❤️AP Bio Unit 8 – Natural Selection

Key Concepts

• Natural selection is the process by which organisms with favorable traits are more likely to survive and reproduce, passing on their genes to future generations
• Variation within a population is necessary for natural selection to occur, as it provides the raw material for selection to act upon
• Heritable traits are those that can be passed from parent to offspring through genetic inheritance (eye color, height)
• Differential survival and reproduction of individuals with certain traits leads to changes in allele frequencies over time
• Fitness refers to an organism's ability to survive and reproduce in a given environment, and is a key factor in natural selection
• Adaptation is the process by which populations become better suited to their environment over many generations through natural selection
• Common descent describes the idea that all living organisms are related and descended from a common ancestor

Historical Context

• Charles Darwin developed the theory of evolution by natural selection in the mid-19th century
• Darwin's observations during his voyage on the HMS Beagle, particularly in the Galapagos Islands, provided key insights into the process of evolution
  • He noticed that similar species on different islands had distinct adaptations suited to their specific environments (finches with different beak shapes)
• Alfred Russel Wallace independently developed similar ideas about evolution and natural selection around the same time as Darwin
• The publication of Darwin's "On the Origin of Species" in 1859 revolutionized scientific understanding of the diversity and relatedness of life on Earth
• Prior to Darwin, the prevailing view was that species were unchanging and had been separately created by a divine being
• The discovery of fossils and the development of geology in the 18th and 19th centuries provided evidence for the great age of the Earth and the extinction of species over time

Mechanisms of Natural Selection

• Variation: Individuals within a population differ in their traits, some of which may be heritable
  • Sources of variation include mutation, recombination during sexual reproduction, and gene flow between populations
• Inheritance: Offspring inherit traits from their parents through genetic mechanisms (DNA, genes)
• Differential survival and reproduction: Individuals with traits that are advantageous in a given environment are more likely to survive and reproduce
  • For example, giraffes with longer necks can reach higher leaves and have a better chance of surviving and passing on their genes
• Accumulation of adaptations: Over many generations, the proportion of individuals with advantageous traits increases, leading to changes in the population
• Speciation: Natural selection can lead to the formation of new species when populations become reproductively isolated and accumulate different adaptations
• Coevolution: Species can evolve in response to changes in other species they interact with (predators and prey, hosts and parasites)

Evidence for Natural Selection

• Fossil record: Fossils provide evidence of the evolutionary history of life on Earth and show changes in species over time
  • Transitional fossils, such as Archaeopteryx, demonstrate the evolution of new traits and the link between different groups of organisms
• Comparative anatomy: Similarities in the structure of different species suggest common ancestry and evolutionary relationships
  • Homologous structures, such as the forelimbs of mammals, birds, and reptiles, have a common evolutionary origin but have been modified for different functions
• Embryology: The development of embryos in different species reveals similarities that suggest common ancestry
  • For example, all vertebrate embryos have gill slits and a tail at some stage of development
• Molecular biology: Comparisons of DNA sequences and proteins across species provide evidence of evolutionary relationships and common descent
• Biogeography: The distribution of species across continents and islands reflects their evolutionary history and the influence of geographic barriers
• Observed instances of natural selection: Studies of living populations have documented natural selection in action (peppered moths, antibiotic resistance in bacteria)

Types of Selection

• Directional selection: Selection favors individuals with traits at one extreme of the population distribution, shifting the mean trait value over time
  • For example, selection for larger body size in a population of fish
• Stabilizing selection: Selection favors individuals with intermediate trait values, reducing variation in the population
  • For example, selection for average birth weight in humans, as both very low and very high birth weights have associated health risks
• Disruptive selection: Selection favors individuals with traits at both extremes of the population distribution, leading to a bimodal distribution
  • For example, selection for very large and very small beak sizes in a population of finches, depending on the availability of different food sources
• Sexual selection: Selection based on traits that affect an individual's ability to attract mates and reproduce
  • Intrasexual selection involves competition between members of the same sex for access to mates (male deer competing with their antlers)
  • Intersexual selection involves the preferential choice of mates based on certain traits (female peacocks choosing males with more elaborate tail feathers)
• Artificial selection: Human-directed selection of traits in domesticated plants and animals for specific purposes (breeding dogs for desired characteristics)

Adaptation and Fitness

• Adaptation refers to the process by which populations become better suited to their environment through natural selection
  • Adaptations can be morphological (physical features), physiological (internal processes), or behavioral
• Fitness is a measure of an individual's ability to survive and reproduce in a given environment
  • Relative fitness compares the survival and reproductive success of individuals with different genotypes in a population
• Adaptations increase the fitness of individuals by enhancing their ability to survive and reproduce in specific environments
  • For example, the thick fur of polar bears is an adaptation that helps them survive in cold Arctic environments
• Trade-offs: Adaptations that are beneficial in one context may be detrimental in another, leading to evolutionary trade-offs
  • For example, the large antlers of male elk are advantageous for attracting mates but can hinder their ability to move through dense vegetation
• Constraints: The evolution of adaptations can be limited by developmental, genetic, and historical constraints
  • For example, the vertebrate eye has a "blind spot" due to the arrangement of nerve cells, which is a result of its evolutionary history

Case Studies and Examples

• Darwin's finches: The diverse beak shapes of Galapagos finches are adaptations to different food sources, exemplifying adaptive radiation
• Peppered moths: The shift in coloration of peppered moths during the Industrial Revolution in England demonstrates natural selection in response to changing environmental conditions
• Antibiotic resistance: The evolution of antibiotic-resistant bacteria is an example of natural selection in action, driven by the overuse of antibiotics
• Sickle cell anemia: The persistence of the sickle cell allele in human populations is an example of heterozygote advantage, as individuals with one copy of the allele have increased resistance to malaria
• Mimicry: Many species have evolved to resemble other species that are toxic or dangerous to predators, a form of adaptation known as mimicry (monarch and viceroy butterflies)
• Coevolution: The relationship between plants and their pollinators, such as flowers and hummingbirds, demonstrates coevolution, as each species has evolved adaptations in response to the other

Applications and Implications

• Agriculture: Understanding the principles of natural selection and adaptation is crucial for developing effective strategies for crop and livestock improvement
  • Selective breeding of plants and animals has been used for thousands of years to enhance desirable traits
• Conservation biology: Knowledge of evolutionary processes is essential for designing effective conservation strategies to protect endangered species and maintain biodiversity
  • For example, understanding the evolutionary history and adaptations of a species can inform decisions about habitat protection and captive breeding programs
• Medicine: Evolutionary biology has important applications in understanding the origins and spread of diseases, as well as in developing new treatments and therapies
  • The evolution of antibiotic resistance in bacteria highlights the need for the judicious use of antibiotics and the development of new antimicrobial strategies
• Biotechnology: Principles of evolution and natural selection are used in the development of new biotechnologies, such as directed evolution of enzymes for industrial applications
• Understanding human evolution: The study of human origins and the evolutionary history of our species provides insights into our biology, behavior, and culture
  • For example, the analysis of ancient DNA from Neanderthals and other extinct hominins has revealed evidence of interbreeding with modern humans and the genetic basis of certain traits
• Science education: Teaching the principles of evolution and natural selection is essential for promoting scientific literacy and understanding the unity and diversity of life on Earth