Natural selection shapes populations over time, favoring traits that enhance survival and reproduction. This process involves variation, inheritance, selection, and time, leading to changes in allele frequencies and potential adaptations. Understanding natural selection is crucial for grasping how species evolve and adapt to their environments.
Various types of selection, including directional, stabilizing, and disruptive, influence populations differently. These mechanisms, along with concepts of fitness and adaptation, demonstrate how organisms become better suited to their surroundings. Examples from nature highlight the diverse ways natural selection manifests across species and environments.
Natural selection and genetic change
Components and mechanisms of natural selection
- Natural selection process organisms with advantageous heritable traits more likely to survive and reproduce, passing these traits to future generations
- Four key components of natural selection variation, inheritance, selection, and time
- Natural selection acts on phenotypes observable characteristics of organisms resulting from their genotypes and environmental influences
- Genetic drift, gene flow, and mutation other evolutionary forces interact with natural selection to shape population genetics
- Natural selection leads to changes in allele frequencies within a population over time, potentially resulting in evolutionary adaptations
- Strength of selection pressure and heritability of traits influence the rate at which natural selection can alter a population's genetic composition
Genetic changes and population dynamics
- Natural selection alters allele frequencies in populations over generations
- Advantageous alleles increase in frequency, while disadvantageous alleles decrease
- Genetic variation within populations provides raw material for natural selection
- Mutation introduces new alleles into populations, expanding genetic diversity
- Gene flow between populations can introduce or remove alleles, affecting genetic composition
- Genetic drift randomly changes allele frequencies, especially in small populations
- Bottleneck effects and founder effects can dramatically alter genetic makeup of populations
Types of natural selection
Directional selection
- Directional selection favors extreme phenotypes at one end of a trait's distribution, shifting the population mean towards that extreme over time
- Examples of directional selection
- Increase in giraffe neck length over evolutionary time
- Development of antibiotic resistance in bacteria
- Evolution of pesticide resistance in insects
- Directional selection can lead to rapid evolutionary changes in populations
- Often occurs in response to consistent environmental pressures or changes
- Can result in the loss of genetic variation at the unfavored end of the trait distribution
Stabilizing selection
- Stabilizing selection favors intermediate phenotypes, reducing variation around the population mean and maintaining the status quo
- Examples of stabilizing selection
- Human birth weight optimal range for infant and maternal survival
- Clutch size in birds balancing energy investment and offspring survival
- Flower size in plants attracting pollinators while conserving resources
- Stabilizing selection maintains traits within an optimal range
- Reduces genetic variation in populations over time
- Often observed in stable environments where current adaptations are well-suited
Disruptive selection
- Disruptive selection favors extreme phenotypes at both ends of a trait's distribution, potentially leading to bimodal distributions or speciation
- Examples of disruptive selection
- Beak size divergence in Galápagos finches
- Shell color polymorphism in land snails
- Seed size variation in some plant species
- Disruptive selection can increase genetic variation within populations
- May lead to the formation of distinct subpopulations or eventual speciation
- Often occurs in heterogeneous environments with multiple ecological niches
Fitness and adaptation
Concept of fitness in evolutionary biology
- Fitness measure of an organism's reproductive success relative to other individuals in the population
- Absolute fitness quantified as the number of offspring an individual produces
- Relative fitness compares an individual's reproductive success to the population average
- Natural selection acts to increase the average fitness of a population over time by favoring traits that enhance survival and reproduction
- Fitness landscapes model the relationship between genotypes or phenotypes and their corresponding fitness values in a given environment
- Trade-offs between different fitness components (survival vs. reproduction) can result in optimal rather than maximal trait values
- Fitness context-dependent and can change as environmental conditions or population dynamics shift over time
Adaptation and its relationship to fitness
- Adaptations heritable traits that increase an organism's fitness in its environment
- Natural selection drives the development and refinement of adaptations over generations
- Adaptations can be structural (physical features), physiological (internal processes), or behavioral
- Examples of adaptations
- Camouflage in animals (Arctic fox fur color changes)
- Drought resistance in plants (deep root systems, waxy leaf coatings)
- Mimicry in insects (viceroy butterfly resembling monarch butterfly)
- Adaptations often involve trade-offs between different aspects of fitness
- Maladaptations traits that were once adaptive but become detrimental due to environmental changes
- Exaptations traits that evolve for one purpose but later serve a different function (feathers for insulation becoming used for flight)
Adaptations from natural selection
Protective adaptations
- Mimicry in butterflies, such as Batesian and Müllerian mimicry, demonstrates how natural selection can favor protective coloration and patterns
- Batesian mimicry non-toxic species resembles toxic species (viceroy butterfly mimicking monarch)
- Müllerian mimicry multiple toxic species evolve similar warning patterns (Heliconius butterflies)
- Camouflage in animals showcases adaptation to specific environmental conditions
- Peppered moths industrial melanism during Industrial Revolution
- Leaf-mimicking insects (walking leaf insects, leaf-tailed geckos)
- Defensive structures and behaviors evolved through natural selection
- Porcupine quills
- Skunk spray
- Hedgehog's ability to curl into a spiny ball
Physiological and behavioral adaptations
- Antibiotic resistance in bacteria illustrates rapid adaptation through natural selection in response to strong selective pressures
- MRSA (Methicillin-resistant Staphylococcus aureus) evolution
- Multi-drug resistant tuberculosis development
- The evolution of echolocation in bats and dolphins represents convergent evolution through natural selection in different lineages
- Bats use high-frequency sound for navigation and prey location
- Dolphins employ similar technique for underwater navigation and hunting
- Plant adaptations to specific pollinators demonstrate co-evolution driven by natural selection
- Long-necked flowers for hummingbirds (trumpet honeysuckle)
- UV patterns on flowers visible to bees but not humans
- Orchids mimicking female insects to attract male pollinators
Adaptive radiation and speciation
- Beak shape variations in Galápagos finches exemplify adaptive radiation and the role of natural selection in speciation
- Different beak shapes adapted to various food sources on different islands
- Led to the evolution of multiple finch species from a common ancestor
- Cichlid fish species in African lakes showcase rapid adaptive radiation
- Over 1000 species evolved in Lake Malawi alone
- Adaptations for different feeding strategies, habitats, and mating behaviors
- Human adaptations illustrate ongoing natural selection in our species
- Lactase persistence in dairy-consuming populations
- High-altitude tolerance in Tibetan and Andean populations
- Sickle cell trait providing malaria resistance in certain African populations