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Plant of the Day
Isoetes andicola
Endemic to central and southern Peru
Found in scattered populations above
4000 m
Restricted to the edges of bogs and lakes
Leaves lack stomata and so CO2 is
obtained, not from the atmosphere, but
from sediment via the roots
Carbon fixation occurs via the C3 pathway
by day, but via a CAM-like process at
night
Members of the quillwort family
(Isoetaceae) are the nearest living
relatives of the ancient tree lycophytes
Definition: Macroevolution
Macroevolution
evolution on the grand scale, and it is mainly
Evolution atmeans
or above
studied in the fossil record. It is contrasted with microevolution, the
theof level
of the
study
evolution
over short time periods, such as that of a human
lifetime
or less. Microevolution therefore refers to changes in gene
species…
frequency within a population ....
Macroevolutionary
are much more likely to take millions of
Also, long-termevents
trends,
years. Macroevolution refers to things like the trends in horse
biases,
orthe
patterns
evolution
... or
origin of major groups, or mass extinctions, or the
(i.e. phylogeny!)
in
Cambrian
explosion .... Speciation
is the traditional dividing line
between
micro- and macroevolution.
the evolution
of
higher taxonomic
levels.
- Mark Ridley, Evolution
Why are some clades more diverse?
Why are some clades more diverse?
Net diversity =
number of speciation events –
number of extinction events
What affects the rates of
speciation and extinction?
There are at least 3 different reasons for variation in
species number between sister groups.
1. Stochasticity: if speciation is random, then
clades with more species are more likely to
be subdivided again than clades with fewer
species
2. Extrinsic Factors: external factors, such as
environment and geology, can affect
speciation and extinction rates
3. Intrinsic Factors: a single character, or
combinations of characters, moves a group
into a new “adaptive zone”, either
geographically or ecologically
There are at least 3 different reasons for variation in
species number between sister groups.
1. Stochasticity: if speciation is random, then
clades with more species are more likely to
be subdivided again than clades with fewer
species
2. Extrinsic Factors: external factors, such as
environment and geology, can affect
speciation and extinction rates
KEY
INNOVATION
3. Intrinsic Factors: a single character, or
combinations of characters, moves a group
into a new “adaptive zone”, either
geographically or ecologically
What are the main extrinsic drivers of
evolutionary change?
• Physical environment?
• Interaction with other species?
• Darwin puts a lot of emphasis on competition
and predation
 Van Valen’s Red Queen hypothesis
Red Queen hypothesis
Red Queen hypothesis
The continual evolutionary change by a species that is necessary to
retain its place in an ecosystem because of ongoing co-evolution by
other species (and it’s environment)
Physical and biotic aspects of the environment of any species are
forever in flux, with subtle changes in annual temperature and
weather cycles, topography, food supply, predators and group
dynamics
 the targets of selection keep changing, and so the organism is never
perfectly adapted.
Red Queen vs. Court Jester
• Van Walen saw evidence for this through his
observation of what he called the ‘law of
constant extinction’: species or genera were as
likely to become extinct at one time as at any
other, irrespective of their geological age
– Paleontologists did not like that
– Other studies showed that extinction risk depended
on taxon age
• Other models, collectively called Court Jester
hypotheses, posit that random changes to the
physical environment (e.g. climate change,
tectonic events) are the key drivers of major
changes in organisms and diversity.
Red Queen vs. Court Jester
• Evidence for either hypothesis from the fossil record?
– Difficult since it is hard to exclude the possibility of changes in physical
environmental conditions
• Barnosky (2001): suggests that the different hypotheses may
operate at different spatial & temporal scales
– Red Queen OK for local, ecosystem scale views, but in the grand schemes of
things ‘random geological events’ matter more
• Biologists tend to favor Red Queen, whereas Paleontologists tend to
favor the Court Jester
Red Queen vs. Court Jester
• Venditti et al (2010):
– Test the Red Queen’s prediction of constant speciation rate
– Used empirical data (trees from different organism groups) and tested five
different statistical models for the frequency distribution of branch lengths
– Result: Best fit is to the simplest model of evolution, the exponential model, in
which new species emerge from single events — each event being infrequent
but individually sufficient to cause speciation.
– Their interpretation: Rare stochastic events that cause reproductive isolation
explain constant rate of speciation.
“Species do not so much ‘run in place’ as simply wait for the next
sufficient cause of speciation to occur.”
So, the saga continues…
• Nonetheless there
are many examples
of particularly
diverse, and species
rich clades!!
• Pictured here are the
flowering plants.
Non-adaptive radiation: rapid speciation in the
absence of ecological diversification
e.g. Aegean Nigella
arvensis complex (12
taxa)
-similar habitats on
different islands
-changes in sea level
allow disperal, selfing
-drift
Comes et al 2008
Adaptive radiation: the evolution of ecological
diversity within a rapidly multiplying lineage
George G. Simpson (famous
paleontologist and founder of the
modern synthesis) first described
this term.
He believed that…
Adaptive radiations resulted from
diversification accelerated by
ecological opportunity, such as
dispersal into a new territory, the
extinction of competitors or the
adoption of a new way of life.
This view is echoed by Verne Grant…
• “When a species succeeds in establishing itself in a new
territory or new habitat, it gains an ecological opportunity for
expansion and diversification. The original species may
respond to this opportunity by giving rise to an array of
daughter species adapted to different niches within the
territory or habitat. These daughter species become the
ancestors of a series of branch lines when they, in turn,
produce new daughter species. The group enters its second
phase of development, the phase of proliferation... Adaptive
radiation is the pattern of evolution in this phase of
proliferation. And speciation is the dominant mode of
evolution in adaptive radiation.” (1977: 309)
This view is echoed by Verne Grant…
Caveats:
1) Few examples rigorously document a causal
relationship between rapid levels of adaptive
diversification and speciation
•
genes underlying any adaptive differences must be linked
to/the same as those responsible for isolation
•
ecological speciation has been documented in plants (e.g.
sago palms, pollinator isolation in Aquilegia )
2) The environment is not divided into predetermined
niches in the absence of the organisms that inhabit
them
“...the greatest living example of adaptive radiation in plants”
Dolph Schluter
The 28 Hawaiian-endemic species in silversword alliance include trees,
shrubs, mat-plants, rosette plants, cushion plants, and vines; all have arisen
in past 5.2 million years
Andean lupines: eighty species have
arisen in last 1.2-1.8 Myr
Andean uplift
- between 2 and 4 Myr ago
- created ecological
opportunities for diversification
Andean lupines: eighty species have
arisen in last 1.2-1.8 Myr
Andean uplift
- between 2 and 4 Myr ago
- created ecological
opportunities for diversification
Intrinsic Factors: Key innovations
Key innovations are novel morphological or behavioral traits
thought either to open new ‘adaptive zones’ or to offer the
ability to rapidly speciate after environmental change.
What about the
angiosperm radiation?
Why are the angiosperms so species rich?
Darwin's abominable mystery: the origin
and diversification of flowering plants
90% land plants are angiosperms
Ecologically widespread
Discovering adaptive radiations…
• Step 1: construct a phylogenetic tree for the family
to find a sister group.
– Sister groups are the descendants of a common and
unique speciation event and therefore must be of equal
age!! (ensure all else is equal)
• Step 2: test for a significant difference in species
richness between sister-groups
– > 90% of the total diversity in the two sister groups in
species-rich genus
– Step 3: which sister-group is unusual in the context
of the whole tree?
Discovering key innovations…
• As described in Hodges & Arnold, traits of
interest must be optimized on the
phylogenetic tree.
• Following the origin of the putative innovation
on the tree, does the diversification rate
increase? (Maximum-Likelihood based tests)
Difficulties…
• - multiple innovations may work together to affect
diversification, a key “complex”
• - the innovation only has an effect after an
environmental change (therefore, change in rates
delayed compared to origin of innovation)
• - the effects of innovations may be obscured over
time by the evolution of other characters
• - require homoplasy (a character shared by a
number of organisms but not present in their
common ancestor) to do statistical tests
Back to the Angiosperms
Gnetales
69
Ceratophyllaceae
6
Rest of the
angiosperms
> 250000
Angiosperm synapomorphies,
possible “key innovations”
Distinguishing Angiosperm
characteristics
(1) Closed (angio-) carpels (sperm)
Distinguishing Angiosperm
characteristics
(2) Double fertilization
Distinguishing Angiosperm
characteristics
(3) Reduced male and female gametes
Distinguishing Angiosperm
characteristics
(4) Stamens with two pollen sacs
Distinguishing Angiosperm
characteristics
(5) Flowers
Back to the Angiosperms
Gnetales
69
Ceratophyllaceae
6
Rest of the
angiosperms
> 250000
Angiosperm synapomorphies,
possible “key innovations”
Three other characteristics have been proposed:
biotic pollination (decreases extinction risk with
more precise pollen transfer)
YES
biotic dispersal of seeds (enhances probability of
speciation as seeds are dispersed over a larger
area/range) NO
herbaceous growth form (shorter generation times
and less stable habitats enhance chances of
speciation)
YES
Dodd et al. 1999
Phylogenetic relationships among the seed plants
remain controversial
Conclusions
There is no single key innovation involved in the
angiosperm radiation, but many.
The picture is more dynamic than a single origin of one
key innovation & subsequent diversification.
Innovations are gained and lost within the Angiosperma.