Species Radiation and Extinction
through Geologic Time
Text Readings:
pages 39-41
Box 3.3 (pg 85)
86-89
224-225
Speciation occurs with a split in a gene pool of a
species such that the separate lines become
reproductively incompatible
Washington Community & Tech. College
How does Speciation Occur?
1.Allopatric Speciation
• microallopatric speciation
2.Sympatric Speciation
3.Parapatric Speciation
1) Allopatric Speciation
- populations become reproductively isolated
during periods of geographic separation (e.g. by
mountains, land bridge disappearance, sea strait).
- this mode is recognized as important for
speciation of many plants and animals;
- the best evidence comes from birds (Darwin's
finches, Hawaiian honeycreepers), mammals,
butterflies and Drosophila (e.g. Hawaii)
One of the best examples is the accepted history
behind Darwin’s Galapagos finches. The initial
colonist is believed to be a finch from Cocos Island,
off the mainland of Costa Rica.
The Cocos finch is the sole finch species on Cocos
Island, whereas many species occupy the Galapagos
islands owing to emigration followed by allopatric
speciation on ‘new’ islands in the Galapagos.
Habitat and food
specialization and
geographic isolation
facilitated species
Cocos Island
Cocos finch
Darwin's finches
Hawaiian Honeycreepers
same process as per
Darwin's finches except
the scale of radiation
and morphological
variation in beak size
was far more
extensive. Process
occurred in <10 million
years
S. Olson (2004) Evolution in Hawaii
Hawaiian
Drosophila
Kevin Edwards,
Illinois State
Univ.
Hawaiian Drosophila: ~1000 diverse species from a single introduction
of an Asian Drosophila species that spread before the current Hawaiian
Islands were formed, and colonized the chain by 'island hopping' about
26 million years ago. New islands were colonized as they became
suitable ecologically. Lava flows and geographic separation fragmented
species' ranges, causing bottlenecks that facilitated evolution of species
Wrasse Speciation
A classic example of allopatric speciation as a result of
separation of populations by the uplifting of the Isthmus of
Panama (<3 million years ago) separating some species
into Atlantic and Pacific populations
2) Sympatric Speciation
- more controversial and in most cases less
common than allopatric speciation
- best examples are from plants with
polyploidy (multiplication of chromosome
number resulting in instantaneous speciation)
- this mode is successful only if new individuals
can interbreed through assortative mating,
inbreed (self-mating), or become polyploid
- believed responsible for ~40% of plant species
diversity, far less in animals, but is found in
parasitic Chalcidoidea, and may occur in parasitic
and hyperparasitic species through shifts in host
selection.
Hawaiian Silverswords
• Hawaiian silversword alliance consists of about 30
species in 3 genera (Wilkesia, Argyroxiphium, Dubautia).
• The species exhibit an extraordinary range of
anatomical, morphological, and ecological adaptations
• genetically very close
• evolved from a single ancestor that colonized Hawaii by
way of long-distance dispersal from North America
Hawaiian Silversord Alliance
3-Spine Sticklebacks in British Columbia Lakes
Limnetic Male
Benthic Male
• The similarity of their mtDNA (<0.5% sequence divergence)
suggests that each pair arose independently by sympatric speciation,
following a single invasion of each lake by ancestral marine
sticklebacks
• Other possibility is an initial invasion from the sea, with individuals
developing benthic feeding mode, followed by a later 2nd invasions,
with individuals focusing on planktonic foods
• Incipient speciation is driven by positive assortative mating
Vamosi, & Schluter. 1999. Evolution
Cichlid Fishes in African Great Lakes
hundreds of species in each of Lakes Victoria, Malawi and
Tanganyika, that have both habitat and feeding specialization, the
latter driven largely by mouth morphology
3) Parapatric model
- speciation occurs among adjacent populations of
the same species as a result of strong and differing
selective pressures in the local environments
- there is only limited evidence, perhaps best
exemplified by local 'races' of plants like
Deschampsia caespitosa that establish on mine
wastes rich in heavy metals like copper and nickel;
these populations are different ecologically and
genetically from adjacent individuals on non-toxic
soils.
Deschampsia caespitosa
• is an extremely variable grass with a wide distribution, though
mainly in the northern hemisphere
• 40 subspecies or varieties
• in North America there exist 5 morphologically different forms
• taxonomy of the group, particularly in Central Europe, is still
puzzling, because of the extreme morphological variability and
ambiguous boundaries between the taxa
• Adaptation to particular environments (metal vs. nonmetal)
may result in the formation of these ecotypes and represent
the beginning of speciation
Chiapella (2000) Biol. J. Linn. Soc.
Parapatric speciation in Orioles?
May occur in zones where distinct species have
range overlap and hybridize
Sibley & Short (1964) Condor
How does speciation occur?
1.Gradual divergence in the face of differing
selective pressures for different, isolated
populations; and/or
2.genetic revolution in which more than 1 genetic
constitution is favoured by selection for
populations with the same environmental
conditions.
Speciation in the latter model occurs if a population
switches from one adaptive peak to another, most
likely occurring because of genetic drift or by
polyploidization, which causes instantaneous
speciation.
Diversification has occurred both continuously
and sporadically, with dramatic increases in
diversity following each of the mass extinctions.
Without repeating the history, we can review the
times of major diversification of particular
phyla…
Cambrian: sea invertebrates: archaeocyathans,
inarticulate branchiopods, trilobites
Family Level Gains and Losses
1900
Rest of Paleozoic:
corals, articulate
branchiopods,
cephalopods (e.g.
squids), ostracods,
crinoids, starfish,
graptolites
(extinct colonial
group), first land
plants; insects in
late Paleozoic
Mesozoic: bivalves, gastropods, malacostracans,
echinoids, bony fishes, marine reptiles, first
gymnosperms and (later) angiosperms; dinosaurs
appear.
Cenozoic: social insects; modern and placental
mammals; hoofed mammals and apes; man.
Most geologic records are based on marine taxa,
specifically invertebrates.
Family-level Diversification:
Benton (1995) used data collated from 90 experts for
taxa including microbes, algae, fungi, protists, plants
and animals for 7186 different families.
Benton selected family level distinctions for a
number of reasons, the main one being that he could
avoid uncertainty regarding species designations by
working at a higher level of resolution.
Family level diversification shows the following
history:
Family Diversification through time based on fossil record
Benton (1995) Science
log (diversity) thru time
Speciation through Time
new families in period
Extinction through Time
# families lost thru time
% families lost thru time
Benton (1995) Science
% families lost in period
Loss of families as a function of
the number that started the
period and the length of the
period
Benton (1995), Science
1) family-level diversity increased in a burst during
the Vendian (Precambrian);
2) diversity fell to 120 families during late Cambrian;
3) diversity increased to 450 during Ordovician;
4) diversity rose to ~600 during remaining Paleozoic;
5) fell to 420 at beginning of Triassic;
6) rose to 2600 during Cretaceous (144 myr) and
2159 during Pleistocene and Holocene (<2 myr);
7) most continental families arose following Silurian
– land plants, insects and vertebrates;
8) by contrast, many marine families arose during
the Vendian and early Cambrian period followed
by massive losses during the late Cambrian and
late-Permian; then exploded in diversity until the
late Cretaceous; further expansion occurred at the
beginning of the Tertiary (65 myr);
9) family diversity increases were clearly
logarithmic for all taxa and for continental taxa,
though a more complex pattern of radiation and
extinction occurred with marine families.
Overall Patterns:
• many of the radiations occurred following
massive extinctions;
• most of the early radiations were marine, most of
the later ones terrestrial;
• huge increases during the Eocene (55 myr)
correspond with radiation of the insects
• insect and flowering plants appeared to undergo
radiation together suggesting coevolutionary
radiations
- mass extinctions occurred during the early
Cambrian, late Ordovician, middle and late
Devonian, late Carboniferous, late Permian, end
Triassic, end-Jurassic, mid-Cretaceous, late Eocene
- 4 of extinction periods were protracted (not fast)
- some extinctions (and radiations) were not well
recorded in the fossil record (referred to as
slippage)
- different measures of extinction (per family, total
rate etc.) each have their own weakness (e.g.
dividing by total extant diversity where the small
number of families early on skews that statistic)
For the 5 accepted mass extinctions:
Time Period
Late Ordovician
Late Devonian
Late Permian
Late Triassic
end-Cretaceous
% of families Extinct
20-22
20-24
31-52
15-24
11-16
Remember, the clock ticks faster for genera or for species
than for families, so large family losses would
correspond with huge species losses
General patterns in extinctions have been identified:
•terrestrial vertebrate taxa that went extinct were
large-bodied (e.g. dinosaurs; end-Pleistocene
megafauna). That would be consistent with the
expected population biology of these species: low
population numbers, long-lived, slow maturing, low
reproductive rates, large home ranges and high food
demands.
•Widespread genera fared extinction periods much
better than those with narrow geographic limits.
• Factors (e.g. species richness) that protected genera
from background extinctions were ineffective
protection during the mass extinction that occurred
at the end of the Cretaceous.
• Tropical marine taxa (corals) were harder hit during
mass extinctions than taxa from higher latitudes.
• Species diversity losses of marine bivalves took, on
average, 10 million years to recoup through new
radiations.
• Species responses to changing environments
(particularly following the Pliocene-onset [<5.2
myr]) were highly individualistic.
Thus, current reserve designs should reflect the
broad requirements and environmental responses
of different taxa if large-scale biodiversity is to
be successfully preserved.
• In many cases, a rich diversity of protected
microhabitats may prove more important in
reserve design than maximizing reserve area.
References:
Axelrod, D.I. 1952. A theory of angiosperm evolution. Evolution 6:29-60.
Axelrod, D.I. 1970. Mesozoic paleogeography and early angiosperm history. Bot.
Rev. 36:277-319.
Benton, M.J. 1995. Diversification and extinction in the history of life. Science
268:52-58.
Futuyma, D.J. 1986. Evolutionary Biology, 2nd Edition. Sinauer, Sunderland, MA.
Jablonski, D. 1991. Extinctions: a palaeontological perspective. Science 253:754-757.
Groom, M.J., Meffe, G.K. and C.R. Carroll. 2005. Principles of Conservation
Biology. Sinauer. 3rd ed.
Primack, R.B. 1994. Essentials of Conservation Biology. Sinauer, Sunderland, MA.
Sisk, T.D., A.E. Launer, K.R. Switky and P.R. Ehrlich. 1994. Identifying extinction
risks. Bioscience 44:592-604.
Sun, G., D.L. Dilcher, S. Zheng, Z. Zhou. 1998. In Search of the First Flower: A
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