New Perspectives from Developmental Biology

    Previously: Evolution is the transformation of adult morphologies
        We visualize fish "morphing" to lungfish
       E.g., Reorganization of biomaterials (e.g., changes in protein amino acid sequences)

    Now: Evolution is the modification of developmental (embryonic) ontogeny
        Developmental programs serve as "blueprints"
       E.g., alteration of gene regulation (where / when / how much gene product?)

Can small, quantitative changes lead to large, qualitative changes?

Same program with different parameters can generate unexpected results
    TURTLES program draws pictures: square  triangle  five-pointed star
    Dawkins "Blind Watchmaker"
       Nine-parameter pattern program produces "biomorphs" resembling living organisms
    Raup (1962) four-parameter model for gastropod snail shell coiling
        "Real" snails occupy one corner of 4-D space
        Many theoretically possible patterns haven't evolved (yet)
    D'Arcy Thompson (1926) "coordinate transformation"
       One fish: pufferfish  sunfish by positive deformation of tail region
            (two fish)  (red fish)  (blue fish)
    See also Dawkins' "The Blind Watchmaker" program

Allometric Equation:
    Let   y = mx^a   where x & y are trait measures
        form of equation implies non-linear growth relationship
        if x is a general size measure and y = 0 when x = 0:
    Then ln(y) = a ln(x) and a = allometric coefficient
        proportional change of x with respect to y
            if a = 1 then y is proportional to x : no allometry, y is isometric wrt x
            if a > 1 then y grows faster than x (positive allometry)
                Ex.: in humans, legs grow faster than torso
            if a < 1 then y grows slower than x (negative allometry)
                Ex.: in humans, head grows slower than torso

    The allometric coefficient can be shown to be
        variable, heritable, & adaptive selectable

Theory of Heterochrony:
    The end-point of ontogeny can be altered
      by selection on initial parameters of development:
          allometric relationship
          relative timing: change in onset & offset of developmental events

  Four processes are recognized:
      Acceleration: relative growth rate changes faster (a increases)
      Neoteny: relative growth rate change more slowly (a decreases)
      Hypermorphosis: relative growth rate is prolonged
      Progenesis: relative growth rate is fore-shortened

        Progenesis and / or Neoteny produce Paedomorphosis ("child form"):
          retention of juvenile characteristics into adult (reproductive) organisms

        Hypermorphosis and / or Acceleration produce Peramorphosis ("elder form"):
          exaggeration of adult characteristics in larger organisms

Hypermorphosis can be produced by positive allometry & delayed offset
    What are the evolutionary consequences of selection to increase body size?
        [Remember that Cope's Rule says this is a common trend]

    Ex.: Irish Elk (Megaloceros) was largest deer ever: 14' antler rack
       Previously: a case of Orthogenesis:
            An evolutionary trend continued past the point of functionality
       Now: Antlers are positively allometric wrt body size
            Irish Elk antlers are expected size for a deer of its (large) size
            At other extreme, miniature neotropical deer have "spike" antlers

    Ex.: Horse evolution from Eocene Hyracotherium ("Eohippus")  Recent Equus
       hypsodont dentition is positively allometric wrt skull length
       facial portion of skull is positively allometric wrt body size

Progenesis can be produced by negative allometry & early offset
    What are the consequences of selection on age / size of sexual maturity?
        In many species, time of metamorphosis is size-dependant

    Ex.: Ascidians (Urochordata) metamorphose from a motile juvenile sessile adult "sea squirt"
        Small advancement of age of sexual maturity
            produces functional gonads in a pre-metamorphosed (motile) form
        Garstang Hypothesis:
            Chordates evolved from motile juvenile urochordates with a notochord
            Evidence: Larvacea are a motile group of "sea squirts"

    Ex.: Thorius salamanders are smallest vertebrates (adults are ~1 cm long)
        Limb structures are variable, new arrangements as adaptations are possible
        Skull structure simplified, roofing bones greatly reduced

        In many species, sexual maturity is achieved at some definite age;
            somatic growth rate may be limited by environmental conditions.

    Ex.: Ambystoma (tiger salamander) has external gills in aquatic juvenile stage,
        Metamorphoses into terrestrial adult with internal gills.
        Metamorphosis is triggered by thyroid growth hormone (thyroxin)
            "Axolotl" (Aztec) lives in high Mexican lakes (cold & low [I₂])
                 Achieves sexual maturity as aquatic form with external gills.
                 Axolotls fed thyroid extract metamorphose into Ambystoma same species

        Developmental shift may be "fixed" evolutionarily as between-species difference
            Among salamanders (Amphibia: Caudata)
                adult Eurycea resembles juvenile Typhlotriton.
                Typhlomulge & Gyrinophilus (cave salamanders) are permanently aquatic.
                Amphiuma (giant salamander) is a separate family (Amphiumidae).

    Ex.: Homo appears to be a neotenic ape.
        Mandibular portion of human skull grows slowly wrt cranium (negative allometry)
         Juvenile apes & humans resemble each other
                => Relatively enlarged cranium of Homo is a retained juvenile trait

Acceleration can be produced by acceleration of somatic growth wrt size
    In many species, pattern formation is dependant on cytodifferentiation:
        where & when do mitoses occur?

    Ex.: In floral bud formation, petal primordia are separated by intercalary cells
            If mitoses in intercalary cells are slow lobate flowers
                                                                    fast  sympetalous floral tubes
            Floral morphology will determine pollinator type:
                bees on lobate flowers versus hummingbirds on sympetalous floral tubes.

    Ex.: Digit formation in vertebrates
        Distinct digits are the result of programmed cell death (apoptosis) of intercallary cells
        Terrestrial species have accelerated rate wrt aquatic species
       e.g., chicken versus duck feet

Can systemic mutations produce "Hopeful Monsters"? (Goldschmidt 1940)
    Single mutations affect entire developmental systems
      dachshund  (German: "badger dog") is an achondroplastic beagle
            Similar mutations are found in cattle & Ancon sheep
            mutation is preserved because dachshund "fills a niche"
    Modification of leg & ankle bones in Gallus
       Hampé experiment restored 250 MY-old pattern

Homeotic mutations in Drosophila
    Cuticular structures (legs, wings, antennae, etc.) are formed from imaginal disks
        Imaginal disks are paired, undifferentiated structures in larvae
        Metamorphosis of disks is under local hormonal control
    Homeotic mutants cause transdetermination of imaginal disk fates
      antennapedia: antenna disks transdetermined to legs
      bithorax: posterior segment of thorax transdetermined to anterior segment
         Dipteran (Di + ptera = two-wing) flies grow two pairs of wings (halteres   wings)
                [Could this be the origin of Diptera from "tetraptera" ancestors?]

All text material © 2010 by Steven M. Carr