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Lecture 21: Macroevolution

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a) Hypermorphosis: dev't extended from to. 1 b) Predisplacement: y ... Wrasse & Angelfish. Skulls of Human, Chimp & Baboon. Evolution of Higher Taxa (Gould) ... – PowerPoint PPT presentation

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Title: Lecture 21: Macroevolution


1
Lecture 21 Macroevolution
  • Last class
  • 1) Peramorphosis addn of extra stages
  • a) Hypermorphosis devt extended from ? to ?1

2
b) Predisplacement
  • y starts growing early rel. to x in descendent
    vs. ancestor

3
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
4
2) Paedomorphosis
  • retention of juvenile features in adult
  • A) Progenesis
  • B) Neoteny
  • C) Postdisplacement

5
a) Progenesis
  • devt stops early

6
b) Neoteny
  • slower rate of growth of y rel. to x in
    descendant vs ancestor

7
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
8
(No Transcript)
9
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
  •  

10
  • 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.

11
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

12
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

13
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

14
Type of Heterochronic Process?
  • Axolotl
  • vs. Tiger Salamander
  • failure to metamorphose
  • thyroxine can be exptally induced
  • external gills in adult (juvenile morphology)

15
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

16
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

17
Hatchetfish
Wrasse Angelfish
Skulls of Human, Chimp Baboon
18
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?

19
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

20
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)
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