Transcript 1. Distinguish between anagenesis and cladogenesis.

LE 24-4ab
Postzygotic barriers prevent a hybrid zygote from
developing into a viable, fertile adult
Reduced
hybrid
viability
Reduced
hybrid
fertility
Hybrid
breakdown
Viable,
fertile
offspring
Fertilization
REDUCED HYBRID
VIABILITY
REDUCED HYBRID
FERTILITY
HYBRID BREAKDOWN
Chapter 24 Reading Quiz
1.
2.
3.
4.
5.
What is the term for the origin of a new
species?
Evolution of many species from one common
ancestor is known as…
____ is the mutant condition when there are
extra sets of chromosomes.
The concept that species diverge in spurts of
relatively rapid change is…
True or False: An evolutionary trend does not
mean that evolution is goal oriented.
1. Distinguish between anagenesis and cladogenesis.
Anagenesis  (phyletic evolution) the
transformation of an unbranched lineage of
organisms, sometimes to a state different enough
from the ancestral population to justify renaming
it as a new species
Cladogenesis  (branching evolution) the budding
of one or more new species from a parent species
that continues to exist
- this is more common and promotes biological
diversity 
2. Define morphospecies and explain how this concept
can be useful to biologists.
“Morphospecies” characterizes species on
the basis of measurable physical features
Species  Latin term meaning “kind” or
“appearance”
Linnaeus described species in terms of
their physical form (morphology) and it is
still the most common method used for
describing species 
3. Define biological species (E. Mayr).
It is a population or group of populations
whose members have the potential to
interbreed with one another in nature and
to produce viable, fertile offspring, but
cannot produce viable, fertile offspring
with members of another species 
4. Describe some limitations of the biological species
concept.
Reproductive barriers prevent
interbreeding between closely related
species
A “species” is the largest unit of population
in which gene flow is possible 
5. Explain how gene flow between closely related
species can be prevented.
Reproductive barriers do not allow gene
flow to occur
Reproductive isolation occurs in natural
environments 
6. Distinguish between prezygotic and postzygotic
isolating mechanisms.
Prezygotic  impedes mating between
species or hinders fertilization of the ova
should members of a different species
attempt to mate
Postzygotic  prevents the hybrid zygote
from developing into a viable, fertile adult

7. Describe five prezygotic isolating mechanisms and
give an example of each.
1.
Habitat isolation  two species living in
different habitats within the same area
may encounter each other rarely
ex: two species of garter snake, one is in
the water, and one is on land 
#7 continued…
2. Behavioral isolation  species-specific
signals and elaborate behavior to attract
mates are important reproductive
barriers among closely related species
ex: fireflies, moths (pheromones) 
#7 continued…
3. Temporal isolation  two species that
breed at different times of the day,
seasons, or years do not mix gametes
ex: brown and rainbow trout
- brown only mate in the fall
- rainbow only mate in the spring 
# 7 continues….
4. Mechanical isolation
 anatomical
incompatibility may
prevent sperm
transfer when closely
related species
attempt to mate
ex: plant floral
anatomy may prevent
pollen from entering
into ovule 
The end of # 7
5. Gametic isolation  gametes of different
species that meet rarely fuse to form a
zygote, but it may not survive in the female
internal environment 
8. Explain why many hybrids are sterile.
If the chromosomes of the two parent
species differ in number or structure,
meiosis cannot produce normal gametes in
the hybrid
Ex: mule and liger 
9. Explain in your own words how hybrid breakdown
maintains separate species even if gene flow occurs.
If species cross-mate, the first generation
hybrid may be viable and fertile, but when
these hybrids mate with one another or
with the parent species, offspring will be
feeble or sterile 
10. Distinguish between allopatric and sympatric
speciation.
Allopatric  speciation that occurs when
the initial block to gene flow is a
geographical barrier that physically
isolates the population (ocean, mountains)
Sympatric  the formation of a new
species within the range of parent
populations
- reproductive isolation evolves without
geographical isolation 
11. Explain, in your own words, the allopatric speciation model
and describe the role of intraspecific variation and geographical
isolation.
Allopatric speciation  speciation that occurs
when the initial block to gene flow is a
geographical barrier that physically isolates the
population
 A small isolated population is more likely to change
substantially enough to become a new species than
a large isolated population
 Small populations may become geographically
isolated from the parent population 
12. Explain why peripheral isolates are susceptible if
geographic barriers arise.
1.
2.
3.
As long as the gene pools are isolated from the parental
population, peripheral isolates are good candidates for
speciation for 3 reasons:
The gene pool of the peripheral isolate probably differs
from that of the parent population from the outset
Genetic drift will continue to cause chance changes in the
gene pool of the small peripheral isolates until a large
population is formed
Evolution caused by a selection is likely to take a different
direction in the peripheral isolate than in the parental
population 
13. Describe the adaptive radiation model and use it to describe how it
might be possible to have many sympatric closely related species even if
geographic isolation is necessary for them to evolve.
Adaptive radiation  the evolution of many
diversely adapted species from a common ancestor
Ex: Galapagos finches
 ancestral finch from South America speciated
on the Galapagos Islands
 can occur quickly if a genetic change results in a
reproductive barrier between the mutants and the
parent population 
14. Define sympatric speciation and explain how
polyploidy can cause reproductive isolation.
Sympatric speciation  the formation of
new species within the range of parent
populations
Polyploidy is a mutant condition which
originated from improper cell division =
extra sets of chromosomes
ex: larger fruits due to polyploidy 
15. Distinguish between autopolyploidy and
allopolyploidy.
Autopolyploid  an organism that has more than 2
chromosome sets, all derived from a single species
ex: nondisjunction, self-fertilization
Allopolyploid  a polyploid hybrid resulting from
contributions by 2 different species
 when two different species interbreed and a
hybrid is made
 these sterile hybrids are more vigorous and can
propagate asexually (plants) 
16. List some points of agreement and disagreement between the two
schools of thought about the tempo of speciation (gradualism vs.
punctuated equilibrium).
 Gradualism  species gradually diverge with each new
species evolving continuously over long spans of time
 Punctuated equilibrium  species appear and disappear
suddenly with a lack of transitional forms in the fossil
record
 “sudden” can refer to 1000’s of years on the geological time
scale
 Stabilizing selection would hold a population at one adaptive
peak to produce long periods of stasis
 Gradualists feel that species continue to change during
these periods in ways undetectable in fossils 
17. Describe the origins of evolutionary novelty.
1.
2.
3.
Most evolutionary novelties are modified versions of older
structures
 one mechanism is the gradual refinement of existing
structures for new functions
 “exaptation” is a term applied to a structure that
evolves in one context and becomes co-opted for another
function (ex: wings & light bones on birds)
Genes that control development play a major role in
evolutionary novelty
 requires large modifications at many gene loci
An evolutionary trend does not mean that evolution is goal
oriented
 the term “progression” is misleading 