i. overview of chordates evolution ii. selection for terrestrial...

I. Overview of Chordates EvolutionI. Overview of Chordates Evolution

II. Selection for Terrestrial LivingII. Selection for Terrestrial Living

III. Evolution of Paired AppendagesIII. Evolution of Paired Appendages

Invasion of Land by VertebratesInvasion of Land by Vertebrates

Phylum Phylum ChordataChordata

•• CephalochordataCephalochordata

•• UrochordataUrochordata

•• CraniataCraniata (vertebrates)(vertebrates)–– craniumcranium–– axial skeletonaxial skeleton–– elaborate brain elaborate brain

Cephalochordata

Urochordata

Larval Urochordate

Ostracoderms:Extinct Jawless Fish

[Source: Radinsky 1987, The Evolution of Vertebrate Design, University of Chicago Press]

500-400 mya

Class Class AgnathaAgnatha

(jawless fish)(jawless fish)

Evolution of the Vertebrate JawEvolution of the Vertebrate Jaw

Evidence for the EvolutionEvidence for the Evolutionof Vertebrate Appendagesof Vertebrate Appendages

•• ComparativeComparative

• Fossil

• Genetic/Developmental

Actinopterygii(ray-finned fish)

SarcopterygiiSarcopterygii(lobe(lobe--finned fish)finned fish)

Hypothetical Transition from Water to LandHypothetical Transition from Water to Land



• Fossil


Ichthyostega

Lobe-finned fish Extinct tetrapod

Acanthostega

Oldest Oldest TetrapodTetrapod Fossil Fossil ((HynerpetonHynerpeton))



• Fossil


Clusters of Clusters of HoxHox Genes in VertebratesGenes in Vertebrates

Carroll (1997)Carroll (1997)

Head Tail

HoxHox Genes and Limb DevelopmentGenes and Limb Development

[Source: Davis et al. 1995, [Source: Davis et al. 1995, NatureNature 375]375]

As in the development of headAs in the development of head--toto--tail axis,tail axis, HoxHox genes are genes are expressed in orderexpressed in order——from the shoulder to the fingers. from the shoulder to the fingers.

Hypothetical Evolution of Hypothetical Evolution of Paired AppendagesPaired Appendages

•• HoxHox clusters were duplicated in an early clusters were duplicated in an early vertebratevertebrate

•• NewNew HoxHox clusters became responsible for clusters became responsible for building limbs (developmental modularity).building limbs (developmental modularity).

SuperclassSuperclass TetrapodaTetrapoda

Class AmphibiaClass AmphibiaClass Class ReptiliaReptiliaClass Class MammaliaMammaliaClass AvesClass Aves

AmniotesAmniotes

Amniotic Egg

AmnionAmnion:: surrounds and protects embryosurrounds and protects embryoChorionChorion:: exchange gases and circulate nutrientsexchange gases and circulate nutrientsYolk sacYolk sac:: contains protein and lipid for energycontains protein and lipid for energyAllentoisAllentois:: stores metabolic wastesstores metabolic wastes

MonotremesMonotremes MarsupialsMarsupials PlacentalsPlacentals

MammalsMammals

i. overview of chordates evolution ii. selection for terrestrial...

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