This document discusses different types of morphological changes that can occur during macroevolution, including: 1. Saltation, where new features arise through major reorganization rather than gradual intermediates. 2. Modification of ancestral features, such as gill arches evolving into jaws. Changes can involve number, size, shape, position or complexity of elements. 3. Serially homologous structures like digits may increase or decrease in number, and differentiation of structures from ancestors can also occur through processes like allometry or heterochrony.

1. 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 reorgan’n-
not progression through intermediates
≠ Punctuated equilibrium ( no specific mech.)
Darwin was an anti-Saltationist

2. 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.)

3. Transformation
Changes in elements:
number
size
shape
position
association with other parts
differentiation (complexity)

4. Examples
• Enlargement of Cerebral Hemispheres
(reptile → mammal)
• Complexity of Lung
( amphib → rept → mamm)
• Reduction of skull bones
( fish → mamm)

5. 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)

6. Serially Homologous Structures
• change more likely when indeterminate
(large #, variable)
• e.g. stamens ( magnolias vs. legumes)

7. • 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

8. 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)

9. Allometry
Equation: y = b xa
Linearized: log y = log b + a log x
• e.g. ontogenetic allometry
Humans Black-headed Godwits

10. Adaptiveness of Allometry
• e.g. intestine scales 3/2 body size
i.e. intestine length = body size1.5
• b/c surface area : volume ratio

11. Interspecific Allometry of brain:body weight:
• Homeotherms: Brw = 0.07 (Bw)0.67
• Poikilotherms: Brw = 0.007 (Bw)0.67

12. Heterochrony
Evolutionary changes in timing of dev’t 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
sp 2
sp 1
brainwt
body wt

13. 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

14. 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

15. Growth curves
log x
logy
α
line of equal growth (m=1)
ancestral trajectory:
α = starts growing
β = stops growing
slope α → β > 1
y grows fast relative to x
β

16. Hypermorphosis
• Type of peramorphosis:
• growth of structure lasts longer during dev’t
• greater y/x ratio at maturity rel. to ancestor
β1
log x
logy
α
β
extended
dev’t
m= 1 β = ancestral cond’n
β1 = descendant cond’n

17. 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!