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

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 x a Linearized: log y = log b + a log x e.g. ontogenetic allometry HumansBlack-headed Godwits

Adaptiveness of Allometry e.g. intestine scales 3/2 body size i.e. intestine length = body size 1.5 b/c surface area : volume ratio

Interspecific Allometry of brain:body weight: Homeotherms: Br w = 0.07 (B w ) 0.67 Poikilotherms: Br w = (B w ) 0.67

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 brain wt 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 early b) Neoteny : development slowed c) Postdisplacement : starts late

Growth curves log x log y  line of equal growth (m=1) ancestral trajectory:  = starts growing  = stops growing slope    > 1 y 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1 log x log y   extended dev’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!