Allopatric and Sympatric
Speciation: A Review
• In allopatric speciation, geographic
isolation restricts gene flow between
populations
• Reproductive isolation may then arise by
natural selection, genetic drift, or
sexual selection in the isolated
populations
• Even if contact is restored between
populations, interbreeding is prevented
Allopatric and Sympatric
Speciation: A Review
• In sympatric speciation, a
reproductive barrier isolates a
subset of a population without
geographic separation from the
parent species
• Sympatric speciation can result
from polyploidy, natural selection,
or sexual selection
Animals
• Don't form polyploids and will
use other mechanisms.
Concept 24.3: Hybrid zones
provide opportunities to study
factors that cause reproductive
isolation
• A hybrid zone is a region in
which members of different
species mate and produce
hybrids
Patterns Within Hybrid Zones
• A hybrid zone can occur in a single
band where adjacent species meet
• Hybrids often have reduced fitness
compared with parent species
• The distribution of hybrid zones can
be more complex if parent species
are found in multiple habitats within
the same region
A narrow hybrid zone for B. variegata
and B. bombina in Europe
EUROPE
Fire-bellied
toad range
Hybrid zone
0.99
Allele frequency (log scale)
Yellow-bellied toad,
Bombina variegata
Yellow-bellied
toad range
Fire-bellied toad,
Bombina bombina
0.9
0.5
0.1
0.01
40
20
30
10
0
10
Distance from hybrid zone center (km)
20
Hybrid Zones over Time
• When closely related species
meet in a hybrid zone, there are
three possible outcomes:
–Strengthening of reproductive
barriers
–Weakening of reproductive
barriers
–Continued formation of hybrid
individuals
Fig. 24-14-1
Formation of a hybrid zone and
possible outcomes for hybrids
over time
Gene flow
Population
(five individuals
are shown)
Barrier to
gene flow
Formation of a hybrid zone and
possible outcomes for hybrids
over time
Isolated population
Fig. 24-14-2
diverges
Gene flow
Population
(five individuals
are shown)
Barrier to
gene flow
Formation of a hybrid zone and
possible outcomes for hybrids
over time
Isolated population
Fig. 24-14-3
diverges
Hybrid
zone
Gene flow
Hybrid
Population
(five individuals
are shown)
Barrier to
gene flow
Formation of a hybrid zone and
possible outcomes for hybrids
over time
Isolated population
Possible
Fig. 24-14-4
diverges
Hybrid
zone
outcomes:
Reinforcement
OR
Fusion
Gene flow
Hybrid
Population
(five individuals
are shown)
OR
Barrier to
gene flow
Stability
Reinforcement: Strengthening
Reproductive Barriers
• The reinforcement of barriers
occurs when hybrids are less fit than
the parent species
• Over time, the rate of hybridization
decreases
• Where reinforcement occurs,
reproductive barriers should be
stronger for sympatric than
allopatric species
Fig. 24-15
Allopatric male
pied flycatcher
Sympatric male
pied flycatcher
28
Pied flycatchers
Number of females
24
Collared flycatchers
20
16
12
8
4
(none)
0
Females mating
with males from:
Own
species
Other
species
Sympatric males
Own
species
Other
species
Allopatric males
Reinforcement
of barriers to
reproduction
in closely
related species
of European
flycatchers
Fusion: Weakening Reproductive
Barriers
• If hybrids are as fit as parents,
there can be substantial gene
flow between species
• If gene flow is great enough, the
parent species can fuse into a
single species
Fig. 24-16
Pundamilia nyererei
Pundamilia pundamilia
The
breakdown of
reproductive
barriers
Pundamilia “turbid water,”
hybrid offspring from a location
with turbid water
Stability: Continued Formation of
Hybrid Individuals
• Extensive gene flow from outside the
hybrid zone can overwhelm selection
for increased reproductive isolation
inside the hybrid zone
• In cases where hybrids have
increased fitness, local extinctions
of parent species within the hybrid
zone can prevent the breakdown of
reproductive barriers
Concept 24.4: Speciation can occur
rapidly or slowly and can result from
changes in few or many genes
• Evolution has two speeds of
change:
–Gradualism or slow change
–Rapid bursts of speciation
• Many questions remain
concerning how long it takes for
new species to form, or how many
genes need to differ between
species
The Time Course of Speciation
• Broad patterns in speciation can
be studied using the fossil
record, morphological data, or
molecular data
Patterns in the Fossil Record
• The fossil record includes
examples of species that appear
suddenly, persist essentially
unchanged for some time, and
then apparently disappear
Patterns in the Fossil Record
• Niles Eldredge and Stephen Jay
Gould coined the term
punctuated equilibrium to
describe periods of apparent
stasis punctuated by sudden
change
• The punctuated equilibrium model
contrasts with a model of gradual
change in a species’ existence
Fig. 24-17
Two models for the tempo of
speciation
(a) Punctuated pattern
Time
(b) Gradual pattern
Gradualism Evolution
• Darwinian style evolution.
• Small gradual changes over long
periods time.
Gradualism Predicts:
• Long periods of time are needed
for evolution.
• Fossils should show continuous
links.
Problem
• Gradualism doesn’t fit the fossil
record very well.
•
(too many “gaps”).
Punctuated Evolution
• theory that deals with the
“pacing” of evolution.
• Elridge and Gould – 1972.
Predictions
• Speciation can occur over a very
short period of time
•
(1 to 1000 generations).
• Fossil record will have gaps or
missing links.
Predictions
• New species will appear in the
fossil record without connecting
links or intermediate forms.
• Established species will show
gradual changes over long
periods of time.
Speciation Rates
• The punctuated pattern in the fossil
record and evidence from lab studies
suggests that speciation can be rapid
• The interval between speciation
events can range from 4,000 years
(some cichlids) to 40,000,000 years
(some beetles), with an average of
6,500,000 years
Possible Mechanism
• Adaptive Radiation, especially
after mass extinction events
allow new species to originate.
• Saturated environments favor
gradual changes in the current
species.
Comment
• Punctuated Equilibrium is the
newest ”Evolution Theory”.
• Best explanation of fossil
record evidence to date.
Rapid speciation in a sunflower
hybrid zone
(a) The wild sunflower Helianthus anomalus
Studying the Genetics of
Speciation
• The explosion of genomics is enabling
researchers to identify specific
genes involved in some cases of
speciation
• Depending on the species in question,
speciation might require the change
of only a single allele or many alleles
Fig. 24-19
Single-gene speciation
From Speciation to Macroevolution
• Macroevolution is the cumulative
effect of many speciation and
extinction events
Origin of Evolutionary Novelty
• How do macroevolution changes
originate?
• Several ideas discussed in
textbook (read them)
–Exaptation
–Heterochrony
–Homeosis
Another idea
• Mutations in developmental or
control genes (Chapter 21)
• Looking very promising as a
source of macroevolution
Exaptation
• When a structure that was
adapted for one context is
co-opted for another function.
• Ex. – feathers and flying
Heterochrony
• Changes in the timing or rate of
development.
–Allometric Growth
–Paedomorphsis
1.
Allometric Growth – changes in
the relative rates of growth of
various parts of the body.
• Ex. – skull growth in primates
2. Paedomorphosis
– when an adult
retains features that are
present in the juvenile form.
Ex. – gills in adult salamanders
Ex - Homeosis
Changes in the basic body design
or arrangement of body parts.
Ex. – Hox gene clusters that gave
rise to vertebrates from
invertebrates.
Gene Duplications
• Allow genes to be used for
other functions such as in the
previous slide.
• Many other examples are known.
Future of Evolution ?
• Look for new theories and ideas
to be developed, especially from
new fossil finds and from
molecular (DNA) evidence.
Evolutionary Trends
• Evolution is not goal oriented.
It does not produce “perfect”
species.
• Remember – species survive
because of their adaptations.
They don’t adapt to survive.
Summary
• Be able to discuss the main
theories of what is a “species”.
• Know various reproductive
barriers and examples.
Summary
• Know allopatric and sympatric
speciation.
• Be able to discuss gradualism
and punctuated equilibrium
theories.
Summary
• Recognize various ideas about
the origin of evolutionary
novelties.
You should now be able to:
1. Define and discuss the
limitations of the four species
concepts
2. Describe and provide examples
of prezygotic and postzygotic
reproductive barriers
3. Distinguish between and provide
examples of allopatric and
sympatric speciation
You should now be able to:
4. Explain how polyploidy can cause
reproductive isolation
5. Define the term hybrid zone
and describe three outcomes for
hybrid zones over time
End of Chapter 24