Viruses and Evolution: Study Notes for General Biology 19

Chapter 19: Viruses

Virus Structure and Characteristics

Viruses are infectious particles that exist at the boundary between living and non-living matter. They are not cells and require a host cell to reproduce and carry out metabolism.

• Virus: An infectious particle consisting of genes (either DNA or RNA) packaged in a protein coat called a capsid.

• Viruses are obligate intracellular parasites—they cannot reproduce or carry out metabolism outside a host cell.

• Viruses are considered to lead a "borrowed life," existing between life-forms and chemicals.

Examples of Virus Structure:

• Tobacco mosaic virus: Helical capsid with RNA.

• Adenovirus: Icosahedral capsid with DNA.

• Influenza virus: Spherical, enveloped, with RNA.

• Bacteriophage T4: Complex structure with head, tail, and tail fibers, containing DNA.

Viruses as Pathogens

Viruses and prions are formidable pathogens in animals and plants, causing a variety of diseases.

• In animals, viral infections may:

  • Damage or kill cells by causing the release of hydrolytic enzymes from lysosomes.

  • Cause infected cells to produce toxins that lead to disease symptoms.

  • Have molecular components, such as envelope proteins, that are toxic.

• In plants, viruses spread disease by two major routes:

  • Horizontal transmission: Virus enters through damaged cell walls.

  • Vertical transmission: Virus is inherited from a parent.

Replicative Cycles of Phages

Bacteriophages (or phages) are viruses that infect bacteria. They have two alternative reproductive mechanisms:

• Lytic cycle: Produces new phages and lyses (breaks open) the host's cell wall, releasing progeny viruses.

• Lysogenic cycle: Replicates the phage genome without destroying the host. The viral DNA integrates into the bacterial chromosome as a prophage.

• Phages that use both cycles are called temperate phages.

Lytic Cycle Steps:

1. Attachment

2. Entry of phage DNA and degradation of host DNA

3. Synthesis of viral genomes and proteins

4. Self-assembly

5. Release (lysis of host cell)

Lysogenic Cycle: The phage DNA integrates into the host genome and is replicated along with it without killing the host. Environmental triggers can induce the lytic cycle.

Prions: Proteins as Infectious Agents

Prions are infectious proteins that cause degenerative brain diseases in animals and humans.

• Examples: Scrapie in sheep, mad cow disease, Creutzfeldt-Jakob disease (CJD) in humans.

• Prions are misfolded proteins that can convert normal proteins into the misfolded prion form.

• They are virtually indestructible, act slowly, and can be transmitted in food.

• Prions may also be involved in diseases such as Alzheimer's and Parkinson's disease.

Chapter 22: Descent with Modification: A Darwinian View of Life

Patterns and Processes of Evolution

Evolution explains both the unity and diversity of life. It can be viewed as a pattern (revealed by scientific data) and a process (mechanisms causing change).

• Descent with modification: Species accumulate differences from their ancestors as they adapt to different environments over generations.

• Shared ancestry results in shared characteristics; accumulation of differences leads to diversity.

Historical Contributions to Evolutionary Theory

Several key figures contributed to the development of evolutionary theory:

• Aristotle: Species are fixed and arranged on a scale of increasing complexity (scala naturae).

• Carolus Linnaeus: Developed a nested classification system and binomial nomenclature (e.g., Homo sapiens).

• James Hutton: Proposed gradual formation of Earth's geologic features.

• Charles Lyell: Proposed that the same geologic processes operate today as in the past.

• Georges Cuvier: Developed paleontology; observed that fossils in older strata are less similar to current organisms.

• Jean-Baptiste de Lamarck: Proposed use and disuse and inheritance of acquired characteristics (not supported by evidence).

• Charles Darwin: Proposed natural selection as the mechanism of evolution.

• Alfred Russel Wallace: Independently conceived a similar theory of natural selection.

Natural Selection and Adaptation

Darwin's observations during the voyage of the Beagle led to the theory of natural selection.

• Adaptations: Inherited characteristics that enhance survival and reproduction in specific environments.

• Natural selection: Individuals with advantageous traits survive and reproduce at higher rates.

• Populations, not individuals, evolve over time.

• Natural selection acts only on heritable traits that vary in a population.

Example: The peppered moth's coloration changed in response to industrial pollution, demonstrating natural selection in action.

Lamarckian vs. Darwinian Views

• Lamarck: Traits acquired during an organism's lifetime are inherited by offspring (e.g., giraffes stretching their necks).

• Darwin: Variation exists in populations; individuals with longer necks survive and reproduce, passing on the trait.

Evidence for Evolution

Four types of data document the pattern of evolution:

• Direct observations

• Homology (similarity due to shared ancestry)

• The fossil record

• Biogeography (geographic distribution of species)

Chapter 23: The Evolution of Populations

Mechanisms of Evolution

Evolution in populations is driven by several mechanisms:

• Natural selection

• Genetic drift (random changes in allele frequencies)

• Gene flow (movement of alleles between populations)

• Mutation (source of genetic variation)

The Hardy-Weinberg Equation

The Hardy-Weinberg equation describes the expected genetic makeup for a population that is not evolving at a particular locus.

• If observed genetic makeup differs from Hardy-Weinberg expectations, the population may be evolving.

Equation:

$p^2 + 2pq + q^2 = 1$

where $p$ and $q$ are the frequencies of two alleles at a locus.

Conditions for Hardy-Weinberg Equilibrium:

Condition	Consequence if Condition Does Not Hold
No mutations	The gene pool is modified if mutations occur or if genes are deleted or duplicated.
Random mating	Random mixing of gametes does not occur and genotype frequencies change.
No natural selection	Allele frequencies change when individuals with different genotypes show differences in survival or reproduction.
Extremely large population size	In small populations, allele frequencies fluctuate by chance over time (genetic drift).
No gene flow	By moving alleles into or out of populations, gene flow can alter allele frequencies.

Chapter 24: The Origin of Species

The Biological Species Concept

A species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring, but do not produce viable, fertile offspring with members of other such groups.

• Gene flow between populations holds a species together genetically.

Reproductive Isolation

Reproductive isolation results when biological barriers prevent members of two species from interbreeding and producing viable, fertile offspring. These barriers limit the formation of hybrids (offspring from interspecific mating).

• Prezygotic barriers: Block fertilization from occurring by:

  • Impeding different species from attempting to mate

  • Preventing the successful completion of mating

  • Hindering fertilization if mating is successful

• Postzygotic barriers: Prevent hybrid zygotes from developing into viable, fertile adults by:

  • Reduced hybrid viability

  • Reduced hybrid fertility

  • Hybrid breakdown

Speciation

Speciation can occur with or without geographic separation:

• Allopatric speciation: Populations are geographically isolated.

• Sympatric speciation: Populations are not geographically isolated.

Hybrid zones may form where the habitats of closely related species meet, sometimes resulting in the formation of hybrids.

Summary Table: Virus Types and Examples

Virus Type	Genetic Material	Structure	Example
Tobacco mosaic virus	RNA	Helical	Plant virus
Adenovirus	DNA	Icosahedral	Respiratory infections
Influenza virus	RNA	Spherical, enveloped	Flu
Bacteriophage T4	DNA	Complex (head, tail, fibers)	Infects bacteria

Additional info: These notes synthesize content from lecture slides and textbook images, expanding on key concepts and providing academic context for exam preparation.