Transcript Speciation - Lake Worth ISD

How did this flightless bird come to live on the isolated Galápagos Islands?
Process of species change
• Speciation, the origin of new species, is at the
focal point of evolutionary theory
Scale of evolution:
• Microevolution consists of changes in allele
frequency in a population over time
• Macroevolution refers to broad patterns of
evolutionary change above the species level
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Concept 24.1: The biological species
concept emphasizes reproductive isolation
• Biological Species Concept (one way to divide species):
– A species is a group of populations whose members can breed
and produce viable, fertile offspring
– Ability to mate = formation of a species
– Gene flow between populations holds together the phenotype of
a population (ongoing exchange of alleles)
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It’s all about ability to breed
(a) Similarity between different species
(b) Diversity within a species
Reproductive Isolation is the driving force
behind Speciation
• A new species will form when reproductive isolation occurs
– Absence of gene flow
• Reproductive Isolation:
– The existence of biological factors (barriers) that prevent two
individuals of a species from mating and producing viable and
fertile offspring
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Figure 24.3_b
Prezygotic barriers
Habitat
Isolation
Temporal
Isolation
Individuals
of
different
species
(a)
MATING
ATTEMPT
(c)
(d)
(b)
Gametic
Isolation
Mechanical
Isolation
Behavioral
Isolation
(e)
(f)
FERTILIZATION
(g)
Figure 24.3_c
Postzygotic barriers
Reduced Hybrid
Viability
Reduced Hybrid
Fertility
Hybrid
Breakdown
VIABLE,
FERTILE
OFFSPRING
FERTILIZATION
(h)
(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)
• The biological species concept emphasizes
absence of gene flow
• However, gene flow can occur between
distinct species
– HYBRIDS: For example, grizzly bears and
polar bears can mate to produce “grolar
bears”
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Figure 24.4
Grizzly bear (U. arctos)
Polar bear (U. maritimus)
Hybrid “grolar bear”
Other Definitions of Species
1. morphological species concept:
– A species is characterized by its body shape
– It applies to sexual and asexual species but relies on
subjective criteria
2. ecological species concept:
– A species is characterized by its ecological niche
– It applies to sexual and asexual species and emphasizes
the role of disruptive selection
3. phylogenetic species concept:
– A species is the smallest group of individuals that share a
common ancestor
– It applies to sexual and asexual species, but it can be
difficult to determine the degree of difference required for
separate species
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Concept 24.2: Speciation can take place
with or without geographic separation
• Speciation can occur in two ways:
– Allopatric speciation
– Sympatric speciation
• What do the route words mean?
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Figure 24.5
(a) Allopatric speciation.
A population forms a
new species while
geographically isolated
from its parent population.
(b) Sympatric speciation.
A subset of a population
forms a new species
without geographic
separation.
Allopatric (“Other Country”) Speciation
• In allopatric speciation, gene flow is
interrupted or reduced when a population is
divided into geographically isolated
subpopulations
– For example, the flightless cormorant of the
Galápagos likely originated from a flying
species on the mainland
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The Process of Allopatric Speciation
• The definition of barrier depends on the ability of a
population to disperse
– For example, a canyon may create a barrier for
small rodents, but not birds, coyotes, or pollen
A. harrisii
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A. leucurus
ALLOPATRIC SPECIATION
A. formosus
A. nuttingi
Atlantic Ocean
Isthmus of Panama
Pacific Ocean
A. panamensis
A. millsae
• What types of regions have the greatest
isolation?
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EXPERIMENT
Initial population
of fruit flies
(Drosophila
pseudoobscura)
Some flies raised on
maltose medium
Some flies raised
on starch medium
Mating experiments
after 40 generations
RESULTS
Female
22
9
8
20
Male
Maltose
Starch
Starch
population 1 population 2
Number of matings
in experimental group
Starch
Starch
population 2 population 1
Starch
Starch
Male
Female
Maltose
Figure 24.10
18
15
12
15
Number of matings
in control group
Sympatric (“Same Country”) Speciation
• In sympatric speciation, speciation takes place
in geographically overlapping populations
• How can this occur if parent species is in same
area? Provide an example.
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Sympatric (“Same Country”) Speciation
• In sympatric speciation, speciation takes place
in geographically overlapping populations
• In a sympatric speciation, various factors can
limit gene flow:
– Polyploidy
– Habitat differentiation
– Sexual selection
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Polyploidy
• Polyploidy is the presence of extra sets of
chromosomes due to accidents during cell division
– Polyploidy is much more common in plants than in animals.
Why do you think?
• An autopolyploid is an individual with more than two
chromosome sets, derived from one species
– When would this originate?
• An allopolyploid is a species with multiple sets of
chromosomes derived from different species (see
diagram on proceeding slides)
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Figure 24.11-1
Species A
2n = 6
Normal
gamete
n=3
Species B
2n = 4
Meiotic error;
chromosome number not
reduced from 2n to n
Unreduced gamete
with 4 chromosomes
Figure 24.11-2
Species A
2n = 6
Normal
gamete
n=3
Species B
2n = 4
Meiotic error;
chromosome number not
reduced from 2n to n
Unreduced gamete
with 4 chromosomes
Hybrid with
7 chromosomes
Figure 24.11-3
Species A
2n = 6
Normal
gamete
n=3
Species B
2n = 4
Meiotic error;
chromosome number not
reduced from 2n to n
Unreduced gamete
with 4 chromosomes
Hybrid with
7 chromosomes
Normal
gamete
n=3
Unreduced gamete
with 7 chromosomes
Figure 24.11-4
Species A
2n = 6
Normal
gamete
n=3
Species B
2n = 4
Meiotic error;
chromosome number not
reduced from 2n to n
Unreduced gamete
with 4 chromosomes
Hybrid with
7 chromosomes
Normal
gamete
n=3
Unreduced gamete
with 7 chromosomes
New species:
viable fertile hybrid
(allopolyploid) 2n = 10
• Many important crops (oats, cotton, potatoes,
tobacco, and wheat) are polyploids
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Habitat Differentiation
• Sympatric speciation can also result from the
appearance of new ecological niches
• For example, the North American maggot fly can
live on native hawthorn trees as well as more
recently introduced apple trees
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Sexual Selection: Sexual selection for mates of different colors has
likely contributed to speciation in cichlid fish in Lake Victoria
EXPERIMENT
P. pundamilia
P. nyererei
Normal light
Monochromatic
orange light
Concept 24.3: Hybrid zones reveal factors
that cause reproductive isolation
• A hybrid zone is a region in which members of
different species mate and produce hybrids
• Can you think of an example of where and how
this would occur?
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Patterns Within Hybrid Zones
• A hybrid zone can occur in a single band where
adjacent species meet
– For example, two species of toad in the genus
Bombina interbreed in a long and narrow hybrid
zone (see next slide)
– Would you expect hybrids to have more, less and
the same fitness as the parents?
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Figure 24.13
EUROPE
Fire-bellied
toad range
Hybrid zone
Fire-bellied toad, Bombina bombina
Yellow-bellied
toad, Bombina
variegata
Frequency of
B. variegata-specific allele
Yellow-bellied
toad range
0.99
Hybrid
zone
0.9
Yellow-bellied
toad range
0.5
Fire-bellied
toad range
0.1
0.01
40
10
0
20
10
20
30
Distance from hybrid zone center (km)
Frequency of
B. variegata-specific allele
Figure 24.13b
0.99
Hybrid
zone
0.9
Yellow-bellied
toad range
0.5
Fire-bellied
toad range
0.1
0.01
40
30
10
0
20
10
20
Distance from hybrid zone center (km)
Hybrid Zones over Time
• When closely related species meet in a hybrid
zone, there are three possible outcomes:
– Reinforcement
– Fusion
– Stability
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Figure 24.14-1
Gene flow
Population
Barrier to
gene flow
Formation of a hybrid zone and possible outcomes for hybrids over time.
Isolated
population
diverges
Gene flow
Population
Barrier to
gene flow
Figure 24.14-3
Isolated
population
diverges
Hybrid
zone
Gene flow
Population
Barrier to
gene flow
Hybrid
individual
Figure 24.14-4
Possible
outcomes:
Isolated
population
diverges
Hybrid
zone
Reinforcement
OR
Fusion
OR
Gene flow
Population
Barrier to
gene flow
Hybrid
individual
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
– For example, in populations of flycatchers, males
are more similar in allopatric populations than
sympatric populations
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Figure 24.15
Females choosing between
these males:
28
Number of females
24
Females choosing between
these males:
Sympatric pied male
Allopatric pied male
Sympatric collared male
Allopatric collared male
20
16
12
8
4
(none)
0
Own
species
Other
species
Female mate choice
Own
species
Other
species
Female mate choice
Fusion: Weakening Reproductive Barriers
• If hybrids are as fit as parents, there can be
substantial gene flow between species
– How would increasing gene flow affect separate
species?
• For example, researchers think that pollution
in Lake Victoria has reduced the ability of
female cichlids to distinguish males of
different species
– How would this affect different cichlid species?
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Figure 24.16
Pundamilia nyererei
Pundamilia pundamilia
Pundamilia “turbid water,”
hybrid offspring from a location
with turbid water
Concept 24.4: Speciation can occur rapidly
or slowly and can result from changes in
few or many genes
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The Time Course of Speciation
• By what tools can we determine rate of
speciation?
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The Time Course of Speciation
• By what tools can we determine rate of
speciation?
– fossil record,
– morphological data, or
– molecular data
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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
• Niles Eldredge and Stephen Jay Gould coined the
term punctuated equilibria to describe periods of
apparent stasis punctuated by sudden change
– Compare punctuated equlibrium to uniformitarianism.
– Which model is more correct?
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Figure 24.17
(a) Punctuated
pattern
Time
(b) Gradual
pattern
Represent these two models in
graphic form
Speciation Rates
• The punctuated pattern in the fossil record and
evidence from lab studies suggest that speciation
can be rapid
– For example, the sunflower Helianthus anomalus
originated from the hybridization of two other
sunflower species
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Figure 24.18
EXPERIMENT
H. annuus
gamete
H. petiolarus
gamete
How does hybridization lead to
speciation in sunflowers??
F1 experimental hybrid
(4 of the 2n = 34
chromosomes are shown)
RESULTS
H. anomalus
Chromosome 1
Experimental hybrid
H. anomalus
Chromosome 2
Experimental hybrid
Average rates of speciation
• The interval between speciation events can range
from 4,000 years (some cichlids) to 40 million
years (some beetles), with an average of 6.5
million years
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Figure 24.UN02
Fill in the
blanks
Original population
_______ speciation
________ speciation