evo bio CH 16-Evolution and development

• Mutations are not the only source for new characters

  ↓

  Natural selection works on phenotypes no matter what the source
• Homeotic Transformations


  Different phenotypes due to developmental change

  • Can be source for evolution
  • Example: extra vertebra
• William Bateson (1894) linked transformations to genetics but did not understand mechanism
• Ontogeny


  Developmental stages of an organism
• Baer’s law: general traits develop before specialized traits


  Ernst Haeckel: ontogeny recapitulates phylogeny
• Heterochrony


  time in the developmental process when a trait is first expressed
• Recapitulation


  an ancestral character that appears earlier in development
• Paedomorphosis


  the retention of juvenile traits in adults of a species.
• The beginning of eve-devo started with the discovery of Hox genes

  • Homeotic selector genes
  • Pattern specific body structures
  • Different genes activated start pathways for different outcomes
  • Mutations in hox genes may have large, immediate effects
• remember for evo-devo

  • Life develops from a single cell, even multicellular organisms
  • Early in development, cells are totipotent
• remember for evo-devo

  • Cells of an organism all contain the same set of genes
  • Differences depend on which genes are activated
  • Enhancers and silencers regulate genes
• Evolution of Gene Regulation - Regulatory modularity

  – Most genes have a number of enhancers

  – Changes in enhancers change expression

  – DNA sequences bind transcription factors

  Level of expression also has evolutionary consequences
• Temporal Co-option of Genes


  (regulatory gene co-opted to promote novel morphology)

  involves genes being utilized at different developmental stages or time points than their original intended role.

  - This can occur when a gene that was originally active during a specific developmental stage is recruited to function at a different stage.
• Spatial Co-Option:

  The process where a gene is utilized in a different spatial location compared to its original function. - This can occur during evolution when a gene that was originally expressed in one specific tissue or organ is recruited to perform a new function in a different tissue or organ.
• Allometry – differential growth rate


  Longer development could mean addition of characters due to added gene expression
• allometry ... longer development = bigger characters
• Developmental Constraints

  1. Physical – materials of structure do not permit
  2. Selective – some things are disadvantageous
  3. Genetic – variation may not be present
  4. Developmental – missing necessary building blocks
• Gene duplication = establishment of multiple copies of a gene


  Fates of duplicate genes

  • Lost due to natural selection
  • Evolve into functionless pseudogene
  • Be expressed (known as a paralog) and possibly create new pathways
• An example of temporal co-option is the Hox genes. These genes are crucial for embryonic development, specifying the body plan along the anterior-posterior axis. However, some Hox genes have been temporally co-opted to regulate processes in adult tissues beyond embryonic development.
• An example of spatial co-option is the evolution of bird feathers. Feathers initially evolved for insulation and display purposes but were spatially co-opted for flight, providing a new function in a different context.
• Paedomorphosis example 


  The axolotl is a salamander species. Unlike other salamanders that undergo metamorphosis and transition from larval to adult forms, axolotls retain their gilled appearance and aquatic lifestyle into adulthood. This retention of juvenile characteristics allows them to reach sexual maturity without undergoing a complete metamorphosis.