The document discusses background extinction and mass extinction in biology. It defines background extinction as the normal rate of species loss, around 1 extinction per million species per year. Mass extinction refers to periods when the extinction rate is much higher, around 40-50% of species dying off over a short period. There have been five major mass extinctions in Earth's history. The most well-known one occurred 65 million years ago at the boundary of the Cretaceous and Tertiary periods, wiping out the dinosaurs. This mass extinction is thought to have been caused by an asteroid impact, based on evidence of anomalously high iridium levels and shocked quartz found in the boundary layer.

1. Extinction Krishnendu Sinha
Assistant Professor in Zoology
Jhargram Raj College
© 6th Mass Extinction by Nat Morley

2. a) Background Extinction
Refers to the permanent loss of a species and is marked by the death or failure to breed of the last individual
Extinction is of two types:
Extinction
b) Mass Extinction
though the distinction between background
and mass extinction can be fuzzy in principle…

3. Background Extinction
a) A long-prevailing rate at which taxa become extinct, in contrast to the highly elevated rates that characterize
mass extinction
b) A standard or “baseline” process of extinction occurring outside a period of mass extinction
c) It is estimated to be around 1 extinction per million species per year, though it varies widely from group to group

4. Background Extinction
a) A long-prevailing rate at which taxa become extinct, in contrast to the highly elevated rates that characterize
mass extinction
b) A standard or “baseline” process of extinction occurring outside a period of mass extinction
c) It is estimated to be around 1 extinction per million species per year, though it varies widely from group to group
Causes of Background Extinction
Major causes includes
a) Predation
b) Competition
c) Disease
These are not mutually exclusive (often) rather had a compounding effect on the victim taxa!
For detail case studies consult page 508-514 of Evolution by Bergstorm and Dugatkin, 1st ed.

5. Mass Extinction
a) It is a large-scale extinction on the order of 40 to 50% of all species in many major taxa—over a broad geographic
range—over a relatively short period of evolutionary time (though there is no exact definition of a mass extinction)
b) The loss of 40 to 50 per cent of species is about the norm, but this is only the upper end of a spectrum of
extinction events
c) There is no set timescale either: some extinctions happen relatively quickly while others take several million years
As per Sepkoski, 1986
“any substantial increase in the amount of extinction (lineage
termination) suffered by more than one geographically wide-
spread higher taxon during a relatively short interval of
geologic time, resulting in an at least temporary decline in their
standing diversity”
Now a clear understanding of GTS is very important to
clearly understand the chronological facts of extinction…

6. Geological time scale
a) The geologic time scale is a system of chronological dating that relates geological strata to time. It is used by
geologists, paleontologists, and other Earth scientists to describe the timing and relationships of events that
have occurred during Earth's history
b) The Geologic Time Scale (GTS) is an arbitrary chronological arrangement or sequence of geologic events, used as
a measure of the relative or absolute duration or age of any part of geologic time, and usually presented in the
form of a chart showing the names of the various rock-stratigraphic, time-stratigraphic, or geologic-time units.
c) The table of geologic time spans presented here agrees primarily with the nomenclature, dantes and standard
color codes as set forth by the International Commission on Stratigraphy. This, however, only goes back as far as
the age of the Earth (4.54 billion years)

7. Geological time scale

8. Geological time scale

9. Extinction rates of
marine families over
time. Spikes represent
mass extinctions
There were five mass extinctions in the history of life and we are not living in the sixth mass extinction which is
now happening
Mass Extinction

10. Extinction rates of
marine families over
time. Spikes represent
mass extinctions
There were five mass extinctions in the history of life and we are not living in the sixth mass extinction which is
now happening
Mass Extinction

11. Extinction rates of
marine families over
time. Spikes represent
mass extinctions
There were five mass extinctions in the history of life and we are not living in the sixth mass extinction which is
now happening
Mass Extinction

12. Extinction rates of
marine families over
time. Spikes represent
mass extinctions
There were five mass extinctions in the history of life and we are not living in the sixth mass extinction which is
now happening
Mass Extinction

13. Dead clades walking
Many clades survive periods of mass
extinction, only to go extinct some time in
the following geological time period. In four
(A, B, C, E) of the extinctions shown here, the
survival rate for genera during the period
before the mass extinction (yellow dot) was
higher than the survival rate during the
period that followed the mass extinction
(blue dot). Only D shows higher survival after
the mass extinction. The abbreviations on
the x-axis indicate geological time periods
going forward in time from left to right.
Adapted from Jablonski (2002)

14. Dead clades walking
Many clades survive periods of mass
extinction, only to go extinct some time in
the following geological time period. In four
(A, B, C, E) of the extinctions shown here, the
survival rate for genera during the period
before the mass extinction (yellow dot) was
higher than the survival rate during the
period that followed the mass extinction
(blue dot). Only D shows higher survival after
the mass extinction. The abbreviations on
the x-axis indicate geological time periods
going forward in time from left to right.
Adapted from Jablonski (2002)

15. Mass Extinctions in a Nutshell

16. K-T(or K-Pa)Extinction

17. K-T(or K-Pa)Extinction
a) The most well-known, and well-studied, of the mass
extinctions occurred approximately 65 million years ago,
at the boundary of the Cretaceous and Tertiary periods
b) This most recent of mass extinctions—often called the K–
T mass extinction
c) It had profound effects on many different taxa, both in
the water and on the land, flora and fauna, invertebrate
and vertebrate
d) The most famous victims were the dinosaurs
e) Conservative estimates report that half of all the genera
alive before the end of the Cretaceous period died off
during this mass extinction

18. K-T(or K-Pa)Extinction
A. Walter Alvarez, a paleontologist and his team,
hypothesized the cause of K-T extinction as an
asteroid, approximately 10 km in diameter, struck
Earth 65 million years ago
B. Their hypothesis was supported by the abnormally
high presence of iridium (an extremely rare element
in earth’s crust) and shocked quartz in the K-T
stratum of earth’s crust
C. Iridium is extremely rare in earth (0.005 ppm) but
relatively abundant in asteroids (0.5 ppm)
D. Shocked quarts can only be formed from extreme
condition like atom bomb detonation or an asteroid
impact!

19. K-T(or K-Pa)Extinction
a. The discovery of Chicxulub crater in the Yucatán peninsula in 1991 of Mexico by Alan Hildebrand first
provides support for the hypothesis
b. The crater is 65 Mya old and have extreme high concentration of iridium and was composed of rock
with mineralogical characteristics that showed evidence of a massive collision
But till now debate about K-T extinction is on…

20. References

21. That’s all Folks…