CHAPTER 14 The Origin of Species

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CHAPTER 14The Origin of Species

• The origin of new species is called speciation

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What is a species?

• Similarities between some species and variation
  within a species can make defining species
  difficult
• The biological species concept defines a species
  as
• a population or group of populations whose
  members can interbreed and produce fertile
  offspring

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The Evolution of Species

• The evolution of new species, a process called
  speciation occurs when members of similar
  populations no longer interbreed to produce
  fertile offspring within their natural
  environment.

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MECHANISMS OF SPECIATION

• 1. Reproductive isolation can result in
  speciation
• Prezygotic and postzygotic reproductive barriers
  prevent individuals of different species from
  interbreeding
• 2. Geographic isolation can lead to speciation
• Geographic isolation occurs whenever a
  physical barrier divides a population

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Geographic isolation occurs whenever a physical
barrier divides a population

• When a population is cut off from its parent
  stock, species evolution may occur
• An isolated population may become genetically
  unique as its gene pool is changed by natural
  selection, genetic drift, or mutation
• This is called allopatric speciation

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Islands are living laboratories of speciation

• On the Galápagos Islands, repeated isolation and
  adaptation have resulted in adaptive radiation of
  14 species of Darwins finches

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Adaptive radiation on an island chain

• The evolution of many diversely-adapted species
  from a common ancestor is called an adaptive
  radiation.

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. The Evolution of Species

• When geographic isolation divides a population
  of tree frogs, the individuals no longer mate
  across populations
• Tree frogs are a single population.

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The Evolution of Species

• The formation of a river may divide the frogs
  into two populations.

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The Evolution of Species

• Over time, the divided populations may become
  two species that may no longer interbreed, even
  if reunited

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New species can also arise within the same
geographic area as the parent species

• In sympatric speciation, a new species may arise
  without geographic isolation
• A failure in meiosis can produce diploid gametes
• Self-fertilization can then produce a tetraploid
  zygote
• Any individual or species with a multiple of the
  normal set of chromosomes is known as a polyploid.

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The tempo of speciation can appear steady or jumpy

• Speciation rates
• 1. Gradualism new species evolve by the gradual
  accumulation of changes brought about by natural
  selection
• However, few gradual transitions are found in the
  fossil record

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Speciation rates

• 2. Punctuated Equilibrium(proposed by Niles
  Eldredge and Stephen J. Gould in 1972)
• The punctuated equilibrium model suggests that
  speciation occurs in spurts
• Rapid change occurs when an isolated population
  diverges from the ancestral stock
• Virtually no change occurs for the rest of the
  species existence