biology 3/11

African Cichlids in Aquaria

  • Commonality of African Cichlids:
    • Found in tropical fish aquaria.
    • Available in pet stores.
    • Favored for their hardiness and small size.
    • Exhibit diverse colorations (oranges, blues, stripes, splotches).
  • Species Radiation:
    • Over the last few million years, various species have evolved from a common ancestor.
    • This radiation in species is relatively recent on the geologic timescale.
    • Species are defined by traits such as reproductive compatibility.
  • Question of Species Differentiation:
    • Why are there multiple species when they share a common ancestry?
    • Answer: Sympatric speciation.
    • All species inhabit the same lake and live near one another.
    • They have become reproductively isolated.
  • Female Preferences:
    • Female cichlids exhibit strong inherited preferences for specific male colorations and patterns.
    • e.g., a female may prefer yellow-striped males.
    • This preference leads to reproductive isolation as the favored coloration continues to be passed to offspring.
    • Inherited preferences contribute to narrowing reproductive choices.
    • This can result in inbreeding among populations, but preference effects can lead to the formation of isolated groups.
  • Role of Sexual Selection:
    • Sexual selection is believed to be a dominant force in the creation of new species from previously unvaried populations.
  • Complex Interactions:
    • Male coloration and female preferences are linked to other behaviors (territoriality and nesting), further complicating speciation.
  • Other Examples of Sympatric Speciation:
    • Discussed polyploidy in plants as another example.
  • Mechanisms of Speciation:
    • Major causes of sympatric speciation include:
    • Ecological Speciation
    • Sexual Selection
    • Polyploidy
    • These differ fundamentally from allopatric speciation, which usually involves physical separation.

Tree of Life and Luca

  • Transition to Discussing Living Organisms:
    • Moving focus from conceptual mechanisms of evolution to different categories of living organisms.
  • Tree of Life Overview:
    • A genetic tree that depicts relationships among all living things excluding viruses.
    • All living organisms descended from a common ancestor known as Luca (Last Universal Common Ancestor).
  • Characteristics of Luca:
    • Presumed to have existed around 4 billion years ago, possessing key characteristics shared by all cellular lifeforms.
    • Key components include a common genetic code, metabolic processes, and cellular structure across all life forms.
  • Evolution of Classification:
    • Original interpretations of the tree of life have evolved with new scientific discoveries and genomic exploration.
    • Currently, it is understood that all cellular life forms can be grouped into two primary domains:
    • Bacteria: Diverse group including many pathogenic and environmental species.
    • Archaea: Similar to bacteria, but with distinct molecular biology and genetics.
  • Eukaryotes:
    • Eukaryotes appear around 2 billion years ago, distinct from bacteria and archaea, marked by cells containing nuclei and organelles.
    • Multicellular eukaryotes are classified within metazoans (animals).
  • Undiscovered Diversity:
    • Many undiscovered species exist across all three domains of cellular life; significant diversity is found mainly in bacteria.

Viruses and Their Role in Cellular Life

  • Viruses Not Included in the Tree of Life:
    • Viruses are not classified as cellular organisms and their exact relationship to cellular life is still uncertain.
    • Study of viruses is primarily focused on pathogenic species due to medical relevance.
  • Prevalence of Viruses:
    • Viruses exist abundantly in various environments, including the human body and natural ecosystems.
    • Each milliliter of seawater contains approximately 10-15 million bacteriophages (viruses that infect bacteria).
    • Global estimates suggest a staggering total of $10^{31}$ viral particles present on Earth.
  • Biomass Contextualization:
    • While viruses are the most abundant entities, they contribute less to biomass compared to cellular microbes.
    • Biomass refers to the total mass of living matter, generally excluding water.
  • Viruses as Regulators of Life:
    • Although often viewed negatively, viruses play vital roles in regulating cellular life and ecosystems.
    • Viruses can control population dynamics and interactions within various microbiomes.
  • Understanding Viruses:
    • A virus is defined as an infectious particle made up of genes encapsulated in proteins.
    • Classified as intracellular parasites due to their dependency on host cells for replication and survival.
  • Virus Structure:
    • Composed of two fundamental components:
    • Genetic Material: Can include DNA or RNA. Different viruses exhibit variations (e.g., single vs. double-stranded nucleic acids).
    • Protein Coat (Capsid): Protects the genetic material; made up of structural proteins.
    • Some viruses also possess a lipid envelope derived from host cellular materials, containing embedded proteins.
  • Viral Life Cycle:
    • Steps in the viral replication cycle:
    1. Attachment: Virus binds to host cell.
    2. Entry: Genetic material enters the host cell.
    3. Genomic Expression: Host cell reads viral genome to produce viral components.
    4. Replication: Copying of viral genetic material.
    5. Assembly: New viral particles are assembled.
    6. Release: New viral particles exit the host to infect additional cells.
  • Viral Morphologies:
    • Viruses can vary significantly in shape and structure (e.g., filamentous, icosahedral).
    • Electron microscopy reveals intricate details of viral morphology and structures.
  • Examples of Viruses:
    • Poliovirus: Infects human nerve cells; can cause paralysis.
    • More examples and details about various human-infecting viruses will be discussed in future sessions.