evo bio CH 18-Major Evolutionary Transitions

• Major Evolutionary Transitions

  • First cells

  • Eukaryotic cells

  • Sexual reproduction

  • Multicellular organisms

  • Developmental complexity

  • Evolution of groups and societies
• Endosymbiotic Theory (Symbiogenesis)


  proposes that eukaryotic cells (cells with a nucleus) evolved from a symbiotic relationship between different prokaryotic cells (cells without a nucleus).
• Endosymbiosis


  a biological process where one organism lives inside another organism, forming a mutually beneficial relationship. In the context of the Endosymbiotic Theory, endosymbiosis plays a crucial role in the evolution of eukaryotic cells.
• Aspects of Endosymbiosis

  • Mitochondria and Chloroplasts have their own DNA, replicate independently

  • Some genes have been exchanged with nucleus (lateral gene transfer)

  • Mitochondria from Protobacterium
  • Chloroplast from Cyanobacterium

  • Symbiosis of mitochondria came first, lineages split before chloroplast symbiosis

  
• Mitochondria are believed to have evolved from an ancient aerobic bacterium that was engulfed by a primitive eukaryotic cell through endosymbiosis.


  Through the symbiotic relationship, the cyanobacterium evolved into chloroplasts, maintaining its own DNA and membrane structures within the eukaryotic cell.
• A few eukaryotes lack mitochondria


  • Evidence of loss since ancestor

  • Mitochondrial genes transferred to nucleus

  • Example Giardia (protozoan)
• Defining Characters of Eukaryotes

  • Nucleus

  • Cytoskeleton

  • Both evolved in ancestral Eukaryote

  • Cytoskeleton is hardest to explain
• Evolution of the Nucleus

  • May have evolved from Archaeal ancestors, not bacterial

  • Probably frequent organelle to nucleus migration

  
• models for evolution of the Three domains

  (a) nucleus first hypothesis

  (b) mitochondrion first hypothesis

  (c) eukaryote first hypothesis
• Benefits of a nucleus (favored by natural selection)

  • Excludes most (but not all) viruses

  • Functions of DNA isolated

  • Replication
  • Transcription
  • RNA processing

  • Isolates translation to cytoplasm
• Evolution of Cytoskeleton

  • Cytoskeleton genes are 40-50% similar to some genes in prokaryotes

  • Some prokaryotes have cytomotive filaments

  • Structure could be advantageous – selected for
• results of phylogenies based on

  the three domain hypothesis or the eocyte hypothesis
• The Three Domain Hypothesis

  categorizes all life forms into three domains: Bacteria, Archaea, and Eukarya.

  It suggests that Eukarya, which includes all eukaryotic organisms, share a more recent common ancestor with Archaea than with Bacteria.
• The Eocyte Hypothesis 


  proposes a different evolutionary relationship, suggesting that Eukarya emerged from within Archaea.

  It specifically proposes that the eukaryotic cell nucleus originated from within an archaeal cell through a process of endosymbiosis.
• results of phylogenies based on

  what hypotheses

  A) three domain

  B) eocyte

  
• How to identify fossils of single cell eukaryotes

  • Biomarkers

  • Cytoskeleton gives uniform shape to individuals
  • Cell wall may be observable
  • Spines and ornamentation
  • Eukaryotic cells mostly larger than prokaryotic
• Evolution of Multicellularity

  Unicellular life may have persisted for 3 billion years

  Multicelluarity evolved convergently more than once

  
• Two Routes to Multicellularity

  • Staying together 

  • Dividing cells stayed together
  • Each cell is a clone

  • Coming together

  Free living cells joined

  Problem is genetic similarity
• Two Routes to Multicellularity

  identify the models

  A) coming

  B) staying

  
• Multicellularity


  a significant evolutionary transition that involves the development of organisms composed of multiple cells working together in a coordinated manner
• multicellularity: Staying Together – evidence from yeast


  • Yeast reproduce and then forced to remain in proximity show “snowflake clustering”

  • Cells then remain attached after cell divisions

  
• multicellularity: Coming together – evidence from slime mold


  single-celled organisms can exhibit multicellular behaviors under specific conditions. Through their ability to come together, differentiate, and cooperate as a multicellular entity, slime molds offer valuable evidence supporting the evolutionary significance of multicellularity
• types of multicellularity

  Aggregations of cells that work together – like slime molds

  Multicellular individuals

  • Cells cannot persist separately

  • Indivisible wholes

  • Differentiation of function

  • Somatic and germ cells
• once multicellular, go from solitary to group living

  • Benefits

  • Foraging efficiency

  • Safety

  • Can have positive effects in fitness

  • Requirements Degree of sociality
• Group living

  Evolved independently many times

  • Example: Theridiidae spiders

  • Some form group cobwebs, Many form individual webs
• Many Eyes Theory


  Protection from predators by groups
• Costs of group living

  • Allows for cheaters

  • Increases competition for limited resources

  • Parasite & disease transmission