Chapter 24 The Origin of Species

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Chapter 24The Origin of Species
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Concepts

• Speciation is the origin of new species and is at
  the focal point of evolutionary theory
• Evolutionary theory must explain how new species
  originate and how populations evolve

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Biological Species Concept

• Species is a Latin word meaning kind or
  appearance
• Archaeopteryx lithographica
• Morphology, physiology, biochemistry, and DNA
  sequences are compared when grouping organisms

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Definition of Species

• Species is a group of populations whose members
  have the potential to interbreed in nature and
  produce viable, fertile offspring they do not
  breed successfully with other populations

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Concept of Species

• Biological species concept reproductive barriers
• Ecological species concept ecological niche
• Pluralistic species concept unique factors
  depending on the species
• Genealogical species concept genetic history

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Biological Species Concept Similarity and
diversity
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Reproductive Isolation

• Reproductive isolation is the existence of
  biological factors (barriers) that impede two
  species from producing viable, fertile offspring
• Hybrids are the offspring of crosses between
  different species
• Reproductive isolation can be classified by
  whether factors act before fertilization
  (prezygote) or after fertilization (postzygote).

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Prezygotic (before the zygote)

• Prezygotic barriers block fertilization from
  occurring by
• Impeding different species from attempting to
  mate
• Preventing the successful completion of mating
• Hindering fertilization if mating is successful

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Types of Prezygotic Barriers

• Habitat isolation
• Behavioral isolation
• Temporal isolation
• Mechanical isolation
• Gametic isolation

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Habitat isolation
One lives in water the other on land
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Behavioral isolation
The dancing and singing are completely different
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Temporal Isolation
One diurnal and the other nocturnal
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Mechanical isolation
Spirals in opposite directions
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Gametic Isolation

• Gametes are not compatible
• Timing of gamete release may be different
• Conditions in the female womb of a different
  species may not be ideal for the sperm
• Gamete recognition hampered due to molecules
  arround the ovum

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Postzygotic Barriers

• Reduced hybrid viability
• Reduced hybrid fertility
• Hybrid brakedown

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Reduced hybrid viability
Ensantia hybrid shows to be frai, may not
survive
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Reduced hybrid fertility
Mule sterile hybrid of donkey and horse
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Hybrid brakedown
Hybrid (center sample) is stunt and may not
survive or reproduce
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Speciation Based on Gene Flow
(a) Allopatric speciation
(b) Sympatric speciation
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Allopatric

• In allopatric speciation, gene flow is
  interrupted or reduced when a population is
  divided into geographically isolated
  subpopulations

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• The definition of barrier depends on the ability
  of a population to disperse
• Separate populations may evolve independently
  through mutation, natural selection, and genetic
  drift

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Allopatric speciation in the Grand Canyon
A. leucurus
A. harrisi
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• Reproductive isolation between populations
  generally increases as the distance between them
  increases
• Barriers to reproduction are intrinsic
  separation itself is not a biological barrier

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Mantellinae (Madagascar only) 100 species
Rhacophorinae (India/Southeast Asia) 310 species
Other Indian/ Southeast Asian frogs
100
80
60
40
20
0
1
2
3
Millions of years ago (mya)
1
2
3
India
Madagascar
88 mya
65 mya
56 mya
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Sympatric Speciation

• Sympatric means same country
• Speciation takes place in geographically
  overlapping populations
• Occurs because of
• Polyploidism
• Habitat differentiation
• Sexual selection

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Polyploidism

• Polyploidy is the presence of extra sets of
  chromosomes due to accidents during cell division
• Autopolyploidy
• Allopolyploidy

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Autopolyploidy in plants
2n6
4n12
Failure of cell division after chromosome
duplication gives rise to tetrapliod cells which
may be viable and self-fertile
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Allopolyploid

• An allopolyploid is a species with multiple sets
  of chromosomes derived from different species
• Many important crops (oats, cotton, potatoes,
  tobacco, and wheat) are polyploids

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Ancestral species
AA
BB
DD
Wild T. tauschii (2n 14)
Triticum monococcum (2n 14)
Wild Triticum (2n 14)
Product
AA BB DD
T. aestivum (bread wheat) (2n 42)
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Habitat differentiation

• Sympatric speciation can also result from the
  appearance of new ecological niches
• North American maggot fly can live on native
  hawthorn trees as well as more recently
  introduced apple trees, so food preference may
  isolate the two populations

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Sexual selection

• Sexual selection can drive sympatric speciation
• Sexual selection for mates of different colors
  has likely contributed to the speciation in
  cichlid fish in Lake Victoria

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Hybrid Zones

• region in which members of different species meet
  and mate
• Patterns
• abnormal, more fragile offspring, less fit (mule
  is sterile)
• more fit than the parents for both environments
  (paco vicuña)

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Reinforced Barriers

• less fit hybrids might show a reinforcement in
  the barriers
• stronger in allopatric than sympatric species
• mule is sterile

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Fusion

• barriers are not strong enough
• hybrids are more likely (red wolf coyote x grey
  wolf)

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Stability

• the hybrid production is stable
• barriers haven't happen or are too weak
• stil hybrids are not the norm because the
  location where they occur are the extremes of the
  normal habitat where the original species exist
• The End