Speciation

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Speciation

• Chapter 18 19
• BIO 21 SJCC

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• Species Concept
• What is a species?
• Species are groups of actually or potentially
  interbreeding natural populations which are
  reproductively isolated from other such groups
  Ernst Mayr
• Speciation process of new species arising
  through evolutionary processes
• What happens during evolution?
• In order for speciation to occur what must happen
  to an existing species?

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• Isolating Mechanisms

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• By definition speciation involves reproductive
  isolation
• Reproductive isolating mechanisms are the
  barriers that prevent genetic exchange between
  species
• Prezygotic isolating mechanisms
• Prevent the formation of zygotes
• Postzygotic isolating mechanisms
• Prevent the proper functioning of zygotes after
  they have formed

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• Prezygotic Mechanisms
• Allopatric speciation spatial isolation
  geographic isolation
• Spread of species into new areas
• Founding population in isolated location
• Geologic changes gradual process
• Sympatric speciation different niches/same
  habitat or geographic area
• Genetic changes (as ploidy levels in plants)
• Disruptive selection divergent genotypes favored

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Live in forest
Live in open grassland
Tiglon

• Hybridization has been successful in captivity
• But it does not occur in the wild

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When DoesSpeciation Occur?

• Generalists example Horseshoe crabs
• Already adapted to various food sources no
  change necessary/do not evolve or speciate
• Specialists finches narrowly adapted to food
  resources and highly evolved
• Gene pool shifts in response to changing food
  resources

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• Adaptive Radiation a form of sympatric
  speciation
• Single ancestral from give rise to multiple
  specialist forms filling various niches

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Postzygotic Mechanisms

• Zygotic mortality
• Hybrid inviability
• Hybrid sterility

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Postzygotic Isolating Mechanisms

• Zygotic mortality
• Hybrid inviability
• Hybrid sterility
• Examples
• Hybridization between sheep and goats produces
  embryos that die in the earliest embryonic stages
• Leopard frogs of the United States are a group of
  similar species, and NOT a single species as was
  long assumed

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• Hybrids between them produced defective embryos
  in the lab

• Their mating calls also differ substantially

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Speciation by Polyploidy
Instant speciation

• Change in chromosome number (3n, 4n, etc.)
• Offspring with altered chromosome number cannot
  breed with parent population
• Common mechanism of speciation in flowering plants

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Polyploid Speciation
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Possible Evolution of Wheat
Figure 18.9Page 299
T. aestivum (one of the common bread wheats)
Triticum monococcum (einkorn)
T. tauschii (a wild relative)
Unknown species of wild wheat
T. turgidum (wild emmer)
42AABBDD
14AA
14BB
14AB
28AABB
14DD
X
X
cross-fertilization, followed by a spontaneous
chromosome doubling
Figure 18.9Page 299
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Parapatric Speciation

• Adjacent populations evolve into distinct
  species while maintaining contact along a common
  border

BULLOCKS ORIOLE
BALTIMORE ORIOLE
HYBRID ZONE
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• Geographic Isolation of Ensatina escholtzii
  populations in CA
• ring species geographically isolated
  populations surround uninhabitable area
• Adjacent populations can interbreed
• Reproductively incompatible at extremes (black)

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Gradual Model

• Speciation model in which species emerge through
  many small morphological changes that accumulate
  over a long time period
• Fits well with evidence from certain lineages in
  fossil record

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Punctuation Model

• Speciation model in which most changes in
  morphology are compressed into brief period near
  onset of divergence
• Supported by fossil evidence in some lineages

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• Speciation is a two-part process
• 1. Identical populations must diverge
• 2. Reproductive isolation must evolve to
  maintain these differences
• Speciation occurs much more readily in the
  absence of gene flow
• This much more likely in geographically isolated
  populations

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Convergent evolution many paths to one goal
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• Analogous structures and Convergent Evolution
• Analogous Structures
• Resemble each other
• result of parallel evolutionary adaptations
  Example Wings of Bird and Wings of Butterfly
• Convergent Evolution
• Different animal groups adapt in different ways
  to solve same problem

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• Patterns of Evolution How and why do new
  species develop?
• Changes in gene pool
• Gene pool all the genotypes within a population
• All the different alleles present
• Changes in allele frequency ? changes in
  appearance of the species

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Extinction

• Irrevocable loss of a species
• Mass extinctions have played a major role in
  evolutionary history
• Fossil record shows 20 or more large-scale
  extinctions
• Reduced diversity is followed by adaptive
  radiation

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How do we classify things?Linnaean Classification

• Kingdom
• Phylum
• Class
• Order
• Family
• Genus
• Species
• Kings Play Chess On Fine Grained Sand
• In order from most inclusive to most specific

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Species Names

• By convention, the binomial name consists
• Genus name
• The first word and is always capitalized
• Species grouping
• The second word which refers to the species and
  is not capitalized
• The two words are written in italics
• Together, they form the scientific name
• Example Apis mellifera

The honeybee
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A cladogram of vertebrate animals
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