🔬General Biology I Unit 19 – The Evolution of Populations

Key Concepts and Definitions

• Evolution the change in allele frequencies in a population over time
• Population a group of individuals of the same species living in a specific area
• Gene pool the total collection of genes and alleles present in a population
• Allele frequency the proportion of a specific allele in a population relative to other alleles of the same gene
• Natural selection the process by which organisms with favorable traits survive and reproduce more successfully than those without these traits
• Adaptation a trait that enhances an organism's survival and reproduction in a specific environment
• Fitness the ability of an individual to survive and reproduce in a given environment
  • Relative fitness compares the fitness of one genotype to another in a population

Mechanisms of Evolution

• Natural selection acts on phenotypic variation within a population
  • Individuals with advantageous traits have higher survival and reproductive success
• Genetic drift random changes in allele frequencies due to chance events
  • More pronounced in small populations (founder effect, bottleneck effect)
• Gene flow the transfer of alleles between populations through migration or interbreeding
• Mutation the ultimate source of genetic variation introduces new alleles into a population
• Non-random mating can alter allele frequencies in a population
  • Assortative mating individuals mate with others who have similar phenotypes
  • Disassortative mating individuals mate with others who have different phenotypes
• Sexual selection occurs when one sex has a preference for certain traits in the other sex

Genetic Variation and Its Sources

• Genetic variation the differences in DNA sequences among individuals in a population
• Mutations changes in DNA sequence can introduce new alleles
  • Point mutations single nucleotide changes (substitutions, insertions, deletions)
  • Chromosomal mutations large-scale changes (duplications, deletions, inversions, translocations)
• Recombination during meiosis shuffles alleles to create new combinations on chromosomes
• Independent assortment of chromosomes during meiosis I contributes to genetic variation
• Sexual reproduction combines genetic material from two parents to produce offspring with unique genotypes
• Migration between populations can introduce new alleles or change existing allele frequencies
• Horizontal gene transfer the transfer of genetic material between organisms without reproduction (bacteria, viruses)

Natural Selection in Action

• Stabilizing selection favors intermediate phenotypes and reduces variation
  • Human birth weight too low or too high reduces survival
• Directional selection shifts the population towards one extreme of a trait
  • Antibiotic resistance in bacteria, pesticide resistance in insects
• Disruptive selection favors extreme phenotypes over intermediate ones
  • Beak size in African seedcracker finches, resource polymorphism in fish
• Sexual selection can lead to exaggerated traits in one sex
  • Peacock tail feathers, lion manes, deer antlers
• Artificial selection human-driven selection for desired traits in domesticated species
  • Dog breeds, crop plants, livestock
• Balancing selection maintains multiple alleles in a population
  • Sickle cell anemia heterozygote advantage in malaria-endemic regions

Population Genetics and Gene Pools

• Hardy-Weinberg equilibrium a mathematical model for predicting genotype frequencies in a non-evolving population
  • Assumptions: no mutation, no migration, no natural selection, large population size, random mating
  • Equation: $p^2 + 2pq + q^2 = 1$ (p and q are allele frequencies)
• Microevolution changes in allele frequencies within a population over generations
• Genetic drift random changes in allele frequencies due to chance events
  • Founder effect a small group establishes a new population with a different allele frequency (Amish, Ashkenazi Jews)
  • Bottleneck effect a population undergoes a drastic reduction in size, reducing genetic variation (cheetahs, northern elephant seals)
• Effective population size ($N_e$) the number of individuals in an idealized population that would experience genetic drift at the same rate as the actual population
  • Accounts for factors like unequal sex ratios, variation in reproductive success, and population fluctuations

Adaptation and Fitness

• Adaptation a trait that enhances an organism's survival and reproduction in a specific environment
  • Camouflage in prey animals (peppered moths, stick insects)
  • Mimicry one species evolves to resemble another (viceroy butterfly mimics monarch)
• Fitness the ability of an individual to survive and reproduce in a given environment
  • Absolute fitness the total number of offspring an individual produces
  • Relative fitness compares the fitness of one genotype to another in a population
• Inclusive fitness considers the reproductive success of an individual and its relatives
  • Explains altruistic behaviors (worker bees, meerkat sentinels)
• Trade-offs occur when one trait's improvement leads to another trait's decline
  • Allocation of resources (energy, nutrients) between growth, reproduction, and maintenance
  • Example: larger brain size in humans led to narrower birth canals and difficult childbirth

Speciation and Reproductive Isolation

• Speciation the formation of new species from existing populations
  • Allopatric speciation occurs when populations are geographically separated (Galápagos finches, Lake Victoria cichlids)
  • Sympatric speciation occurs without geographic separation (apple maggot fly, hawthorn fly)
• Reproductive isolation barriers prevent interbreeding between populations
  • Prezygotic barriers prevent fertilization (habitat isolation, behavioral isolation, mechanical isolation, gametic isolation)
  • Postzygotic barriers reduce the fitness of hybrids (reduced hybrid viability, reduced hybrid fertility, hybrid breakdown)
• Reinforcement selection for prezygotic barriers in sympatric populations to avoid producing unfit hybrids
• Hybrid zones areas where two distinct species meet and interbreed
  • Provide insights into the speciation process and the strength of reproductive barriers
  • Example: Ensatina salamanders in California form a ring species with varying degrees of reproductive isolation

Real-World Examples and Case Studies

• Antibiotic resistance in bacteria a classic example of directional selection
  • Overuse of antibiotics selects for resistant strains, leading to "superbugs" (MRSA, MDR-TB)
• Peppered moth evolution during the Industrial Revolution in England
  • Dark-colored moths were better camouflaged on soot-covered trees, demonstrating natural selection in action
• Galápagos finches studied by Charles Darwin
  • Adaptive radiation led to 15 species with diverse beak shapes adapted to different food sources
• Sickle cell anemia and malaria resistance
  • Heterozygotes (carriers) have a selective advantage in malaria-endemic regions, maintaining the allele in the population
• Domestication of crops and animals through artificial selection
  • Maize (corn) was selectively bred from wild teosinte grass
  • Dogs were domesticated from wolves and selectively bred for various traits (herding, hunting, companionship)
• Speciation in Darwin's finches
  • Peter and Rosemary Grant documented the formation of a new species of Galápagos finch in real-time