25.4 Continental Drift, Mass Extinctions, & Adaptive RadiationsThe rise and fall of dominant groups

Continental Drift• The continents are part of great plates of the

Earth’s crust that essentially float on the underlying portion of the mantle

• When these plates move it is called Continental Drift

• Geologists are able to measure the rate at which the plates move today, usually only a few centimeters per year

Continental Drift (cont.)• Many important geological processes, formation of

mountains and islands, occur at plate boundaries.• Some cases two plates move away from each other

– Ex. North America and Eurasian plates, currently moving apart at a rate of about 2 cm per year

• Some cases two plates slide past each other and this is where earthquakes are common– Ex. San Andreas Fault

• Some cases two plates colide– Ex. Himalayan mountains when the Indian plate crashed into

the Eurasian plate

Consequences of Continental Drift

• Although plate movement is slow, the overall effects are dramatic

• Effects of Continental drift:– Alters the habitats in which organisms live• It’s said that millions of years ago the plate moved

separated land masses together to form a supercontinent, Pangea. The Pangea destroyed the habitat of the animals. Driving some animals to extinction

Consequences cont.• Effects of Continental drift:

– Climate change that results when a continent shifts northward or southward• Ex. Southern tip of Labrador, Canada. It was once located near the

equator but has moved 40 degrees north. • This causes organisms to adapt, move, or become extinct.• Some believe that the shifting positions of the continents have rerouted

the ocean currents causing the global climate to be colder & contributes to the formation of ice caps.

– Promotes allopatric speciation on a grand scale• When supercontinents break apart, regions that were connected now

become geographically isolated. • As the continents have drifted apart, each continent began to get their

own lineages of plants and animals.

Consequences cont.• Effects of Continental drift:– It can help explain puzzles about geographic

distribution of extinct organisms– It also helps explain the distribution of living

organisms.• Marsupials used to exist in multiple places

orginating most likely in Asia and North America.• Continental drift carried marsupials on Australia

where they diversified while marsupials became extinct on other continents.

Mass Extinctions• Mass Extinction- abnormally large numbers of

species become extinct throughout Earth.• A majority of species that have existed are now

extinct.• This is shown by the fossil record.• Reasons for extinction could be:– Destruction of habitat– An unfavorable change to the environment

Mass Extinctions cont.The “Big Five” Mass Extinctions

• In each mass extinction 50% of Earth’s marine species became extinct.

• The “Big Five”– Ordovician– Late Devonian– Permian*– Triassic-Jurassic– Cretaceous*

Mass Extinction cont.Permian extinction

• Modern day Serbia• Enormous Volcanic Eruptions• Most extreme episode of volcanism in the past half

billion years• Reduced temperature differences between the

poles and the equator slowed mixing of ocean water.

• This reduced the amount of oxygen available to marine organisms.

Mass Extinction cont.Cretaceous extinction

• Took out more than 1/2 of all marine species, and many land plants

and animals (including most dinosaurs)

• Possible cause: iridium which is an element that is very rare on Earth

but common in meteorites and other extraterrestrial objects

• A cloud of debris came into the atmosphere when an asteroid or a

large comet collided with Earth. The cloud would have also blocked

sunlight changing the climate for many months.

• Chicxulub crater: off the coast of Mexico; 10 km wide (over 6 miles

wide)

Results of Mass Extinctions

• Completely changes an ecological community– Organisms found in the ecosystems will change

• When an evolutionary lineage disappears, it cannot reappear which changes the course of evolution

• It takes about 5-10 million years for the diversity of life to recover from a mass extinction.

• Paves the way for adaptive radiation.

Adaptive Radiation• Periods of evolutionary change in which groups of organisms form

many new species whose adaptations allow them to fill different ecological roles, or niches, in their communities

• Adaptive radiations occur when a few organisms make their way into new areas, when novel adaptations arise, or when extinction opens up ecological niches for surviving species.

• Large-scale adaptive radiations followed each of the five mass extinctions

• Adaptive radiations occurred in groups of organisms that possessed major evolutionary innovations, such as armored body coverings.

• Organisms colonizing regions with little competition from other species have undergone major adaptive radiations.

Adaptive Radiation cont.• In some adaptive radiations, groups of organisms increased in

diversity as they came to play entirely new ecological roles in their communities

• Examples include the rise of photosynthetic prokaryotes; the evolution of predatory animals during the Cambrian explosion; and the plants, insects, and tetrapod radiations that followed the colonization of land

• Each of these radiations was associated with major evolutionary innovations that facilitated life on land

• Also organisms that arise in an adaptive radiation may serve as a new source of food for still other organisms

World-Wide Adaptive Radiations

Millions of years ago

Monotremes(5 species)

250 150 100200 50

ANCESTRALCYNODONT

0

Marsupials(324 species)

Eutherians(placentalmammals;5,010 species)

Ancestralmammal

Mammals underwent a dramatic adaptive radiation following the extinction of the dinosaurs 65 million years ago. Before they were restricted to smaller sizes as a result of competition from the larger dinosaurs, but with the disappearance of the dinosaurs, mammals diversified to fill the ecological roles once occupied by dinosaurs.

Regional Adaptive Radiation Adaptive radiation can also occur over a more limited geographic

area

Such radiations can be initiated when a few organisms make their way to a new, often distant location with limited competition from other organisms

The Hawaiian archipelagos showcases this type of radiation because each island was born “naked” with immense physical diversity and was gradually populated by stray organisms from the wind or ocean

Multiple invasions, followed by speciation events, have ignited an explosion of adaptive radiations of novel species with many of the thousands of species there found nowhere else on Earth

Close North American relative,the tarweed Carlquistia muirii

Argyroxiphium sandwicense

Dubautia linearis

Dubautia scabra

Dubautia waialealae

Dubautia laxa

HAWAII0.4

millionyears

OAHU3.7

millionyears

KAUAI5.1

millionyears

1.3millionyears

MOLOKAI MAU

ILANAI

Hawaiian Islands Regional Adaptive Radiations

Close North American relative,the tarweed Carlquistia muirii

Argyroxiphium sandwicense

Dubautia linearisDubautia scabra

Dubautia waialealae

Dubautia laxa

HAWAII0.4

millionyears

OAHU3.7

millionyears

KAUAI5.1

millionyears

1.3millionyears

MOLOKAIMAUI

LANAI