Lecture 21: Macroevolution

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Lecture 21 Macroevolution

• Last class
• 1) Peramorphosis addn of extra stages
• a) Hypermorphosis devt extended from ? to ?1

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b) Predisplacement

• y starts growing early rel. to x in descendent
  vs. ancestor

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c) Acceleration

• faster growth of y rel. to x in descendent vs.
  ancestor

Descendant Ancestor
?1
?
log y
Larger (or more devd) y (not x) at maturity
?
log x
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2) Paedomorphosis

• retention of juvenile features in adult
• A) Progenesis
• B) Neoteny
• C) Postdisplacement

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a) Progenesis

• devt stops early

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b) Neoteny

• slower rate of growth of y rel. to x in
  descendant vs ancestor

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c) Postdisplacement

• y starts growing late rel. to x in descendant vs.
  ancestor

Ancestor Descendant
?
?1
log y
- same allometry - late start of y means smaller
y (not x) at maturity
?
?1
log x
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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. isoln
•  

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• 3. May release lineages from phylogenetic
  constraints
• - e.g. paedomorphosis descendant no longer
  passes through the same developl stages as
  ancestor
• - can free the sp. from the constraint imposed
  by that structure
• - only affects existing structures.

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Genetic Basis of Heterochrony

• Homeotic (Hox) genes
• 1st discovered in Drosophila spp.
• involved in gross alterations in phenotype
• Affect developt of cuticular structures from
  imaginal disks
• in all animal phyla
• share of common
• characteristics
• e.g. antennapedia

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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 devt than 5' end
  

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Hox Genes contd

• 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

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Type of Heterochronic Process?

• Axolotl
• vs. Tiger Salamander
• failure to metamorphose
• thyroxine can be exptally induced
• external gills in adult (juvenile morphology)

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So whats going on?

• not postdisplacement age at maturity other
  salamanders
• not progenesis body size at maturity other
  salamanders (progenesis ?tiny adult)
• Neoteny somatic devt slows is overtaken by
  normal sexual maturity ? giant juvenile

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DArcy 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 devt
• shown as deformations of grid system
• now partial warp analysis

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Hatchetfish
Wrasse Angelfish
Skulls of Human, Chimp Baboon
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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 millipede-like
  ancestor?
• e.g. chordates ?larval condn of tunicates?

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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

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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)