Lecture 20: Morphological Changes in Macroevolution

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Lecture 20 Morphological Changes in
Macroevolution

• Microevolution evolution w/i species
• Macroevolution evolution at or above sp. level
• Macroevolution often major morph changes
• How do they occur?
• Saltation new features arise by major
  reorgann - not progression through intermediates
• ? Punctuated equilibrium ( no specific mech.)
• Darwin was an anti-Saltationist

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Morphological Changes

• Completely new features rare
• usually modification of ancestral feature
• e.g. branchial basket (agnathans)
• ? gill arch (bony fish)
• ? jaw (reptile)
• ? ear bones (mammal)
• (N.B. not direct descendants, C.A.)

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Transformation

• Changes in elements
• number
• size
• shape
• position
• association with other parts
• differentiation (complexity)

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Examples

• Enlargement of Cerebral Hemispheres
• (reptile ? mammal)
• Complexity of Lung
• ( amphib ? rept ? mamm)
• Reduction of skull bones
• ( fish ? mamm)

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Serially Homologous Features

• e.g. petals, scales, stamens, digits etc.
• May increase in number
• (vertebrae in snakes body segments in
  millipedes)
• More frequently reduced
• (teeth, vertebrae, digits in most vertebrate
  lineages)

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Serially Homologous Structures

• change more likely when indeterminate (large ,
  variable)
• e.g. stamens ( magnolias vs. legumes)

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• Differentiation of structures from ancestor
• e.g. leaves ? tendrils spines etc.
• e.g. appendages of trilobites ?mouthparts,
  reproductive, locomotary
• Structures may become homogeneous
• e.g. toothed whales

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Allometry

• differential rates of growth of body parts
• comparisons may be inter- or intraspecific
• intraspecific w/i inds
• (ontogenetic - different ages)
• among inds
• (static - same age)

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Allometry

• Equation y b xa
• Linearized log y log b a log x
• e.g. ontogenetic allometry
• Humans Black-headed Godwits

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Adaptiveness of Allometry

• e.g. intestine scales 3/2 body size
• i.e. intestine length body size1.5
• b/c surface area volume ratio

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• Interspecific Allometry of brainbody weight
• Homeotherms Brw 0.07 (Bw)0.67
• Poikilotherms Brw 0.007 (Bw)0.67

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Heterochrony

• Evolutionary changes in timing of devt of
  feature
• e.g. compare ontogeny of 2 spp.
• Brain size changes faster (rel. to body) in sp. 2
  vs. sp. 1
• Brain cell lines in sp. 2 develop faster than in
  sp. 1

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Types of Heterochrony

• 1) Peramorphosis addn of extra stages beyond
  adult stage of ancestor
• a) Hypermorphosis more stages, longer time
• b) Acceleration more stages, same time
• c) Predisplacement starts earlier

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Types of Heterochrony

• 2) Paedomorphosis retention of juvenile
  features in adult (opposite of peramorphosis)
• a) Progenesis development stops early
• b) Neoteny development slowed
• c) Postdisplacement starts late

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Growth curves
line of equal growth (m1)
?
ancestral trajectory ? starts growing ?
stops growing
log y
slope ? ? ? gt 1 y grows fast relative to x
?
log x
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Hypermorphosis

• Type of peramorphosis
• growth of structure lasts longer during devt
• greater y/x ratio at maturity rel. to ancestor

?1
extended devt
m 1
? ancestral condn ?1 descendant condn
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Irish Elk

• e.g. of Hypermorphosis
• extinct 10,000 years ago
• antlers 13 ft span 100 lb!
• metabolic costs of antler prodn
• implicated in extinction
• to grow 40 kg antlers in 150 days
• 60 g calcium 30 g phosphorus per day!