19.1 Population Evolution
3 min read•june 14, 2024
explores how genetic variation changes within populations over time. It's a crucial aspect of evolution, helping us understand how species adapt and diversify. Factors like , , , and all play key roles.
The serves as a baseline for detecting evolutionary change. By comparing observed genetic patterns to this model of equilibrium, scientists can identify when and how populations are evolving. This knowledge is vital for understanding everything from disease resistance to speciation.
Population Genetics and Evolution
Changes in allele frequencies over time
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- Various evolutionary forces cause to change in populations over time
- favors individuals with advantageous traits, increasing the frequency of associated alleles across generations (peppered moth coloration)
- Genetic drift leads to random changes in allele frequencies, particularly in small populations, potentially causing (100% frequency) or loss of alleles (, )
- Mutation introduces new alleles into a population at low rates, but these can accumulate over time and contribute to genetic variation (sickle cell anemia)
- Gene flow transfers alleles between populations due to movement of individuals or gametes (pollen), increasing or decreasing genetic variation within populations (Great Lakes sea lamprey)
Application of Hardy-Weinberg principle
- The Hardy-Weinberg principle describes a non-evolving population at genetic equilibrium, assuming no mutation, gene flow, natural selection, random mating, and an infinitely large population size
- When these assumptions are met, allele frequencies remain constant across generations
- Hardy-Weinberg equations:
- , where and are allele frequencies
- , where , , and are genotype frequencies
- Deviations from indicate that the population is evolving
- Observed genotype frequencies differing significantly from expected frequencies suggest evolutionary forces at work ( in sickle cell anemia)
- Changes in allele frequencies across generations also imply evolution (antibiotic resistance in bacteria)
Factors affecting genetic variation
- Mutation introduces new alleles into a population, increasing genetic variation, although most mutations are neutral or deleterious (albinism)
- (gene flow) involves movement of individuals or gametes between populations
- Can increase genetic variation by introducing new alleles (invasive species)
- Can decrease genetic variation if immigrants are genetically similar to the recipient population (island populations)
- results in differential survival and reproduction of individuals based on their traits
- favors one extreme of a trait distribution, shifting the population mean and reducing genetic variation by eliminating less favorable alleles (antibiotic resistance)
- favors intermediate trait values, reducing variation around the population mean and maintaining genetic variation by preserving alleles near the optimum (birth weight in humans)
- favors both extremes of a trait distribution, increasing variation and potentially leading to population divergence and speciation (beak size in Galápagos finches)
Population Genetics and Fitness
- Population genetics studies the distribution and change of allele frequencies within populations
- refers to the number of individuals in a population that contribute offspring to the next generation, which can be smaller than the actual population size
- is a measure of an individual's reproductive success relative to other individuals in the population
- occurs when a single ancestral species gives rise to multiple descendant species adapted to different ecological niches, often in a relatively short period of evolutionary time
Key Terms to Review (29)
(microevolution): Microevolution is the change in allele frequencies within a population over a relatively short period of time. It is driven by mechanisms such as natural selection, genetic drift, mutation, and gene flow.
Adaptive radiation: Adaptive radiation is the evolutionary process in which a single ancestral species rapidly diversifies into a wide variety of forms to adapt to different environments and ecological niches. This phenomenon often occurs after the introduction of new habitats or following mass extinctions, enabling species to exploit various resources and reduce competition. It highlights the relationship between environmental factors and the diversification of life forms.
Allele frequencies: Allele frequencies refer to how often a particular allele appears in a population compared to all alleles for that gene. This concept is crucial in understanding population evolution as it provides insights into genetic diversity, natural selection, and how traits are passed on over generations. By studying allele frequencies, scientists can monitor changes in populations, track evolutionary processes, and assess the impacts of environmental factors on genetic variation.
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.
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.
Effective Population Size: Effective population size is a concept in population genetics that refers to the number of individuals in a population who contribute to the gene pool of the next generation. This measure is important because it helps in understanding genetic drift, inbreeding, and the overall genetic health of a population, especially in the context of evolutionary processes.
Evolutionary (Darwinian) fitness: Evolutionary (Darwinian) fitness is the measure of an organism's ability to survive and reproduce in its environment. It directly correlates with the number of viable offspring an individual contributes to the next generation.
Fitness: Fitness refers to the ability of an organism to survive and reproduce in its environment. It is not just about physical strength or speed but encompasses a range of factors that contribute to reproductive success, such as the organism's behavior, health, and adaptability to environmental changes.
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.
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.
Gene pool: A gene pool is the complete set of genetic information within all individuals in a population. It encompasses all the alleles present in the population's individuals.
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 structure: Genetic structure refers to the distribution of genetic variation within and between populations. It includes the frequencies of alleles, genotypes, and phenotypes influenced by evolutionary processes.
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.
Hardy-Weinberg principle: The Hardy-Weinberg principle is a fundamental concept in population genetics that describes the genetic equilibrium within a population. It states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of evolutionary influences. This principle provides a baseline to understand how and why populations evolve over time, emphasizing factors like genetic drift, mutation, migration, and natural selection that can disrupt this equilibrium.
Heterozygote advantage: Heterozygote advantage refers to a situation in genetics where individuals who are heterozygous for a particular gene have a higher fitness than those who are homozygous. This concept is important in understanding how genetic diversity can be maintained in populations, as it provides a selective advantage that can influence evolutionary dynamics and population evolution.
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.
Modern synthesis: Modern synthesis is the fusion of Mendelian genetics with Darwinian evolution that resulted in a unified theory of evolution. It emphasizes the role of natural selection, genetic variation, and population genetics.
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.
Population genetics: Population genetics is the study of genetic variation within populations and the factors that influence this variation over time. This field connects evolutionary theory and genetic principles, highlighting how evolutionary processes such as natural selection, genetic drift, and gene flow shape the genetic makeup of populations. Understanding these genetic dynamics is crucial for exploring the mechanisms behind evolution and speciation.
Selection pressure: Selection pressure refers to the environmental factors that influence the survival and reproductive success of individuals within a population. It drives natural selection by favoring certain traits over others, leading to changes in the genetic makeup of the population over time. This concept is crucial for understanding how populations evolve in response to their surroundings and how specific adaptations can arise in species as they face varying challenges.
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.