time without affecting an organism's fitness.
Synonymous mutations: Synonymous mutations are genetic alterations in a DNA sequence that do not change the amino acid sequence of the resulting protein. These mutations occur in the coding regions of genes but leave the protein function unchanged, often leading to a silent effect on the organism's phenotype. This feature is crucial in understanding how genetic variation can arise without affecting an organism's fitness, which plays a key role in molecular evolution.
Wright-Fisher model: The Wright-Fisher model is a foundational mathematical framework used to describe how allele frequencies in a population change over time due to genetic drift. This model assumes a finite population size and random mating, allowing researchers to study the effects of chance events on genetic variation. It provides insights into the processes of genetic drift and how they can lead to phenomena like the founder effect and support the neutral theory of molecular evolution.