🐾General Biology II Unit 12 – Mechanisms of Evolution

Key Concepts

• Evolution involves changes in the genetic composition of populations over time
• Evolutionary forces include mutation, genetic drift, gene flow, and natural selection
• Genetic variation arises from mutations and sexual reproduction
  • Mutations can be beneficial, neutral, or harmful
  • Sexual reproduction shuffles alleles, creating new combinations in offspring
• Natural selection acts on phenotypic variation, favoring traits that enhance survival and reproduction
  • Differential survival and reproduction of individuals with certain traits leads to changes in allele frequencies over generations
• Population genetics studies the distribution and changes in allele frequencies within populations
  • The Hardy-Weinberg equilibrium describes a non-evolving population
  • Deviations from the equilibrium indicate evolutionary forces at work
• Speciation is the formation of new species through reproductive isolation
  • Allopatric speciation occurs when populations are geographically separated
  • Sympatric speciation occurs without geographic isolation
• Evidence for evolution comes from various fields, including fossils, comparative anatomy, and molecular biology
• Evolutionary principles have applications in medicine, agriculture, and conservation biology

Evolutionary Forces

• Mutation introduces new genetic variation into populations
  • Point mutations involve changes in single nucleotides (substitutions, insertions, deletions)
  • Chromosomal mutations involve larger-scale changes (duplications, inversions, translocations)
• Genetic drift is the random change in allele frequencies due to chance events
  • Founder effect occurs when a small group establishes a new population
  • Bottleneck effect occurs when a population undergoes a drastic reduction in size
• Gene flow is the transfer of alleles between populations through migration or interbreeding
  • Gene flow can introduce new alleles or change existing allele frequencies
• Natural selection is the differential survival and reproduction of individuals with certain traits
  • Directional selection favors one extreme of a trait (antibiotic resistance in bacteria)
  • Stabilizing selection favors intermediate values of a trait (human birth weight)
  • Disruptive selection favors both extremes of a trait (beak sizes in finches)
• Sexual selection is a type of natural selection based on mating success
  • Intrasexual selection involves competition within a sex (male deer antlers)
  • Intersexual selection involves mate choice by the opposite sex (peacock tail feathers)

Genetic Variation

• Genetic variation is the foundation for evolutionary change
• Mutations are the ultimate source of new genetic variation
  • Spontaneous mutations occur naturally due to DNA replication errors or environmental factors
  • Induced mutations are caused by mutagens (UV radiation, chemicals)
• Sexual reproduction generates new combinations of alleles through meiosis and fertilization
  • Independent assortment of chromosomes during meiosis I
  • Random fertilization of gametes
• Recombination during meiosis I creates new combinations of alleles on chromosomes
  • Crossing over between homologous chromosomes
• Polyploidy (having more than two sets of chromosomes) can lead to instant speciation in plants
• Horizontal gene transfer can introduce new genetic material from one species to another
  • Bacterial conjugation, transformation, and transduction
  • Viral-mediated gene transfer in eukaryotes

Natural Selection in Action

• Natural selection acts on phenotypic variation, which is often based on genetic variation
• Fitness is the ability of an individual to survive and reproduce in a given environment
  • Relative fitness compares the fitness of one genotype to another
• Adaptation is a trait that enhances an organism's fitness in a specific environment
  • Camouflage in prey animals (peppered moths)
  • Mimicry in harmless species to resemble harmful ones (viceroy butterfly mimicking monarch butterfly)
• Convergent evolution is the independent evolution of similar traits in unrelated species
  • Wings in birds, bats, and insects
  • Streamlined body shapes in aquatic animals (sharks, dolphins, ichthyosaurs)
• Coevolution is the reciprocal evolutionary change in interacting species
  • Predator-prey relationships (cheetahs and gazelles)
  • Host-parasite relationships (humans and influenza viruses)
• Artificial selection is human-directed selection for desired traits
  • Domestication of plants and animals (corn, dogs)
  • Selective breeding for specific characteristics (high-yielding crops, docile behavior in livestock)

Population Genetics

• Population genetics studies the distribution and changes in allele frequencies within populations
• The Hardy-Weinberg equilibrium describes a non-evolving population
  • Assumptions: large population size, no mutation, no migration, no selection, random mating
  • Allele frequencies remain constant across generations when the assumptions are met
• The Hardy-Weinberg equation ($p^2 + 2pq + q^2 = 1$) calculates genotype frequencies from allele frequencies
  • $p$ is the frequency of the dominant allele, $q$ is the frequency of the recessive allele
• Deviations from the Hardy-Weinberg equilibrium indicate evolutionary forces at work
  • Changes in allele frequencies over time
  • Excess or deficiency of heterozygotes compared to expected frequencies
• Genetic drift has a greater impact on small populations
  • Alleles can be lost or fixed more rapidly by chance events
• Effective population size ($N_e$) is the number of individuals that contribute genes to the next generation
  • Smaller than the actual population size due to factors like unequal sex ratios or fluctuating population sizes
• Inbreeding increases the frequency of homozygotes and can lead to inbreeding depression
  • Reduced fitness due to the expression of deleterious recessive alleles

Speciation Processes

• Speciation is the formation of new species through reproductive isolation
• Prezygotic barriers prevent the formation of a zygote
  • Habitat isolation: species occupy different habitats (different soil types for plants)
  • Temporal isolation: species have different breeding seasons or times (diurnal vs. nocturnal)
  • Behavioral isolation: species have different courtship behaviors or mating rituals (bird songs)
  • Mechanical isolation: incompatible reproductive structures (flower shapes and pollinator mouthparts)
  • Gametic isolation: gametes fail to attract, recognize, or fuse with each other (sea urchin sperm and eggs)
• Postzygotic barriers affect the survival or reproduction of hybrids
  • Hybrid inviability: hybrids do not develop properly or survive to adulthood (mule)
  • Hybrid sterility: hybrids are viable but cannot produce functional gametes (liger)
  • Hybrid breakdown: hybrids are fertile, but their offspring have reduced fitness (second-generation hybrids)
• Allopatric speciation occurs when populations are geographically separated
  • Vicariance: a physical barrier divides a population (formation of the Isthmus of Panama)
  • Dispersal: individuals colonize a new area (Galápagos finches)
• Sympatric speciation occurs without geographic isolation
  • Polyploidy: instant speciation through genome duplication (many plant species)
  • Habitat or host shift: populations adapt to different niches within the same area (apple maggot fly)
  • Sexual selection: divergent mate preferences lead to reproductive isolation (cichlid fish in Lake Victoria)

Evidence for Evolution

• Fossils provide a record of past life forms and evolutionary changes over time
  • Transitional fossils show intermediate stages between ancestral and derived forms (Archaeopteryx)
  • Fossil sequences demonstrate gradual changes in morphology (horse evolution)
• Comparative anatomy reveals similarities and differences among species
  • Homologous structures have a common evolutionary origin but may serve different functions (human arm, bat wing, whale flipper)
  • Analogous structures have similar functions but evolved independently (bird wing, insect wing)
  • Vestigial structures have lost their original function but are retained in reduced form (human appendix, whale hip bones)
• Comparative embryology shows similarities in early developmental stages among related species
  • Pharyngeal pouches in vertebrate embryos (human, chicken, fish)
• Molecular biology provides evidence of common ancestry and evolutionary relationships
  • DNA and protein sequence similarities (cytochrome c in various species)
  • Phylogenetic trees based on molecular data (mitochondrial DNA, ribosomal RNA)
• Biogeography studies the distribution of species across space and time
  • Endemic species are unique to a specific geographic area (kangaroos in Australia)
  • Convergent evolution in similar environments (succulent plants in deserts worldwide)
• Experimental evolution demonstrates evolutionary changes in real-time
  • Bacterial resistance to antibiotics
  • Artificial selection experiments (Drosophila, E. coli)

Applications and Implications

• Medicine: understanding the evolution of pathogens and the development of drug resistance
  • Antibiotic resistance in bacteria (MRSA)
  • Antiviral drug resistance in viruses (HIV, influenza)
• Agriculture: crop improvement through artificial selection and genetic engineering
  • High-yielding, disease-resistant crop varieties (Green Revolution)
  • Genetically modified organisms (Bt corn, Golden Rice)
• Conservation biology: preserving biodiversity and managing endangered species
  • Genetic diversity as a measure of population health
  • Evolutionary potential and adaptation to changing environments
• Bioremediation: using evolved microorganisms to clean up environmental pollutants
  • Oil-degrading bacteria (Deepwater Horizon oil spill)
  • Heavy metal-tolerant plants (phytoremediation)
• Evolutionary psychology: understanding human behavior and cognition in an evolutionary context
  • Mate selection preferences (facial symmetry, waist-to-hip ratio)
  • Altruism and cooperation (kin selection, reciprocal altruism)
• Evolutionary algorithms: applying evolutionary principles to solve complex problems in computer science
  • Optimization and machine learning (genetic algorithms, evolutionary programming)
• Science education: promoting scientific literacy and critical thinking skills
  • Teaching the evidence for evolution and the nature of science
  • Addressing misconceptions and pseudoscientific claims (intelligent design, creation science)