11. Patterns of speciation and extinction
The rise and fall of biodiversity
Mass extinctions
800
700
Four major mass extinctions of marine
organisms:
600
500
End of Silurian Devonian, Permian, and
Cretaceous)
400
300
200
Rise in diversity during Cambrian, Silurian,
Cretaceous, and Paleogene
100
0
-600
-500
E
K
-400
O
S
D
-300
C
P
-200
T
J
-100
Kr
0
Pa N
2500
Eliminating all groups known only from a
single stage (5-6 mya):
Rise in diversity during Cambrian, and
Ordovicium and in the Paleogene
Decline of longer lasting taxa from
Ordovicium to Triassic
Mass extinctions
z
The rise and fall of biodiversity
Number of families
Number of families z
900
2000
1500
1000
500
0
-600
-500
E
K
-400
O
S
D
-300
C
P
-200
T
J
-100
Kr
0
Pa N
What is a species in the evolutionary context?
Corvus corax
Corvus corone
Corvus frugilegus
The biological species concept states that species are actually or potentially interbreeding
natural populations that are genetically isolated from others
The evolutionary species concept states that species are ancestor – descendent lineages of
organisms that have their own evolutionary fate.
The phylogenetic species concept states that a species is the smallest monophyletic group
of organisms of common ancestry (a lineage from one node to another).
The genetic species concept states that a species is a genetically sufficiently distinct group
of organisms as identified by a genetic fingerprint.
The ecological species concept states that a species is a group of organisms (population)
that are ecologically distinct from other groups.
The heuristic species concept states that a species is a group of organisms that are
practically clustered together for the aims of a certain study.
Does any species concept fit?
Thelytokous waps
Morphologically divergent races
Dog races
Meteorus pulchricornis
from New Zealand
Heliconius butterflies
Salmonella typhi
Presexual species
Genetical fingerprint „species”
How do species emerge?
A classical example
1. Large cactus finch (Geospiza conirostris)
2. Large ground finch (Geospiza magnirostris)
3. Medium ground finch (Geospiza fortis)
4. Cactus finch (Geospiza scandens)
5. Sharp-beaked ground finch (Geospiza difficilis)
6. Small ground finch (Geospiza fuliginosa)
7. Woodpecker finch (Cactospiza pallida)
8. Vegetarian tree finch (Platyspiza crassirostris)
9. Medium tree finch (Camarhynchus pauper)
10. Large tree finch (Camarhynchus psittacula)
11. Small tree finch (Camarhynchus parvulus)
12. Warbler finch (Certhidia olivacea)
13. Mangrove finch (Cactospiza heliobates
Darwin finches, Geospiza spp.)
Genetic distance
Lineage A
Speciation is the divergence of genetic structure
between subpopulations until new separate populations
emerge.
Lineage B
Any mechanism that promotes the emergence of
sublineages is therefore a potential speciation mechanism
Basal population
Divergence can be triggered by premating and postmating mechanisms:
Premating mechanisms are those that keep populations isolated before mating occurs.
Postmating mechanisms prevent hybrids to develop or breed.
Premating examples are:
Postmating examples are:
spatial isolation
behavioural isolation
temporal isolation (separated generations)
host switch in parasites and herbivores
selective habitat choice
genetic incompatibility
morphological incompatibility
early death of hybrids
sterility
Speciation due to ecological or spatial or temporal isolation
Barriers of gene flow or genetical isolation
Allopatric barrier
Peripatric barrier
Ancestral population
Ancestral population
Allopatric speciation
Lineage
A
Spatial
barrier
Peripatric speciation
Lineage
B
Lineage
A
Lineage
B
Founder effect
Barriers of gene flow or genetic isolation
Sympatric lineage emergence
Parapatric lineage emergence
Ancestral population
Ancestral population
Sympatric speciation
Parapatric speciation
Genetic differences within the same
geographical region result in genetic
isolation and lineage divergence.
Differential selection pressures cause
lineage divergence even within narrow
spatial ranges.
How fast is speciation?
Time to
genetic
isolation
Are species reproductively independent
lineages?
Seed plants
Time to
ecological
isolation
Lineage
length
Mammals
Insects
Ferns
Genetic isolation
Frogs
N.A. songbirds
Taxon
Fishes
Birds
Drosophila
Lineage length
Anthozoa
Bivalvia
Amphibia
Horses
Cichlids
0
Hawaiian Drosophila
Mammals
Angiosperms
Trees
0
2
4
6 8 10 12 14
Mya
It seems that evolutionary speed is not correlated
with generation length
and body size
0.2
0.4
0.6
0.8
1
Proportion of taxonomic species that
represent reproductively independent
lineages
Many ‘species’ do not represent genetically
isolated lineages. However ecological,
morphological or spatial mating barriers
exist
Adaptation or species selection?
Adaptive trend
Time
Time
Species selection
Morphological
divergence
Morphological
divergence
Species selection means that evolution
proceeds via differential extinction of species
with certain characteristic features.
Adaptive trends imply differential speciation
rates of better adapted lineages.
Examples of fast evolutionary speed
Minotetrastichus
frontalis (=ecus)
Cameraria ohridella
The Aesculus miner C. ohridella was first
described in 1984 in Albania as a rare new
species.
Since then it colonized whole Europe and
became a dominant mining species on Aesculus
hippocastanus.
Mus musculus
The Faroer Island house mouse originated
from the Western European House Mouse
(Mus domesticus).
During 250 years of colonization it has
evolved three distinct isolated island
populations.
Nevertheless it is a good example how an
evolutionary novelty can trigger dispersion.
The Nólsoy House Mouse is a sub-species
called (Mus musculus faeroensis) and the
Mykines House Mouse is also a sub-species
called (Mus musculus mykinessiensis).
This dispersion initiated host switches and
lineage divergence of its major parasite
Minotetrastichus frontalis.
Its closest relative was the now extinct St
Kilda House Mouse (Mus musculus muralis).
It is unknown what caused the rapid spread.
Natura non facit saltus?
Gradualism in Pliocene snails, 10 to 3 Mya.
Species A
Genetic divergence
Gradual speciation
Species B
Stasis
Species A
Speciation event
Stasis
Species B
Saltatorial speciation
Species C
Saltatorial speciation
means sudden rapid
evolutionary change that
is manifest in genetic
isolation.
Speciation event
Time
The classic view of speciation
Classical Darwinian selection
implies a continuous
(graduate) change in species
characters.
Ernst Mayr, 1904-2005
Phyletic gradualism asserts that
•
•
•
•
John B. S. Haldane,
1892-1964
The combination with
population genetics gave rise
to the neodarwinean
synthetic theory of
evolution formulated mainly
by Ernst Mayr and J.B.S.
Haldane.
Species arise by the transformation of an ancestral population into its modified descendants.
The transformation is even and slow.
The transformation involves large numbers, usually the entire ancestral population.
The transformation occurs over all or a large part of the ancestral species' geographic range
This implies that
• Ideally, the fossil record for the origin of a new species should consist of a long sequence of
continuous, insensibly graded intermediate forms linking ancestor and descendant.
• Morphological breaks in a postulated phyletic sequence are due to imperfections in the
geological record.
Tempo and mode of evolution reconsidered
Stephen Jay
Gould, 1941-2002
Niles Eldredge
1943-
The theory of punctuated equilibrium of Niles Eldredge and Stephen Jay Gould states that
•
•
•
•
•
•
•
•
The fossil record is relatively complete.
Most speciation occurs via peripatric speciation.
Widespread species usually change slowly, if at all, during their time of existence.
Daughter species usually develop in a geographically limited region.
Daughter species usually develop in a stratigraphically limited extent.
Sampling of the fossil record will reveal a pattern of most species in stasis, with abrupt
appearance of newly derived species being a consequence of ecological succession and
dispersion.
Adaptive change in lineages occurs mostly during periods of speciation.
Trends in adaptation occur mostly through the mechanism of species selection.
Genetic distance
Punctuated equilibrium
Speciation
Stasis
Stasis
Stasis
Speciation
Evolution is assumed to proceed via fast
genetic transitions within an peripatric
speciation framework.
Speciation
Stasis Subspeciation
Time
140
Cladogenesis
Thorax width
z
130
Stasis
120
Stasis
110
Cladogenesis
100
Homo sapiens
-1000000
-2000000
Homo erectus
Homo habilis/ergaster
-3000000
-4000000
Australopithecus
Cladogenesis
Cladogenesis
Stasis
Mean thorax width
of Trilobite species
90
80
-1800
Time before present
0
-5000000
0
500
1000
1500
2000
Cranial capacity
-1600
-1400
-1200
Core depth [cm]
-1000
1000
The evolution of man is a good example of
punctuated equilibrium.
Does evolution need hopeful monsters? Or evolution above the species level
Classical Darwinian theory assumes
character evolution to be a gradual process.
Richard
Goldschmidt,
1878-1958
However higher taxa are of often
distinguished without any intermediate
fossils (fossil gaps).
Did major evolutionary branches evolved
very fast or is our fossil record too
incomplete?
Goldschmidt assumed that major evolutionary transitions are caused by mutations in
regulatory genes giving rise to major morphological changes.
Most of these highly altered creatures have no chance to survive, but few succeed and
are ‘hopeful monsters’ that are ancestors of new higher taxa.
Punctuated equilibrium is a modern form of this saltationism.
150
mya
Paleocene
65
mya
The history of whales: Gradualism or
saltationism?
First
feathers
Sinosauropteryx prima
Eocene
40
mya
Ambulocetans
natans
Jura
50
mya
46
mya
The history of birds: Gradualism or
saltationism?
Caudipteryx
zoui
Rhodocetus
kasrani
Dorudon atrox
135
mya
Protarchaeopteryx robusta
Evolutionary trends and major questions
Evolutionary constraints
Major evolutionary trends
•
Rising number of genes across
clades?
•
What made vertebrates prone to
evolve large brains?
•
Rising morphological complexity
across clades?
•
Why did insects never get large?
•
Why did plants never evolve
nerves and muscles?
•
Rising hierarchical organization?
•
Rising physiological and ecological
flexibility?
•
Why did Dinosaurs not become
small?
•
Rising efficiacy in design?
•
•
Rising evolvability (the ability to cope
with changing environmental
conditions)?
Why did marine taxa stop
evolving since the Cambrian?
•
Why did major taxa (phyla) only
evolve in the late Proterozoic?
•
Did life appear only once?
•
Decreasing evolvability due to trends
for specialization?
•
Rising complexity of ecological
interactions?
Discuss these questions using the many internet sources!
zs f
20
Extinctions of families
Extinctions
16
18
Marine taxa
Mass
extinctions
14
12
R2 = 0.21
8 p(t) < 0.0001
10
6
4
2
0
-600
E
K
O
S
D
-200
-300
-400
-500
C
P
T
J
0
-100
Kr
Pa N
Trade off between extinction and speciation
dS
 c(t)S  e(t)S  (c  e)S  St  S0e[c(t ) e(t )]t
dt
The background extinction rate e(t) of marine taxa decreased!
Stand. numbers of families
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
Origination rate of genera z
z
Trade off between extinctions and speciations
Mass extinctions might be followed by high
origination rates
Extinction and origination
rates are connected.
Peaks in speciation of
marine taxa occurred often
after mass extinctions.
0
-600
-500
Extinctions of families
zs f
E
K
-400
O
S
D
-300
C
P
-200
T
J
-100
Kr
0
Pa N
600
Mass
extinction
500
Bivalvia
Mass extinctions might also
change ecological dominance.
400
300
Bivalvia raised after the mass
extinction of the ecologically
similar Brachiopoda.
200
100
Brachiopoda
0
-600
-500
E
K
-400
O
S
D
-300
C
P
-200
T
J
-100
Kr
0
Pa N
zs f
Proportion of marine genera
1
Buffered
species
0.9
0.8
Mass extinctions are not equally
distributed among taxa.
0.7
0.6
Advanced species that are physiologically
more buffered against environmental
changes increased in frequency after
mass extinctions
0.5
0.4
0.3
0.2
0.1
Mass
extinction
Unbuffered
species
0
Proportion of marine genera
zs f
-600
-500
E
K
-400
O
S
D
-300
C
P
-200
T
J
-100
Kr
0
Pa N
1
Motile
species
0.9
0.8
0.7
Motile species were often less affected
than sessile species
0.6
0.5
0.4
0.3
0.2
Mass
extinction
Sessile
species
0.1
0
Proportion of marine genera
zs f
-600
-500
E
K
-400
O
S
D
-300
C
P
-200
T
J
-100
Kr
0
Pa N
1
0.9
Prey
0.8
0.7
Predator species richness increased after
mass extinctions
0.6
0.5
Mass
extinction
0.4
0.3
0.2
0.1
Predators
0
-600
-500
E
K
-400
O
S
D
-300
C
P
-200
T
J
-100
Kr
0
Pa N
Adaptation to herbivory and promiscuity might cause high rates of speciation
Change in feeding style
Cucujoidea
Curculionoidea
Trichoptera
Lepidoptera
< 10000 species > 200000 species
< 10000 species > 300000 species
Herbivory
Herbivory
Predators
Detritivorous
Change in mating system
Manucodes
5 species
Birds of paradise
33 species
Swifts
103 species
Hummingbirds
319 species
Promiscuity
Promiscuity
Pair bonds
Pair bonds
The interplay between extinction and origination
0.5
Correlations between actual diversity and speciation
r(D-S) and extinction rates r(D-E) (mid Jurassic to
Neogene) show that
r (D-E)
0.3
0.1
-0.3
-0.1
-0.1
0.1
-0.3
0.3
high diversities were correlated with high extinction
rates but not with high speciation rates.
This points to a density dependent effect in speciation.
-0.5
r (D-S
c t  c0 (
Smax  St
)
Smax
S S
dS
 (c0 max
 e(t))S
dt
Smax
The equilibrium number of species is then
S*  (
c0  e(t)
e(t)
)Smax  (1 
)Smax
c0
c0
Hence, the realized number of species is always lower
than the maximum possible number.
Ecological systems cannot be saturated with species.
Species richness and taxon age
Species richness increases with taxon age.
Speciation rates l are independent of species
richness but decrease with taxon age.
Younger taxa have higher speciation rates l.
Total species richness is also determined by
species survival rates.
Insecta and Vertebrata
Chordata
Arthropoda
Mollusca
Data from Mc Peek, Brown (2007)
Extinction rates seem to be pylogenetically clustered
Extinction rates were
phylogenetically
clustered in time
Extinction rates of
mesozoic Bivalvia
were stable in time
From Roy et al. 2009)
z
The Red Queen hypothesis
Positive lineage age
dependent rate
of extinction
1000
taxa
Number of surviving
10000
Constant rate
of extinction
100
Leigh M. Van
Valen 1935-
10
Negative lineage age
dependent rate of extinction
1
Number of genera z z
0
200
400
600
Time
1000
y = 633e-0.038x
100
10
1
0
50
100
150
200
Survival time [mya]
Survival times for extinct genera of
Echinoidea (sea urchins).
Extinction rates (probabilities) are
roughly constant through time.
One explanation for this is the Red
Queen hypothesis (after Lewis
Carroll’s Through the Looking Glass).
Each species has to run as far as
possible (to evolve continuously) only
to stay in the same place.
Its competitors, predators and
parasites also evolve continuously.
Under these circumstances extinction
probabilities will remain roughly
constant.
Today’s reading:
Speciation: http://en.wikipedia.org/wiki/Speciation
Observed instances of speciation: http://www.talkorigins.org/faqs/faq-speciation.html
The origin of species: http://bill.srnr.arizona.edu/classes/182/Lecture%202007-03.htm
Punctuated equilibrium: http://en.wikipedia.org/wiki/Punctuated_equilibrium
Punctuated equilibrium: http://www.mun.ca/biology/scarr/2900_Fossils.htm