Transcript Acquired characteristics - University of West Alabama

Extinction
More than 99% of
the species that have
existed since the
Cambrian are now
extinct.
Hanging on that long
is tough to do.
There are
exceptions.
A marine organism (brachiopod)
occupying vertical burrows in sand
and mud has survived
morphologically unchanged since
the Silurian.
The horseshoe crab, an inhabitant of marine shores, has lived
morphologically unchanged since the Ordovician. A horseshoe
crab in ventral (underside) view is shown.
Extinction is the symmetrical partner of
speciation….
…and is just as important in determining the
nature of life today.
We distinguish….
Catastrophic, or mass, extinctions
Include the loss of many species from
different taxonomic groups occurring
relatively abruptly over short period of
geologic time.
Uniform, or background, extinctions
The gradual loss of members of taxonomic
groups over long time periods without
abrupt loss of large numbers.
Most extinctions have been uniform.
Why do species go extinct?
Often, the answer is unclear.
Horses provide an interesting example.
Horses
evolved in
North
America and
spread to
Eurasia and
Africa.
They went
extinct in
North
America
some 15,000
years ago.
They were reintroduced
about 400 years ago by
the conquistadors.
How did they do?
Pretty well.
So, why did they go
extinct in the first
place?
We’re not sure.
Uniform Extinctions – what causes them?
Coevolutionary interactions can explain some
extinctions.
Remember the dodo and the Calavaria tree?
A similar tight
association
exists between
the Australian
koala and the
eucalyptus that
it feeds on.
The disappearance
of the eucalyptus
would threaten
the continued
existence of
koalas.
Clearly, however, extinctions are not always
this easily explained.
Islands
The study of life on
islands may also
provide some clues.
It’s often easier to understand events on islands,
since they are typically simpler than those on
larger bodies of land.
MacArthur
Wilson
A landmark
work published
in 1967 helped
provide some
answers.
If the size of an island is plotted against the number of species present
on it, a direct relationship usually holds—the more area, the more
species. Reptile and amphibian species in the West Indies are plotted
here. (Data from MacArthur and Wilson 1967.)
Immigration and Extinction on Islands
(a) Immigration curve. As colonists fill the island, the rate of arrival of
new species drops. (b) Extinction curve. As colonists fill up the island,
the rate at which species disappear increases. (After MacArthur and
Wilson 1967.)
Where immigration and extinction curves cross, an equilibrium number of
species is reached. In this example, there are 10 species on the nearby
mainland, making it possible for up to 10 species to be on the island.
However, in this example the equilibrium sustainable by the island is 6.
Distance and Area Effects on Species Equilibrium
(a) Distance effect. If our island (from figure 13.4) were moved farther from the mainland,
the equilibrium would shift to the left, settling at 2, and some species would become
extinct on the island. (b) Area effect. A large island reaches a higher equilibrium than a
small island.
It doesn’t have to
be an island to be
an “island”.
Applied island biogeography
Principles of island biogeography applied to the management of nature reserves helps
make decisions about survival of organisms living in the reserve. In each comparison,
design A is superior to B.
Some taxonomic or ecological
groups have higher rates of
extinction than others.
Small, herbivorous mammals
have lower extinction rates than
large, carnivorous ones.
Tasmanian “tiger” – now extinct
Mathematical models
have been developed
to use demographic
factors to predict the
susceptibility of a
species to extinction.
One thing is clear.
When populations
become small, the
chance of extinction
resulting from purely
random factors goes
up.
It appears that the
chance of extinction
increases non-linearly
as population size
decreases.
Results of a study employing
demographic factors to predict time
to extinction.
Recent extinctions
Over the last 200 years,
humans have directly or
indirectly caused the
extinction of thousands of
species…. that we know of.
Case in point – the passener
pigeon (Ectopistes
migratorius).
Read the discussion on page
216 in your text.
Others….
Carolina parakeet
Stellar’s sea cow
Great auk
The American chestnut (Castanea
dentata) has been virtually wiped
out by an introduced pathogenic
fungus (Endothia parasitica) which
reached the U.S. accidentally in
1904. The resulting blight
destroyed most existing trees, and
threatens the species with
extinction.
The damming of the
Chagres River during
the building of the
Panama Canal led to
the formation of Barro
Colorado Island.
Since the formation of
the island, perhaps as
many as 50 species of
birds have become
extinct on the island.
It appears that the
small populations that
were present on the
island died out and
were not replaced by
colonists from the
mainland.
So, what does island biogeography tell us about
extinction?
Habitat size is a critical component.
Why?
And, there’s the Red Queen.
The idea came from
Leigh van Valen.
Van Valen
looked at the
fossil record of
different
groups.
Given is a family of fossil organisms. To follow declines in numbers over
time, we pick a starting point and then follow these species forward in
time (thick lines). Starting with 10 species (A–J) in this family, some
become extinct, leaving 7, 5, 4, 2, and 1 remaining species on successive
time intervals. Below the phylogeny, these numbers are plotted (semilog
plot) showing the constant decline through time.
He envisioned an evolutionary “race”, which a
species must ultimately lose.
Evolutionary Survivorship Curves
(a) Protists: foraminifera. (b) Mammals: artiodactyla—deer, elk, and
related species. (c) Pteridophyta: ferns and related species. (d)
Mammals: perissodactyla—horses and related species. M.Y.: millions of
years passed. (After Van Valen 1973.)
Mass Extinctions
How many have
there been?
Of more than 19,000 genera, 10 samples of 1,000 genera were chosen
at random and the percentage becoming extinct was plotted. Major
peaks of mass extinction are indicated.
Extinction Episodes—Marine Animals
Plotted a slightly different way, the standing diversity of marine
invertebrate and vertebrate animals is shown. Five abrupt drops in
diversity are numbered (1-5). The relative magnitudes of these drops
were determined by measuring from the stage before to the stage after
the extinction event. These relative magnitudes are given in parenthesis.
(After Raup and Sepkoski 1982.)
Extinction episodes—marine animals
The standing diversity of marine invertebrate and vertebrate animals is shown. Five abrupt
drops in diversity can be identified.
One major mass
extinction occurred
about 250 million
years ago at the
boundary between the
Permian and Triassic
Periods.
A number of possible
causes have been
suggested, focusing
primarily on periods
of global warming or
cooling.
A reconstruction of
the ancient seabed in
southern China
before and after the
Permo-Triassic mass
extinction event.
Extinction Episodes
Tetrapods
The number of tetrapod
families, the standing
diversity, is plotted beginning
with the first tetrapods in the
Devonian. Six mass
extinctions are indicated
where diversity abruptly
drops. Notice that the curve
starts sharply upward during
the Tertiary—this is a
reflection of some increase in
diversity, especially among
birds and mammals, but also
an artifact of the better
preservation of fossils as we
approach the present. (After
Benton 1989.)
FIGURE 13.11 Extinction Episodes—Families of Marine Animals
The number of marine animals going extinct per million years is plotted. The solid line
(regression line) expresses the average—fewer than 8 species extinctions per million years.
The parallel dashed lines are the statistical confidence limits that bound the “background”
extinction rate. Elevated levels of extinction above background are “mass extinctions,”
indicated with the five points (1–5) corresponding to geological time intervals. (After Raup
and Sepkoski 1982.)
Origin and extinction
The rise and fall of various groups of animals are shown relative to the geologic time scale.
The Permo-Triassic and Cretaceous extinctions are evident.
What about the dinosaurs?
Extinction of dinosaurs
After their demise at the end of the Cretaceous, dinosaurs were replaced as dominant land
vertebrates by birds and mammals. [The geologic dates given here are now revised, but
provide a relative picture of groups.]
Mesozoic Tetrapods-Ancestors and Survivors
Together, the Ornithischia* plus the Saurischia* constitute the
“dinosaurs.” Notice that birds and mammals are early contemporaries
of the dinosaurs.
Cosmic collision
At the end of the Cretaceous, an asteroid or comet struck the Earth in the location of
present day Yucatan Peninsula in Mexico. Although such a collision certainly occurred, it is
debated whether or not this collision was directly responsible for the dinosaur extinctions.
Gravity map of the
Chicxulub crater
Luiz and Walter Alvarez
at a clay outcrop in
Italy. High iridium
layers at the K-T
boundary led to their
hypothesis about an
asteroid impact.
Is an asteroid impact the only explanation for the
demise of the dinosaurs?
Perhaps not.
Recall that dinosaurs were not typical reptiles.
Other possibilities include:
1. Inability to compete with developing
mammals.
2. Development of angiosperms.
After the dinosaurs
Extinction of the dinosaurs left may ecological niches empty. In part, the subsequent
flourishing of mammals and birds represents an adaptive radiation into many of these
vacated life styles.
Another possible cause of extinctions –
mingling of species
Plate tectonics
The crustal surface of the Earth is broken up into plates that abut against each other and
move about carrying their continents to every changing positions. Present day plates are
shown.
Collision zones
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.
Continents and extinctions
At the end of the
Permian (Late
Paleozoic), the
major continents of
the world were
joined together into
a super continent,
Pangaea. At the
end of the
Cretaceous (end of
the Mesozoic), the
great land mass of
Pangaea had begun
to break apart into
the basic continents
we recognize today.
FIGURE 13.14 The Great Exchange — 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.
FIGURE 13.15 Continental Drift
(a) Pangaea formed at the end of the Permian as the result of previously separate
continental landmasses fusing together into this single supercontinent. (b) Continents
broke up again at the end of the Cretaceous.
FIGURE 13.16 Perimeter Area
When a landmass (a) is broken in two (b), this adds area along the perimeter where they
split. This adds to the intertidal area. When two landmasses (b) are brought together (a),
this results in loss of available intertidal area.
FIGURE 13.17 Continental Shelves
Because of the sloped geometry of the continents, a drop in sea level along a continental
shelf eliminates large areas of intertidal habitat (dark shaded) compared to a similar drop
in sea level against a steeply sloped drop-off area (light shaded).
FIGURE 13.18 Sea Level Drop, Permo-Triassic
Note that as the sea level dropped (a) at the end of the Permian, there was a
corresponding loss of marine animals (b). Sea level is expressed as a percentage of
possible coverage, which began falling about 30 to 35 million years ago (solid arrow)
before the end of the Permian. Estimates of absolute drop in level range from 300 to 600
feet (100Ð200 m). Tick marks along horizontal axes represent geologic stages of varying
duration. (After Schopf, 1974.)
FIGURE 13.19 Phanerozoic Extinctions and Ice Ages
Ice ages are indicated along this geologic time line for comparison to five mass extinction
episodes. Note that there is no tight correlation between ice ages and mass extinctions.
Some species have shown a
remarkable ability to come back from
the brink of extinction.
The northern elephant seal (Mirouga
angustirostris) and the sea otter
(Enhydra lutris) were both hunted to
near extinction in the early part of
the 20th Century.
Once protected, both have recovered
to large, healthy populations.
The gray whale (Eschrichtius robustus)
is also recovering under protection.
A number of bird species (sandhill
cranes, trumpeter swans, snowy
egrets, and others) have also made
significant recoveries.
Extinction in the Fossil Record
In some cases, the
fossil record is
complete enough to
show that many
species became
extinct over a
relatively short time
period. These
events have come to
be known as mass
extinctions.
They apparently
result from some
drastic, widespeard
environmental
change.
The six largest mass
extinctions have been
particularly
significant.
Not only did they
eliminate large
portions of the
existing biological
diversity.
They also seem to
have been precursors
to significant
adapative radiations
as organisms rapidly
filled the newly
available niches.
One of the most recent extinction
events was the disappearance of the
Pleistocene “megafauna” (primarily
large mammals, between ~ 15,000 and
~ 8,000 years ago.
This led to the disappearance of a large
number of mammalian species,
particularly in North America.
A variety of causes have
been suggested. A
number of lines of
evidence are now
suggesting that humans
may have played a large
role.