Speciation

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Speciation

• The evolution of new species.

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• A review of Natural Selection. . .
• More individuals are produced than can live to
  reproduce.
• Individuals of a species show variation in any
  generation.
• Some variations are genetically determined.
• Individuals with certain genes that give them a
  more adapted phenotype are more likely to live
  and reproduce.
• Over time, certain characteristics (and the genes
  that cause them) become more common in a
  population.

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A species is a group of organisms with a common
gene pool which is usually isolated from the gene
pools of other species. A species is the largest
unit in which gene flow is possible. Species are
capable of interbreeding in the wild to produce
fertile offspring. Much classification has been
done with morphology (appearance). Morphology is
not enough. Dogs are all the same species, but
have very different appearances.
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Usually each species is composed of populations
that are very distinct and have their own gene
complexes. Subspecies or varieties are connected
by gene flow and can interbreed to produce viable
offspring.
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Biological Isolating Mechanisms. . . Most species
have two or more isolating mechanisms. Isolating
mechanisms preserve the integrity of each
species gene pool, because gene flow between
them is prevented. These isolating mechanisms can
work between populations of the same species - if
they are isolated from each other long enough to
diverge sufficiently in adapting to their own
environments, they may become different species.
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• Prezygotic Isolating Mechanisms. . .
• Temporal Isolation - mating occurs at different
  times (day, season, year). This mechanism works
  for both plants and animals.
• Ecological Isolation - the organisms live and/or
  breed in different habitats. Example - wood
  frogs in temporary woodland ponds and bullfrogs
  in larger, permanent bodies of water.
• Behavioral Isolation - Courtship, songs, dances
  and displays. Mating signals are important to
  individual species.
• spider mating behavior
• Starling mating behavior

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• Mechanical Isolation - Incompatible genital
  structures. This mechanism prevents breeding
  between certain insect species. (Plants may have
  different floral structures).
• Gametic Isolation - Mating may occur, but if the
  sperm and egg are chemically different, they
  wont join. (In aquatic animals, eggs may have
  proteins that will not connect with incompatible
  sperm.)

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• Postzygotic Isolating Mechanisms. . .These insure
  reproductive failure after fertilization. They
  may form hybrids that have many disadvantages.
• Hybrid Inviability - Generally the hybrid zygote
  is aborted. The embryo simply does not develop
  normally.
• Hybrid Sterility - The F1 survives and matures
  but is sterile. Male donkey (2n 62) X Female
  horse (2n 64) mule (2n 63)
• Hybrid breakdown - The F1offspring are fertile,
  but the F2 or F3 become defective. An example of
  this is the Sunflower Layia.

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New species do arise even with isolating
mechanisms in place. . . Allopatric Speciation -
This type of speciation is caused by geographic
isolation and is the most common. Barriers such
as glaciers, mountains, deserts, and changes in
continental formation cause allopatric
speciation. What acts as a barrier to one species
may not be a barrier to another species. This
usually occurs with small populations (example -
Galapagos Islands and Hawaiian Islands). Genetic
drift occurs (minor differences are already there
and are amplified by selection to a new
environment). Most populations that are isolated
from the parent population and subjected to
selective pressures become extinct.
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Some populations survive. Over time, adaptive
changes cause the isolated populations gene pool
to diverge from the original so much that they
cannot interbreed again, even if their range were
to become continuous again. There is a more
rapid change in a gene pool in small populations.

• There are three possibilities if a barrier is
  removed after a period of time
• The population has diverged to the point that it
  cannot mate with the parent population -
  Speciation has occurred.
• Mating can occur with the parent population and
  gene flow is restored.
• Speciation has not quite occurred, but the
  isolated population is very different from the
  parent population. There is little gene flow
  between the two groups and more divergence will
  occur.

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A good example
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Sympatric Speciation - Formation of a new species
within the same geographic region as the parent
species. Far more common in plants. A doubling
of the chromosomes occurs before meiosis, this
can sometimes result in polyploidy - especially
in plants. When this occurs with mating from
more than one species, it is called
allopolyploidy and can result in fertile
offspring. Sterility is common when two parent
species have different chromosome numbers.
Allopolyploids can mate with themselves
(self-fertilization) or with similar individuals
but cannot breed with the parents. The
chromosome numbers are too different. When a new
species of allopolyploids does arise, selective
pressures will cause one of three possible
outcomes...
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• The population cant compete and becomes extinct.
• The population will fill a new niche and will
  coexist with both parent species.
• If the new species is more fit than both parent
  species, it will replace them.
• Hybridization/polyploidy is rare in animals but
  is a significant factor in the evolution of
  plants. Just less than 1/2 of all flowering
  plants are thought to be polyploid. (Most are
  allopolyploids).
• This is a cause of rapid speciation.

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Sympatric speciation
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Anagenesis - In this type of speciation, an
entire population is converted over time to a
form that is so different from the original that
it is considered to be a new species. Anagenesis
can be used to describe the evolution of the
horse from the Eohippus to its present form.
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More common than anagenesis is Cladogensis, a
type of diversifying speciation in which one or
more new species are derived from the parent
species, which continues to exist.
Cladogenesis is the only process that increases
biological diversity (increases total number of
species). Adaptive Radiation is repeated
cladogenesis. It can lead to many new species
coming from a single ancestral species. Darwins
finches are an excellent example of adaptive
radiation.
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