Chapter 18Chapter 18

Mass Extinctions,Mass Extinctions,Opportunities andOpportunities and

Adaptive RadiationsAdaptive RadiationsFigure CO: Dinosaur fossil © Styve Reineck/ShutterStock,

Inc.

Extinction

Overview• 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 re-open niches for new species to invade and occupy

• To understand extinctions, we need to identify rates, patterns and causes

Extinction• 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 fossils

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 morphologically unchanged since the Ordovician

Survivors — Cycads and Horsetails

Survivors — My Favorite!

Periplaneta americana:the cockroach

Extinctions

• 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

Extinctions• Habitat Disruption

– Volcanic Eruptions– Asteroid Impacts– Sea Level Change

• Habitat Modification– Climate Change– Mountain-Building– Sea Level Change– Precipitation Change– Toxic Materials

• “Exotic” Species Introductions– Continental Drift

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 tentative

Extinctions• 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 the 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

Source: Raup, D.M. and J.J. Sepkoski, Jr., Science 231 (1986): 833–835.

Table T01: Details of the Five Major Mass Extinction Events Since the Cambrian

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)

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 extinction

#1

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 affected

• The immediate cause of extinction appears to have been the tectonic movement of Gondwana into the south polar region

The Ordovician Extinction Event

• Gondwana drifting south led to global cooling, glaciation and falling sea levels

• 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

#2

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 reef-building 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 organic decay and allowed the preservation of sedimented organic matter as petroleum and coal

• The impact of a comet or another extraterrestrial body has also been suggested, but the evidence is weak

Late Devonian / Early Carboniferous

continents drifting together to form pangaea

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

#3

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

events– 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 rapidly in less than 10,000 years just before Pangaea started to break apart

• This extinction 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 oceans, a whole class (conodonts) and 20% of all marine families disappeared– Conodonts were early eel-like chordates

#4

The Triassic Extinction Event• On land, all large crurotarsans (non-dinosaurian archosaurs)

other than the crocodilians, some remaining therapsids, and many of the last 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

#5

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 activityImpact 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 species.

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

Deccan traps

The Late Cretaceous Extinction Event• These geological events may have reduced sunlight

and decreased 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 climate

What Happened to the Dinosaurs?

• Sediments were deposited by enormous tsunamis (tidal waves) along the coastline

• 70% of known fossil species, 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 but 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

Figure 01: Percentage of marine animal extinctions Adapted from Fox, W. T., Paleobiology 13 (1987): 257-271.

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%

backgroundrate

Mass Extinction Probable CausesTo the degree that mass extinctions are real, rather than artifacts of a poor fossil

record, the causes are probably complex and multifactorial

Abiotic Causes for Mass Extinctions

1. Plate Tectonics1. Intermingling of Biotas / Introduced species effects2. Trophic Stability3. Changes in Sea Level and ocean chemistry4. Volcanic Activity changing atmospheric gases and dust

levels5. Ice Ages with glaciations, falling sea levels, increased

tropical aridity

2. Planetary Collisions3. 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

oceans– 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 extinctions

iceage

iceage

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 star)

– 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

Life Sciences-HHMI Outreach. Copyright 2006 President and Fellows of Harvard College.

Major Indirect Causesfor 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

The Impact ofthe 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

Adapted from Janis, C.M., Ann. Rev. Ecol. Syst., 24 (1993): 467-500.

Figure 02B: Early Oligocene

The CenozoicThe Last 65 My

• Here you can see how plate tectonics gradually rearranged the continental plates

• 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 changes

Figure 02C: Late Miocene

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 opportunities

• 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 insectivores?

• Would dinosaur lineages continued their advances in intelligence and social behavior?

The Impact of Extinctions

The Butterfly Effect?

The Impact of Extinctions

Figure B02A: Xenarthrans

Adapted from Steel, R., and A.P. Harvey. The Encyclopaedia of Prehistoric Life. Mitchell-Beazley, 1979.

South America: Island Continent

Figure B02B: Ungulates

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

the largest was ~3

meters tall

Figure B01A-D: Continental drift

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 American species managed to expand to the north– Here extinction resulted

from competition for the same habitats & niches

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 large-seeded plants are thought to have once been dispersed by megafauna

• 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

Figure 03: Flying fishCourtesy of Shannon Rankin, NMFS, SWFSC/NOAA

Gliding Locomotion

Gliding has evolved more often than actual flying!

marsupialhoneyglider

placental flying squirrel

• 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

Invading the Air: Flying Reptiles

Figure 04A: Pterodactyloids

Figure 04B: Rhamphorhynchoids

Figure 05: Pteranodon© Paul B. Moore/ShutterStock, Inc.

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 ground-dwelling 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

Figure 07A: Compsognathus

A and B adapted from Carroll, Robert. Patterns and Processes of Vertebrate Evolution. Cambridge University Press, 1997.

The Origin of Feathers

Figure 07B: Archaeopteryx

C adapted from Dingus, L. and Rowe, T. The Mistaken Extinction: Dinosaur Evolution and the Origin of Birds. W. H. Freemanm, 1998.

Figure 07C: Gallus

Figure 09: 4-winged Microraptor

The evidence favors origin from ground-dwelling ancestors

However, the next stage may have been to move upward in the environment and then glide down

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

Megachiroptera Microchiroptera

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, explain the comparative ease with which individual segments can be altered in a body plan that consists of serially repetitive elements

Arthropod Phylogeny

Some of the largest prehistoric flying insects were dragonflies whose wingspan

could be as much as 0.75 meters

Insects

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 evidence)

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 "Paleoptera“

• 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

Basic motion of the insect wing in insect with an direct flight mechanism. Scheme of dorsoventral cut through a thorax segment with wingsa wingsb jointsc dorsoventral musclesd longitudinal muscles

Paleopteran insects

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 a 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

Basic motion of the insect wing in insect with an indirect flight mechanism. Scheme of dorsoventral cut through a thorax segment with wingsa wingsb jointsc dorsoventral musclesd longitudinal muscles

Neopteran insects

Insect Social Organization• Social Insects are one of

Wilson’s pinnacles of Social Behavior– Division of labor

• different morphological types (castes)

• diffusible hormones (pheromones)

• special foods, chemical signals

– Kin or Group selection? Figure 12A: Swarm of caterpillars© Joy Stein/ShutterStock, Inc.

Eusocial Insects

Figure 12B: Termites mound© Imagex/Dreamstime.com Bees Ants Wasps

Increased Complexity?• Has complexity increased

during organismal evolution?– Not a “Great Chain of

Being”• Measuring complexity

– genome– gene (copy) number– increase in the size of

organisms – number of genes that

encode proteins– number of parts or units

in an organism

• Measuring complexity– number of cell types

possessed by an organism;

– increased compartmentalization, specialization, or subdivision

– number of gene, gene networks or cell-to-cell interactions

– number of interactions between the parts of an organism

Increased Complexity?– Increased genome? Yes.– Increased gene (copy)

number? Yes, but highly variable.

Increased Complexity?– Increased number of

genes that encode proteins? Yes, but highly variable.

– Increased number of parts or units in an organism? Yes, in the 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-to-cell 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.

Figure B03: Dentary bone of the rodent lower jaw

Modularity as a Form of Complexity

Figure B04: 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.

Courtesy of Brian Hall

Figure B06: Bone of the mammalian lower jaw

Developmental modules allow for adaptation of subunits in structures under somewhat separate levels of regulatory gene control

In the honeypossum, we have an example of a reduction in complexity due to a homeotic gene change

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

latitudes– 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 PotentialSixth 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~100/day~1 species every 15 minutes

Is There a PotentialSixth 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

Habitats– Contamination of Habitats

• mining wastes• salts from irrigation and aquifer depletions

– Global Warming / Climate Change

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) Mass Extinction

• Human-caused extinctions of the last 10,000 years:• Excessive Predation (food, fur, collecting, exotic

pets, pest eradication, Chinese medicine, 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

#6

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 germinate

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 (range 7.4 – 10.6)

• [7.0 billion in 2011]• nearly 11 billion people by 2100• > 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

Life Sciences-HHMI Outreach. Copyright 2006 President and Fellows of Harvard College.

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

Our Future Earth – 250 Million Years Ahead

Plate tectonic maps and Continental drift animations by C. R. Scotese, PALEOMAP Project (www.scotese.com)

A Simple Cladogram of the Tree of LifeIn 2100, I expect all these higher taxa to be alive, but the destruction of individual species will be enormous and inevitable in the mean time

In 250 million years, I expect most of these higher taxa to be alive, but I’m less confident for Homo sapiens

Chapter 17End

Victim of its own fecundity and adaptability

Radiations of the Metazoans in the Phanerozoic

What Happened at the Big Five Mass 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."

A map of the world in 2007, with colours to highlight the population density of each country or territory. Numbers on the

legend are in people per km2, and all countries smaller than 20,000 km2 are represented by a dot.