SPECIATION -The Origin of Species
What is speciation?
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Recap:
Species = a population or group of populations
whose individual members can interbreed to
produce viable, fertile offspring
Speciation = the formation of a new species from
existing species
Sometimes called macroevolution
Process of speciation:
Parent
species
I. Barrier
2 separate species
II. Diverge
III. Don’t interbreed
Reproductive Isolation May Occur With or Without
Geographic Isolation
Allopatric speciation
occurs when
geographic isolation
creates a reproductive
barrier (an extrinsic
mechanism).
Sympatric speciation
occurs when a
reproductive barrier is
created by something
other than geographic
isolation (intrinsic
mechanisms).
Allopatric Speciation
Harris’ antelope squirrel
White-tailed antelope squirrel
Two species of ground squirrel are postulated to have descended from
a common ancestral population that was separated by formation of the
Grand Canyon.
Intrinsic Reproductive Isolating Mechanisms Are Always
Required for Speciation
Harris’ antelope squirrel
White-tailed antelope squirrel
Intrinsic mechanisms involve changes to organisms that prevent
interbreeding.
In allopatric speciation, intrinsic mechanisms come into play
once populations are physically separated.
In sympatric speciation, intrinsic mechanisms are the only
ones involved.
Isolating Mechanisms
PREZYGOTIC BARRIERS
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Prevents mating or
egg fertilization if
members of different
species try to mate
POST ZYGOTIC BARRIERS
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Prevents viability of
offspring or their
ability to reproduce
Many Intrinsic Reproductive Isolating Mechanisms Drive Speciation
(different habits within an overlapping range)
Many Intrinsic Reproductive Isolating Mechanisms Drive Speciation
Courtship rituals, like these, are critical for mating within a
species, but ineffective for attracting members of other
species.
Many Intrinsic Reproductive Isolating Mechanisms Drive Speciation
A summary of reproductive
barriers between closely related
species
Many Intrinsic Reproductive Isolating Mechanisms Drive Speciation
Tigon
Result of male tiger and female lion
mating in captivity. Offspring are infertile.
Separated both geographically and
ecologically.
Liger
Result of male lion and female tiger
mating in captivity. Offspring are
infertile.
Adaptive Radiation
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The diversification of a common ancestral species
into a variety of differently adapted species
Studies on islands – great "laboratories" to see how
a population changes in response to new
environmental conditions in isolation
Adaptive Radiation
An example of adaptive radiation – these
species all diverged from a common ancestor
(founder species)
FOUNDER SPECIES
Types of Evolution
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Divergent – species that were once similar diverge
or become increasingly distinct
 Ex:
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finch beaks
Convergent – two unrelated species share similar
traits because each has independently adapted to
similar environmental conditions
 Ex:
bird and bee wings
Speciation Occurs at Widely Differing Rates
A slow rate of speciation evidenced by a living horseshoe crab
(13 extant species) and a 300 million year-old fossil species
A rapid rate of speciation evidenced by Galapagos finches which
have diversified into 13 species within the last 100,000 years.
Speciation Rates
Generalists, like the horseshoe crab, tend to remain as stable
species.
Specialists, like the Galapagos finch, tend to
be unstable as species.
Species Come and Go
Best estimates from the fossil record indicate that greater than 99%
of species that have exited are now extinct.
A typical “lifetime” for a
species is about 1 million
years.
Mass Extinctions Are a Fact of Life
Speciation Dynamics - Gradualism or Punctuated Equilibrium?
Slow and steady change
Punctuated equilibrium
appears to be a more
accurate view of speciation
dynamics.
Does Evolution Create the Perfect Organism?
No, only better organisms as evolution is
constrained by history and buffeted by
random events.
Essentially, every organism on earth is in
significant part a sum of accidents.
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P. 373 #1-4, 7, 8, 11
Review Questions: text p. 387 #2, 3, 5-23, 25-27,
29-33, 43-47, 52, 53