Biology Chapter 22: Descent with Modification

Descent with Modification

• Scientists who influenced Darwin: Hutton, Cuvier, Lyell, Wallace, etc.
• Darwin's 5-year voyage on the HMS Beagle as a naturalist, published "The Origin of Species" in 1859
• Definition of evolution: the process by which species accumulate differences from their ancestors as they adapt to different environments over time
• Populations change over time genetically, not individuals

Natural Selection

• The process in which individuals with certain inherited traits tend to survive and reproduce at higher rates because of those traits
• 4 observations (or inferences) of natural selection:
  • Individuals in populations vary in their inherited traits
  • All species can produce more offspring than the environment can support, and many of these offspring fail to survive and reproduce
  • Individuals with inherited traits that increase survival and reproduction in an environment tend to produce more offspring than other individuals
  • The unequal ability of individuals to survive and reproduce will lead to the accumulation of favorable traits in the population over generations

Evidence of Evolution

• Direct observation: antibiotic resistance, birds' beak size and fruit size
• Homology: similarity from common ancestry, including:
  • Anatomical homology (homologous structures)
  • Comparative embryology
  • Vestigial structures (e.g. goosebumps, wisdom teeth, ear muscle, tailbone of human)
  • Molecular homology (DNA)
• Fossil records: 3.5 billion years old
• Biogeography: the scientific study of the geographic distribution of species

Evolution of Populations

• Microevolution vs. Macroevolution
• Three mechanisms that directly cause allele frequency change:
  • Natural selection
  • Genetic drift
  • Gene flow
• Genetic variation: the differences in genes or other DNA sequences among individuals
• Sources of genetic variation: mutation, gene duplication, sexual reproduction
• Gene pool: consists of all copies of every allele in all members of the population

Hardy-Weinberg Equilibrium

• If a population is not evolving, genotype and allele frequencies will be constant from generation to generation
• Equilibrium conditions: no mutation, random mating, no natural selection, extremely large population size, no gene flow
• Hardy-Weinberg equation: p + q = 1, p^2 + 2pq + q^2 = 1

Natural Selection (continued)

• Natural selection is the only mechanism that consistently causes adaptive evolution
• Relative fitness: the contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals
• Patterns of natural selection: directional, disruptive, and stabilizing
• Sexual selection: a process in which individuals with certain heritable traits are more likely to obtain mates than other individuals of the same sex

Genetic Drift

• A process in which chance events cause allele frequencies to fluctuate unpredictably from one generation to the next
• Tends to reduce genetic variation through the random loss of alleles
• The smaller the sample, the greater the chance of random deviation from a predicted result
• Founder effect: occurs when a few individuals become isolated from a larger population
• Bottleneck effect: occurs when there is a drastic reduction in population size due to a sudden change in the environment

Gene Flow

• Consists of the movement of alleles among populations

The Origin of Species

• Speciation: the process by which one species splits into two species
• Microevolution vs. Macroevolution
• Biological species concept: defined by the populations' ability to interbreed and produce fertile offspring
• Morphological species concept: mainly depends on anatomical physical features
• Ecological species concept: defines species by its ecological niche

Reproductive Isolation

• Results when biological barriers impede members of two species from interbreeding and producing fertile offspring
• Prezygotic and postzygotic isolations
• Prezygotic isolation: blocks fertilization, including:
  • Habitat isolation
  • Temporal isolation
  • Behavioral isolation
  • Mechanical isolation
  • Gametic isolation
• Postzygotic isolation: prevents hybrid zygotes from developing into fertile offspring, including:
  • Reduced hybrid viability
  • Reduced hybrid fertility
  • Hybrid breakdown

Speciation

• Two ways of speciation: Allopatric vs. Sympatric speciation
• Allopatric speciation: populations are geographically isolated
• Sympatric speciation: speciation occurs in populations that live in the same geographic area
• Polyploidy: instant speciation, common in plants
• Sexual selection: speciation is driven by mate choice for certain traits
• Habitat differentiation: the exploitation of new habitats or resources

The History of Life on Earth

• The formation of Earth: about 4.6 billion years ago
• The oldest prokaryote fossils: 3.5 billion years ago
• The oldest eukaryote fossils: 1.8 billion years ago
• The first genetic material on Earth: likely RNA
• Radiometric dating: how fossils are dated, using radioactive isotopes
• Geologic record: Cambrian explosion, many animal phyla appear suddenly in the fossils of the Cambrian period

Endosymbiosis

• Origin of eukaryotic organisms
• Both mitochondria and plastids are thought to have descended from bacteria
• Serial endosymbiosis: the hypothesis stating that mitochondria evolved before plastids through a sequence of endosymbiotic events

Mass Extinctions

• Occur when large numbers of species rapidly become extinct worldwide
• Five mass extinctions have been documented
• Paved the way for adaptive radiation

Phylogeny and the Tree of Life

• Phylogeny: the evolutionary history of a species or group of related species
• Phylogenetic tree: represents the evolutionary history of a group of organisms
• Biological classification system:
  • Domain-Kingdom-Phylum-Class-Order-Family-Genus-Species
  • Binomial nomenclature
• Homology vs. Analogy:
  • Homology: similarity due to shared ancestry
  • Analogy: similarity due to convergent evolution

Bacteria and Archaea

• Prokaryotes: single-celled organisms that make up domains Bacteria and Archaea
• First organisms to inhabit Earth
• Most are unicellular, but some species form colonies
• Smaller and simpler than eukaryotic cells
• Shapes: cocci, bacilli, spirals
• Structures: nucleoid, plasmid, pili, fimbriae, capsule, ribosome
• Reproduce quickly by binary fission
• Genetic recombination by transformation, transduction, conjugation
• Different ways to obtain energy: phototrophs, chemotrophs, autotrophs, heterotrophs
• Two domains with prokaryotes: Bacteria and Archaea

The Commonalities and Differences between Bacteria and Archaea

• Both have a plasma membrane, no nuclear envelope
• Different composition of cell wall (peptidoglycan in bacteria, not in archaea)
• The roles of prokaryotes: pathogens, mutualistic organisms, decomposers

Antibiotic Resistance

• The cause of antibiotic resistance: natural selection, evolution of bacteria