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!