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Chapter 26

BIOLOGY 1M03 Chapter 26: Chapter 26

Course Code
Jon Stone

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 the evolution of two or more distinct species from a single
ancestral species
oproduced the great diversity of life
If gene ow (movement of alleles from one population to another) ends,
allele frequencies in isolated species are free to diverge
If  cause 
to suciently, then process of speciation takes place (distinct
species form)
Speciation is splitting event that creates two or more distinct species from
single ancestral group
oComplete speciation results in new branch in tree of life
 ! "
Species distinct because genetic characteristics di!er
Genetic distinctions occur because mutation, selection and drift act on
each species independently of what is happening in other populations
Four criteria for identifying species:
1. The Biological Species Concept

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2. The Morphospecies Concept
3. The ecological Species Concept
4. The phylogenetic Species Concept
The Biological Species Concept:
oCritical criterion: 
oGroups that both naturally/potentially interbreed and are
reproductively isolated from other groups (do not breed with other
oReproductively isolated populations (those who do not interbreed or
mate) are evolutionary independent
oTwo forms of isolation:
i. #$%prevents ind. of di!erent species from
ii. #$% o!spring of matings b.w members of
di!. species do not survive or reproduce
oDisadvantages to theory:
Cannot be evaluated in fossils
Cannot be evaluated in species that reproduce asexually
Dicult to apply when closely related populations do not
happen to overlap with each other geographically (in this
case, have to predict if interbreeding and gene ow would
occur if populations came in contact)
Evident that many species DO HYBRIDIZE to limited extent in
o& ' 
The Morphospecies Concept:
oIdentify evolutionarily independent lineages by di!erences in size,
shape, or other morphological features

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oDistinguishing features are most likely to arise if populations are
independent and isolated from gene ow
oDisadvantages to concept:
i. Cannot identify cryptic species (individuals that are
morphologically identical but from di!erent species)
ii. Morphological features used to distinguish species are
Researchers may disagree on the characteristics that
distinguish species
The Ecological Species Concept:
oDe<nes a species a set of organisms exploiting a single set of
resources, having same range of environmental tolerances, and
facing the same predators and parasites
oEmphasizes on role of natural selection
Favours characteristic traits that bene<t organism in using
speci<c resources and coping with speci<c set of envion.
oMost useful for identifying species of bacteria, Archaea, or asexual
eukaryotes because cannot be de<ne by reproductive isolation
Instead, distinguish species by their adaptation to di!erent
ecological conditions
The Phylogenetic Species Concept:
oSpecies are de<ned as the smallest monophyletic groups on the
tree of life
Populations that share one or more unique synapomorphies
%% Trait found in certain groups of
organisms that exist in no others
A homologous trait that is unique to population
)%* consists of ancestral population and
all of its descendants and ONLY those descendants
oAdvantages to concept:
i. Can be applied to ANY population (fossils, asexual or sexual)
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