Speciation

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Speciation
Culex pipiens, Culex molestus

• Campbell 14.1-14.9 Monday, 26
  January 2004

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1. What is a species?

• Defining a species is not easy!
• Most definitions recognize that species share a
  distinguishing characteristic
• Evolutionary independence

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Genetic Change and Speciation

• Genetic divergence
• Differences accumulate between the gene pools of
  genetically separate populations
• Genetic changes between populations can only be
  countered by gene flow (migration)

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What is a species?

• Linnaeus used physical appearance to identify
  species when he developed the binomial system of
  naming organisms
• Known as the morphological species concept
• This system established
• the basis for taxonomy
• What is taxonomy?

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What is a species?

• But appearance alone does not always define a
  species

e.g Mature leaves of Sagittaria
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What is a species?

• Similarities between some species and variation
  within a species can make defining species
  difficult
• Humans exhibit extreme
• physical diversity

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What is a species?

• The biological species concept defines a species
  as

a population or group of populations whose
members 1. can interbreed and produce fertile
offspring and 2. are reproductively isolated from
other such populations
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2. The Biological Species Concept

• A species comprises one or more populations of
  individuals that can interbreed and produce
  fertile offspring
• Populations of a species have a shared genetic
  history, maintain genetic contact over time, and
  evolve separately from other species

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What are problems with the BSC?

• The biological species concept is not
• applicable to fossils
• applicable to asexual organisms
• Other problems? (discussion in class)
• Most organisms are classified using the
  morphological species concept because this
  concept is more convenient

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3. What keeps species separate?

• Simply reproductive isolating mechanisms
• Any heritable feature of body, form,
  functioning, or behavior that prevents breeding
  between one or more genetically divergent
  populations
• Occurs at one of two stages
• Prezygotic or Postzygotic
• zygote first cell of a new individual a
  fertilized egg

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Pre-Zygotic Isolation

• Mating or zygote formation is blocked by
• Temporal Isolation
• Behavioral Isolation
• Mechanical Isolation
• Ecological Isolation
• Gamete Mortality
• What do you think each of the above terms mean.?

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Examples of prezygotic barriers

• Behavioral isolation
• Courtship ritual in albatross
• Mechanical isolation
• flower structures that are adapted
• to specific pollinators (zebra orchid
• and wasp species)

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Post-Zygotic Isolation

• Prevents development of fertile hybrids
• Hybrid inviability - Egg is fertilized but zygote
  or embryo dies
• Hybrid breakdown- First generation hybrid forms
  but shows low fitness
• Hybrid infertility - Hybrid is fully or partially
  sterile

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Example of postzygotic barriers

• Hybrid sterility is one type of postzygotic
  barrier
• A horse and a donkey may produce a hybrid
  offspring, a mule
• Mules are sterile

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4. How does speciation occur?

• Speciation occurs through two main mechanisms
• Allopatric speciation
• (allos other, patra fatherland)
• Sympatric speciation
• (syn together)

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Allopatric speciation

• 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

Time 0
X
Time 0 t
Population 1
Population 2
Population 2
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Allopatric Speciation

• Mechanism physical barrier prevents gene flow
  between populations of a species

Blue-headed wrasse
ISTHMUS OF PANAMA
Cortez rainbow wrasse
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1
A few individuals of a species on the mainland
reach isolated island 1. Speciation follows
genetic divergence in a new habitat.
3
2
4
Later in time, a few individuals of the new
species colonize nearby island 2. In this new
habitat, speciation follows genetic divergence.
1
2
Speciation may also follow colonization of
islands 3 and 4. And it may follow invasion of
island a by genetically different descendants of
the ancestral species.
1
3
2
4
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Example of Allopatric speciation

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

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Sympatric Speciation

• New species forms without geographic isolation
• Different mechanisms
• ecological or behavioral separation
• polyploidy

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Example of sympatric speciation

• Ecological separation in Malawi cichlids
• 700 000 species in 2 million years

Crevice feeder
Piscivore
Planktivore
Egg Robber
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Example of sympatric speciation

• Behavioral separation
• Lake Malawi rock-dwelling cichlids (mbuna)
• species flocks

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Example of Sympatric speciation

• Polyploidy
• Many plants are polyploid
• products of hybridization
• bread wheat

Triticum monococcum (einkorn)
Unknown species of wild wheat
T. tauschii (a wild relative)
T. aestivum (one of the common bread wheats)
T. turgidum
CROSS-FERTILIZATION FOLLOWED BY SPONTANEOUS
CHROMOSOME DOUBLING
X
X
42AABBDD
14AA
14BB
14AB
28AABB
14DD
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6. Patterns of speciation

• The timing, rate and direction of speciation
  varies within and between lineages.
• The extinction of some number of species within
  lineages is usually a natural process

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Evolutionary trees and rates of change

• Gradualist (unbranching)
• Changes in allele frequencies and morphology
  accumulate over time
• little evidence in fossil record

new species
branch point (a time of divergence, speciation)
a single lineage
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Evolutionary trees and rates of change

• Punctuated equilibrium model
• Lineage splits within populations,
• results in genetic isolation
• evolution occurs in spurts and trees are
  branching
• we see many adaptive radiations in the fossil
  record

a new species
branch point (a time of divergence, speciation)
a single lineage
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eg Adaptive Radiation of Mammals

• Remember the K-T boundary?
• Burst of divergences gave rise to many new
  species of mammals
• an adaptive radiation is a burst of divergences
  that give rise to many new species

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Conservation species concept
Redefinition of subspecies based on genetic data
Current and historical ranges
The case of the Florida Panther
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Study Guide lecture structure

• Intro
• speciation in the underground
• genetic change and speciation
• What is a species?
• the morphological species concept
• the biological species concept
• The biological species concept
• problems with the concept

• What keeps species separate?
• reproductive isolation
• mechanisms of reproductive isolation, pre and
  post zygotic
• How do species arise?
• allopatric speciation
• sympatric speciation
• Patterns of speciation
• graduated model
• punctuated equilibrium model
• e.g. adaptive radiations

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Study Guide

• There are many species definitions. What
  distinguishing characteristic is recognized by
  most concepts?
• Populations of a species share a genetic history
  and speciation is the process by which species
  form from a population of a parent species. What
  is genetic divergence? How does genetic
  divergence occur? What prevents genetic
  divergence?
• Define the morphological species concept. What is
  one problem with the use of this concept?
• Define the biological species concept. Name
  advantages associated with this concept. What are
  problems associated with the concept?

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Study Guide

• What isolating mechanisms contribute to
  speciation? Name these mechanisms and give
  examples.
• What are the two main models of speciation? Give
  an explanation for how speciation occurs under
  each of these models. Give examples of each type
  of speciation.
• What is polyploidy? How does speciation occur
  through polyploidy?
• What does the fossil record tell us about the
  patterns of speciation?
• What is an adaptive radiation? Give an example of
  an adaptive radiation.