Speciation prezygotic and postzygotic

speciation lesson for AP Biology

1. Warm up question
•What is a zygote?

2. Quick Write @ end of class
•Normal watermelon plants are diploid (2n = 22),
but breeders have produced tetraploid (4n = 44)
watermelons. If tetraploid plants are hybridized
with their diploid relatives they produce triploid
(3n = 33) seeds. These offspring can produce
triploid seedless watermelons and can be further
propagated by cuttings. Are the diploid and
tetraploid watermelon plants different species?
Explain.

3. CH 22: The Origin of SpeciesCH 22: The Origin of Species
Overarching Question:Overarching Question:
How does How does speciationspeciation (the development of a (the development of a
new species) fit into evolutionary theory?new species) fit into evolutionary theory?

4. Biological Species Concept
Decide whether the organisms in each of the following
examples belong to the same species:
Example A Example B Example C
Harris’ antelope squirrel
vs. white-tailed antelope
squirrel
Various breeds of dogs
Donkey, horse, mule

5. The biological species concept =population whose
members have the potential to naturally produce viable
offspring

6. What are some things that might keep What are some things that might keep
organisms from breeding successfully??organisms from breeding successfully??
Modes of reproductive isolation (blocking gene flow between
populations)
Achieved through mechanisms that can be classified into two main
categories:
1. Prezygotic barriers- impede mating before fertilization can even
take place
2. Postzygotic barrier- prevent a hybrid zygote from developing into
a viable, fertile adult.

The Grand Canyon separating
land animals-pre/postzygotic?
Pre/postzygotic?

8. Prezygotic BarriersPrezygotic Barriers
Habitat/geographic
isolation: - live in different
places
e.g. Two snakes of the same
genus live in the same area, but
one is aquatic & the other is
terrestrial-they just don’t have
many encounters because they
spend their time in different
places

9. Temporal isolation – timing is different
these skunks live in the same area, but
they are reproductively active during
different seasons
Do a little dance….
Behavioral isolation – mating rituals
different e.g. certain courtship rituals are
performed before mating takes place
Prezygotic BarriersPrezygotic Barriers

10. Prezygotic BarriersPrezygotic Barriers
Mechanical Isolation – parts don’t fit or come
together e.g. physical differences inhibit
cross-fertilization2 flowers of the same
genus have different colors and therefore
attract different pollinators
Gametic Isolation: biochemical mechanisms
may prevent gametes from fusing, or the
reproductive tract in the
female may be an unfavorable
environment for sperm

12. Postzygotic BarriersPostzygotic Barriers
•Reduced hybrid viability- hybrids
do not complete development
Hybrid breakdown- F
1
s are viable,
but later generations are not
Reduced hybrid fertility- the hybrid
offspring is sterile

13. Quick Think
1.Define REPRODUCTIVE ISOLATION
2.List the 5 pre-zygotic and 3 post-zygotic
barriers to reproduction

14. Quick Think
•2 species of bird in a forest are not known
to interbreed. One species feeds and mates
in the treetops and the other on the ground.
But in captivity, the 2 species can
interbreed and produce viable, fertile
offspring. What type of reproductive barrier
most likely keeps these species separate?
Explain your reasoning.

15. Limitations of the Biological Species Concept
•You can’t define a species by reproductive
isolation if the organism reproduces
asexually, as is the case in many bacteria
•What if you’re studying 2 extinct
organisms??? How are you going to be
able to tell whether or not they can mate &
produce viable offspring??
Are these the
same
species?
Are these the same species?

16. Other Definitions of SpeciesOther Definitions of Species
•Morphological species concept~ a species is defined by body shape,
size, & structure
Pros: can be applied to fossils & asexual organisms
Cons: the criteria are subjective
•Paleontological species concept~ Classifies fossils based solely on
morphological differences.
Pros/cons exist, but this is our only option with extinct species…unless you can
get a DNA sample
•Ecological species concept~ Distinguishes between species based on
their ecological niche-its biological role in the community
Pros: can be used for sexual & asexual organisms bacteria can be classified
based on their food source
Cons: two organisms may have a distinct niche, but still be able to interbreed
successfully
•Phylogenetic species concept~ a species is a set of organisms that
share a unique genetic history. Compares physical characteristics as
well as molecular sequences.
Pros: can distinguish between species that are morphologically similar. Often
times all you need is a DNA sample
Cons: hard to say how genetically different something has to be before it can be
considered a different species

17. The Bottom LineThe Bottom Line
•Species is somewhat of a vague term in that there is no
single system for classifying one group of organisms as
separate from another…although there is an effort to
find one “uniform species concept” 
•Researchers are often forced to use a different definition
for “species” based on the circumstances & info they
have
Although the biological species concept is still
heavily used, the advent of DNA technology
reinvented the phylogenetic species concept and
cladistics. In fact, genetic
similarity/dissimilarity is leading to massive
reclassifications. Example= in plants, the
classification as a dicot is being phased out

18. Modes of SpeciationModes of Speciation
•Allopatric Speciation VS Sympatric SpeciationAllopatric Speciation VS Sympatric Speciation
Allopatricpart of the
population is
geographically isolated
from the rest of the
population & forms a
new species since it is
reproductively isolated
Sympatric speciation
occurs while the
organisms are still
cohabiting.

19. Allopatric SpeciationAllopatric Speciation
•Example of geographic barriers: Mountain
ranges, rivers, land bridges (the Isthmus of
Panama), a big lake becoming a bunch of
smaller lakes, the Grand Canyon...

20. Allopatric SpeciationAllopatric Speciation
•An example of Allopatric Speciation in the Grand
Canyon: Antelope Squirrels. The separated gene
pools diverge via mutations, natural selection, sexual
selection, genetic drift, etc.

21. Allopatric Speciation: Allopatric Speciation:
A Non-ExampleA Non-Example
•These Florida Keys natives are a
non-example. Why is this a
non-example???

22. Sympatric SpeciationSympatric Speciation
•“Barriers” in sympatric speciation include
chromosomal changes, habitat differentiation, &
sexual selection.
–Polyploidy:
Tetraploids can only produce viable
offspring with themselves (self-
pollination) or other tetraploids=
reproductive isolation

23. Sympatric SpeciationSympatric Speciation
•“Barriers” in sympatric speciation include chromosomal changes,
habitat differentiation, & sexual selection.
–Habitat Differentiation:
In soapberry bugs, beak
length dictates which fruit
on a plant the insects eat.
Even though the 2 species
live on the same plant, they
don’t run into each other
often since they have different
food preferences = decreased
gene flow.

24. Sympatric SpeciationSympatric Speciation
•“Barriers” in sympatric speciation include chromosomal changes,
habitat differentiation, & sexual selection.
–Sexual Selection: Cichlids in Lake Victoria. These 2
similar species vary in color. Females of the 2 species
preferentially mate with males who have
the same
coloration
that they
do.

25. Quick Think
•How are sympatric and allopatric speciation
similar and how are they different?

26. Quick Think
•Explain why allopatric speciation would be
less likely to occur on an island close to the
mainland than on a more isolated island of
the same size.

27. The Tempo of Speciation: How Fast Does All
This Happen?
•A couple of suggestions: Gradualism VS
Punctuated Equilibrium
Gradualism: species P.E.: new species
diverge slowly over emerge suddenly
time then stabilize for a
while before
another divergence
What type of
speciation do you
think we might see
with the finches on
the Galapagos?
How is speciation
of this type related
to the environment?

28. Adaptive RadiationAdaptive Radiation
•Diversely adapted species stemming from a common
ancestor
–Often occurs when a founder population settles in a new
environment, or when extinction opens up an ecological
niche.
–These species
of silver sword share
a common ancestor,
but have diverged
after colonizing
different
Hawaiian islands

29. Macroevolutionary Changes Can Accumulate
Through Many Speciation Events
•Evolutionary
Novelty: in most
cases, complex
structures
evolved in
increments from
simpler
prototypes in
other species

31. •Exaption: An existing structure that is
co-opted after having previously served
a different function. Ex: feathers may
have been favored
by sexual selection
before being used
to assist in flight

32. Evolution of the Genes That Control Development Evolution of the Genes That Control Development
•Heterochrony-evolutionary change in the rate/timing of
developmental events; this can result in a change in the
overall appearance of a species. A chimp’s
jawbone elongates at a faster
rate relative to the rest of its
skull, thus giving it its
characteristic shape. Note that
early in development, the
human &chimp skull look similar.

33. Evolution of the Genes That Control DevelopmentEvolution of the Genes That Control Development
•Paedomorphosis: Sexual maturity is accelerated
–Ex: most salamander species undergo metamorphosis
before sexual maturity. A relatively small genetic
change accelerates sexual development to the point
where this salamander is an adult
even though it still has
larval features such as gills

34. Changes in Spatial PatternChanges in Spatial Pattern
•Recall: Homeotic Genes determine where limbs
grow on the general body plans of organisms
–HOX genes are specific homeotic genes in animal
embryos
–Here, turning on additional
HOX genes allows the buds
to give rise to digits in
chickens
Determine the
fate of whole
groups of cells

35. A “Rough Sketch” of the Evolution A “Rough Sketch” of the Evolution
of Vertebrates from Invertebratesof Vertebrates from Invertebrates
•Alterations are
associated with HOX
genes as well
Most inverts only have one cluster
of Hox genes
A duplication mutation could have
led to the evolution of the first verts.
A duplication mutation or a
nondisjunction mutation may have
given rise to the 4 sets of Hox genes
seen in verts today

36. Remember, Evolution is NOT Goal Remember, Evolution is NOT Goal
OrientedOriented
•Species are not trying to become better…

37. Check for Understanding
1.Two species of birds in a forest are not
known to interbreed. One species feeds
and mates in the treetops and the other on
the ground. But in captivity, the two
species can interbreed and produce viable,
fertile offspring. What type of
reproductive barrier most likely keeps
these species separate?

38. Check for Understanding
•Normal watermelon plants are diploid (2n = 22),
but breeders have produced tetraploid (4n = 44)
watermelons. If tetraploid plants are hybridized
with their diploid relatives they produce triploid
(3n = 33) seeds. These offspring can produce
triploid seedless watermelons and can be further
propagated by cuttings. Are the diploid and
tetraploid watermelon plants different species?
Explain.

39. Quick Write
•How can the Darwinian concept of descent
with modification explain the evolution of
such complex structures as the vertebrate
eye or heart?