Lecture 20: Morphological Changes in Macroevolution
• Microevolution = evolution w/i species• Macroevolution = evolution at or above sp. level
Macroevolution: often major morph changesHow do they occur?
• Saltation: new features arise by major reorgan’n- not progression through intermediates Punctuated equilibrium ( no specific mech.)
Darwin was an anti-Saltationist
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.)
Transformation
Changes in elements:
number
size
shape
position
association with other parts
differentiation (complexity)
Examples
• Enlargement of Cerebral Hemispheres
(reptile mammal)
• Complexity of Lung
( amphib rept mamm)
• Reduction of skull bones
( fish mamm)
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)
Serially Homologous Structures
• change more likely when indeterminate (large #, variable)
• e.g. stamens ( magnolias vs. legumes)
• 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
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)
Allometry
Equation: y = b xa
Linearized: log y = log b + a log x
• e.g. ontogenetic allometry
Humans Black-headed Godwits
Adaptiveness of Allometry
• e.g. intestine scales 3/2 body size
i.e. intestine length = body size1.5
• b/c surface area : volume ratio
Interspecific Allometry of brain:body weight:
• Homeotherms: Brw = 0.07 (Bw)0.67
• Poikilotherms: Brw = 0.007 (Bw)0.67
HeterochronyEvolutionary changes in timing of dev’t of featuree.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
sp 2
sp 1
brai
n w
t
body wt
Types of Heterochrony
1) Peramorphosis: add’n of extra stages beyond adult stage of ancestor
a) Hypermorphosis: more stages, longer time
b) Acceleration: more stages, same time
c) Predisplacement: starts earlier
Types of Heterochrony
2) Paedomorphosis : retention of juvenile features in adult (opposite of peramorphosis)a) Progenesis : development stops earlyb) Neoteny : development slowedc) Postdisplacement : starts late
Growth curves
log x
log
y
line of equal growth (m=1)
ancestral trajectory: = starts growing = stops growing
slope > 1y grows fast relative to x
Hypermorphosis• Type of peramorphosis:
• growth of structure lasts longer during dev’t
• greater y/x ratio at maturity rel. to ancestor
1
log x
log
y
extendeddev’t
m= 1 = ancestral cond’n
1 = descendant cond’n
Irish Elk
• e.g. of Hypermorphosis
• extinct ~ 10,000 years ago
• antlers: 13 ft span ~ 100 lb!
• metabolic costs of antler prod’n
• implicated in extinction:
• to grow 40 kg antlers in 150 days :
60 g calcium; 30 g phosphorus per day!
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