Nerve activates contraction

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CHAPTER 24 THE ORIGIN OF SPECIES

• 1. The biological species concept emphasizes
  reproductive isolation, and has some major
  limitations
• Alternative species concepts exist, and may be
  getting more popular
• Allopatric and sympatric speciation elsewhere
  vs. together
• Punctuated equilibrium how fast does evolution
  happen?

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Introduction

• Darwin recognized that new species formed in the
  Galapagos Islands ( existed nowhere else).
• Evolutionary theory must also explain
  macroevolution, the origin of new taxonomic
  groups (new species, new genera, new families,
  new kingdoms)
• Speciation is the keystone process in the
  origination of diversity of higher taxa.

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• Two patterns of speciation anagenesis and
  cladogenesis.
• Evidence fossil record.
• 1. Anagenesis - cumulative changes, gradual
  transition or transformation of one species into
  another.

Fig. 24.1a
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• Cladogenesis branching of one or more new
  species from a parent species.
• Cladogenesis yields more species

Fig. 24.1b
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• Morphological differences were used for a long
  time to distinguish species.
• Today, differences in body function,
  biochemistry, behavior, and genetic makeup are
  also used to differentiate species.

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1. The biological species concept emphasizes
reproductive isolation

• Ernst Mayr (1942) the biological species concept
• A species is a population or group of populations
  whose members have the potential to interbreed
  with each other in nature to produce viable,
  fertile offspring, but who cannot produce viable,
  fertile offspring with members of other species.
• A biological species is the largest set of
  populations in which genetic exchange is possible
  and is genetically isolated from other
  populations.

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• Species interfertility, not physical
  similarity.
• Example eastern / western meadowlarks - similar
  shapes and coloration, but song differences
  prevent interbreeding.
• Humans considerable diversity,but one species
  because ofour capacity to interbreed.

Fig. 24.2
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• Many species are genetically sequestered by
  multiple barriers.
• Barriers often intrinsic to the organisms, not
  simple geographic separation.
• Prezygotic or postzygotic reproductive barriers
• Prezygotic Postzygotic
• habitat isolation hybrid viability
• behavioral isolation reduced hybrid fertility
• temporal isolation hybrid breakdown
• mechanical isolation
• gametic isolation

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• Reproductive barrierscan occur beforemating,
  betweenmating andfertilization, orafter
  fertilization.

Fig. 24.5
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but has some major limitations

• BSC has been important in evolutionary theory,
  but its limited when applied to species in
  nature.
• Reproductive isolation of morphologically-similar
  fossils?
• Living species do we know about interbreeding
  for all?
• Asexual species (e.g., bacteria)?

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2. Alternative species concepts exist, and may
be getting more popular

• Ecological SC based on ecological niche
• Pluralistic SC - invokes reproductive isolation
  and/or ecological niche.
• Morphological SC old and practical unique set
  of structural features.
• Genealogical SC - a species a set of organisms
  with a unique genetic history
• All are attempts to categorize a moving spectrum.

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3. Allopatric and sympatric speciation
elsewhere vs. together

• How is gene flow among populations initially
  interrupted.?

Fig. 24.6
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• Allopatric the significance of a barrier depends
  on the ability of organisms to move about.
• The Grand Canyon is a significant barrier 2
  closely-related species of ground squirrels on
  opposite sides.
• Not so for birds.

Fig. 24.7
Fig. 24.9
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• The odds of allopatric speciation increase when a
  population is both small and isolated.
• Genetic drift and natural selection are more
  likely.
• But most small, isolated populations fail.
• A test

Fig. 24.8
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• Evolution of many species from a common ancestor
  is called an adaptive radiation.

Fig. 24.11
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• Geographic isolation promotes allopatric
  speciation, but ? reproductive isolation.
• Reproductive isolation must also evolve.
• Natural selection and genetic drift both act as
  allopatric populations evolve separately.
• Drosophila pseudoobscura cultures (D. Dodd)
• Grew cultures separately on starch or maltose
  media
• Several generations
• Starch cultures improved on starch, maltose
  cultures improved on maltose (natural selection)

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• Females raised on a starch medium preferred
  starch males over males raised in maltose medium.
• Prezygotic barrier to interbreeding developed
  after only several generations of isolation.

Fig. 24.12
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• Ditto for postzygotic reproductive barriers
• R. Vickery cross-pollinated allopatric
  populations of monkey flower (Mimulus
  glabratus).
• Planted hybrid seeds - develop into fertile
  plants?
• Cross plants from different regions proportion
  of fertile offspring decreased as distance from
  the source populations increased.
• Some hybrids almost sterile hybrid breakdown, a
  postzygotic barrier.

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• Allopatric speciation - new species form when
  geographically isolated populations evolve
  reproductive barriers as a byproduct of genetic
  drift and natural selection to its new
  environment.
• Sympatric speciation new species arise within
  the range of the parent populations.
• Reproductive barriers must evolve between
  sympatric populations.
• Polyploidy - common in plants (some animals) -
  accidents during cell division result in extra
  sets of chromosomes.
• Also in animals - gene-based shifts in habitat or
  mate preference.