Evidence for Evolution

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Evidence for Evolution

• Chapter 2

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Evidence of change through timeLiving species

• Soapberry bugs
• Vestigial structures
• Vestigial genes (pseudogenes)

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Evolution of beak length in soapberry bugs
microevolution

• Native to southern US
• In Florida, native host plant is balloon vine
• which occurs primarily on southern tip of
  Florida, on the Florida Keys, and less commonly,
  in central Florida
• Bugs feed by piercing the fruit with beak and
  inserting it into the seeds
• Flat-podded golden rain tree introduced to
  central Florida in 1926
• Rain tree fruit are much thinner than balloon
  vine fruit

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Evolution of beak length in soapberry bugs
microevolution

• Soapberry bugs in c. Florida started using rain
  trees as a host plant
• Populations of bugs living on thin-fruited rain
  trees evolved shorter beaks

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Fig. 2.1(a) Evolutionary change in soapberry bugs
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Fig. 2.1(b) Evolutionary change in soapberry bugs
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More on soapberry bugs 1

• Beak length is a variable quantitative phenotype
  (also heritable)
• Phenotypic distributions on the two host plant
  species overlap
• But some bugs found on rain trees have much
  shorter beaks than any bugs found on balloon
  vines (and vice versa)
• Evolution of mean phenotype and the range of the
  phenotype
• Standing heritable variation enables
  populations to make evolutionary responses to
  changing environments (dont have to wait for the
  right mutations)

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More on soapberry bugs 2

• How do we know that this is really an
  evolutionary change, rather than simply a plastic
  growth response?
• Why does natural selection favor shorter beaks in
  populations that use rain trees as hosts?
• a long-beaked bug can exploit both thick and thin
  fruit
• See Exploring the Literature 11 at the end of
  this chapter
• This appears to be an example of relatively
  modest morphological change (microevolution), but
  could this also be an example of speciation in
  progress?

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Vestigial structures are evidence for evolution

• Reduced wings in flightless birds (e.g., kiwi)
• Reduced or missing eyes in cave organisms (e.g.,
  Mexican tetras)
• Eye development can be stimulated by
  transplantation of lens tissue from an eyed
  relative
• Vestigial digits during embyronic development in
  chickens
• Pelvises without hind limbs in whales

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Fig. 2.8(a) Vestigial structural traits
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Molecular vestiges pseudogenes

• Pseudogenes non-functional DNA sequences that
  share similarity with functional genes
• Arise through gene duplication followed by
  accumulation of mutations that cause one copy to
  become non-functional
• Example a-globin gene and ya-globin
• members of the globin gene family

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Evidence of change through timeThe fossil record

• Extinction
• Appearance of new forms in the fossil record
  Law of Succession
• glyptodonts and armadillos in S. America
• Diprotodon and wombats in Australia
• Transitional forms
• Archaeopteryx

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Archaeopteryx fossil 19th Cent. Engravings of
the London specimen

• http//www.geo.ucalgary.ca/macrae/t_origins/archa
  eopteryx/
• http//www.talkorigins.org/faqs/archaeopteryx/info
  .html

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Archaeopteryx vs. modern birdswww.geologyrocks.co
.uk/ tut.php?id13
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Evidence of common ancestry Homology

• Originally, and literally, the study of likeness
• Similarities among species that are not the
  result of functional necessity
• Example the tetrapod forelimb

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Fig. 2.11 Structural homologies tetrapod
forelimb
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Homology is explained by common ancestry

• Darwin argued that homologous similarities were
  the result of inheritance from common ancestors
  humans, moles, horses, dolphins and bats share
  similar limb skeletons because they all inherited
  their limbs from a common ancestor with a similar
  number and arrangement of bones

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Tree Thinking

• If
• New species come from already existing species
• Some species go extinct without leaving
  descendants
• The total number of species does not decline over
  time
• Then
• Some species must leave two or more descendants
• And, the history of life is a branching process
  that results in an evolutionary tree that
  describes relationships among species

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Fig. 2.9 Darwins hypothetical evolutionary tree
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More Tree Thinking

• Evolutionary trees define related groups of
  species in a nested hierarchy (more closely or
  more distantly related)
• Darwin realized that our ability to erect a
  hierarchical classification system (e.g., genus,
  family, order, class) was a direct consequence of
  branching evolution and common ancestry

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Fig. 2.10 An evolutionary tree for eight species
of cats
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A phylogeny (evolutionary tree) is an hypothesis
about relationships

• In the cat example, no one was around to see it
  happen
• Other investigators reach different conclusions
  about the details (e.g., lions and tigers are
  each others closest relative sister taxa)

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Molecular homology processed pseudogenes

• Processed pseudogenes are pseudogenes that lack
  the introns and promoter regions found in their
  functional equivalents
• Thought to result from reverse transcription of
  processed mRNA
• The age of pseudogenes can be estimated by
  comparing the their nucleotide sequences to those
  of their functional partners (more differences
  more time)
• In any phylogeny, pseudogenes that arose more
  recently (i.e., younger) will be shared by fewer
  taxa than older pseudogenes

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Fig. 2.17(a) Processed pseudogenes used to test
Darwins hypothesis of common ancestry
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Fig. 2.17(b) Processed pseudogenes used to test
Darwins hypothesis of common ancestry
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Fig. 2.17(c) Processed pseudogenes used to test
Darwins hypothesis of common ancestry
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Fig. 2.18 The geological time scale
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Radiometric dating 1

• Radioactive isotopes decay at known rates (
  half-life)
• Relative amounts of parent and daughter isotopes
  in a rock sample indicate the age of the rock
• Important assumptions
• Amount of daughter isotope at time rock was
  formed is known
• Parent and daughter isotopes have not entered or
  left rock since its formation

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Fig. 2.19 Radioactive decay
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Radiometric dating 2

• http//www.earthsci.unimelb.edu.au/Thomas/lteng/en
  geimg/enge0507a.GIF
• See Table 2.1 of Freeman and Herron (p. 61)