Transcript General Ecology: EEOB 404

General Ecology:
EEOB 404
Evolutionary ecology:
speciation and extinction
Topics for this class:
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Species richness of communities depends on both speciation
and extinction processes
Nature of speciation concepts, mechanisms
Example: beetle herbivores & speciation on angiosperm
plants
Nature and impact of extinction on species richness
Impact of humans on contemporary rates of extinction
Species richness depends on
both speciation and extinction
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Species richness is another form of biological
diversity than genetic diversity within a species
Species richness is a characteristic of
evolutionary clades, and of ecological
communities
High rates of speciation can boost species
richness over time, just as high rates of
extinction can decimate it
Species concepts
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Evolutionary biologists still debate which of a
number of species concepts apply most widely in
nature
Ernst Mayr’s Biological Species Concept probably
applies most widely (at least in animals):
Accordingly species are reproductively isolated
units
No one species concept applies to all organisms
Mechanisms of speciation
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Similarly, mechanisms of speciation are still poorly
understood by evolutionary biologists
Several are well documented:
 Allopatric
speciation (probably most vertebrates)
 Sympatric speciation (especially insects)
 Polyploidy and chromosome rearrangements in small
peripheral populations (especially plants)
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We need not concern ourselves in this class with
particular species concepts or mechanisms to get
the main points about the importance of evolutionary
phenomena to understand ecological questions
Effect of speciation on
species richness: an example
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Outstanding question: “Why are there so many
beetle species?”
Background:
 Insects
are overwhelmingly rich in species: 751,000
species, ca. 50% of all described species
 Of insects, beetles are overwhelmingly dominant:
>50% of all insects are beetles
 Of beetles, leaf-eating families are dominant groups
(e.g., Chrysomeloidea = leaf beetles, Curculionoidea
= weevils)
Famous anecdote
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J.B.S. Haldane is a well known evolutionary
biologist, one of the architects of “The Modern
Synthesis”
A theologian reputedly asked Haldane, late in life,
“What can we conclude about the nature of the
creator from looking at the creation?”
Haldane replied: The Creator has an inordinate
fondness for beetles”
Question of beetle diversity was recently
addressed by phylogenetic study
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Study by Farrell,B.D. 1998. Science 281:555.
(copy on class reserve)
Study first devised phylogeny of 115
phytophagous beetles (representative families)
Sequenced entire 18s ribosomal subunit, 2117
nucleotide positions-->355 informative characters
(i.e., those that varied among the species)
2nd data set,scored 212 morphological characters
Parsimony analysis-->most likely “tree” of
evolutionary relationships (= cladistic analysis)
Adaptive radiations of beetles began in
late Jurassic, continued to diversify up
until today;
Herbivores diversified disproportionately
20000
Number of beetle genera
400
300
200
Herbivores
Carnivores
Saprophages
10000
100
0
Permian Triassic Jurassic CretaceousTertiary
Recent
0
Next step in analysis...
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Superimpose onto phylogeny feeding habitats of
living species, timing at which groups show up in
fossil record, number of extant species
Results (next slide):
 Five
independent cases in which phytophagous
beetles became associated with angiosperm plants
 Each of these cases associated with increased
species richness
 This pattern general, because we see it replicated 5
times, independently in beetles’ evolutionary history
 These beetle radiations took place in Cretaceous &
Tertiary, corresponding with adaptive radiation of
angiosperms (flowering plants)
Nemonychidae(85)
Attelabinae(800)
Rhynchitinae(1200)
Antliarhininae(24)
Apioninae(1500)
Rhynchophoridae(1100)
Curculionidae(40502)
Oxycoryninae(10)
Allocoryninae(20)
Belinae(150)
Prioninae(770)
Aseminae(75)
Spondylinae(3)
Cerambycinae(10000)
Lamiinae(14000)
Lepturinae(1000)
Palophaginae(3)
Megalopodinae(350)
Zeugophorinae(50)
Orsodacninae(8)
Aulacoscelidinae(18)
Synetinae(11)
Eumolpinae(3200)
Megascelidinae(60)
Lamprosomatinae(190)
Chlamisinae(360)
Clytrinae(947)
Cryptocephalinae(2290)
Chrysomelinae(2000)
Galerucinae(5300)
Alticinae(8000)
Cassidinae(3000)
Hispinae(3000)
Donaciinae(165)
Criocerinae(1400)
Pachymerinae(77)
Amblycerinae(400)
Bruchinae(3000)
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5
Tr
Ju
Cr
Ter
early Oligocene
Interpretation of Farrell’s study?
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Angiosperms diverged greatly in plant chemistry.
Each plant group provided new “island archipelago”
with opportunities for colonization, diversification in
how the insects could use these new plants (e.g.,
chew leaves, mine leaves, eat seeds, mine stems,
chew flowers)
Basically flowering plant speciation appears to have
triggered massive adaptive radiation of plant-feeding
insects, i.e., co-evolutionary arms-race
Thus, we see evolutionary tools applied to
addressing ecological question (distribution &
abundance of species)
Extinction is the other
evolutionary process that
influences species richness
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Rates of extinction have varied enormously in
Earth’s history from negligible (background) rates to
a number of mass extinctions (5--class overhead)
 E.g.,
extinction of dinosaurs (and many other birds,
mammals, marine reptiles, etc.) 65 MYA, response
to asteroid impact
 Extinctions opened opportunities for subsequent
radiations, such as mammals and birds following
dinosaur mass extinctions
 Extinction is the rule, rather than the exception
Humans causing extinctions today,
higher rate than mass-extinctions!
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Increasing rate of extinction (e.g., birds, mammals)
correlated with increasing human population
Disproportionate vulnerability of island populations
 Entire
clades wiped out by human over-hunting (e.g.,
Moas of New Zealand; native flightless birds of
Hawaii)
 Almost all islands world-wide are missing species due
to human-caused extinctions (on order of 50% of spp.)
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However mainland species also vulnerable--e.g.,
passenger pigeon, Carolina parakeet, ivory-billed
woodpecker, Bachman’s warbler = U.S. birds extinct
since 19th century
What are some lessons about
extinction & its causes?
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Causes of known animal extinctions (484 spp.):
 Introduced
animals (17%)--e.g., brown tree snake, Guam
 Habitat destruction (16%)
 Over-hunting (10%)
 Other (1%) or unknown (56%) causes
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Different causes of extinction in different taxa,
different locations
 E.g.,
Hawaiian organisms particularly affected by
introduced organisms
 Hawaiian birds particularly affected by disease (avian
malaria)
 Many island populations overexploited (e.g., hunted)
Conclusions
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Understanding evolutionary processes (such as
speciation, extinction) provides critical information on
present-day patterns of organism distribution and
abundance
Speciation and extinction processes have both had a
profound impact on ecological patterns and
processes observed today
Extinction and speciation processes affect taxa
differently, and geographic locations differently
Humans today, and deep into our evolutionary past,
have profound impacts on ecological systems...why?