1. Haeckel and Heterochrony
    1. Biogenetic law: Ontogeny recapitulates phylogeny
      1. Phylotypic stage ( http://zygote.swarthmore.edu/evo5.html )
      2. Heterochrony
        1. Change in the order of the developmental appearance of organs. This idea developed as a model for the cause of exceptions to the biogenetic law
          1. Siren
          2. Human
            1. http://www.mun.ca/biology/scarr/Neoteny_in_humans.htm
  2. Gould (following de Beer)
    1. Perturbations in offset times
    2. Changes in rates
    3. Changes in timings
    4. So for Haeckel, heterochrony was the change in the ordering of the development of two or more traits in a descendent relative to its ancestor but in more modern (until very recently) uses, this has changed to the change in the rate and timing of development of an organ in a descendent relative to its ancestor (the growth relative a second organ is not relevant in the more modern usage).

  1. Trajectory model (see above image)
    1. Processes
      1. Reduction in offset time (hypomorphosis)
      2. delay of offset time (hypermorphosis)
      3. Acceleration of rate (acceleration)
      4. Retardation of rate (deceleration)
      5. Reduction of onset time (Pre-displacement)
      6. delay of onset time (Post-displacement)
    2. Patterns
      1. Paedomorphosis
      2. Paramorphosis
    3. Other terms
      1. Neoteny. originally the retention of larval traits in a reproductively mature adult, generally by a slowing down or delay in somatic development,  so equal to paedomorphosis. Since has been limited to more and more restrictive definitions including one equivalent  deceleration. Best to abandon this term.
      2. Progenesis. A larva becoming reproductively mature generally by acceleration of gonad maturity. Has been defined variously but including the equivalent of hypomorphosis Ð best to abandon this.
    4. Important
      1. Traits are paedo or para- morphic - not organsisms!
      2. The phrase "humans are paedomorphic apes" implies that humans have a suite of conspicuous traits that are paedomorphic
      3. A phylogeny is critical to determine the direction of the perturbation!
        1. Urochordates macclade file
        1. This tree shows the distribution of metamorphosis to a sessile adult (note that this metamorphosis differs from that in amphibians, which metamorphose from a swimming animal to a 4-legged animal). From this distribution, we can infer that that metamorphosis is derived and the primitive trait is to stay a swimming animal throughout life. So Garstang's model of vertebrate orgins, which suggested that vertebrates are paedomorphic tunicates, is wrong. Tunicates are peramorphic chordates.
      4. Offset time is key and offset time is determined by the cessation of growth of the trait (not reproductive maturity, etc.)
    5. Event-pairing model
      1. Over the last 10 years, more biologists have returned to Haeckel's idea that heterochrony is a changing in the timing of development of one organ relative to another - or what is now called sequence heterochrony. One way to quantify this and analyze this is with the event-pairing method.
      2. We do this by comparing the timing of every pair of developmental events in some list and coding the event pair
        1. 0 if event A occurs before event B
        2. 1 if event A occurs at the same time as event B
        3. 2 if event A occurs after event B
      3. We then use MacClade to look at the distribution of these event timing codes on a tree. Here are the methods and some results for amniotes in a nutshell: