Speciation: the origin of species

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• Speciation the origin of species
• How many species?
• Number described 1.5 1.6 million
• Number estimated 5-6 million to 100 million.
• Species smallest evolutionarily independent unit
  (fundamental unit of biodiversity).
• Species boundaries
• geographical
• genetic
• How do species originate?
• Speciation mechanisms

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Speciation mechanisms 1. Polyploidization A.
Autoploidy doubling of homologous chromosomes B.
Alloploidy stage in the speciation
process Plants
Nicotiana
Alloploidy
Autoploidy
n 96
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• 1. Polyploidization
• Animals
• Occasionally produces a new
• species

Hybridization Reticulate evolution
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• Speciation Mechanisms
• Most animal speciation is visualized as lineage
  splitting.
• Y
• Basic speciation models require separation of
  gene pools.
• Darwinian idea slow accumulation of genetic
  differences.
• Goldschmitian idea saltation rapid, big effects
  from small changes in developmental pathways.
• 1. Dispersal either setting up peripheral
  isolates or island hopping.
• 2. Vicariance population is subdivided by
  extraneous geological or climatic events.
• 3. Habitat segregation

typically messy
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DNA sequence divergence Morphological differences
Reproductive isolation
Snapping shrimp species
Speciation by Vicariance Asynchronous closure (3
mya)
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How many species of African elephants?
Sample 195 elephants from 21 populations Four
genes sequenced genetic distances used to
construct a phylogeny Conservation
implications Two species (using a phylogenetic
species concept)
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Sympatric Speciation

• Flies (Rhagoletis pomonella)
• (1) Larvae develop in hawthorns (Crataegus)
• Native to NE U.S.
• (2) Larvae develop in apples (Pyrus)
• Apples introduced c. 300 ya
• H0 The flies belong to the same species there
  is phenotypic plasticity in use of hosts.
• H1 Speciation has occurred each species of fly
  adapted to one host species.
• Since hawthorns and apples are both within the
  range of Rhagoletis, this would represent an
  example of sympatric speciation.

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• Flies using the two types of fruit cannot be
  distinguished morphologically (cryptic).
• But, evidence of lineage splitting
• 1. Allele frequency differs for 6 different
  proteins therefore can be distinguished
  genetically.
• 2. Flies imprint on fruit from which they
  developed.
• Mating takes place on the fruit.
• Provides some degree of physical segregation.
• Only c. 6 of matings are between misimprinted
  flies.

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• Flies are diverging because of natural selection
• Selective agent timing of fruit ripening.
• Apples ripen early.
• Larvae in apples
• Selected to develop slowly.
• Prevents emergence of adults prior to winter.
• Hawthorns ripen 3 weeks later.
• Larvae in hawthorn fruit
• Selected to develop rapidly.
• Enables pupation prior to winter.
• Appears to be speciation nearing completion.

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Speciation by Dispersal
Representative Hawaiian Drosophila diversity
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Speciation problem
Aspidoscelis carmenensis
A. picta
A. danheimae
A. franciscensis
A. espiritensis
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Grismer, L. Lee. 1999. Phylogeny, taxonomy, and
biogeography of Cnemidophorus hyperythrus and C.
ceralbensis (Squamata Teiidae) In Baja
California, Mexico
Unresolved maybe not species?
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226 specimens scored for 9 phenotypic
characters Canonical variate analysis (pair-wise
F-values) Pair-wise F-values converted to
Mahalanobis distances (D2) Additive tree
constructed from a matrix of D2
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Speciation nearing completion? Northwest
Arizona Aspidoscelis tigris
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Sample 4 in the middle of a step
cline. Phenotypically intermediate. Why?
center
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Assortative mating?