Transcript palaeobiology lecture

Palaeobiology: how extreme
environments drive
evolutionary change in large
organisms
Robin Allaby
[email protected]
Lecture content
• DEEP TIME
– the palaeontological record
• mass extinctions
• speciation explosions
• possible mechanims for speciations
• SHALLOW TIME
– Life coming out of the ice: the last ice age
• palaeogenetics
• archaeogenetics
What is an extreme
environment?
• temperature (either hot or cold)
• chemistry (unfavourable conditions:
CO2, H20, 02 etc.)
• violent
The palaeontological record
The palaeontological record
oxygen
levels over
time
carbon
dioxide
levels over
time
The 5 mass extinctions
mass extinctions occur at period boundaries
Mass radiations followed
extinctions.
Extinction 1. Ordovician
Extinction2. Devonian
Extinction 3. Permian - the big
one
Extinction 4. Triassic
Extinction 5. Cretaceous
Impact of a 6 mile diameter
asteroid
The
Chicxulub
crater is
pretty big!
Mass extinction causes
1. Ordovician Ice age (unknown)
2. Devonian Ice age (biotic probably)
3. Permian Global warming (unknown cause)
4. Triassic Global warming (volcanic activity possibly)
5. Cretaceous Asteroid impact
What did mass extinctions
kill?
• Large organisms (e.g. dinosaurs)
• Specialized organisms (e.g. climax community of
Carboniferous forests, such as lycopod trees Lepidodendron;
e.g. no entirely carnivorous or herbivorous fauna after the
Cretaceous event)
Species turnover and extreme
environments: the tale of
therapsids vs dinosaurs
Dimetrodon
Pristeroognathus
Replacement rather than
competition?
extreme
environment
Does Darwinian evolution happen at all ?
How do mass extinctions increase
evolutionary divergence?
• Decrease predation pressure, allowing novelties to
become established
• Decrease competition, allowing previously noncompetitive species to rise (more ecological space)
• Extreme environmental (stressful) conditions can
be associated with increased genetic variability
• Change in the ‘fitness landscape’ caused by
extreme environment
TE expansions linked to
punctuated equilibrium and
‘evolvability’
high TE content, low
diversity = evolvable
Myotis
Branchiostomus
high TE content, high
diversity = stasis
low TE content = stasis
Coelocanthus
Does environmental extremity
determine the mode of evolution?
Phyletic gradualism
(as Darwin expected)
Punctuated equilibrium
(caused by extreme
environments?)
Phyletic gradualism in the
palaeontological record
Sheldon 1987 Nature 330:561-563
Punctuated equilibrium in the
palaeontological record
Williamson 1981 Nature 293:437-43
BUT see Van Bocxlaer et al 2008.
Different environments have
different evolutionary rates
Near shore: horseshoe crabs (300 Mya)
Onshore: rich in fossil species (see
Hoffmann and Parsons 1997 p.187)
Dry habitats: origin of angiosperms
(Coiffard et al 2007.)
See Mestre et al 2009 for
deep sea colonization
Highly specialized, lots of
gradual co-evolution e.g.
predator prey, pollinating
systems (Parsons 1994)
How does environment drive
evolution?
• evolutionary patterns determined by intensity of
biotic interactions which differ in different
environments
• fluctuating environments can clear ecological
space
• continuous fluctuating conditions can prevent
adaptation
• intermittant stresses can increase genetic
variability normally unexpressed
Extreme environments in recent
history: evidence from
Palaeogenetics and archaeogenetics
• directly examine evolutionary change
• are morphological changes associated with speciation?
• is there more going on that we cannot see due to
morphological stasis?
Pleistocene-Holocene
Megafaunal extinctions
Hofreiter and Stewart 2009 Current Biology 19:R584-94
Recolonization from refugia
Hewitt 2000 Nature
405:907-913
Brown bear recolonization
Barnes et al 2002 Science 295:2267-70
Arctic foxes did not contract with
glaciers
Dalén et al 2007 Proc. Natl.
Acad. Sci USA 104:6726-9
Cave bears - multiple
species?
• look like different species from DNA
Hofreiter et al 2004 Curr. Biol. 14:40-3
Hofreiter et al 2007 Curr. Biol. 17:R122-123
• we see replacement of one group by the
other - not competition.
Competition in
mammoths?
Gilbert et al 2008 Proc. Natl. Acad. Sci USA 105:8327-32
Mammoth haemoglobin
is cold adapted
Campbell et al
2010 Nature
Genetics 42:536-
540
Extreme environments in the
Holocene: drought tolerance
This barley has the transcription factor for 6-row, but
has evolved back into 2 row by another means to
cope with drought stress.
Palmer et al 2009 PLoS One 4:e6301
Punctuated evolution in cotton
during the Holocene