Lecture 21: Macroevolution
Last class:
1) Peramorphosis: add’n of extra stages
a) Hypermorphosis: dev’t extended
from  to 1
b) Predisplacement:
y starts growing early rel. to x in descendent
vs. ancestor
log y
1

1

log x
Descendant
Ancestor
- same allometry
(relationship of y to x)
- early start of y means
greater y (not x) at maturity
c) Acceleration
• faster growth of y rel. to x in descendent vs.
ancestor
Descendant
1
log y

Ancestor
Larger (or more dev’d)
y (not x) at maturity

log x
2) Paedomorphosis
• retention of juvenile features in adult
A) Progenesis
B) Neoteny
C) Postdisplacement
a) Progenesis
• dev’t stops early

log y
1

log x
Ancestor
Descendant
Smaller y, smaller x at
maturity vs. ancestor
- Allometry unchanged
- Compare: hypermorphosis
b) Neoteny
• slower rate of growth of y rel. to x in
descendant vs ancestor

log y
1

log x
Ancestor
Descendant
- Smaller or less
developed y rel. to x
at maturity
c) Postdisplacement
log y
• y starts growing late rel. to x in descendant vs.
ancestor
 1
log x

Ancestor
1
Descendant
- same allometry
- late start of y means
smaller y (not x) at
maturity
Paedomorphosis Peramorphosis
underdevelopment
Rate
overdevelopment
Slower
Faster
(Neoteny)
(Acceleration)
Onset Time
Later
Earlier
(Postdisplacement) (Predisplacement)
Offset Time
Earlier
Later
(Progenesis)
(Hypermorphism)
Evolutionary Significance of
Heterochrony?
1. Large changes in phenotypes easily accomplished
- mutations at one or several loci may be involved
2. Likely important in speciation
- gene pools w diff. heterochronic mutations
 repro. isol’n
3. May release lineages from phylogenetic
constraints
- e.g. paedomorphosis: descendant no longer
passes through the same develop’l stages as
ancestor
- can “free” the sp. from the constraint
imposed by that structure
- only affects existing structures.
Genetic Basis of Heterochrony
Homeotic (Hox) genes:
• 1st discovered in Drosophila spp.
• involved in gross alterations in phenotype
• Affect develop’t of cuticular structures from
imaginal disks
• in all animal phyla
• share # of common
characteristics
• e.g. antennapedia
Hox Genes
1. organized in gene complexes
- probably involves gene duplication
2. spatial & temporal collinearity:
- 3' end expressed anterior; 5' end expressed
posterior
- 3' end expressed earlier in dev’t than 5' end
Hox Genes cont’d
3. contain highly-conserved 180 bp region
- involved in binding
Hox genes are regulators - control timing and
expression of other genes
e.g. Ubx (ultrabithorax) in Drosophila:
controls expression of 85 - 170 genes
Type of Heterochronic Process?
Axolotl
vs. Tiger Salamander
• failure to metamorphose
• [thyroxine] : can be exp’tally induced
• external gills in adult (juvenile morphology)
So what’s going on?
• not postdisplacement : age at maturity ≈ other
salamanders
• not progenesis : body size at maturity ≈ other
salamanders (progenesis tiny adult)
• Neoteny: somatic dev’t slows & is overtaken by
normal sexual maturity  giant juvenile
D’Arcy Thompson
• early 20th century
• comparative anatomist
• “On Growth & Form”: transformation grids:
explain changes in shape & determine allometric growth
• measurements made & plotted on rectangular coordinates
• same measurements made in a related organism or a
different stage in dev’t
• shown as deformations of grid system
• now : partial warp analysis
Hatchetfish
Wrasse & Angelfish
Skulls of Human, Chimp
& Baboon
Evolution of Higher Taxa (Gould)
• new groups often arise from neotenic or
progenetic ancestors
• e.g. flightless birds
• e.g. insects: from larval form of millipedelike ancestor?
• e.g. chordates larval cond’n of tunicates?
Saltationists
• distinctive features of higher taxa arise through
“systemic mutation” (complete reorganization)
• Argument:
- few intermediates among higher taxa
- little selective advantage to incipient
structures
- results in dramatic, discontinuous effects
Neodarwinists
Counter-argument:
- characters of higher taxa evolve mosaically
- many intermediate forms
e.g. Archaeopteryx, Lepidoptera
- early stages of complex structures selectively
advantageous
- mutations with disruptive pleiotropic effects
usually fatal (no change in rate)