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Chapter 24
The Origin of Species
Overview: That “Mystery of Mysteries”
•
In the Galápagos Islands Darwin discovered plants and animals found
nowhere else on Earth
–
•
Darwin later realized that these species, along with the islands on
which they resided, were relatively new
Speciation, the origin of new species, is at the focal point of evolutionary
theory
–
Evolutionary theory must explain how new species originate and how
populations evolve
–
Speciation forms a conceptual bridge between microevolution and
macroevolution
• Microevolution consists of adaptations that evolve within a
population, confined to one gene pool
• Macroevolution refers to evolutionary change above the species
level
Video: Galápagos Tortoise
Concept 24.1: The biological species concept
emphasizes reproductive isolation
• Species is a Latin word meaning “kind” or
“appearance”
– Biologists compare morphology, physiology,
biochemistry, and DNA sequences when
grouping organisms into discrete groups
The Biological Species Concept
•
The biological species concept states that:
•
•
A species is a group of populations whose members have the potential
to interbreed in nature and produce viable, fertile offspring – but do not
produce viable, fertile offspring with members of other such groups
Members of a species often resemble one another because their
populations are connected by gene flow
Fig. 24-2
•
Populations located near one another
exchange alleles relatively often
(a) Similarity between different species
• Researchers have also shown that
a low level of gene flow occurs
between even widely separated
populations of various plant and
animal species
(b) Diversity within a species
•
In one experiment, researchers studied gene flow in a species of bird called
the grey-crowned babbler
–
•
Gene flow in this species was thought to disperse only short distances
Researchers sequenced a segment of DNA from birds in 12 widely
separated populations (A-L) located throughout Australia and Papua New
Guinea
–
They then used this data to construct evolutionary trees called gene
trees that showed patterns of relatedness among the alleles at a
particular locus
• If a gene tree showed that some birds in one population had an
allele that shared a recent
common ancestor with alleles
found in a different population, EXPERIMENT
Example of a gene tree for population pair A-B
it was reasoned that gene
Allele Population
Gene flow event
1
B
flow must have occurred
Allele 1 is more closely related to
2
A
between those populations
alleles 2, 3, and 4 than to
Fig. 24-3a
–
In this way, allele relatedness
for various combinations of the
12 study populations was
analyzed
3
A
4
A
5
B
6
B
7
B
alleles 5, 6, and 7.
Inference: Gene flow occurred.
Alleles 5, 6, and 7 are more closely
related to one another than to
alleles in population A.
Inference: No gene flow occurred.
•
Results:
–
•
Among the 12 study populations, researchers inferred that gene flow
occurred in 7 population pairs
Conclusion:
–
Because gene flow was detected between population separated by
over 1000 km, researchers concluded that gene flow can potentially
hold the grey-crowned babbler gene pool together even though
individuals were thought to disperse only short distances
Fig. 24-3b
• The long-distance
movement of alleles could
result from a series of
shorter movements by
individual birds, or from
chance events
– Ex) Perhaps a storm
transported the birds
to a distant location
RESULTS
Pair of
populations
with detected
gene flow
Estimated minimum
number of gene flow
events to account for
genetic patterns
Distance between
populations (km)
A-B
5
340
K-L
3
720
A-C
2–3
1,390
B-C
2
1,190
F-G
2
760
G-I
2
1,110
C-E
1–2
1,310
Reproductive Isolation
• The formation of new species hinges on
reproductive isolation, the existence of biological
factors (barriers) that impede two species from
producing viable, fertile offspring
– Such barriers block gene flow between species
and limit the formation of hybrids
• Hybrids are the offspring of crosses
between different, but often closely related,
species
– Although a single barrier may not prevent all
gene flow, a combination of several barriers
can effectively isolate a species’ gene pool
• Reproductive isolation can be classified by whether factors
act before or after fertilization
– Prezygotic (“before the zygote) barriers block
fertilization from occurring by:
• Impeding different species from attempting to mate
• Preventing the successful completion of an
attempted mating
• Hindering fertilization if mating is successful
•
Habitat isolation: Two species encounter each other rarely, or not at all,
because they occupy different habitats, even though not isolated by physical
barriers
–
•
Fig. 24-4a
Ex) Although 2 species of garter snake live in the same geographic Prezygot
area, one lives mainly in water and the
Habitat Isolation
Temporal Isolation
other is primarily terrestrial
Temporal isolation: Species that breed at
different times of the day, different seasons,
or different years cannot mix their gametes
–
Individuals
of
different
species
(a)
Ex) In North America, the eastern
spotted skunk mates in late winter but
the western spotted skunk mates in
late summer
(c)
(d)
(b)
• Behavioral isolation: Courtship rituals and other behaviors unique
to a species are effective barriers
–
Such behavior rituals enable mate recognition, a way to identify
potential mates of the same species
Fig. 24-4a
Prezygotic
barriers
• Ex) Blue-footed boobies mate only after
a courtship
display
unique to their species, part of which
Habitat Isolation
Temporal Isolation
Behavioral Isolation
calls for the male to “high-step” to bring
the female’s attention to his bright
Individuals
blue feet
of
Mating
different
species
(a)
attemp
(c)
Video: Albatross Courtship Ritual
Video: Giraffe Courtship Ritual
Video: Blue-footed Boobies Courtship
Ritual
(d)
(e)
•
Mechanical isolation: Morphological differences can prevent successful
mating
–
•
ation
Ex) The shells of 2 species of snails spiral in different directions,
meaning that the snails’ genital openings are not aligned and mating
cannot be completed
Gametic isolation: Sperm of one species may not be able to fertilize eggs
of another species
–
Prezygotic barriers
Ex) Gametes of different
species of sea urchins are unable to fuse
because
proteins on
the surfaces Mechanical Isolation
Behavioral Isolation
Temporal Isolation
Gametic Isolation
of the eggs and sperm cannot
bind to each other
Mating
attempt
(c)
(e)
(f)
Fertilizat
(g)
• Postzygotic barriers prevent the hybrid zygote
from developing into a viable, fertile adult:
– Reduced hybrid viability
– Reduced hybrid fertility
– Hybrid breakdown
•
Reduced hybrid viability: Genes of the different parent species may
interact and impair the hybrid’s development
–
•
Most hybrids of salamander subspecies do not complete development,
and those that do are frail
Reduced hybrid fertility: Even if hybrids are vigorous, they may be sterile
–
Ex) The hybrid offspring of a donkey and a horse is a robust but sterile
mule
Fig. 24-4i
•
Prezygotic barriers
Hybrid breakdown: Some first-generation
hybrids are fertile, but when they mate with
another species or with either parent
species, offspring of the next generation
are feeble or sterile
Gametic Isolation
Postzygotic barriers
Reduced Hybrid Viability Reduced Hybrid Fertility
Hybrid Breakdown
Viable,
fertile
offspring
Fertilization
(g)
–
(h)
Ex) Although 1st generation hybrids if
2 strains of cultivated rice are vigorous and
fertile, plants in the next generation that carry
too many mutant recessive alleles are
small and sterile
(i)
(j)
(k)
(l)
Limitations of the Biological Species Concept
• The biological species concept cannot be applied to fossils
or asexual organisms (including all prokaryotes)
– In addition, species are designated by the absence of
gene flow according to the biological species concept
• However, there are many pairs of morphologically
and ecologically distinct species between which
gene flow occurs
• These species continue to remain separate despite
this gene flow
– Because of the limitations to the biological species
concept, alternative species concepts are useful in
certain situations
Other Definitions of Species
• Other species concepts emphasize the unity within a
species rather than the separateness of different species
– The morphological species concept defines a
species by structural features, such as body shape
• Advantages of this concept include that:
– It can be applied to sexual and asexual species
– It can be useful even without information on the
extent of gene flow
• One disadvantage, however, is that it relies on
subjective criteria
– Researchers may thus disagree on which structural
features distinguish a species
• The ecological species concept views a species in terms of
its ecological niche
–
This refers to the sum of how members of the species
interact with the nonliving and living parts of their
environment
• Ex) Two species of amphibians might be similar in
appearance but differ in the foods they eat or in their
ability to tolerate dry conditions
–
As with the morphological species concept, it can also be
applied to sexual and asexual species
• In addition, it emphasizes the role of disruptive
selection as organisms adapt to different
environmental conditions
• The phylogenetic species concept defines a species as the
smallest group of individuals that share a common ancestor
–
Biologists trace the phylogenetic history of a species by
comparing characteristics such as morphology or molecular
sequences with those of other organisms
• These analyses can distinguish groups of individuals
that are sufficiently different to be considered separate
species
–
Though it can be applied to sexual and asexual species,
one disadvantage of this concept is that it can be difficult to
determine the degree of difference required for separate
species
Concept 24.2: Speciation can take place with or
without geographic separation
• Speciation can occur in two ways, depending
on how gene flow is interrupted between
populations of existing species:
Fig. 24-5
– Allopatric speciation
– Sympatric speciation
(a) Allopatric speciation
(b) Sympatric speciation
Allopatric (“Other Country”) Speciation
• In allopatric speciation, gene flow is interrupted or reduced when a
population is divided into geographically isolated subpopulations
–
Ex) The water level in a lake may subside, resulting in 2+ smaller
lakes that will become home to separated populations
• Allopatric speciation can also occur without geological remodeling
–
Ex) If individuals colonize a remote area, their descendents may
become geographically isolated from the parent population
Fig. 24-1
• The flightless cormorant
likely originated in this
way from an ancestral
flying species that
migrated to the
Galapagos Islands
The Process of Allopatric Speciation
•
The definition of barrier depends on the ability of a population to move about
–
•
Ex) Large or flight-capable mammals, as well as windblown pollen of plants,
can cross rivers or canyons that may be formidable barriers to smaller animals
Once geographic isolation has occurred, these separate populations may evolve
independently through mutation, natural selection, and genetic drift
–
Reproductive isolation may then arise as selection or drift cause the
populations to diverge genetically
•
Ex) Selection has favored the evolution of copper tolerance in populations
of monkey flowers living near copper mines, where
soil copper concentrations can reach lethal levels to nontolerant individuals
– When members of copper-tolerant populations interbreed with
individuals from other populations, the offspring survive poorly
Fig. 24-6
–
Genetic analyses have shown that either the gene for copper
tolerance itself or an allele
genetically linked to this
gene are responsible for
the poor survival of these
A. harrisi
hybrid offspring
A. leucurus
Evidence of Allopatric Speciation
•
Many studies provide evidence that speciation can occur in allopatric
populations
–
Ex) Data suggests that 2 modern groups of frog species began to
diverge ~88 mya during the separation of the island of Madagascar
from the Indian landmass
• It appears that these 2 frog groups shared a common ancestor that
lived on the Madagascar-India landmass before it began to break
apart
Fig. 24-7
• Following the breakup,
allopatric speciation occurred
within the separated
populations of this common
ancestor, resulting in the
formation of many new
species in each location
Mantellinae
(Madagascar only):
100 species
Rhacophorinae
(India/Southeast
Asia): 310 species
Other Indian/
Southeast Asian
frogs
100
60
80
1
2
40
20
0
3
Millions of years ago (mya)
1
3
2
India
Madagascar
88 mya
65 mya
56 mya
Evidence of Allopatric Speciation
The importance of allopatric speciation is also suggested by the fact that regions with
many geographic barriers typically have more species than do regions with fewer
barriers
–
•
Ex) An unusually large number of bird species are found in the mountainous
regions of New Guinea
Laboratory and field tests also provide evidence that reproductive isolation between
2 populations generally increases as the distance between them increases
–
Ex) In one study of dusky salamanders, biologists brought individuals from
different populations into the lab and tested their ability to produce viable, fertile
offspring
Fig. 24-8
•
•
The researchers observed little
reproductive isolation in
salamanders from neighboring
populations
In contrast, salamanders from
widely separated populations
often failed to reproduce
Degree of reproductive isolation
•
2.0
1.5
1.0
0.5
0
0
50
200
250
100
150
Geographic distance (km)
300
Evidence of Allopatric Speciation
• One possible explanation for these results is that long-distance
gene flow is not occurring between the dusky salamander
populations
– Alternatively, long-distance gene flow between salamander
populations may be outweighed by the effects of natural
selection or genetic drift, causing the populations to diverge
• In other studies, researchers have tested whether intrinsic
reproductive barriers develop when populations are isolated
experimentally and subjected to different environmental
conditions
– In these cases, the results also provide strong support for
allopatric speciation
• Experiment: Researchers divided a fruit fly population, raising some
on a starch medium and others on a maltose medium
–
After 1 year (~40 generations), natural selection resulted in
divergent evolution
• Populations raised on starch digested starch more efficiently,
while those raised on maltose digested maltose more
efficiently
–
Researchers then put flies from the same or different populations
in mating cages
and
Fig. 24-9
EXPERIMENT
measured mating
frequencies
Initial population
Some flies
raised on
starch medium
Mating experiments
after 40 generations
Some flies
raised on
maltose medium
• Results: When these 2 populations of flies were interbred, the flies
24-9 partners
tended to mate withFig.like
EXPERIMENT
–
In the control group, flies from different populations adapted to
starch or maltose were about as likely mate
with each other as
Initial population
with flies from their own population
–
This strong preference of flies Some
to mate
with like-adapted flies
flies
Some flies
raised on
raised on
indicates that a reproductive starch
barrier,
though
incomplete,
was
medium
maltose
medium
Mating experiments
after 40 generations
forming between the divergent populations
of flies
RESULTS
Female
Female
Starch Maltose
9
8
20
Mating frequencies
in experimental group
Starch
Starch
population 2 population 1
22
Starch
Starch
population 1 population 2
Male
Male
Maltose Starch
• This occurred as a
result of differing
selective pressures as
the allopatric
populations adapted
to different
environments
18
15
12
15
Mating frequencies
in control group
• It must be emphasized that barriers to
reproduction are intrinsic
– Separation itself is not a biological barrier to
reproduction
– Other barriers can prevent interbreeding even
when members of different populations come
into contact with one another
Sympatric (“Same Country”) Speciation
• In sympatric speciation, speciation takes place in
populations that live in the same geographic area
– Although contact and thus gene flow between these
populations does occur, making it less common than
allopatric speciation, certain factors can bring about
this kind of speciation:
• Polyploidy
• Habitat differentiation
• Sexual selection
Polyploidy
•
Polyploidy is the presence of extra sets of chromosomes due to accidents
during cell division
–
There are 2 distinct forms of polyploidy:
• Autopolyploidy
• Allopolyploidy
–
An autopolyploid is an individual with more than two chromosome
Fig. 24-10-3
sets, derived from one
species
• This can occur if cell division fails, resulting in a doubling of a cell’s
chromosome number from the diploid number (2n) to a tetraploid
number (4n)
2n = 6
4n = 12
Failure of cell
division after
chromosome
duplication gives
rise to tetraploid
tissue.
2n
Gametes
produced
are diploid..
4n
Offspring with
tetraploid
karyotypes may
be viable and
fertile.
Polyploidy
• Any triploid (3n) offspring produced by mating between these
tetraploid individuals and diploid individuals of the original population
have reduced fertility
–
Thus, the tetraploid becomes reproductively isolated, the first
step towards speciation
• These tetraploids can, however, produce fertile tetraploid offspring by
self-pollinating (plants) or by mating with other tetraploids
–
Thus, in just a single generation, autoploidy can generate
reproductive isolation without any geographic separation
•
A second form of polyploidy can occur when 2 different species interbreed
and produce hybrid offspring
–
Most hybrids are sterile because the set of chromosomes from one
species cannot pair during meiosis with the set of chromosomes from
the other species
• An infertile hybrid may, however, be able to reproduce asexually
(plants)
–
In future generations, various mechanisms can change a sterile hybrid
into a fertile allopolyploid
• An allopolyploid is a species with multiple sets of chromosomes
derived from different species
Fig. 24-11-4
–
Allopolyploids represent a new species because, although they are
B
Unreduced
fertile when mating with each Species
Unreduced
2n = 4
gamete
gamete
with 4
Hybrid
other, they cannot interbreed
with 7
chromosomes
with 7
chromosomes
chromosomes
Meiotic
with either parent species
error
Species A
2n = 6
Normal
gamete
n=3
Normal
gamete
n=3
Viable fertile
hybrid
(allopolyploid)
2n = 10
•
Polyploidy is much more common in plants than in animals
–
•
Botanists estimate that more than 80% of the plant species alive today
are descended from ancestors that formed by polyploid speciation
Many important crops (oats, cotton, potatoes, tobacco, and wheat) are
polyploids
–
The wheat used for bread is an allohexaploid (6 sets of chromosomes,
2 sets from 3 different species)
• The first of the polyploidy event that led to modern wheat probably
occurred ~8,000 years ago in the Middle East as a spontaneous
hybrid of an early cultivated wheat species and a wild grass
–
Today, plant geneticists generate new polyploids in the lab by using
chemicals that induce meiotic and mitotic errors
• Researchers can thus produce new hybrid species with desired
qualities
– Ex) A wheat hybrid can combine the high yield of wheat with
the hardiness of rye
Habitat Differentiation
•
Sympatric speciation can also result from the appearance of new ecological
niches, including new habitats or resources not used by the parent
population
–
Ex) The North American maggot fly can live on native hawthorn trees
as well as more recently introduced apple trees
• Because apples mature more quickly than hawthorn fruit, natural
selection has favored apple-feeding flies with rapid development
– These apple-feeding populations now show temporal isolation
(different breeding times) from the hawthorn-feeding population
– This provides a prezygotic restriction to gene flow between the
2 populations
• Researchers have also identified alleles that benefit the flies that
use one host plant but harm the flies that use the other host plant
– Natural selection operating on these alleles therefore also
provides postzygotic barriers to reproduction, further limiting
gene flow
Sexual Selection
• There is evidence that sexual selection can drive sympatric
speciation
– Sexual selection for mates of different colors has likely
contributed to the speciation in cichlid fish in Lake Victoria
• Genetic data indicates that the ~600 species of cichlids
found in this single lake have evolved in the last
100,000 years from a small number of colonist species
• Researchers have studied 2 closely related sympatric
species of cichlids that differ mainly in the coloration of
breeding males
– Their results suggest that mate choice based on
male breeding coloration is the main reproductive
barrier that normally keeps the gene pools of these 2
species separate
Sexual Selection
•
Experiment: Researchers placed males and females of 2 different cichlid
species together in 2 aquarium tanks
–
One of these tanks was put under natural natural, while the other tank
was placed under a monochromatic orange lamp
• Under normal light, the 2 species are noticeably different in male
breeding coloration
• Under monochromatic orange light, the 2 species appear very
similar in color
EXPERIMENT
Fig. 24-12
–
The researchers then
observed the mate
choices of the females
in each tank
Normal light
P.
pundamilia
P. nyererei
Monochromatic
orange light
Sexual Selection
• Results: Under normal light, females of each species strongly
preferred males of their own species
–
Under orange light, females of each species responded
indiscriminately to males of both species
• These resulting hybrids were viable and fertile
• Conclusion: Mate choice by females based on male breeding
coloration is the main reproductive barrier that normally keeps the
gene pools of these 2 species separate
–
Since the species could still interbreed when this prezygotic
behavioral barrier is breached artificially, the genetic divergence
between the species is likely to be small
• This suggests that speciation in nature has occurred
relatively recently
Allopatric and Sympatric Speciation: A Review
• In allopatric speciation, geographic isolation restricts gene flow
between populations
– Other reproductive barriers may then arise by natural
selection, genetic drift, or sexual selection in the isolated
populations
– Because of this reproductive isolation, interbreeding is
prevented even if contact is restored between populations
• In sympatric speciation, a reproductive barrier isolates a subset
of a population without geographic separation from the parent
species
– Sympatric speciation can occur when gene flow to and
from the isolated subpopulation is blocked
• This may be the result of polyploidy, natural selection
due to habitat differentiation, or sexual selection
Concept 24.3: Hybrid zones provide opportunities
to study factors that cause reproductive isolation
If allopatric populations happen to come back into contact with one another,
one possible outcome is the formation of a hybrid zone
–
•
A hybrid zone is a region in which members of different species mate
and produce hybrids
Hybrid zones exhibit a variety of structures
–
A hybrid zone can occur in a single band where adjacent species meet
–
Ex) The hybrid zone for 2
species of toads extends
for 4,000 km but less than
10 km wide in most places
Fig. 24-13
EUROPE
Fire-bellied
toad range
Hybrid zone
Yellow-bellied toad,
Bombina variegata
Yellow-bellied
toad range
Fire-bellied toad,
Bombina bombina
0.99
Allele frequency (log scale)
•
0.9
0.5
0.1
0.01
40
20
30
10
0
10
20
Distance from hybrid zone center (km)
•
The pattern of allele frequencies across a hybrid zone can change:
–
The frequency of alleles specific to yellow-bellied toads is close to
100% at the edge where only this species of toad is found
–
This number decreases to 50% in the central portion of the zone
–
At the edge where only fire-bellied toads are found, the frequency of
yellow-bellied toad alleles decreases all the way down to 0%
Obstacles to gene flow that produce such patterns of allele frequencies
across a hybrid zone may result from geographic or reproductive barriers
–
Fig. 24-13
In the case of the toads, it
was found that hybrids often
had reduced fitness
compared with parent
species
• As a result, hybrids
rarely serve as a
stepping stone from
which alleles are passed
from one species to
another
EUROPE
Fire-bellied
toad range
Hybrid zone
Yellow-bellied toad,
Bombina variegata
Yellow-bellied
toad range
Fire-bellied toad,
Bombina bombina
0.99
Allele frequency (log scale)
•
0.9
0.5
0.1
0.01
40
20
30
10
0
10
20
Distance from hybrid zone center (km)
•
Other hybrid zones have more complicated spatial pattern
–
The distribution of hybrid zones can be more complex if parent species
are found in multiple habitats within the same region
• Ex) Two species of ground crickets found in the Appalachian
Mountains are distributed based on temperature
– One species is more successful at colder temperatures (higher
elevations), while the other prefers warmer temperatures (lower
elevations)
– In addition, the topography of this region is complex, with many
hills and valleys, resulting in many areas where patches of
both species are closely interspersed
• As a result, populations of the 2 parent species come into contact
and hybrids are formed
– The fitness of these hybrids varies from year to year,
sometimes even exceeding that of either parent
Hybrid Zones over Time
•
When closely related species meet in a hybrid zone, there are three possible
outcomes:
–
Strengthening of reproductive barriers
• This would limit the formation of hybrids
–
Weakening of reproductive barriers
• This may lead to the fusion of 2 distinct species into a single
species
Fig. 24-14-4
–
Continued formation of hybrid individuals
• This would create
a long-term stable
hybrid zone
Isolated population
diverges
Possible
outcomes:
Hybrid
zone
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
–
•
Natural selection thus strengthens prezygotic barriers to reproduction,
reducing the rate of formation of unfit hybrids
Fig. 24-15
Where reinforcement occurs, reproductive barriers
should be stronger for sympatric than allopatric
species
–
–
In allopatric populations of flycatchers, males
of 2 species closely resemble one another
In sympatric populations of these birds, the
males of these 2 species look very different
Sympatric male
pied flycatcher
28
Allopatric male
pied flycatcher
Pied flycatchers
24
Number of females
•
Collared flycatchers
20
16
12
8
4
(none)
0
Females mating Own
Other
with males from: species species
Sympatric males
Own
Other
species species
Allopatric males
Reinforcement: Strengthening Reproductive
Barriers
• Female flycatchers do not select males of the other species
when given a choice between males in sympatric populations
Fig. 24-15
– On the contrary, female
flycatchers frequently make
mistakes when selecting between
males from allopatric populations
28
Allopatric male
pied flycatcher
Pied flycatchers
24
Number of females
• Thus, barriers to reproduction
appear to be stronger in birds
from sympatric populations
than in birds from allopatric
populations
Sympatric male
pied flycatcher
Collared flycatchers
20
16
12
8
4
(none)
0
Females mating Own
Other
with males from: species species
Sympatric males
Own
Other
species species
Allopatric males
Fusion: Weakening Reproductive Barriers
•
Another possible outcome of 2 species contacting one another in a hybrid
zone is that gene flow may occur if reproductive barriers are weak
–
So much gene flow may occur that these barriers weaken even further
and the gene pools of the 2 species become increasingly alike
–
If gene flow is great enough, the parent species can even fuse into a
single species
• Ex) In the past 30 years, 200 of the former 600 species of Lake
Victoria cichlids have vanished
Fig. 24-16
– Researchers think that murky waters
caused by pollution may have reduced
the ability of females to use color to
distinguish males of their own species
– As a result of these “accidental”
matings, many hybrids have been
produced, leading to fusion of the
parent species’ genes
Pundamilia nyererei
Pundamilia pundamilia
Pundamilia “turbid water,”
hybrid offspring from a location
with turbid water
Stability: Continued Formation of Hybrid
Individuals
• A third possible outcome of 2 species contacting one another in a
hybrid zone is that hybrids will continue to be produced, creating a
stable hybrid zone
–
This may occur if the hybrid zone is very narrow, even when
hybrid offspring are less fit than either parent species, as is seen
in the fire-bellied and yellow-bellied toad hybrid zone
• In this case, extensive gene flow from outside the zone
resulting in the continued production of hybrids may
overwhelm selection for increased reproductive isolation
inside the hybrid zone
– If the hybrid zone were wider, this would be less likely to
occur, since the centers of the zone would receive little
gene flow from distant populations outside the hybrid
zone
Stability: Continued Formation of Hybrid
Individuals
•
In cases where hybrids have increased fitness, local extinctions of parent
species within the hybrid zone can prevent the breakdown of reproductive
barriers
–
Though hybrids of the 2 ground cricket species sometimes have higher
fitness than the parent species, data shows that gene flow is not
extensive enough to fuse the species
• This is because the parent species meet at locations where one or
both of them are near the limit of the environmental conditions they
can tolerate
• As a result, even a slight change in the local environment can
cause one or the other parent species to disappear from that
location
• Because location where hybrids are formed may appear and
disappear rapidly, hybrids remain uncommon and fusion of the
gene pools is prevented by insufficient time for reproductive
barriers to break down
Concept 24.4: Speciation can occur rapidly or slowly
and can result from changes in few or many genes
•
Many questions remain concerning how long it takes for new species to
form, or how many genes need to differ between species
–
•
To gather information about how long it takes for speciation to occur,
broad patterns in speciation can be studied using the fossil record,
morphological data, or molecular data
The fossil record includes many examples of species that appear suddenly,
persist essentially unchanged for some time, and then apparently disappear
–
Niles Eldredge and Stephen Jay Gould coined the term punctuated
equilibrium to describe these periods of apparent stasis punctuated by
sudden change
Fig. 24-17
–
All species do not show a
punctuated pattern; rather they
change more gradually over
long periods of time
(a) Punctuated pattern
Time
(b) Gradual pattern
Speciation Rates
•
The punctuated pattern in the fossil record and evidence from lab studies
suggests that speciation can be rapid or relatively slow
–
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
• This extreme variability in the time it takes for speciation to occur
indicates that organisms do not produce new species at regular
time intervals
– Rather, speciation begins only after gene flow between
populations has been interrupted, perhaps by an unpredictable
event that causes geographic isolation
• Once this gene flow has been interrupted, the populations must
diverge genetically to such an extent that they become
reproductively isolated
Studying the Genetics of Speciation
•
The explosion of genomics is enabling researchers to identify specific genes involved
in some cases of speciation
–
This has allowed them to address the question of how many genes change
when a new species is formed
–
In a few cases, speciation is due to a change in a single gene
•
–
Ex) In one species of Japanese snail, alleles of a single gene can induce a
mechanical barrier to reproduction, causing their shells to spiral in different
directions and thus orienting their genitals in a manner that prevents
mating
In other organisms, the speciation process in influenced by a larger number of
genes and gene interactions
Fig. 24-19
– Ex) Hybrid sterility between 2
subspecies of Drosophila
results from gene interactions
among at least 4 loci
•
Overall, studies conducted to date
suggest that a few or many genes
can influence the evolution of
reproductive isolation and thus
speciation
From Speciation to Macroevolution
• Macroevolution is the cumulative effect of many speciation
and extinction events
– Speciation may begin with differences as small as the
color on a cichlid’s back
• As speciation occurs again and again, however,
these differences accumulate, eventually leading to
species the differ greatly from their ancestors
– In addition, as these new groups form, other groups
may shrink, losing species to extinction
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
4. Explain how polyploidy can cause
reproductive isolation
5. Define the term hybrid zone and describe
three outcomes for hybrid zones over time