Mechanisms of Evolution

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Mechanisms of Evolution
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Macroevolution

• Speciation

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• MICROEVOLUTION - A change in the frequency of
  alleles. Review population genetics Ch. 23.
• MACROEVOLUTION - Speciation (or emergence of
  higher taxonomic levels)
• Speciation, or the origin of new species, is the
  central process of macroevolution because any
  higher taxon originates with a new species novel
  enough to be the first member of that taxon.

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• Darwin explored the Galápagos Islands
• And discovered plants and animals found nowhere
  else on Earth

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• The origin of new species, or speciation
• Is at the focal point of evolutionary theory,
  because the appearance of new species is the
  source of biological diversity
• Evolutionary theory
• Must explain how new species originate in
  addition to how populations evolve
• Macroevolution
• Refers to evolutionary change above the species
  level

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• There are two patterns of speciation in the
  fossil record anagenesis and cladogenesis
• Anagenesis (phyletic evolution) - The
  transformation of an unbranched lineage of
  organisms, sometimes to a state different enough
  from the ancestral population to justify renaming
  it as a new species.
• Cladogenesis (branching evolution) - The budding
  of one or more new species from a parent species
  that continues to exist is more important than
  anagenesis. It is more common and can promote
  biological diversity.

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• Two basic patterns of evolutionary change
• Anagenesis
• Cladogenesis

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Defining a Species

• Species - Latin term meaning kind or
  appearance
• Linnaeus (founder of modern taxonomy) - described
  species in terms of their physical form
  (morphology). Morphology is still the most
  common method used for describing species.
• Modern taxonomists also take into account genetic
  makeup and functional and behavioral features
  when describing species.

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The Biological Species Concept

• The biological species concept relies on
  reproductive isolation (proposed by Ernst Mayr,
  1942)
• Biological species - A population or group of
  populations whose members have the potential to
  interbreed with one another in nature and to
  produce viable, fertile offspring, but cannot
  produce viable, fertile offspring with members of
  other species.
• 1. Largest unit of population in which gene flow
  is possible
• 2. Defined by reproductive isolation from other
  species in natural environments (hybrids may be
  possible between two species in the laboratory or
  in zoos)

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Species
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Reproductive Isolation

• Reproductive isolation
• Is the existence of biological factors that
  impede members of two species from producing
  viable, fertile hybrids
• Is a combination of various reproductive barriers

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Prezygotic and postzygotic barriers to
reproduction

• The gene pools of different species are isolated
  from those of others by more than one type of
  reproductive barrier. The barriers that isolate
  gene pools are either prezygotic or postzygotic,
  depending on whether they occur before or after
  fertilization.

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• Prezygotic barriers
• Impede mating between species or hinder the
  fertilization of ova if members of different
  species attempt to mate
• Postzygotic barriers
• Often prevent the hybrid zygote from developing
  into a viable, fertile adult

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Prezygotic Barriers

• 1. Habitat isolation Two species living in
  different habitats in the same area encounter
  each other rarely, even though they are not
  technically geographically isolated. Example
  two species of garter snakes occur in the same
  areas but one species lives mainly in water and
  the other is mainly terrestrial and they seldom
  come into contact.
• 2. Behavioral isolation Species-specific
  signals and behaviors that attract mates are
  barriers among closely related species.
  Example Male fireflies of different species
  signal to females of the same species by blinking
  their lights in a characteristic pattern females
  discriminate among the different signals and
  respond only to flashes of their own species.
  Includes behavioral responses to chemical
  attractants, courtship rituals, bird and insect
  song, etc.
• 3. Temporal isolation Two species that breed at
  different times of day, seasons, or years dont
  mix their gametes. Example brown trout and
  rainbow trout cohabit the same streams, but brown
  trout breed in the fall and rainbow trout breed
  in the spring.
• ATTEMPT TO MATE
• 4. Mechanical isolation Morphological
  differences prevent mating. Example Male
  dragonflies use a pair of special appendages to
  clasp females during copulation. The males
  clasping appendages do not fit the form of the
  females of other species.

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• Prezygotic and postzygotic barriers

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Prezygotic Barriers

• ATTEMPT TO MATE
• 5. Gametic isolation Gametes of different
  species that do meet rarely complete
  fertilization (do not form a zygote).
• For animals that use internal fertilization the
  sperm of one species may not be able to survive
  the internal environment of the female
  reproductive tract of a different species.
• Cross-specific fertilization is also uncommon for
  animals that utilize external fertilization due
  to a lack of gamete recognition. Gamete
  recognition is based on specific molecules on the
  coats of the egg that adhere only to
  complementary molecules on sperm of the same
  species.

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Postzygotic Barriers

• When prezygotic barriers fail and a hybrid
  zygote forms, postzygotic barriers prevent
  development of a viable, fertile hybrid.
• 1. Reduced hybrid viability (hybrid inviability)
• Genetic incompatibility may cause the abortion
  of the hybrid at an embryonic stage. 
• Hybrids generally do not complete development,
  and those that do are frail and soon die. 
• 2. Reduced hybrid fertility (hybrid sterility)
• If two species mate and produce viable hybrid
  offspring, reproductive isolation is maintained
  if the hybrids are sterile. This prevents gene
  flow between the parent species.
• One cause of this barrier is that if
  chromosomes of the two parent species differ in
  number or structure, meiosis cannot produce
  normal gametes in the hybrid. Mules.
• 3. Hybrid breakdown
• In some crosses, the first generation hybrids
  are viable and fertile, but when these hybrids
  mate with each other, or with either parent
  species, the next generation is feeble or
  sterile.
• Example Different cotton species can produce
  fertile hybrids, breakdown occurs in the next
  generation when progeny of the hybrids die in
  their seeds or grow into weak defective plants.

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Limitations of the Biol. Species Concept

• The biological species concept cannot be applied
  to 
• 1. Organisms that are completely asexual. Some
  protists and fungi, some commercial plants
  (bananas), and many bacteria are exclusively
  asexual. 
• 2. Extinct organisms represented by fossils -
  must be classified by morphology.
• 3. Sexual organisms about which little is known.
• The species problem may never be completely
  resolved. It is unlikely that a single
  definition will ever apply in all cases.

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Other Definitions of Species

• The morphological species concept
• Characterizes a species in terms of its body
  shape, size, and other structural features
  useful in the field sometimes difficult to
  apply.
• The paleontological species concept
• Focuses on morphologically discrete species known
  only from the fossil record
• The ecological species concept
• Views a species in terms of its ecological niche
• The phylogenetic species concept
• Defines a species as a set of organisms with a
  unique genetic history

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Modes of Speciation

• The evolution of reproductive barriers that keep
  species separate is the key biological event in
  the origin of new species.
• An essential episode in the origin of a
  species occurs when the gene pool of a population
  is separated from other populations of the parent
  species. 
• This genetically isolated splinter group can
  then follow its own evolutionary course as
  changes in allele frequencies caused by
  selection, genetic drift, and mutations occur
  undiluted by gene flow from other populations.
• There are two general modes of speciation
  allopatric speciation and sympatric speciation.

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• Speciation can take place with or without
  geographic separation
• Allopatric speciation
• Sympatric speciation

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Allopatric (Other Country) Speciation

• In allopatric speciation
• Gene flow is interrupted or reduced when a
  population is divided into two or more
  geographically isolated subpopulations

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Geographic Barriers

• 1. Geological processes can fragment a
  population into two or more allopatric
  populations (having separate ranges).
• This can include emergence of mountain
  ranges, movement of glaciers, formation of land
  bridges, subsidence of large lakes. 
• 2. Small populations may become geographically
  isolated when individuals from the parent
  population travel to a new location. (splinter
  populations) 
• 3. The extent of geographical isolation
  necessary to separate two populations depends on
  the ability of the organisms to disperse due to
  the mobility of animals or the dispersibility of
  spores, pollen and seeds of plants.
• Example the Grand Canyon is an impassable
  barrier to small rodents, but is easily crossed
  by birds. As a result, the same bird species
  populate both rims of the canyon, but each rim
  has several unique species of rodents.

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• Once geographic separation has occurred
• One or both populations may undergo evolutionary
  change during the period of separation

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• In order to determine if allopatric speciation
  has occurred
• Reproductive isolation must have been established

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Conditions Favoring Allopatric Speciation

• When populations are separated, speciation can
  occur as isolated gene pools accumulate
  differences by microevolution. These differences
  may cause a divergence in phenotype between the
  isolated populations.
• 1. A small isolated population is more likely to
  change substantially enough to become a new
  species than is a large isolated population.
  There is more effect from genetic drift.
• 2. The geographic isolation of a small
  population usually occurs at the fringe of the
  parent population's range (peripheral isolate).
  As long as the gene pool is isolated from the
  parent population, a peripheral isolate is a good
  candidate for speciation for three reasons
• a. The gene pool of the peripheral isolate
  probably differs from that of the parent
  population from the outset. Fringe inhabiters
  usually represent the extremes of any genotypic
  and phenotypic clines in an original sympatric
  population. With a small peripheral isolate,
  there will be a founder effect with chance
  resulting in a gene pool that is not
  representative of the gene pool of the parental
  population.

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• b. Genetic drift will continue to cause chance
  changes in the gene pool of the small peripheral
  isolate until a large population is formed. New
  mutations or combinations of alleles that are
  neutral in adaptive value may become fixed in the
  population by chance alone, causing phenotypic
  divergence from the parent population.
• c. Evolution caused by selection is likely to be
  different in the peripheral isolate than in the
  parent population. The peripheral isolate
  inhabits a frontier with a somewhat different
  environment, and it will probably be exposed to
  different selection pressures than the parent
  population.
• Because of the severity of a fringe environment
  (Its already at the edge of the species range.),
  most peripheral isolates do not survive long
  enough to undergo speciation. Though most
  peripheral isolates go extinct, a small
  population can accumulate enough genetic change
  to become a new species in only hundreds to
  thousands of generations.
• NOTE Geographic barriers by themselves are NOT
  biological mechanisms of reproductive isolation
  and do not define species.