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Chapter 18
Mass Extinctions,
Opportunities and
Adaptive Radiations
Figure CO: Dinosaur fossil
© Styve Reineck/ShutterStock,
• Extinctions are as important in the history of
life as are the evolution of new species
• Explaining extinctions is just as challenging a
scientific question as explaining the evolution
of new species
• Extinctions are opportunities for adaptive
radiations because extinctions open or reopen niches for new species to invade and
• To understand extinctions, we need to identify
rates, patterns and causes
• Georges Cuvier is credited with establishing the reality of
extinction for the scientific community in a lecture to the
French Institute in 1796
• G.G. Simpson and many other evolutionary biologists have
estimated that 99% of all species are already extinct
• The only drawback to accepting that number is our lack of
knowledge of how many species are actually living today,
much less how many were alive in the past
• But not all species have gone extinct; there are some living
Survivors — Lingula
• A marine organism
(brachiopod) occupying
vertical burrows in sand and
mud has survived
morphologically unchanged
since the Silurian
Survivors — Horseshoe crabs
• The horseshoe crab
(Limulus), an
inhabitant of marine
shores, has lived
unchanged since the
Survivors — Cycads and Horsetails
Survivors — My Favorite!
Periplaneta americana:
the cockroach
• We know very little about natural
extinctions, especially the precise causes
• Fossil records demonstrate that
extinctions have occurred repeatedly in
the past
• But physical evidence of causative agents
are rarely preserved
• Cause and Effect is hard to establish
• Habitat Disruption
– Volcanic Eruptions
– Asteroid Impacts
– Sea Level Change
• Habitat Modification
Climate Change
Sea Level Change
Precipitation Change
Toxic Materials
• “Exotic” Species Introductions
– Continental Drift
Co-Evolution & Niches
Co-Evolution & Niches
• Any species living in a niche has evolutionary
relationships with other species; some casual,
some crucial
• Therefore, the extinction of a species will have
repercussions in the niches of all species
which have co-evolutionary relationships with
the newly extinct species
Rates of Extinction
• There is much debate about the degree and the
importance of different rates of extinction
• Once again, the incomplete fossil record makes
answering the question far more difficult
• The simple comparison is between a background
rate of “uniform” extinctions, and the occasional
episodes of “mass” extinctions
Uniform/Background Extinctions
• Species Average Survival Time
– marine invertebrates – 30 million years
– mammals – 2 to 3 million years
– one estimate for all fossil species – 4 million years
• Rate of Species Extinction
– One estimate for the background extinction rate is for
one to seven species to die each year
– The same source estimated mass extinction rates as
being 3 to 4 times the background rate with 75 to 95% of
species dying; this is a rate of perhaps 15 to 30 species
per year
Uniform/Background Extinctions
• A major problem in estimating the rate of fossil
species extinction is the incomplete fossil record
• Approximately one quarter of a million fossil species
have been identified so far (in ~35,000 genera from
~4,000 families)
Uniform/Background Extinctions
• Based on current biodiversity (~40 million living
species?) and average species extinction rates,
there may well have been 5 to 50 billion species
present on earth since the origin of life on earth
• So all our conclusions should be considered very
• Extinction is the converse of speciation; species
arise and species disappear
• Extinction can be considered at levels of
increasing severity and impact:
– Extinction may be local and limited to demes
– Extinction may eliminate an entire species
– Extinction may eliminate most or all of the species in a
region, habitat, or ecosystem
– Extinction may be of much larger scale, eliminating
most of the species on a continent or on Earth –these
are mass extinctions
• At least five mass extinctions have occurred in the history of
life on earth
• Each mass extinction has been followed by the successful
adaptive radiations of new organisms
Table T01: Details of the Five Major Mass
Extinction Events Since the Cambrian
Source: Raup, D.M. and J.J. Sepkoski, Jr., Science 231 (1986): 833–835.
The Five Major Mass Extinction Events
• Why are Mass Extinction Events defined by the
relatively abrupt disappearance of at least 75%
of marine animal species?
• Because over the 500 million years of metazoan
existence, the most complete fossil records are
for marine animals
• Geologists tend to divide that time into Eras and
Periods by mass extinctions, which are followed
by adaptive radiations of new forms; or simply
by major adaptive radiations of new forms
(index fossils)
Radiations of the Metazoans in the Phanerozoic
Mass Extinction Events
Note that this chart tracks all animal families,
not just marine animals, and at the family level, not the species level
The Ordovician Extinction Event
• The second-largest of the five major extinction
events in Earth's history in terms of percentage
of genera that went extinct and the second
largest overall in the overall loss of life
• Between about 450 Ma to 440 Ma, two bursts
of extinction occurred, separated by one
million years
• This was the second biggest extinction of
marine life, ranking only below the Permian
The Ordovician Extinction Event
• At the time, all known metazoan life was
confined to the seas and oceans
• More than 60% of marine invertebrates died;
brachiopods, bivalves, echinoderms,
bryozoans and corals were particularly
• The immediate cause of extinction appears to
have been the tectonic movement of
Gondwana into the south polar region
The Ordovician Extinction Event
• Gondwanan drift south led to global cooling,
glaciation and consequent sea level fall
• The falling sea levels disrupted or eliminated
expansive shallow marine habitats along the
continental shelves
• The event was preceded by a fall in
atmospheric CO2, a global cooling, and the
newly forming Appalachian Mountains may
have been the CO2 sink
Middle Ordovician
The Devonian Extinction Event
• The third-largest of the five major extinction
events in Earth's history in terms of percentage
of genera that went extinct
• The timing is less well understood
• Conflicting hypotheses propose from as few as
two to as many as seven related bursts of
extinction centered on 365 Ma to 440 Ma, over
as little as one half to as many as 25 million years
• The extinction seems to have primarily affected
marine life
The Devonian Extinction Event
• By the late Devonian, there were massive reefs built by
stromatolites and corals in the oceans, while the land had
been colonized by plants and insects
• Vascular plants were becoming tall and changing the soils as
well as the co-evolving biota
• Euramerica and Gondwana were beginning to converge into
what would become Pangea
• Hard-hit groups include brachiopods, trilobites, and reefbuilding organisms; the latter almost completely disappeared,
with coral reefs only returning upon the evolution of modern
corals during the Mesozoic
• Surprisingly, jawed vertebrates seem to have been unaffected
by the loss of reefs, while agnathans were in decline long
before the end of the Devonian
The Devonian Extinction Event
• The causes of the Devonian extinctions are unclear
• The extinction of ~20% of all animal families and 70-80% of
all animal species
• Leading theories include changes in sea level and ocean
anoxia, possibly triggered by global cooling (glaciation on
Gondwana) or oceanic volcanism
• The widespread oceanic anoxia prohibited decay and
allowed the preservation of sedimented organic matter as
• The impact of a comet or another extraterrestrial body has
also been suggested, but the evidence is weak
Late Devonian / Early Carboniferous
The Permian Extinction Event
• The Earth's most severe mass extinction
event, with up to 96% of all marine species
and 70% of terrestrial vertebrate species
becoming extinct
• It is the only known mass extinction of insects
• Some 57% of all families and 83% of all genera
were killed
• Because so much biodiversity was lost, the
recovery of life on Earth took significantly
longer than after other extinction events
The Permian Extinction Event
• There were from one to three distinct pulses
of extinctions that occurred about 245-251
million years ago
• There are several proposed mechanisms for
the extinctions
• The earlier phase was likely due to gradual
environmental change, while the latter phase
may has been due to a catastrophic event
The Permian Extinction Event
• Suggested mechanisms for the latter
catastrophic extinction pulse include:
– large or multiple bolide (meteor/comet) impact
– increased volcanism and sudden release of
methane clathrate from the sea floor
– gradual changes include sea-level change, anoxia,
increasing aridity, and a shift in ocean circulation
patterns driven by climate change
– Excess dissolved CO2 acidified the oceans,
contributing to the decline of shelled organisms
The Permian Extinction Event
• Most fossil insect groups found after
the Permian–Triassic boundary
differ significantly from those that
lived prior to the P–Tr extinction
• Over two-thirds of terrestrial
labyrinthodont amphibians,
sauropsid ("reptile") and therapsid
("mammal-like reptile") families
became extinct
• Large herbivores suffered the
heaviest losses
Late Permian
The Triassic Extinction Event
• The first of the final two more modest of
the five major extinction events
• The extinction occurred around 208
million years ago and happened in less
than 10,000 years just before Pangaea
started to break apart
• This extinctions struck marine life and
terrestrial life profoundly
• At least half of the species now known to
have been living at that time went extinct
• In the seas, a whole class (conodonts) and
20% of all marine families disappeared
– Conodonts were early eel-like chordates
The Triassic Extinction Event
• On land, all large crurotarsans (non-dinosaurian archosaurs)
other than the crocodilians, some remaining therapsids, and
many of the large amphibians were wiped out
• This event vacated terrestrial ecological niches, allowing the
dinosaurs to assume the dominant roles in the Jurassic period
• Statistical analysis of Triassic marine losses suggests that the
decrease in diversity was caused more by a decrease in
speciation than by an increase in extinctions
The Triassic Extinction Event
• Several explanations for this event have been
suggested, but all have unanswered challenges:
– Gradual climate change or sea-level fluctuations during the
late Triassic; however, this does not explain the suddenness of
the extinctions in the marine realm
– Asteroid impact, but no impact crater has been dated to
coincide with the Triassic–Jurassic boundary; the largest late
Triassic impact crater occurred about 12 million years before
the extinction event
– Massive volcanic eruptions (known from the central Atlantic
magmatic province -- an event that triggered the opening of
the Atlantic Ocean) that the would release CO2 or sulfur
dioxide and aerosols, which would cause either intense global
warming (from the former) or cooling (from the latter)
Late Triassic / Early Jurassic
The Late Cretaceous Extinction Event
• The second of two more modest extinction
events, the fifth and final of the five major
extinction events
• There is agreement that it was a relatively rapid
extinction event dated to 65.5 million years
• Widely known as the K–T extinction event, it is
associated with a geological signature known
as the K–T boundary, usually a thin band of
iridium-rich sedimentation found in various
parts of the world
The Late Cretaceous Extinction Event
• The event marks the end of the Mesozoic Era and the
beginning of the Cenozoic Era
• Essentially all non-avian dinosaurs, mosasaurs,
plesiosaurs, pterosaurs and many species of plants and
invertebrates became extinct
• Stem mammalian clades passed through the boundary
with few extinctions and began their remarkably
successful adaptive radiations across the globe
The Late Cretaceous Extinction Event
• Scientists theorize that the K–T extinctions were
caused by one or more catastrophic events, such as
massive asteroid impacts
• Like the Chicxulub impact, a 10km diameter
meteorite, leaving a crater ~200 Km in diameter
• or increased volcanic activity
Impact caused acid rain, ash that
blocked out the sun for months,
severe global cooling (nuclear winter).
Increase in atmospheric CO2, resulting
in global warming, the final blow to
dinosaurs & many other Cretaceous
The Late Cretaceous Extinction Event
• Several bolide impacts may have contributed to
massive volcanic activity, such as the Deccan traps of
west-central India, one of the largest volcanic features
on Earth, have been dated to the approximate time of
the extinction event
The Late Cretaceous Extinction Event
• These geological events may have reduced sunlight
and hindered photosynthesis, leading to a massive
disruption in Earth's ecology
• Other researchers believe the extinction was more
gradual, resulting from slower changes in sea level or
What Happened to the Dinosaurs?
• Sediments were deposited by enormous tsunamis (tidal
waves) along the coastline
• 70% of known fossils, including non-avian dinosaurs, were
wiped out
What Happened to the Dinosaurs?
The Late Cretaceous Extinction Event
• Before the end of the Cretaceous, flight evolved
independently three times:
– Insects, flying reptiles, birds (avian dinosaurs)
• By the end of the Cretaceous 65 Mya, most dinosaurs
along with other large marine reptiles and various
invertebrates died out
• No land vertebrate larger than a large dog survived
the KT boundary event
• The angiosperm radiation was well underway during
the Cretaceous, but the shift from gymnosperm to
angiosperm dominated forests may have been
triggered by the Late Cretaceous Extinction
Late Cretaceous
The Five Major Mass Extinction Events
Note in the left chart that global temperatures
have fluctuated dramatically over the time of
life on earth and those dramatic changes do not
always correlate with mass extinction events
The Five Major Mass Extinction Events
• Plants are relatively immune to mass extinction,
with the impact of all the major mass extinctions
"negligible" at the family level
• Even the reduction observed in species diversity (of
50%) may be mostly due to differential preservation
of plant fossils
• However, a massive rearrangement of ecosystems
does occur, with climax communities and dominant
plants, plant abundances and distributions changing
profoundly after mass extinctions
Mass Extinctions
It is important to remember that mass extinctions are just temporary increases in
extinction rates that are significantly more severe than the average background
extinctions rates (illustrated in green) which also fluctuate through time
Percentage of Species Wiped Out
Ordovician-Silurian - 85%
Late Devonian - 82%
Permian-Triassic - 96%
End Triassic - 76%
Cretaceous-Tertiary - 76%
Figure 01: Percentage of marine animal extinctions
Adapted from Fox, W. T., Paleobiology 13 (1987): 257-271.
Mass Extinction Probable Causes
To the degree that mass extinctions are real, rather than artifacts of a poor fossil
record, the causes are probably complex and multifactorial
What Happened at the Big Five Mass
Abiotic Causes for Mass Extinctions
1. Plate Tectonics
Intermingling of Biotas / Introduced species effects
Trophic Stability
Changes in Sea Level
Volcanic Activity changing atmospheric gases and dust
5. Ice Ages with glaciations, sea levels fallings, increased
tropical aridity
2. Planetary Collisions
3. Cosmic Forces & Periodic Galactic Cycles
Trophic Stability
• When a landmass (a) is broken in two (b), this adds area
along the perimeter where they split; this adds to the
intertidal area which is a species and nutrient rich habitat
• When two landmasses (b) are brought together (a), this
results in loss of available intertidal area
Trophic Stability
• The larger the land mass, the less climatic buffering from
the oceans, which are heat sinks
• Therefore, during the time of Pangaea’s stability, there may
have been more extremes of hot and cold, and wet and dry,
in the supercontinent’s interior
Changes in Sea Level
• Movement of the Earth’s crustal plates results in their slow
collision with each other
• Usually one plate over rides another, as shown here
• Note how the ocean basin between them changes in size
and depth, thereby changing sea level against the side of
the continents
Volcanic Activity
• Locally, lava flows sterilize and reform the surface,
start fires, and their explosive blasts may also create
damage within the surrounding habitat
Volcanic Activity
• Large volumes of volcanic dust enter the atmosphere and
become a possible cause for the cooling of the earth by
blocking out the sun’s rays and reducing photosynthesis
Ice Ages
• Vulcanism or other forces may
contribute to the cooling of the
earth and the formation of glaciers
that covers a part of the earth’s
surface for periods of time
• Formation of glaciers causes:
– Ocean levels drop (due to water
trapped in glaciers as ice)
– Decrease in O2 levels
– Increase in salt (mineral) content of
– Changes in natural environments
Ice Ages and Extinctions
• Ice ages (blue) are indicated along this geologic time line
for comparison to five mass extinction episodes (red)
• There is no tight correlation between ice ages and mass
Extraterrestrial Impacts
• Extraterrestrial impacts are known to have battered
the moon and Earth repeatedly since 4 Bya
• More than 100 large craters are known on Earth:
– Ten meteors, each one km in diameter, are estimated to
have each produced 20-km–wide craters at a frequency of
one every 400,000 years
– A 50-km–wide crater is produced every 12.5 My
– A 150-km–wide crater is produced every 100 My
• Extraterrestrial impacts frequently cause extinctions,
but other than the K-T Cretaceous event, are probably
not the single major cause of mass extinctions
Extraterrestrial Impacts
Cosmic Forces &
Periodic Galactic Cycles
• Supernova (explosion of a
– Influence on earth radiation levels and
destroys ozone layer
– May have influenced extinctions (no
scientific proof as yet)
• Changes in the properties
of the Milky Way as the
Solar System orbits around
the galaxy’s center
Major Indirect Causes
for Mass Extinctions
1. Continental-Flood Basalt Lava (3 of 5)
2. Abrupt Falls in Sea Levels (1 of 5)
3. Asteroid/Bolide Impacts (4 of 5)
4. Changes in CO2, H2S, and other green
house gases and changes in O2 levels
may also play a role
Life Sciences-HHMI Outreach.
Copyright 2006 President and Fellows
of Harvard College.
The Impact of
the Late Cretaceous Extinction
The loss of the non-avian dinosaurs left
many open niches and within 15 My,
the mammals had radiated widely,
occupying some of those niches, and
finding others which had never existed
before because there were also large
changes in the plant communities
Figure 02A: Continental landmasses: Early Eocene
Figure 02B: Early Oligocene
Figure 02C: Late Miocene
Adapted from Janis, C.M., Ann. Rev. Ecol. Syst., 24 (1993): 467-500.
The Cenozoic
The Last 65 My
• Here you can see how plate
tectonics gradually
rearranged the continental
• Continental drift and
Climate change increased
the diversity of habitats
• Especially reduced were the
scope of tropical forests
where dinosaurs had lived
• Angiosperms radiated
widely adapting to these
The Radiations of Mammals
• Traits, such as small size, that were of benefit
during the Cretaceous extinction might not
have been the traits that were most
advantageous before extinction, when large
dinosaurs dominated
• Changing climates and habitats created new
• Mammalian endothermy also permitted
expansion into colder latitudes, altitudes, and
into active nocturnal lifestyles
The Impact of Extinctions
• There has been much speculation about the
impact of the loss of the dinosaurs.
• Had dinosaurs not gone extinct, would the
mammals have remained a minor component
of the earth’s fauna ― small nocturnal
• Would dinosaur lineages continued their
advances in intelligence and social behavior?
The Impact of Extinctions
The Butterfly Effect?
The Impact of Extinctions
The Impact of Extinctions
• "The picture's pretty bleak, gentlemen . . . the world's
climates are changing, the mammals are taking over, and
we all have a brain about the size of a walnut."
South America: Island Continent
One of the more interesting stories of
the Cenozoic mammalian radiations is
that of South American where
placentals and marsupials evolved in
isolation for about 30 My
Birds also radiated widely in South
America at this time, producing some
of the top carnivores
Figure B02A: Xenarthrans
the largest
was ~3
meters tall
Figure B02B: Ungulates
Adapted from Steel, R., and A.P. Harvey. The Encyclopaedia of Prehistoric Life. Mitchell-Beazley, 1979.
South America: Island Continent
• Many of the South American
marsupials and placentals
became extinct as invading
North American placental
mammals diversified rapidly
and took their place; a few S
Am species managed to
expand to the north
– Here extinction resulted
from competition for the
same habitats
Figure B01A-D: Continental drift
Intermingling of Biotas — North
and South America
• Separate faunas and floras
evolved on these continents
when they were separate during
the Cenozoic
• About 2 to 3 million years ago,
the Isthmus of Panama formed,
providing a land bridge between
the continents that became a
route of migration and exchange
between the continents
Among the placental mammals, many arising in North America
dispersed south, and many originating in South America dispersed north
• This led to many extinctions, more in the South than in the North
Co-Evolution — Niches
— Extinction
osage orange
Co-Evolution — Niches — Extinction
• Avocados and Osage Oranges only make sense in the
light of megafauna
• That is because American gomphotheres (related to
elephants) and ground sloths ate and dispersed
those large-seeded fruits
• While those megafauna went extinct around 10,000
years ago, many large-seeded plants in the Americas
are still around today
• If those plants once relied on those large creatures to
disperse their seeds, why have they not gone they
way of the dispersers?
Co-Evolution — Niches — Extinction
• Approximately 100 species of these New World largeseeded plants are thought to have once been dispersed by
• Scientists conclude that many large-seeded plant species,
that once relied on bygone American elephants and
gomphotheres, now rely on present-day small and
medium-sized mammals such as primates, tapirs, along
with domestic pigs and cows, for seed dispersal
• As those medium-sized mammal species become
threatened, the large-seeded plants face possible
extinction again
Australia: Island Continent
Equally interesting
adaptive radiations of
plants, invertebrates, and
vertebrates occurred in
Australia, New Zealand,
and New Guinea
Gliding Locomotion
Figure 03: Flying fish
Courtesy of Shannon Rankin, NMFS, SWFSC/NOAA
Gliding has evolved more often than actual flying!
placental flying squirrel
Invading the Air: Flying Reptiles
• Known adaptations for sustained powered
flight have appeared only three times in the
evolution of terrestrial vertebrates: in
pterosaurs, birds and bats.
– Pterosaurs and Pterodactyls
Figure 05: Pteranodon
© Paul B. Moore/ShutterStock, Inc.
Figure 04A: Pterodactyloids
Figure 04B: Rhamphorhynchoids
Avian Dinosaurs: Birds
• Based on cladistic classification, all birds nest
within the dinosaur lineage
Figure 06A & B: Archaeopteryx
Reproduced from Heilmann, G. The Origin of Birds. Appleton, 1927 (Reprinted Dover Publication, 1972)
Figure 08: Phylogenetic relationships between
birds and sauropod and ornithischian dinosaurs
The Origin of Feathers
• Feathers evolved from reptilian scales
for insulation and display
– Three major hypotheses for their origin:
• Feathers were an adaptation for insulating the
(presumed) warm-blooded and grounddwelling reptilian ancestors of birds
• Ancestral birds were tree-dwelling reptiles
that used their developing wings to glide from
branch to branch
• Ancestral birds were ground-dwelling runners
whose feathers formed planing surfaces
increasing their speed
The Origin of Feathers
Figure 07A: Compsognathus
Figure 09: 4-winged Microraptor
The evidence favors origin from
ground-dwelling ancestors
Figure 07B: Archaeopteryx
Figure 07C: Gallus
C adapted from Dingus, L. and Rowe, T. The Mistaken Extinction:
Dinosaur Evolution and the Origin of Birds. W. H. Freemanm, 1998.
However, the next stage may have
been to move upward in the
environment and then glide down
A and B adapted from Carroll, Robert. Patterns and Processes of Vertebrate Evolution. Cambridge University Press, 1997.
The Origin of Flight
Evolutionary Reversal: Flightlessness
• The extinction of the
dinosaurs opened up a large
number of vacant terrestrial
niche spaces
• The Ratites filled some of
those niches
Figure 10: Large flightless birds
Adapted from Feduccia, A. The Age of Birds. Harvard University Press, 1980.
The Final Vertebrate Fliers – The Bats
The oldest known bat fossil (53 Mya), Onychonycteris
finneyi , discovered in Wyoming, has wings like a modern
bat but lacks adaptations for echolocation
• The two sub-orders of bats are Megachiroptera
(“megabats”) and Microchiroptera (“microbats”)
• Megabats most commonly eat fruit; have no echolocation,
larger eyes than the microbats, and a longer snout
• The microbats possess echolocation (except for Rousettes
and relatives), eat insects, blood, small mammals, and fish,
lack the claw at the second forelimb, have poor eyesight, and
possess a broader snout than the megabats
The First Fliers – The Insects
• Arthropods exploit almost every conceivable
ecological habitat
• Insects evolved from crustaceans and the first
fossil dates to the Devonian, ~400 Mya
• Insects underwent what may be the most
explosive radiation of any animals since the
Cambrian, diversifying into 900,000 extant
species and perhaps as many as eight million
undescribed species
The Insects
• The enormous diversification of insects has
been attributed to the modular organization
of the insect body in which antennae can
evolve independently of wings, mouthparts
independently of legs, and so forth — a
process known as mosaic evolution
• Homeotic mutations, so common in insects,
attest to the comparative ease with which
individual segments can be altered in a body
plan that consists of serially repetitive
Arthropod Phylogeny
Some of the largest prehistoric flying
insects were dragonflies whose wingspan
could be as much as 0.75 meters
The Evolution of Insect Flight
• The Paranotal hypothesis suggests that the insect's wings developed from
paranotal lobes, a preadaptation found in insect fossils that is believed to
have assisted stabilization while hopping or falling (not well supported by
The Evolution of Insect Flight
• The Epicoxal hypothesis suggests that the wings developed from movable
abdominal/ tracheal gills found in many aquatic insects; these tracheal gills started
as extensions of the respiratory system and over time were modified into
locomotive purposes, eventually developed into wings; the tracheal gills are
equipped with little winglets that perpetually vibrate and have their own tiny
straight muscles
The Evolution of Insect Flight
• The Endite-Exite hypothesis suggests that the wings developed from the
adaptation of endites and exites, appendages on the respective inner and outer
aspects of the primitive arthropod limb; the innervation, articulation and
musculature required for the evolution of wings are already present in podomeres
(perhaps the strongest evidence)
The Evolution of Insect Flight
• The Paranota plus Leg Gene Recruitment hypothesis suggests that the wings
developed from mostly immobile winglike projections from the back of the thorax.
Then, once these projections were in place, already-existing genes for limb
development were expressed on the back as well as in the legs, resulting in the
formation of the joints and musculature needed to make functioning wing
• There have been some other hypotheses
Insect Flight – Direct
• Two living orders with direct flight muscles
(mayflies and odonates - dragonflies and
damselflies) and a variety of extinct insects
that cannot fold their wings over their
abdomen form a paraphyletic grade, the
• The wing muscles of Paleopterans insert
directly at the wing bases, which are hinged so
that a small movement of the wing base
downward, lifts the wing itself upward like
rowing through the air
• In mayflies, the hind wings are reduced,
sometimes absent, and play little role in their
flight, which is not particularly agile
• In odonates, , the fore and hind wings are
similar in shape and size, and operated
independently, and as aerial predators
evolved more advanced flight ability
Paleopteran insects
Basic motion of the insect
wing in insect with an direct
flight mechanism. Scheme of
dorsoventral cut through a
thorax segment with wings
a wings
b joints
c dorsoventral muscles
d longitudinal muscles
Insect Flight - Indirect
• Other than two orders with direct flight
muscles (mayflies and odonates "Paleoptera“), all other living winged insects
fly using a different mechanism, involving
indirect flight muscles
• This mechanism evolved once, and is
synapomorphy for the infraclass Neoptera
• It corresponds with the appearance of a
wing-folding mechanism, which allows
Neopteran insects to fold the wings back
over the abdomen when at rest
• This ability has been lost secondarily in
some groups, such as all butterflies
• The wing muscles of Paleopterans insert
directly at the wing bases, which are hinged
so that a small movement of the wing base
downward, lifts the wing itself upward
Neopteran insects
Basic motion of the insect
wing in insect with an
indirect flight mechanism.
Scheme of dorsoventral cut
through a thorax segment
with wings
a wings
b joints
c dorsoventral muscles
d longitudinal muscles
Insect Social Organization
• Social Insects are one of
Wilson’s pinnacles of
Social Behavior
– Division of labor
• different morphological
types (castes)
• diffusible hormones
• special foods, chemical
– Kin or Group selection?
Figure 12A: Swarm of caterpillars
© Joy Stein/ShutterStock, Inc.
Eusocial Insects
Figure 12B: Termites mound
© Imagex/
Increased Complexity?
• Has complexity increased • Measuring complexity
during organismal
– number of cell types
possessed by an
– Not a “Great Chain of
• Measuring complexity
– genome
– gene (copy) number
– increase in the size of
– number of genes that
encode proteins
– number of parts or units
in an organism
– increased
specialization, or
– number of gene, gene
networks or cell-to-cell
– number of interactions
between the parts of an
Increased Complexity?
– Increased genome? Yes.
– Increased gene (copy)
number? Yes, but highly
Increased Complexity?
– Increased number of
– Increased number of parts or
genes that encode
units in an organism? Yes, in the
proteins? Yes, but highly transition from Prokaryotic to
Eukaryotic cells; Yes, in the
development of Metazoans, but
with little change in 500 My;
Much less in multicellular plants,
and very little in Fungi.
– Many exceptions, especially
among parasites and pathogens;
many of their organelles or
organs become simplified or
disappear altogether (reversals).
– Increased number of interactions
between the parts of an
organism? Yes, probably, but
very difficult to measure.
Increased Complexity?
• Increase in Numbers of
Types of Cells? Yes
• Increased specialization,
compartmentalization, or
subdivision? Yes, but not
for the last 500 My.
• Increased number of gene,
gene networks or cell-tocell interactions? Yes, but
with many exceptions,
especially the streamlined
genomes of Eubacteria.
Figure 13: Time of origin of various animals
Adapted from Valentine, J. W., A. G. Collins, and C. P. Meyer, Paleobiology 20 (1994): 131-142.
Modularity as a Form of Complexity
Developmental modules allow for
adaptation of subunits in structures
under somewhat separate levels of
regulatory gene control
Figure B04: Dentary bone of the rodent lower jaw
Figure B03: Dentary bone of the rodent lower jaw
Figure B05: Western Australian honey possum
Reproduced from Parker, W. K., Stud. Mus. Zool. Univ. Coll. Dundee (1890): 79-83.
In the honeypossum,
we have an example
of a reduction in
complexity due to
homeotic gene
Courtesy of Brian Hall
Figure B06: Bone of the mammalian lower jaw
Increase in Organismal Size?
• Increase in organismal
size in a lineage is not
routinely used as a
criterion of complexity
because no sustained
size increase occurs
within many lineages
• In many lineages, dwarf
and giant forms evolve
in response to specific
ecological situations
• Dwarf and Giant forms:
– Especially on islands
– At high altitudes and
– Since dwarf and giant
forms are neither more
nor less successful than
their average sized
related taxa, and often
live contemporaneously,
size is a poor criterion
for demonstrating trends
in increasing complexity
Increase in Organismal Size?
Is There a Potential
Sixth Major Extinction?
• Increased extinctions began around 1700 AD
– Sixth Extinction by Richard Leakey and Roger
Lewin (1995)
• Are we creating a mass extinction to rival the
other major events in the geologic past?
• Species are becoming extinct at a rate of:
~ 4,000 – 30,000 species/year
~1 species every 15 minutes
Is There a Potential
Sixth Major Mass Extinction?
• Why are species becoming extinct so rapidly?
• Human population growth
• Human impact on the environment
– Deforestation and Desertification
– Fragmentation and Destruction of Natural
– Contamination of Habitats
• mining wastes
• salts from irrigation and aquifer depletions
– Global Warming
Global Warming
• Increasing global temperatures
• Rising ocean levels as polar ice caps
and glaciers melt
• Changing seasonal weather patterns
• More frequent occurrence of
weather extremes (e.g., stronger
storm systems, increase in droughts
& floods)
• Global migration of pathogens and
disease vectors (HIV, malaria,
bilharzia, cholera, etc.)
typhoon Haiyan Philippines 2013
Human-Caused Holocene
(Anthropocene) Extinction
• Human-caused extinctions of the last 10,000 years:
• Excessive Predation (food, fur, collecting, pest
eradication, etc.)
• Destruction of keystone species
• Introduction of Exotic Species
– Competitors, Predators
– Diseases
– Exotic Pet Trade
• Air and Water pollution
• Soil and Ocean Pollution
Golden Toad of Costa Rica
described 1866, extinct 1989
Extinction — Commensalism
• The flightless dodo lived on
the island of Mauritus off the
coast of Africa
• It was first described to
science ~1600
• The last Dodo bird was killed
in 1662
• It fed on plants and seeds,
including the seeds of the
Calvaria/Tambalcoque tree
Extinction — Co-Extinction
• The Calvaria/Tambalcoque
tree’s seeds had evolved thick
coats to survive the passage
through the grinding gizzard of
the dodo
• With the extinction of the dodo,
these seeds no longer made
such an abrading trip through
the digestive tract, the coat
remained thick, and the young
tree embryo could not so easily
There Goes the Neighborhood
Humans arrive and megafaunas and many other
species go extinct
• Australia 40,000 years ago
• Pacific Islands 30,000 years ago
• Americas 15,000 years ago
• Madagascar 2000 years ago
• New Zealand 1500 years ago
• Indian Ocean Islands 1500 years ago
Recent Extinctions
Auroch (1627) & Dodo (1662)
Stellar’s Sea Cow (1768)
Mascarene Island Giant Tortoise (1795)
South African Cape Lion (1858)
Quagga (1883)
Passenger Pigeon (1914)
Tasmanian Wolf (1936)
Bali Tiger (1937) / Javan Tiger (1976)
Kaua’i ‘O’o (1987)
Golden Toad (1989)
Baiji White Dolphin (2006)
Chinese Paddlefish (2007)
Christmas Island Pipistrelle (2009)
Vietnamese Rhinoceros (2010)
Pinta Island Tortoise (2012)
Where Will You Be in 2050?
• By 2050, it is estimated that
the earth's human
population will be ~9.1
billion (7.4 – 10.6 range)
• [7.0 billion in 2011]
• > 60% of those people will
live in Africa, Southern Asia
and Eastern Asia
• 50% of all species on the
planet will be either
endangered or extinct
By 2050 - 2100?
• 50% of all species on the planet will be either
endangered or extinct
– Habitat destruction
– Global Warming
• 25% mammalian species
• 15% bird species
• In The Future of Life (2002), E.O. Wilson of Harvard
calculated that, if the current rate of human
disruption of the biosphere continues, one-half of
Earth's higher lifeforms will be extinct by 2100
Life Sciences-HHMI Outreach.
Copyright 2006 President and Fellows
of Harvard College.
Our Future Earth – 250 Million Years Ahead
Plate tectonic maps and Continental drift animations by C. R.
Scotese, PALEOMAP Project (
A Simple Cladogram of the Tree of Life
In 2100, I expect all
these higher taxa to be
alive, but the destruction
of individual species will
be enormous and
inevitable in the mean
In 250 million years, I
expect most of these
higher taxa to be alive,
but I’m less confident for
Homo sapiens
Chapter 17