Download Time Number of species

The Evolution of Biological Diversity
All living organisms are descended from an ancestor
that arose between 3 and 4 billion years ago.
The diversity of life on earth currently includes some
5 to 50 million species!
Before discussing the observed patterns, it is worth
thinking about the possible ways in which the number
of species present at any point in time may have
changed over the history of the earth:
Number of species
Millions
One
3.5 BYA
Present
Time
Number of species
Millions
One
3.5 BYA
Present
Time
Number of species
Millions
One
3.5 BYA
Present
Time
The patterns and rates of diversification reflect the
rates of speciation (S) and extinction (E) of taxa.
At the simplest conceptual level, the number of
species (N) present on earth will change over time
according to the formula:
∆N = (S - E) N ∆ t,
where ∆N is the change in number of species over an
amount of time ∆t and where S and E are rates of
speciation and extinction per species.
If S and E remain approximately constant over time,
exponential growth will result.
Both speciation and extinction rates may vary over
time and from species to species depending on:
• resource and habitat availability ("niche space")
• interactions among species
• key adaptations (changing "adaptive zones")
• climactic changes
• catastrophes
Diversity-dependent growth: An alternative
possibility is that either the rate of speciation or
extinction (or both) depends on diversity levels (N).
Why might this be true?
If (S - E) goes down as N increases, then logistic
growth in the number of species is expected,
potentially resulting in a global equilibrium.
The number of species is difficult to determine from
the fossil record.
• Are allopatric populations the same or different species?
• Are populations from different points in time the same
or different species?
• Are there sufficient fossil remains of most species?
Consequently, paleontologists tend to study diversity
by counting the number of higher taxa (e.g. families).
Two major databases have been compiled:
• John Sepkoski (1984) compiled data on the
temporal ranges of more than 4000 marine
skeletonized families throughout the 543 MY of the
Phanerozoic.
• Michael Benton (1993) compiled the Fossil Record 2,
containing 7186 families of all groups of microbes,
algae, fungi, protists, plants, and animals.
The pattern of diversification depends on the taxa
examined.
For several groups, an exponential rise in the number
of families best fits the data.
(1) All of the families in the Fossil Record 2 database:
Time (MYA)
Geological Time
(2) Tetrapod families:
Number of Families
Time (MYA)
Mammals
Birds
Amphibians
Reptiles
Geological Time
Number of Families
(3) Insect families:
Geological Time
(REMEMBER: Extinctions are happening throughout.)
For other groups, the patterns are more complicated.
For example, the growth of marine families appears to
show a number of steps:
Number of Families
800
400
543
251
Geological Time (MYA)
65
Sepkoski interpreted this as the result of the radiation
of three distinct "evolutionary faunas":
• Cambrian fauna with broad habitat and trophic
requirements (e.g. trilobites, inarticulate
brachiopods...)
• Palaeozoic fauna with more specialized
requirements (e.g. crinoids, cephalopods, soft
corals...)
• Modern fauna with predatory and defensive
strategies (e.g. bivalves, hard corals,
malacostracan crustaceans, fish...)
Why might marine fauna have increased rapidly and
then reached a plateau between 400 and 250 MYA
whereas tetrapods and insects have not plateaued?
Number of Species
Similarly, the number of species of vascular land
plants appears to have increased in steps:
Geological Time (MY)
Knoll and colleagues (1984, 1985) interpreted this
as the appearance and radiation of new major
baupläne (= body forms):
• Devonian flora of early vascular plants
• Carboniferous flora of club mosses, ferns, conifers
• Triassic flora of gymnosperms
• Cretaceous flora of angiosperms
The subsequent rise of angiosperms has proceeded
exponentially.
Surprisingly, these data suggest that the number of
species on earth has not reached a global equilibrium.
If anything, the most prevalent pattern is of
exponential growth.
Can exponential growth of species be sustained?
How?
Earth
Earth
That is, exponential growth of species may occur by
changing patterns of "species packing", with
generalists being replaced by more specialized forms
and with an increase in the complexity of the
ecological community.
Are the rates of speciation and extinction the same
for different organisms?
S >> E
(S-E) per million years
Trilobites
Ammonites
Graptoloids
Birds
Mammal families
Insects
Freshwater Fishes
S~E
Low E
High E
Average species duration (MY)
No!
Interestingly, those taxa with high rates of increase
(S-E) also tend to have high rates of extinction.
What might explain this odd result?
• Specialists may be more likely to speciate
because of their patchy distribution but may also
be at higher risk of extinction.
• Species with small population sizes may be more
likely to speciate (if drift is important) but are at
higher risk of extinction.
• Species with low dispersal rates may be more
likely to speciate (lower gene flow) but may be
more likely to go extinct following local
environmental changes.
Factors Affecting the Origin of Biological Diversity
Speciation rates are higher in some lineages than
others and at certain times over others. Here we
explore several possible explanations.
(1) Ecological Opportunity:
Living organisms present numerous examples of
spectacular radiations following the colonization of
isolated islands or bodies of water, e.g.:
• honeycreepers on Hawaii
• drosophilids on Hawaii
• cichlids in African Great Lakes
• amphipod crustaceans in
Lake Baikal in Siberia
In these cases, the fauna was locally depauperate
before the arrival of the original colonist.
"Vacant niches" existed into which the newly
arrived organisms diversified.
Similarly, there are several examples in the fossil
record where the decreased representation of one
group is followed or accompanied by a proliferation of
another group.
The new group may cause the extinction of the
former group (displacement)
The new group may be released from
competition by the extinction of the former group
(incumbent replacement)
For example, rodent-like, non-placental mammals
(multituberculates) decreased in diversity in North
America following the appearance of placental rodents.
MYA
34
56
65
Multituberculates
Rodent Genera
The correlated pattern of increase and decrease
suggests displacement or incumbent replacement?
Lineages often are seen to radiate following the mass
extinction of another group.
For example, mammals radiated in the Tertiary period
following the mass extinction of dinosaurs near the
K-T boundary (Cretaceous-Tertiary boundary ~65MYA).
Does this suggest displacement or incumbent
replacement?
(2) Key Adaptations:
Speciation rates within a group may rise after the
evolution of a new adaptive trait.
How can we tell whether a trait increases
speciation rates?
Replicated Sister-Group Comparisons
For example, Ehrlich and Raven (1964) suggested
that defenses against herbivores (e.g. latex and resin
canals) promoted diversification in plants.
Mitter, Farrel and colleagues (1988,1991) tested this
hypothesis by identifying 16 sister groups of plants
with and without these canals.
In 13/16 cases, the canal-bearing groups contained
more species than their sister clades.
Conifers
(+ canals)
Gingko
(- canals)
559
1
Fig family
(+ canals)
1703
Elm family
(- canals)
150
Number of living species
Similarly, the fossil record suggests that key adaptations
in marine organisms (specialization, predation,
swimming, hard shells) promoted their diversification.
Key adaptations allow organisms to evolve into a
greater variety of niches, creating a more complex and
tiered community structure.
(3) Provinciality:
Speciation rates will depend on the extent to which
organisms are distributed through space.
There has been a general trend over the last 250MY
from wide-spread distributions to more localized
distributions.
As Pangaea began to break apart during the Triassic,
land and ocean masses became more spatially
separated.
Ocean currents also changed, leading to a more
pronounced temperature gradient.
These changes have increased the number of
biological "provinces" (= a self-contained region
wherein speciation rather than colonization
dominates the appearance of new taxa).
An example (in reverse) is the extinction caused when
the separate land provinces of North and South
America became connected by the Isthmus of
Panama (~2MYA).
23 families of mammals were endemic to South
America (incl. sloths, armadillos, opossums, raccoons),
25 to North America (incl. mammoths, mastodons,
saber-toothed cats, and camels), and 2 occurred in both.
Migrants from
the South
The "Great American
Interchange" describes the
resulting migration of
animals across the
Isthmus of Panama.
Migrants from
the North
Diversity, supported by provinciality, was lost following
the Interchange: only 38/50 families remained, 24%
went extinct.
Extinctions and The Decline of Biological Diversity
The tree of life has been severely and often pruned.
The vast majority of species that have ever lived have
gone extinct.
Extinction rates vary over
time, but are dominated
by the "big five" mass
extinction events.
Percent Extinct
Geological Age
3
1
2
[Note: Peak in Cambrian probably reflects low
diversity at that time.]
4
5
Extinction Event MYA Family Loss Species Loss
End-Cretaceous
65
~14%
~76%
End-Triassic
208
~30%
~80%
End-Permian **
245
~60%
~95%
Late Devonian
367
~30%
~83%
End-Ordovician
439
~23%
~85%
[Family extinctions reflect all organisms in the Fossil
Record 2 database. Species extinctions reflect loss of
marine species estimated by Jablonski (1991,1995).]
Climate change has been cited as a major factor
involved in each mass extinction event.
The largest of the extinction events, at the end of the
Permian, is associated with a number of catastrophic
climate changes (the "world-went-to-hell" hypothesis)
including:
• major sea level regression
• ocean anoxia (= decreased oxygen)
• Siberian flood basalts (=magma flows) over
1.5 million km2
• increased CO2
• global warming
Major climate changes also surround the
end-Cretaceous extinction (K-T), possibly resulting
from a massive asteroid hitting the earth.
(A buried crater has been detected in the Yucatan
peninsula of Mexico with a diameter of 180 km!)
These mass extinctions have played a major role in
shaping the biota we see today.
Regardless of how adapted a species may be in
"normal" times, if it succumbs to extinction during a
massive climate change, it will play no further role in
evolution.
For instance, at the K-T boundary, insects,
amphibians, crocodilians, mammals, and turtles
suffered few extinctions, whereas several bird
species, ammonites, dinosaurs, and other large
reptiles went extinct.
Although we generally do not know why extinction
events are so selective, some patterns have emerged.
% of Genera Extinct
Jablonski (1986) found that bivalves and gastropods
with wider geographic distributions were less likely
to go extinct at the end of the Cretaceous.
Number of Provinces Inhabited
Without these mass extinctions, the world would be a
very different place.
For one thing, the rise and diversification of mammals
may never have happened.