Speciation

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• Speciation

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The Biological Species Concept

• based primarily on the actual or potential
  ability to produce fertile offspring by members
  of a group of organisms and results in
  reproductive isolation from other such groups (by
  Ernst Mayr).
• Dependent on the existence of reproductive
  isolating mechanisms

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Prezygotic isolating mechanisms are those that
prevent breeding between members of two different
species or prevent the fertilization and include

• habitat isolation some geographic barrier
  (different continents) or when microhabitat
  partitioning occurs within the same geographic
• behavioral isolation Mating behaviors that are
  used to attract or signal potential mates are
  often different in the various species
• temporal isolation mating at different times of
  day or seasons
• mechanical isolation due to structural
  incompatibility
• gametic isolation it is possible that the gametes
  will not form a zygote

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Postzygotic reproductive isolating mechanisms are
those that occur after fertilization and include

• reduced hybrid viability. Because of genetic
  incompatibilities the embryo may cease
  development.
• reduced hybrid fertility. Many hybrids between
  two different species are not fertile or are
  largely sterile. For examples mules.
• hybrid breakdown. Even if the hybrid is not
  sterile and is able to produce offspring, those
  offspring are often sterile or feeble (often
  affects the F2).

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major problems with the BSC

• the BSC is not applicable for those organisms
  that reproduce asexually or those that produce
  fertile hybrids (e.g., some plants). It also
  does not apply to extinct species or to those
  that are geographically separated (or allopatric)

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Examples of other species concepts

• Ecological Species Concept identifies a species
  in terms of its ecological niche (the role that
  it plays in its environment).

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• Pluralistic Species Concept defines a species
  based on some key unifying factor(s) that vary
  for some the unifying factor is the niche, for
  others it is reproductive isolation, and for some
  a combination of the two.

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• Morphological Species Concept identifies
  evolutionarily independent lineages by
  differences in size, shape, and other
  morphological features. The logic behind this
  species concept is that distinguishing features
  are most likely to arise if populations are
  independent and isolated from gene flow.

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• The Phylogenetic Species Concept is based on
  reconstructing the evolutionary history of
  populations

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• Paleontological Species concept is the only way
  that some species (e.g., some extinct) can be
  classified and involves the use of the fossil
  record

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Definition and Conservation
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• Speciation or the origin of new species is the
  outcome of isolation and divergence. Isolation
  is created by reductions in gene flow and
  divergence is created when mutation, genetic
  drift, and selection act on the separated
  populations

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Allopatric speciation

• A population becomes geographically separated,
  then as selection pressures change for the 2
  populations, there are changes in allele
  frequency that can ultimately lead to formation
  of organisms that are different enough to be
  considered to be of different species.

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Sympatric speciation

• A new species arises without any geographic
  separation of a population. An example in plants
  would be the evolution of a new species due to
  polyploidy occurring. This can happen in a
  hybrid plant resulting from the interbreeding of
  two parental plant species.

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Gradualism and Punctuated Equilibrium
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• Phylogeny and Systematics (an overview) Chapter 26

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Phylogeny is the evolutionary history of a
species or group of species. Phylogenies are
based on

• fossil records
• morphological similarities
• molecular homologies (homologies are similarities
  that are due to shared ancestry whereas analogies
  are similarities that are due to convergent
  evolution instead of shared ancestry).

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• Systematics is an analytical approach to
  understanding the diversity and relationships of
  organisms (extant as well as extinct). Now the
  use of molecular systematics (using comparisons
  of DNA, RNA, proteins etc.) allows for more
  realistic evolutionary relationships.

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• Recall that scientists strive to categorize
  living organisms into increasingly inclusive
  taxa. The goal is to show the evolutionary
  relationships of the various species. One of the
  common methods for studying phylogenetic
  systematics (classification based on evolutionary
  history) is cladistic analysis

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Cladistics

• A phylogeny is a phylogenetic diagram that shows
  patterns of shared characteristics based on
  cladistics (the analysis of how species may be
  grouped into clades).

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• Each branch of the phylogeny is called a clade
  but note that clades are like the taxa (genus,
  family etc.) and thus can be nested within larger
  clades.
• A clade is defined as a group of species that
  includes an ancestral species and all of its
  descendents.

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• Identifying shared primitive and shared derived
  characters also helps with cladistic analyses.
  An outgroup is useful for the comparisons.
• Note that data on the taxa being studied are
  input into computer programs which often generate
  multiple cladograms and that the cladogram or
  cladograms that is/are most parsimonious are the
  ones that are considered to be the ones that are
  most likely to reflect the phylogeny.

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