General Biology: Evolution, Viruses, Prokaryotes, and Eukaryotes – Structured Study Notes

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Chapter 19 – Descent with Modification

Introduction to Evolution

Evolution describes the unity and diversity of life, explaining how adaptations arise and how current species are descended from ancestral ones.

Descent with modification: The process by which species change over time, giving rise to new species.

Historical Context

Aristotle: Proposed fixed species and the scala naturae (natural ladder of complexity).
Linnaeus: Developed binomial nomenclature and classified organisms by creation.
Cuvier: Advocated catastrophism, suggesting extinctions occurred via sudden events.
Hutton & Lyell: Introduced gradualism and uniformitarianism, proposing that Earth changes slowly over time.
Lamarck: Suggested use/disuse and inheritance of acquired traits (now known to be incorrect, but influential).

Darwin's Research

Voyage of the Beagle (1831–36): Studied fossils, finches, and adaptations in the Galápagos Islands.
Concluded that Earth is ancient.
Influenced by Malthus: Competition for limited resources shapes populations.

Natural Selection

Observations: Heritable variation and overproduction of offspring.
Inferences: Survival of the fittest leads to accumulation of adaptations.
Populations evolve, not individuals.
Traits depend on the environment.

Evidence for Evolution

Direct Observation: Examples include MRSA (antibiotic resistance) and soapberry bugs.
Homology: Anatomical, embryological, molecular, and vestigial similarities among species.
Convergent Evolution: Analogous traits arise independently (e.g., gliders vs. squirrels).
Fossil Record: Transitional forms show evolutionary links (e.g., whale ancestors).

Chapter 21 – Evolution of Populations

Population Genetics

Populations, not individuals, evolve. Microevolution refers to changes in allele frequencies within a population.

Microevolution: Small-scale changes in gene frequencies.

Variation Sources

Mutation: Random changes in DNA sequence.
Gene duplication: Can create new functions.
Sexual reproduction: Increases variation via recombination, assortment, and fertilization.
Discrete vs. quantitative traits: Traits may be categorical or continuous.

Gene Pool & Hardy-Weinberg Principle

Gene pool: All alleles in a population.
Hardy-Weinberg equation:

Equilibrium conditions: No mutation, random mating, no selection, large population, no gene flow.

Mechanisms of Change

Natural selection: Leads to adaptive evolution.
Genetic drift: Random changes in allele frequencies, including founder effect (new colony) and bottleneck effect (population shrinkage).
Gene flow: Movement of alleles between populations.

Selection & Adaptation

Relative fitness: Contribution to the next generation.
Modes of selection: Directional, disruptive, and stabilizing selection.
Balancing selection: Heterozygote advantage and frequency-dependent selection.
Sexual selection: Leads to sexual dimorphism.

Chapter 17 – Viruses

Overview

Viruses are acellular particles composed of nucleic acid and a protein coat. They cannot metabolize or reproduce outside a host cell.

Acellular: Not made of cells.
No metabolism or independent reproduction.

Structure

Genome: DNA or RNA, single-stranded (ss) or double-stranded (ds).
Capsid: Protein shell made of capsomeres.
Envelope: Host-derived membrane with glycoproteins (in some viruses).
Bacteriophages: Infect bacteria; often have complex tails.

Replication

Entry → replication → assembly → exit
Lytic cycle: Virus replicates and kills host cell.
Lysogenic cycle: Viral DNA (prophage) integrates into host genome and can reactivate.

Defenses

Host defenses include receptor mutations, restriction enzymes, and CRISPR-Cas systems.

Animal Viruses

Classified by genome type and presence of envelope.
Enveloped RNA viruses: Bind to host via glycoproteins.
Retroviruses (e.g., HIV): RNA genome is reverse-transcribed to DNA, which integrates as a provirus.

Disease & Control

Vaccines: Prevent infection.
Antibiotics: Ineffective against viruses.
Antivirals: Slow viral replication.
Emerging viruses: Examples include Ebola, Zika, H1N1.
Plant viruses: Spread via horizontal or vertical transmission.
Prions: Infectious proteins causing brain degeneration (e.g., Mad cow disease).

Chapter 24 – Early Life & Prokaryotes

Early Earth & First Cells

Life began on Earth approximately 4.6 billion years ago, with the first fossils dating to 3.5 billion years ago. Prokaryotes are the earliest and most ancient forms of life.

Earth age: 4.6 billion years; first fossils: 3.5 billion years.
Prokaryotes: Only life for over 2 billion years.
Origin steps: Abiotic molecules → macromolecules → protocells → RNA world hypothesis.

Fossil & Metabolic Evidence

Cyanobacteria: Produced oxygen, leading to the Great Oxidation Event.
Aerobes evolved; anaerobes persisted.

Structure

Shapes: Cocci, bacilli, spirals.
Cell walls: Peptidoglycan (bacteria), none (archaea).
Gram-positive/negative: Differences in cell wall structure.
Capsule, endospores, fimbriae, pili: Specialized structures.
Movement: Taxis and flagella.

Internal Organization

No organelles; circular chromosome and plasmids.
Ribosomes: Target for antibiotics.

Metabolism & Ecology

Photoautotrophs, chemoautotrophs, heterotrophs.
O2 relations: Obligate aerobes/anaerobes, facultative anaerobes.
Nitrogen fixation: Conversion of N2 to NH3.
Cooperation: Biofilms, symbiosis (e.g., Anabaena).

Reproduction & Diversity

Binary fission, fast mutation rates, gene exchange (transformation, transduction, conjugation).
Horizontal gene transfer complicates phylogeny.

Roles

Decomposers, symbionts, pathogens, biotechnology (CRISPR, bioremediation).

Chapter 25 – Origin & Diversification of Eukaryotes

Rise of Eukaryotes

Eukaryotes originated about 1.8 billion years ago, with evidence from chemical fossils dating to 2.7 billion years ago. They possess organelles and a cytoskeleton.

1.8 billion years ago: Fossil evidence for eukaryotes.
Organelles and cytoskeleton are defining features.
Endosymbiotic theory: Mitochondria and plastids originated from engulfed prokaryotes.
Own DNA (circular), double membrane, bacterial-like ribosomes, binary fission.
Secondary endosymbiosis: Algal cell engulfed, leading to red/green plastids.

Fossils & Innovations

Red algae: 1.2 billion years ago (oldest eukaryote fossil).
Multicellularity and photosynthesis are major evolutionary steps.

Multicellularity Evolution

Started as colonies; cell differentiation (e.g., Volvox).

Animal Origins

Choanoflagellates: Closest relatives to animals; share adhesion/signaling genes and have collar cell morphology like sponges.

Takeaways

Prokaryotes are ancient, diverse, and vital to ecosystems.
Eukaryotes arose via endosymbiosis; multicellularity evolved multiple times.
Symbiosis and cooperation are core themes in the evolution of life.