🧬AP Biology Unit 7 – Natural Selection

Key Concepts and Definitions

• Natural selection process by which populations become adapted to their environment over the course of generations
• Adaptation a trait that increases an organism's fitness and arises through natural selection
  • Can be morphological (physical), physiological, or behavioral
• Fitness the ability to survive and reproduce in a specific environment
• Variation the presence of differences among individuals in a population
  • Variation is necessary for natural selection to occur
• Inheritance the passing of traits from parents to offspring
• Differential reproduction occurs when individuals with certain traits leave more offspring than individuals without those traits
• Speciation the formation of new species as a result of evolution by natural selection

Historical Context and Darwin's Contributions

• Charles Darwin developed the theory of evolution by natural selection in the mid-19th century
• Darwin's voyage on the HMS Beagle exposed him to diverse flora and fauna and geological evidence that influenced his thinking
• Darwin observed that organisms seemed well-suited to their environments and that similar species often differed across geographic regions
• Alfred Russel Wallace independently developed a theory of evolution by natural selection and corresponded with Darwin
• Darwin's book "On the Origin of Species" (1859) outlined his evidence for evolution and proposed natural selection as the mechanism
  • The book was controversial at the time but eventually gained widespread acceptance in the scientific community
• Darwin's theory built upon earlier ideas, such as Lamarck's concept of the inheritance of acquired characteristics
• Modern evolutionary theory synthesizes Darwin's ideas with Mendelian genetics and other scientific discoveries

Mechanisms of Natural Selection

• Overproduction more offspring are produced than can survive due to limited resources
• Genetic variation exists within populations for traits that affect survival and reproduction
  • Variation arises through mutation, recombination during sexual reproduction, and gene flow between populations
• Differential survival and reproduction individuals with traits that are advantageous in a given environment tend to survive and reproduce more than those without those traits
  • Over time, this leads to a change in the frequency of alleles in the population
• Heredity traits that confer an advantage are passed down to offspring
• Accumulation of changes over many generations, the advantageous traits become more common in the population
• Adaptation to the environment results from the gradual accumulation of beneficial traits
• Speciation can occur when populations become reproductively isolated and diverge due to different selective pressures

Evidence for Natural Selection

• Fossil record shows gradual changes in organisms over time and the existence of extinct species
  • Transitional fossils demonstrate the evolution of one species into another
• Comparative anatomy reveals structural similarities among related species, suggesting common ancestry
  • Homologous structures (e.g., human arm, bat wing, whale flipper) have similar underlying anatomy due to shared ancestry
  • Analogous structures (e.g., bird wing, insect wing) have similar functions but different underlying anatomy due to convergent evolution
• Embryology early developmental stages of different species show similarities, supporting common ancestry
• Molecular biology DNA and protein sequences show similarities among related species
  • The degree of similarity reflects the relatedness of species
• Biogeography the distribution of species across geographic regions reflects their evolutionary history
  • Island species often resemble nearby mainland species but with unique adaptations
• Observed instances of natural selection (e.g., antibiotic resistance in bacteria, beak shape changes in Galápagos finches)

Types of Selection

• Directional selection favors individuals at one extreme of a trait distribution
  • Leads to a shift in the population's mean trait value over time
• Stabilizing selection favors individuals with intermediate trait values
  • Reduces variation in the population and maintains the mean trait value
• Disruptive selection favors individuals at both extremes of a trait distribution
  • Can lead to the formation of distinct subpopulations or even new species
• Sexual selection occurs when traits that enhance mating success are favored
  • Can result in the evolution of exaggerated traits (e.g., peacock tail)
• Artificial selection human-driven selection for desired traits in domesticated plants and animals
  • Demonstrates the power of selection to alter populations

Adaptation and Fitness

• Adaptation an inherited trait that enhances an organism's fitness in a specific environment
• Fitness the ability to survive and reproduce in a given environment
  • Measured by an individual's relative contribution to the next generation
• Adaptations can take many forms, such as morphological, physiological, or behavioral traits
  • Examples: camouflage, antibiotic resistance, courtship rituals
• Adaptations are specific to the environment in which they evolved
  • A trait that is adaptive in one environment may be neutral or maladaptive in another
• Fitness is always relative to the environment and other individuals in the population
• Trade-offs often exist between different adaptations
  • Allocating resources to one trait may come at the expense of another

Natural Selection in Action: Case Studies

• Peppered moth evolution in response to industrial pollution in England
  • Light-colored moths were better camouflaged on light-colored trees before pollution
  • Dark-colored moths became more common as pollution darkened tree bark
• Galápagos finches' beak adaptations for different food sources
  • Finches with larger, stronger beaks are better adapted for cracking tough seeds
  • Finches with smaller, pointier beaks are better adapted for catching insects
• Antibiotic resistance in bacteria
  • Bacteria with mutations conferring resistance survive and reproduce in the presence of antibiotics
  • Resistant strains become more common over time
• Sickle cell anemia and malaria resistance
  • Having one copy of the sickle cell allele confers resistance to malaria
  • The allele persists in populations where malaria is prevalent despite the negative effects of having two copies

Challenges and Misconceptions

• Evolution is often misunderstood as a purposeful or directed process
  • Natural selection is a result of random variation and differential survival and reproduction
• Adaptation is sometimes misinterpreted as an individual actively trying to adapt
  • Adaptations arise through the selection of inherited traits over many generations
• The phrase "survival of the fittest" is often misinterpreted to mean the strongest or most aggressive individuals
  • Fitness refers to the ability to survive and reproduce, which can involve cooperation and other strategies
• Evolution is sometimes wrongly equated with progress or improvement
  • Natural selection favors traits that enhance fitness in a specific environment, not inherently "better" traits
• The role of chance and random events in evolution is often underestimated
  • Genetic drift, founder effects, and bottlenecks can significantly influence evolutionary outcomes
• Misconceptions about the relationship between evolution and religion persist
  • Many religious individuals and organizations accept evolution as a scientific explanation for the diversity of life