Extinction
Extinction occurs when all individuals of a species are gone and
have left no descendants. If all the species within a genus are
extinct then the genus is extinct.
extinct If all genera in a family are
extinct then the family is extinct. Extinction removes a potential
branch on the evolutionary tree.
We can conclude a species has gone extinct when the last
member
b off that
h species
i has
h died.
di d
Tecopa pupfish - 1977
Dodo - 1581
Elephant Bird
(Madagascar) ~ 1500
Moa (New Zealand) –
4 species ~ 1400
Quagga - 1883
Passenger Pigeon – 1914
Once the most common
bird in North America.
Tasmanian Tiger or Thylacine - 1936
Caribbean Monk Seal - 1952
Golden Toad - 1989
Pyrenean Ibex - 2000
Baiji River Dolphin - 2006
It is often difficult to prove that no individuals of a species
remain
i in
i the
h wild.
ild It
I is
i easier
i to prove for
f large
l
animals.
i l
Ivorybilled Woodpecker - ?
The fossil record shows that many kinds of organisms that
were once common no longer
l
exist.
i
A fossil is the remains or traces (e.g. tracks, dung, nest, etc.) of
an organism that is no longer alive and usually applies to
remains that are over 10,000 years old.
The fossil record is not a complete
p
record because:
• Fossilization is a rare event. Very few, if any, individuals of
most species ever leave remains that become fossilized.
• Species with hard body parts are best preserved – shells,
shells
plates, bones fossilize well - soft bodied organisms are not
well represented in the fossil record
• Erosion, weathering, metamorphic processes destroy fossils
and have made the fossil record more incomplete - older time
pperiods are less well represented
p
than newer time periods
p
The process of fossilization requires special conditions
Sedimentary Rocks
– formed by
deposition
p
and
solidification of
sediments are the
onlyy fossil-bearingg
rocks
Igneous
g
rock is
formerly molten
rock
Metamorphic rock
has been heated
beyond the point
where fossils would
be destroyed.
Other special conditions:
Plant sap becomes amber
Tar pits (like La Brea) also kill and
preserve
50 myo
47 myo
3.5 myo
How is the age of
a fossil estimated?
600 myo
The geologic time periods were described by the kinds of organisms that lived
within each period. The fossils of some organisms are commonat many sites
worldwide in a very limited number of geological strata – these are index
fossils. Index fossils in rocks at different geographic locations are likely to be
of about the same age.
The Principle (or Law) of
Superposition - upper
sedimentary layers represent
more recently deposited
sediments – allows the relative
dating
g of fossils in different
strata.
The principle of superposition can be
violated where rocks have been inverted
through geologic upheavals.
upheavals
Radiometric dating:
radioisotopes
di i
decay
d
exponentially
i ll at measurable
bl rates
• Nt = N0*ert
• Gives rate of decayy or ½ life ((t ½)
18
Using: Nt = N0*ert
after
f one half-life:
h lf lif
rt 1
Nt 1
=
=e 2
N0 2
( )
rt 1
⎛1⎞
ln ⎜ ⎟ = ln e 2
⎝2⎠
⎛1⎞
ln ⎜ ⎟ = rt 12
⎝2⎠
ln ( 12 )
r=
t 12
The numerator is a constant and the
denominator is measurable. Plug this
term into the first equation for r and
rearrange to get
⎛N ⎞
ln ⎜ t ⎟ * t1/ 2
N0 ⎠
⎝
t=
⎛1⎞
ln ⎜ ⎟
⎝2⎠
⎛ N0 ⎞
ln ⎜
⎟ * t1/ 2
Nt ⎠
OR
⎝
t=
ln ( 2 )
14C
Dating:
14C decays to 14N with ½ life of 5700 years
14C/12C ratios constant in living things. (about 1.3 * 10-12)
After death 14C is lost: ratio 14C/12C becomes smaller as time
passes and eventually 14C becomes undetectable.
Example:
p If the amount of 14C in a woolyy mammoth tusk
is 1.3 * 10-13, how old is the tusk?
⎛N ⎞
ln ⎜ 0 ⎟ * t1/ 2
Nt ⎠
⎝
t=
ln ( 2 )
14C
⎛ 10 ⎞
ln ⎜ ⎟ *5700 yr
1⎠
⎝
= 19,021 yr
t=
ln ( 2 )
dating is useful for dating organic material less than 80,000
80 000 years old
K/Ar dating can be used for dating igneous rock above or below
a fossil.
f il This
Thi can bracket
b k the
h age off the
h fossil
f il
e.g 40K decays to 40Ar
t ½ = 1.3
1 3 * 109 years
Sum of decay product and remaining undecayed atoms gives
total amount before decay began (N0 = sum(40Ar+ 40K), Nt = 40K)
In the case of K/Ar dating an estimate of the original amount of
40K in the rock can be made by adding the amount present now to
the
h amount off 40Ar
A now di
divided
id d by
b 0.112.
0 112 The
h division
di i i by
b 0.112
0 112
accounts for the fact that only 11.2% of the original 40K decays to
40Ar. ((The other decay
y pproduct is 40Ca.))
⎛
ln ⎜1 +
t= ⎝
Ar
1 ⎞
*
⎟ * t1/ 2
40
K 0.112
0 112 ⎠
ln(2)
40
Use this modified
f
formula
l for
f K/Ar
K/A
dating
21
K - Ar datingg is useful for datingg igneous
g
rock from 1/2 to 10 byo
y
The skeptic should ask: How do we know that there was no Ar in
the rock when it was formed? If there was this would increase the
age estimate.
Since Ar is a gas and leaves heated rock there should be little or
no Ar in igneous rock to begin with. This expectation has been
verified.
Radiometric dating has allowed age estimates to be placed on
geologic time periods.
Read: Paleomagnetic Dating
22
What causes extinctions?
Extinctions of single species
or small groups of species
are sporadic and occur at a
rate that is called
background extinction
rate.
Mass extinctions are large
spikes in the rate of
extinction.
ti ti
There
Th is
i no
defined spike magnitude for
mass extinctions but the fact
th t spikes
that
ik have
h
occurredd is
i
clear in the fossil record.
Mass extinctions mark the
end
d off many geologic
l i time
i
periods.
Extinctions are most easily seen in species that have restricted
distributions – endemic species.
Predators reduce the population size of prey species. Predators
that are very effective can reduce the size of prey populations
to the point of extinction.
The introduction of cats to areas that formerly lacked
mammalian predators has caused the extinction of many
species of birds, reptiles, and mammals. Introduced rats have
caused the extinction of many birds and egg-laying
egg laying reptiles.
reptiles
Lake Victoria had over 150
endemic species of fish belonging
to one family. Each had
specialized feeding adaptations.
The introduction of a large predator,
the Nile perch, has caused the
extinction of about half the species.
An indirect effect is a change in the numbers of one species
through the interaction of two or more other species.
For example, ants are one of the primary consumers of seeds in
deserts of the southwest. Small mammals and birds also depend
on seeds. Predators on ants, or ant diseases, reduce ant
populations and allow small mammal and bird numbers to
increase.
Read: Predation, Extinction and Indirect Effects.
Extinction and Disease
Amphibian populations have been on the decline world-wide
for several decades. This has resulted in the extinction of
many species.
i
There
Th are severall factors
f t that
th t have
h
contributed
t ib t d
to the decline, including habitat loss due to climate change and
human development. The effect of disease has also been
documented.
Amphibian Diversity
Threatened or
extinct species.
A fungus, Batrachobytrium
d d b i was discovered
dendrobatis,
di
d to be
b
contributing to the decline. A skin
infection interferes with
amphibians’ ability to
osmoregulate.
Mass Extinction
Wholesale loss of many
species over a relatively
short period of time has
occurred at least 5 times
in Earth’s history.
Mass extinctions result in
loss of diversity of types
within groups in terms of
morphology, biochemistry
and behavior.
Following mass extinctions, species
diversity continues to decrease. The
cause of continued decrease is
uncertain.
K/T Extinction:
Severall hhypotheses
S
h
hhave been
b
proposed
d
to explain the K/T event but the best
supported
pp
is that it was the result of an
asteroid impact.
Iridium levels
within K/T
boundary strata
at 160 times that
found elsewhere
on Earth suggest
th t an Ir
that
I rich
i h
asteroid struck
the Earth.
Read: ((K-T)) Mass
Extinction for
additional asteroid
evidence.
evidence
There are many large craters on
Earth that have been caused by
asteroid impacts. Only one has
been discovered of the right size
and age to be a candidate for the
K/T event – the Chicxulub crater
on the edge
g of the Yucatan.
The crater is rich in Iridium and
the region shows many other
signs of an impact.
impact
Such an impact would have caused nearly immediate devastation
off coastal
t l areas in
i North
N th andd South
S th America.
A
i
It may have
h
putt
up to 18 km3 of dust into the atmosphere, reducing temperatures
and photosynthesis and destroying the food chain. It may have
also set off world-wide volcanic activity that likely would have
contributed to dust in the atmosphere. Mass extinction followed.
Late Permian Extinction
Near the end of the Permian (251 mya) nearly 90% of all marine
species disappeared along with 70% of terrestrial species. The
Earth was nearly barren.
barren
The mass extinction is correlated
with a massive volcanic eruption
in the area now called Siberia.
At 2,000,000 km3 of molten rock
was deposited.
Dust likely caused global cooling
and when it settled CO2 emitted
during eruptions likely caused
heating to as much as 6 C greater
than before the eruptions.
eruptions
Other gases caused decreased O2
Likely chemical and biological effects of Permian volcanic
eruptions: