lecture 21: macroevolution last class: 1) peramorphosis: add’n of extra stages a) hypermorphosis:...
TRANSCRIPT
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Lecture 21: Macroevolution
Last class:1) Peramorphosis: add’n of extra stages
a) Hypermorphosis: dev’t extended from to 1
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b) Predisplacement:
y starts growing early rel. to x in descendent vs. ancestor
log x
log
y
1
1
- same allometry (relationship of y to x)- early start of y meansgreater y (not x) at maturity
Descendant
Ancestor
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c) Acceleration
• faster growth of y rel. to x in descendent vs. ancestor
log x
log
y
1
Larger (or more dev’d) y (not x) at maturity
Descendant
Ancestor
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2) Paedomorphosis
• retention of juvenile features in adultA) ProgenesisB) Neoteny C) Postdisplacement
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a) Progenesis • dev’t stops early
log x
log
y
1
Smaller y, smaller x at maturity vs. ancestor- Allometry unchanged- Compare: hypermorphosis
Ancestor
Descendant
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b) Neoteny
• slower rate of growth of y rel. to x in descendant vs ancestor
log x
log
y
1
Ancestor
Descendant
- Smaller or less developed y rel. to x at maturity
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c) Postdisplacement• y starts growing late rel. to x in descendant vs.
ancestor
log x
log
y
1
1
Ancestor
Descendant- same allometry- late start of y meanssmaller y (not x) at maturity
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Paedomorphosis Peramorphosisunderdevelopment overdevelopment
Rate Slower Faster(Neoteny) (Acceleration)
Onset Time Later Earlier(Postdisplacement) (Predisplacement)
Offset Time Earlier Later(Progenesis) (Hypermorphism)
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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. isol’n
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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.
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Genetic Basis of HeterochronyHomeotic (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
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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 dev’t than 5' end
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Hox Genes cont’d3. contain highly-conserved 180 bp region
- involved in bindingHox genes are regulators - control timing and
expression of other genese.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 exp’tally induced• external gills in adult (juvenile morphology)
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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
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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
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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 cond’n 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)