19.3 Adaptive Evolution
3 min read•june 14, 2024
shapes populations, favoring traits that enhance survival and reproduction. Directional, stabilizing, and drive evolutionary change, while influences mate choice and competition. These processes lead to adaptations that help organisms thrive in their environments.
However, evolution faces constraints. Genetic limitations, developmental pathways, and environmental factors can restrict adaptive potential. Understanding these constraints, along with mechanisms like and , helps explain the complex patterns of biodiversity we observe in nature.
Natural Selection and Adaptation
Types of natural selection
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- occurs when individuals at one end of a trait spectrum have higher shifting the population mean toward the favored extreme (antibiotic resistance in bacteria)
- favors intermediate phenotypes and selects against extreme variations maintaining the population mean and reducing variation (human birth weight, where both low and high weights have reduced fitness)
- Disruptive selection favors individuals at both extremes of a trait spectrum over intermediate phenotypes potentially leading to the formation of two distinct subpopulations or even speciation (beak size in African finches, where medium-sized beaks are less efficient for feeding on available food sources)
Impact of sexual selection
- Sexual selection arises from competition for mates and mate choice
- involves competition within one sex (usually males) for access to mates leading to the evolution of traits that enhance an individual's ability to compete, such as larger body size, weapons, or aggression (male elk competing with their antlers during the breeding season)
- involves mate choice by one sex (usually females) based on preferences for specific traits leading to the evolution of traits that make an individual more attractive to potential mates, such as bright colors, elaborate ornaments, or courtship displays (peacock's elaborate tail feathers, which are preferred by females)
Limitations of adaptive evolution
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- Limited genetic variation in a population can hinder its ability to adapt to new environmental conditions
- , where one gene influences multiple traits, can limit the evolution of one trait if it negatively impacts another
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- Existing developmental pathways and processes can limit the range of possible evolutionary changes (vertebrate limb structure is conserved across species, limiting the potential for drastic modifications)
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- Evolutionary history of a lineage can influence its future evolutionary trajectory (evolution of the mammalian jaw from reptilian ancestors constrained the number of jaw bones and the attachment of jaw muscles)
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- Adaptive changes in one trait may come at the expense of another trait (allocating more energy to reproduction may reduce an individual's growth or survival)
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- Physical and biotic factors in an organism's environment can limit the range of possible adaptations (aquatic mammals are constrained by the need to surface for air, limiting their ability to exploit deep-water habitats)
- allows organisms to adjust their phenotype in response to environmental conditions, potentially reducing the need for genetic
Mechanisms of evolutionary change
- : The process by which heritable traits that increase become more common in successive generations
- Genetic drift: Random changes in allele frequencies within a population, especially in small populations
- : The ultimate source of new genetic variation, introducing novel alleles into a population
- Gene flow: The transfer of genetic variation between populations through migration or interbreeding
Population genetics
- Studies the changes in allele frequencies within populations over time
- Provides a mathematical framework for understanding how evolutionary forces shape genetic variation
- Helps predict the likelihood of based on population size, selection pressures, and genetic diversity
Key Terms to Review (31)
Adaptation: Adaptation refers to the process through which organisms become better suited to their environment over time, leading to changes in their traits that enhance survival and reproduction. This concept is fundamental to understanding how species evolve, how new species can emerge from common ancestors, and how organisms adjust to specific ecological niches and climates across different environments.
Adaptive evolution: Adaptive evolution is the process by which populations become better suited to their environment through changes in genetic traits. It results from natural selection acting on heritable variation within a 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.
Developmental constraints: Developmental constraints refer to the limitations and restrictions in the way organisms can develop, often due to their evolutionary history and genetic makeup. These constraints can influence how traits evolve over time, shaping the potential adaptations of species. Understanding developmental constraints is essential for grasping the complexities of adaptive evolution, as they can limit the range of possible phenotypic variations available for natural selection to act upon.
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.
Diversifying selection: Diversifying selection, also known as disruptive selection, is a type of natural selection that favors extreme phenotypes over intermediate ones. This process can lead to the development of two or more distinct forms within a population.
Environmental constraints: Environmental constraints refer to the limitations and challenges imposed by external environmental factors that affect the survival, reproduction, and adaptation of organisms. These constraints can include factors like climate, resource availability, competition, and predation, which shape the evolutionary pathways of species over time. Understanding these constraints is essential for grasping how adaptive evolution occurs in response to changing conditions.
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.
Evolutionary fitness: Evolutionary fitness refers to an organism's ability to survive and reproduce in its environment, contributing its genetic material to the next generation. It encompasses not just survival, but also the effectiveness of an organism in passing on its genes, which often varies based on environmental pressures. This concept is crucial for understanding how certain traits become more common in a population over time as those traits enhance reproductive success.
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.
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 constraints: Genetic constraints refer to the limitations on evolutionary change that arise from the genetic architecture of populations. These constraints can restrict the range of possible adaptations and the ability of organisms to respond to selective pressures due to factors such as gene interactions, genetic correlations, and the availability of genetic variation. Understanding genetic constraints is essential for grasping how adaptive evolution operates within the context of natural selection and evolutionary dynamics.
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.
Good genes hypothesis: The good genes hypothesis suggests that certain traits are favored in mate selection because they indicate superior genetic quality. These traits increase the likelihood of offspring survival and reproduction.
Handicap principle: The handicap principle is a hypothesis that suggests individuals of higher fitness can afford to produce costly signals that are otherwise detrimental. These signals serve as honest indicators of their genetic quality to potential mates or rivals.
Historical constraints: Historical constraints refer to the limitations imposed on evolution and adaptation due to an organism's evolutionary history and the pre-existing traits that affect its ability to respond to new environmental pressures. These constraints can shape the paths of evolutionary change, as they determine which adaptations are possible or advantageous based on what has previously evolved.
Honest signal: An honest signal is a trait or behavior that provides truthful information about an individual's fitness or quality to potential mates or rivals. These signals are costly to produce and maintain, ensuring their reliability.
Intersexual selection: Intersexual selection is a form of natural selection where individuals of one sex, typically females, choose mates based on certain desirable traits. This process influences the evolution of species by favoring traits that enhance attractiveness and improve reproductive success, leading to the development of pronounced differences between sexes, known as sexual dimorphism. Intersexual selection is a key driver in adaptive evolution as it shapes the genetic variation within populations.
Intrasexual selection: Intrasexual selection is a form of sexual selection where members of one sex compete among themselves for access to mates. This process often leads to the evolution of traits that enhance an individual's ability to win these contests, such as increased size, strength, or weaponry. It plays a significant role in shaping mating behaviors and can lead to the development of secondary sexual characteristics that are beneficial in these competitions.
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.
Phenotypic Plasticity: Phenotypic plasticity is the ability of an organism to change its phenotype in response to environmental conditions. This trait allows organisms to exhibit different physical and behavioral characteristics based on varying external factors, enhancing their adaptability and survival. It plays a crucial role in how species respond to changing environments and can influence evolutionary processes by allowing populations to exploit new niches.
Pleiotropic effects: Pleiotropic effects occur when a single gene influences multiple phenotypic traits in an organism. This concept is crucial for understanding how certain genes can have far-reaching impacts on an organism’s development, physiology, and behavior, highlighting the complexity of genetic interactions in evolution. Pleiotropy can shape adaptive evolution by affecting traits that are subject to natural selection, leading to changes in population characteristics 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.
Relative fitness: Relative fitness measures the reproductive success of an individual or genotype compared to others in the population. It is often expressed as a ratio or percentage, indicating how well an organism passes on its genes relative to others.
Sexual selection: Sexual selection is a form of natural selection where certain traits increase an individual's chances of attracting mates, leading to reproductive success. This process can drive the evolution of traits that may not necessarily enhance survival but improve mating opportunities, such as bright colors or elaborate courtship behaviors. Sexual selection can lead to the development of new species by creating distinct reproductive strategies and preferences within populations, as well as promote adaptive evolution by favoring traits that enhance mating success in specific environments.
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.
Trade-offs: Trade-offs refer to the compromises that organisms make when adapting to their environment, where gaining one beneficial trait often comes at the expense of another. This concept is crucial in understanding how adaptations shape an organism's fitness and survival, highlighting the balance between different traits like reproduction, growth, and survival. Through natural selection, these trade-offs can influence the evolutionary pathways that species take over time.