Palaeobiology: how extreme environments drive

evolutionary change in large organisms

Robin Allaby

r.g.allaby@warwick.ac.uk

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 non-competitive 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

high TE content, high diversity = stasis

low TE content = stasis

Coelocanthus

Branchiostomus

Myotis

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

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

Near shore: horseshoe crabs (300 Mya)

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?

Hofreiter et al 2004 Curr. Biol. 14:40-3

Hofreiter et al 2007 Curr. Biol. 17:R122-123

• look like different species from DNA

• 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

Palmer et al 2009 PLoS One 4:e6301

This barley has the transcription factor for 6-row, but has evolved back into 2 row by another means to cope with drought stress.

Punctuated evolution in cotton during the Holocene