19.2 Population Genetics
3 min read•june 14, 2024
is the spice of life in populations. Mutations, , and create a diverse genetic buffet, while and shape the menu. These processes determine which traits stick around and which ones fade away.
Small populations face unique challenges. Genetic drift can cause big changes by chance, while bottlenecks and founder effects limit diversity. Understanding these forces helps us grasp how populations evolve and adapt to their ever-changing environments.
Sources and Impacts of Genetic Variation in Populations
Sources of genetic variation
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- Mutations introduce changes in DNA sequence
- affect single nucleotides
- do not change amino acid sequence
- change amino acid sequence
- create premature stop codons
- alter reading frame by inserting or deleting nucleotides
- involve large-scale changes
- Deletions remove segments of chromosomes
- Duplications copy segments of chromosomes
- Inversions reverse orientation of chromosomal segments
- Translocations move segments between non-homologous chromosomes
- affect single nucleotides
- Recombination during meiosis shuffles genetic material
- Independent assortment randomly distributes maternal and paternal chromosomes
- Crossing over exchanges segments between homologous chromosomes
- or gene flow introduces new alleles from one population to another
- Sexual reproduction and random fertilization create new allele combinations in offspring
Natural selection's impact on traits
- Fitness measures an individual's ability to survive and reproduce in a given environment
- shifts population mean by favoring one extreme phenotype (antibiotic resistance in bacteria)
- reduces variation by favoring intermediate phenotypes (human birth weight)
- increases variation by favoring extreme phenotypes over intermediate ones (beak size in African seedcracker finches)
- occurs when fitness depends on phenotype frequency relative to other phenotypes (predator-prey relationships, mimicry)
Genetic Drift, Bottlenecks, and Diversity
Effects of drift and bottlenecks
- Genetic drift causes random changes in allele frequencies due to chance events
- More pronounced in small populations
- Can lead to (all individuals have same allele) or loss of alleles
- occurs when small group establishes new population with reduced genetic diversity
- Population bottlenecks drastically reduce and genetic diversity (Northern elephant seals, cheetahs)
- Can increase
- describes non-evolving population
- Assumes no , , selection, or drift
- Allele frequencies remain constant across generations
- Equation: , where and are allele frequencies
Forces shaping genetic diversity
-
- Increases frequency of beneficial alleles
- Decreases frequency of deleterious alleles
- Adapts populations to their environments
- Genetic drift causes random changes in allele frequencies, more pronounced in small populations
- Gene flow (migration) transfers alleles between populations
- Increases genetic diversity within populations
- Reduces genetic differences between populations
- Mutation introduces new alleles, increasing genetic diversity
- Most mutations are neutral or deleterious, some may be beneficial
- Non-random mating affects genetic diversity
- (mating between closely related individuals) increases , exposes recessive deleterious alleles
- (mating between phenotypically similar individuals) increases homozygosity for involved traits, can lead to divergence within population
Factors Influencing Genetic Variation
- Natural selection acts on existing genetic variation, shaping the genetic makeup of populations over time
- Mutation rates affect the introduction of new alleles, contributing to overall genetic variation
- Migration patterns influence gene flow between populations, impacting genetic diversity
- Population size affects the strength of genetic drift and the maintenance of genetic variation
- Genetic variation within a population is crucial for adaptation to changing environments
Key Terms to Review (40)
Allele frequency: Allele frequency refers to how often a particular allele appears in a population relative to the total number of alleles for that gene. This measure is essential in population genetics as it provides insights into the genetic diversity of populations and how evolutionary processes, such as natural selection, genetic drift, and gene flow, affect allele distribution over time.
Assortative mating: Assortative mating is a non-random mating pattern where individuals with similar phenotypes or genotypes are more likely to pair up than those with dissimilar traits. This behavior can lead to an increase in genetic similarities within a population and can affect allele frequencies over time, influencing the overall genetic structure and evolution of the population.
Back mutations: Back mutations are genetic changes that restore the original sequence and function of a gene that had previously undergone mutation. These can occur naturally or be induced in a laboratory setting.
Bottleneck effect: The bottleneck effect occurs when a population's size is significantly reduced for at least one generation, leading to a loss of genetic diversity. This phenomenon often results from environmental events, such as natural disasters, that drastically decrease the number of individuals. The reduced population may not represent the genetic diversity of the original population, which can have long-term impacts on evolution and adaptation.
Chromosomal mutations: Chromosomal mutations refer to alterations in the structure or number of chromosomes, which can lead to significant changes in an organism's genetic makeup. These mutations can result from various processes, such as errors during DNA replication, exposure to radiation, or chemical agents. The impact of chromosomal mutations extends to essential biological processes, influencing gene expression and contributing to genetic diversity within populations.
Cline: A cline is a gradual change in a trait or characteristic of a species over a geographic area. This variation occurs due to differences in environmental conditions across the region.
Directional selection: Directional selection is a type of natural selection where one extreme phenotype is favored over others. This leads to a shift in the population's trait distribution toward that extreme.
Directional Selection: Directional selection is a mode of natural selection in which a single phenotype is favored, causing the allele frequency to shift in one direction. This process often occurs in response to environmental changes or selective pressures, leading to an increase in the prevalence of certain traits within a population over time. It plays a critical role in shaping population evolution, influencing genetic variation and promoting adaptive evolution.
Disruptive selection: Disruptive selection is an evolutionary process where extreme traits are favored over intermediate traits, leading to increased diversity within a population. This type of selection can result in the emergence of two or more distinct phenotypes, which may enhance the population's adaptability to varying environments. Disruptive selection often occurs in heterogeneous environments where individuals with extreme traits have a survival advantage over those with average traits.
Fixation: Fixation refers to the process through which a specific allele becomes the only variant in a population, meaning all individuals carry that allele. This can occur due to genetic drift, natural selection, or other evolutionary forces, leading to the complete dominance of one genetic variant over others. Fixation is an important concept in understanding how populations evolve over time and how genetic diversity can be affected.
Founder effect: The founder effect is a genetic phenomenon that occurs when a small group of individuals establishes a new population, leading to reduced genetic diversity compared to the original population. This can result in certain alleles becoming more common or entirely absent, significantly influencing the evolutionary trajectory of the new population. The founder effect can play a crucial role in the emergence of new species, population evolution, and the understanding of population genetics.
Frameshift mutations: Frameshift mutations are genetic alterations that occur when nucleotides are added to or deleted from the DNA sequence in a number that is not a multiple of three. This results in a shift in the reading frame of the gene, leading to changes in the amino acid sequence of the resulting protein. Such mutations can have severe effects on protein function and contribute to various genetic disorders.
Frequency-dependent selection: Frequency-dependent selection is a type of natural selection where the fitness of a phenotype depends on its frequency relative to other phenotypes in a given population. This concept plays a crucial role in shaping genetic diversity within populations, as it can favor rare traits or phenotypes over more common ones. The dynamic interactions between different traits create a feedback loop that can influence evolutionary trajectories and lead to balanced polymorphism.
Gene flow: Gene flow is the transfer of genetic material between populations, which can occur through the movement of individuals or their gametes. This process can introduce new alleles into a population, impacting genetic diversity and potentially affecting evolutionary pathways, such as the formation of new species and adaptive traits within populations.
Genetic drift: Genetic drift is a mechanism of evolution that involves random changes in the frequency of alleles (gene variants) in a population over time, primarily due to chance events. This process can lead to significant changes in small populations, impacting their genetic diversity and potentially leading to the fixation or loss of certain alleles, which influences evolutionary dynamics and speciation.
Genetic variability: Genetic variability is the measure of the tendency of individual genotypes within a population to vary from one another. It is a fundamental aspect of evolution and adaptation in populations.
Genetic variation: Genetic variation refers to the differences in DNA sequences among individuals within a population. This variation is crucial for the process of evolution as it provides the raw material for natural selection, influencing traits such as physical characteristics, disease resistance, and adaptability to environmental changes.
Geographical variation: Geographical variation refers to differences in the genetic composition of separate populations of a species. These variations are often influenced by environmental factors and can result in adaptations to local conditions.
Hardy-Weinberg equilibrium: Hardy-Weinberg equilibrium is a principle that describes the genetic variation in a population that remains constant from one generation to the next in the absence of evolutionary influences. This concept is essential for understanding how populations evolve, as it provides a baseline model to compare against real-world scenarios where forces like natural selection and genetic drift are at play.
Heritability: Heritability is a measure used in genetics that quantifies how much of the variation in a trait can be attributed to genetic differences among individuals in a population. It is expressed as a proportion ranging from 0 to 1, where higher values indicate greater genetic influence on the trait.
Heterozygosity: Heterozygosity refers to the presence of different alleles at one or more loci on homologous chromosomes. This genetic variation is crucial for understanding population diversity, adaptability, and evolutionary processes. High levels of heterozygosity in a population often indicate a greater ability to adapt to changing environments, as it can lead to increased fitness and survival rates.
Homozygosity: Homozygosity refers to the genetic condition in which an individual has two identical alleles for a particular gene. This condition plays a significant role in population genetics as it can influence traits, susceptibility to diseases, and overall genetic diversity within a population. High levels of homozygosity can indicate reduced genetic variation, which may have implications for the adaptability and survival of species in changing environments.
Inbreeding: Inbreeding is the mating of closely related individuals within a population. It increases homozygosity and can lead to a higher chance of offspring being affected by recessive or deleterious traits.
Inbreeding: Inbreeding is the mating of individuals who are closely related genetically, which can lead to an increase in homozygosity and the expression of deleterious traits. This genetic practice can significantly affect population genetics by reducing genetic diversity and increasing the likelihood of offspring inheriting harmful genetic conditions. Additionally, inbreeding has implications for behavior as it may influence social structures and mating behaviors within a population.
Inbreeding depression: Inbreeding depression is the reduced biological fitness in a population due to inbreeding, or breeding between closely related individuals. It results from an increase in the frequency of homozygosity for deleterious alleles.
Migration: Migration is the seasonal movement of animals from one region to another. It is often driven by the need for food, breeding grounds, or suitable living conditions.
Migration: Migration is the movement of individuals or groups from one location to another, often driven by factors such as environmental changes, availability of resources, or social conditions. This process can significantly influence genetic diversity, population dynamics, and the evolutionary trajectories of species. Migration plays a crucial role in connecting populations and impacting ecological relationships.
Missense mutations: Missense mutations are a type of point mutation where a single nucleotide change results in the coding of a different amino acid in a protein sequence. This can alter the structure and function of the protein, potentially leading to various effects on an organism's phenotype, which is essential in understanding genetic variation within populations.
Mutation: A mutation is a change in the DNA sequence of an organism's genome, which can lead to alterations in traits or characteristics. Mutations can occur spontaneously or be induced by environmental factors, and they play a crucial role in genetic diversity and evolution. They can be beneficial, harmful, or neutral, impacting an organism's ability to survive and reproduce in its environment.
Natural selection: Natural selection is the process by which organisms better adapted to their environment tend to survive and produce more offspring. Over time, this leads to the evolution of species as advantageous traits become more common in a population.
Natural selection: Natural selection is the process by which certain traits become more or less common in a population based on their impact on the survival and reproduction of individuals. It serves as a key mechanism of evolution, driving adaptation and influencing the genetic makeup of populations over time.
Nonrandom mating: Nonrandom mating occurs when individuals choose their mates based on specific traits rather than at random. This can affect allele frequencies and genetic variation within a population.
Nonsense mutations: Nonsense mutations are a type of genetic mutation that change a codon in the DNA sequence into a stop codon, leading to premature termination of protein synthesis. This can result in truncated proteins that are often nonfunctional, which can have significant effects on an organism's phenotype and fitness. In the context of population genetics, the occurrence of nonsense mutations can influence genetic variation and evolution within a population.
Point mutations: Point mutations are changes in a single nucleotide base pair in DNA, which can lead to alterations in gene expression or function. These mutations can occur due to various factors, such as errors during DNA replication or exposure to environmental mutagens. The significance of point mutations extends into critical areas like DNA repair mechanisms and the study of genetic variation within populations, highlighting their role in evolution and adaptation.
Population size: Population size refers to the total number of individuals within a defined species or group in a specific area at a given time. This measurement is essential for understanding genetic diversity, species interactions, and evolutionary processes within populations. Changes in population size can impact gene flow, genetic drift, and natural selection, which are all crucial components of population genetics.
Population variation: Population variation refers to the differences in genetic and phenotypic traits among individuals within a population. These variations provide the raw material for evolution through mechanisms like natural selection and genetic drift.
Recombination: Recombination is the process by which genetic material is exchanged between homologous chromosomes during meiosis, resulting in new combinations of alleles. This process plays a vital role in increasing genetic diversity within populations, as it allows for the shuffling of genes, creating unique combinations that can be passed down to offspring and influencing traits across generations.
Silent mutations: Silent mutations are changes in the DNA sequence that do not result in a change to the amino acid sequence of a protein. These mutations occur in coding regions of genes, often affecting the third nucleotide of a codon, which can lead to redundancy in the genetic code. Despite their name, silent mutations can have implications for gene expression and protein folding, influencing how traits are inherited in a population.
Stabilizing selection: Stabilizing selection is a type of natural selection that favors intermediate phenotypes over extreme variations within a population. This process reduces variation and maintains the status quo for a particular trait, leading to populations becoming more adapted to their environment. It is essential in understanding how populations evolve, the genetic structure of populations, and the mechanisms driving adaptive evolution.
Wright-Fisher model: The Wright-Fisher model is a mathematical model used in population genetics to describe how allele frequencies in a population change over time due to random sampling effects during reproduction. It provides a framework for understanding genetic drift and how the genetic composition of small populations evolves through generations, particularly under conditions of finite population size. This model is crucial for analyzing the impact of genetic variation and inheritance on evolutionary processes.