New Perspectives from Developmental Biology
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)
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
Let y = mxa 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
can be shown to be
variable, heritable, & adaptive selectable
The end-point of ontogeny can be altered
by selection on initial parameters of development:
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
retention of juvenile characteristics into adult (reproductive) organisms
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]
Elk (Megaloceros) was largest deer ever: 14'
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
from Eocene Hyracotherium
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
adult "sea squirt"
Small advancement of age of sexual maturity
produces functional gonads in a pre-metamorphosed (motile) form
Chordates evolved from motile juvenile urochordates with a notochord
Evidence: Larvacea are a motile group of "sea squirts"
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.
(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
evolutionarily as between-species
Among salamanders (Amphibia: Caudata)
adult Eurycea resembles juvenile Typhlotriton.
Typhlomulge & Gyrinophilus (cave salamanders) are permanently aquatic.
Amphiuma (giant salamander) is a separate family (Amphiumidae).
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
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
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.
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
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
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