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Speciation
Speciation
Speciation is the origin of new species
A species is a population or group of
populations whose members have the potential
to interbreed with one another in nature and
produces viable, fertile offspring
This definition works only for sexually-reproducing
individuals
Diversity Within A Species
 There is much
diversity within a
species
A businesswoman in
Manhattan could
produce offspring with a
dairyman in Mongolia
Reproductive Barriers
 Reproductive barriers prevent organisms from
two different species from producing viable,
fertile offspring
 Prezygotic Barriers
Prevent mating between species
Do not allow the egg to be fertilized if members of
different species attempt to mate
 Postzygotic Barriers
Prevent the zygote from developing into a viable, fertile
adult if the egg does become fertilized
Prezygotic Barriers
 Habitat Isolation
Species that live in
different habitats within
the same area will
rarely (if ever)
encounter one another
 Behavioral Isolation
Special signals that
attract mates are
species-specific
Ie. courtship rituals
Prezygotic Barriers
 Temporal Isolation
Species that breed during different times of the day,
different seasons, or different years cannot mix their
gametes
 Mechanical Isolation
Anatomically incompatible species
 Gametic Isolation
Sperm of one species can’t survive in the female
reproductive tract of another species
Ova may only recognize sperm of their own species
Postzygotic Barriers
Reduced Hybrid Viability
Offspring dies during embryonic development
Reduced Hybrid Fertility
Even if hybrids survive, they are usually sterile
Mule – horse & donkey
Hybrid Breakdown
Even if the first generation is fertile and viable,
offspring of the first generation is infertile
Modes of Speciation
1. Allopatric Speciation
 Speciation that occurs based on the
geographic relationship of a new species to
its ancestral species (Geographic barrier
separates organisms).
2. Sympatric Speciation
 Speciation that occurs when a subpopulation
becomes reproductively isolated in the midst
of its parent population (No geographic
barrier)
Modes of Speciation
Allopatric Speciation
 Occurs as a result of
geographical
isolation
Mountain range, river,
etc.
 Example: different
species of closelyrelated squirrels on
opposite rims of the
Grand Canyon
Adaptive Radiation
 Adaptive radiation is
the evolution of many
diversely adapted
species from a
common ancestor
Darwin’s finches
Especially common on
island chains
(Galapagos, Hawaiian
islands)
Sympatric Speciation
 Sympatric speciation
occurs when new
species arise within
the range of the
parent population
 Reproductive isolation
evolves without
geographical isolation
Sympatric Speciation
Animals may become reproductively
isolated within the geographical range of a
parent population if genetic factors cause
them to become fixed on resources not
used by the parent population
Different food source, different breeding
grounds, etc.
This sets the stage for speciation to occur
The Tempo of Speciation
 There are 2 alternate
views of how quickly
speciation occurs:
Gradualism
Punctuated Equilibrium
Gradualism
 Species, descended
from a common
ancestor, diverge
more and more in
morphology as they
acquire unique
adaptations
Punctuated Equilibrium
 A new species
changes most as it
“buds” from a parent
species and then
changes little for the
rest of its existence