Evolution Morphological Innovation

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Lecture 17

• Evolution Morphological Innovation

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Macroevolution

• Large scale patterns in evolution
• -How have changes occurred long term?
• Origin of whole groups of organisms
• When , where
• -species to kingdoms and domains
• Domains Archaea, Bacteria, Eukarya
• Novelties innovations
• Do new innovations and new species or groups
  arise at the same time?

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MacroEvolutionary Changes

• Punctuated Equilibrium-
• Rapid evolutionary change followed by long
  periods of stasis
• Marked changes in fossil record
• Also gradualism
• Same evolutionary processes (variation, natural
  selection etc.) acting at population levels but
  viewed over long time

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Geologic Time Scale w Extinct
Humans
mammals
K-T Boundary
Flowering plants
Dinosaurs extinct
Cambrian Explosion
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Cambrian Explosion
Most phyla or living animals appeared within a 35
my period lt1 of earths history
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How did Evolutionary Diversity Arise?

• Most new features arise from preexisting cell
  types in new location or timing
• Arrival of new phyla during Cambrian consistent
  with invention of new developmental programs
• By early Cambrian all animals had evolved a
  common genetic tool kit of developmental
  programs
• Changes in these program novel structures and
  morphology

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Development-a key to evolutionEVO-DEVO

• Few, small genetic differences between species
  produce large phenotypic differences
• humans and chimpanzees
• similar genetic and cellular mechanisms of
  development
• dissimilar adult phenotypes
• alleles responsible for new traits must be active
  early in development---gtadult phenotype
• Mutants that change adult phenotype subject to
  selection
• Developmental regulation-key to morphological
  innovation
• Controls when and where organs develop and how
• How cells know where they are and what to do
• genes and gene clusters-Homeotic loci

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Homeotic Loci

• HL Genes whose products provide positional
  information in a multicellular embryo
• HOM-generally invertebrates
• HOX-vertebrates
• MADS-plants
• Molecular palenontology
• -common ancestor had genes before
  diversification

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Homeotic Loci

• Code for regulatory proteins,
• control transcription of other genes
• bind to DNA
• Each locus within the complex contains a highly
  conserved 180-bp sequence-the homeobox.
• Homologous origin

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HOMeotic loci

• Gene complexes
• Related genes found together
• Produced bygene duplication
• Unique to each class or phylum
• Temporal and spatial colinearity
• 3 5 expression
• 3-gthead, 5-gtposterior, timing of expression

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Fig 17.1 HOX genes Drosophila
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5
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Hox (HOM, MADS)Regulation

• Define where cells are in embryo
• In time and space
• Example Drosphila HOM
• two main clusters, bithorax and Antennepedia
  complexes (Fig 18.1)
• Bithorax cause defects in posterior half of
  embryo
• Antennepedia affects anterior
• Mutants
• flies missing one or more gene products can
  produce segment-specific appendages in the wrong
  place.
• -cells misunderstand where they are (location)
• -transformed to different developmental fate

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Fig 17.2 Homeotic mutants
Legs growing from head
Extra wings
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Hox changes to Phenotype changes

• Each major clade of bilateral animals is
  characterized by a particular suite of Hox genes
• Specific changes in Hox cluster are associated
  with particular animal clades.
• Ex Abdominal B is associated with the evolution
  of bilateral animals.
• Entire Hox complex duplicated several times in
  the lineage leading to vertebrates. May have been
  important in the divergence of vertebrates from
  other deuterostomes.

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Fig 17.3 Hox genes animals
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Complexity of metazoan body plans

• Number of loci in the gene complex correlates
  with the overall complexity
• Duplication and elaboration of genes in the Hox
  complex was a genetic innovation made Cambrian
  explosion possible
• sponges and cnidarians have just 3-4 loci
• common ancestor of all bilateral animals had 10
  loci
• Diversification of animal body plans involved
  changes in the timing or spatial location of Hox
  gene expression

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Deep Homology

• Ancestral homeotic complexes
• Sister group to tetrapods is the lobe-finned
  fishes of late Devonionmany structural
  homologies between limbs of lobe-finned fish and
  tetrapods.
• ? the tetrapod limb is derived from the fins of
  lobe-finned fish. Fig 17.7
• ? tetrapod limbs have a common ground plan,
  resulting from a shared developmental program
• ? Loci in the Hox family are critical to limb
  development-tell where along the limb the cells
  are as well as anterior to posterior in body
• the shared developmental program observed in
  tetrapod limbs is produced by homologous genes
• ZPA, AER

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Fig 17.7 Fin bones, lobe fins
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At the genetic level, every type of
sticky-outey appendage found in animals appears
to be homologous!

• Arthropod limbs determined by same genes as
  vertebrate limbs
• instruct cells to form an outgrowth with
  proximal-distal polarity (Distal-less)
• Tetrapods may have gained hands and feet because
  of a change in the timing and location of
  homeotic gene expression
• in the pattern of expression of shh and Hox genes
  that changed the direction of the progress zone.

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Deep Homology-Development and Evolution

• Deep homologies exist in a series of important
  structures and organs.
• The genetic similarities that underlie otherwise
  dissimilar structures -
• -prediction about the kinds of structures that
  should be found in the common ancestor from which
  these genetic control networks were inherited.

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Ancestor of animals

• ? Serial repetition of some body parts without
  complex differentiation of segments
• ? A simple type of appendage, but not a complex
  limb
• ? Clusters of simple photo receptors but not an
  image-forming eye
• ? A contractile blood vessel rather than a heart,
  and
• Condensations of nerve cell bodies into nerve
  cords, but not a brain

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Evolutionary Novelty

• Cell location relative to other cells and time of
  activity must be specified precisely
• polarity, spatial orientation etc.
• Changes in the specification of cell fates
• Major evolutionary mechanism for creation of
  different forms
• One basis for understanding origins of
  evolutionary novelty in multi-cellular organisms

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Insights from Evo-Devo

• Evolution of very different morphologies relies
  on reutilization of a small set of master
  regulatory genes
• The primacy of regulatory change as the driver of
  evolution of new morphologies