“Seed- shadows” how far do propagules or progeny disperse away from the parent(s)? C. J. Clark, et al., 2005 Ecology: 86, e.g. forest trees, Cameroon, Africa
Dispersal of seeds by ocean currents: from the Caribbean to Ireland “Tropical” seeds found on Irish beaches
Dispersal of seeds by animals Seeds with wings: transported by wind Barbed seeds: transported in animal fur Fleshy fruits : transported in animal gut Seeds catapulted by explosions N. American flora Hawai’ian flora
Dispersal of seeds by animals e.g. germinable seeds in horse dung* (Shetland Islands, Scotland) Median density: 700 seeds/pat Median number of plant species: 27 Total number of plant species: 67 *14 samples Cosyns, E. and Hoffmann, M Basic & Applied Ecol., 6,
Stepping-stone? dispersal e.g. giant tortoises (Dipsochelys); Aldabra extant; populations on Seychelles ( ) and Malagasy now extinct 400 km
Island flightlessness Why are ALL the endemic crickets on the Hawai’ian islands flightless (they only have rudimentary wings)?, and Why are the seeds of many plants in the sunflower family in Hawai’i barbless (e.g. Bidens spp.) ? Mainland Hawaii
Heteroblasty (“blast” = bud/sprout) (a.k.a. “Don’t put all your eggs in one blastket”) Crepis sancta, a dandelion-like weed, commonly produces two kinds of seeds. In urban environments the heavier, non wind-dispersed form is dominant*. Why? *3 March 2008 | Nature | doi: /news
Dispersal: overcoming barriers dispersal corridor dispersal filter sweepstakes dispersal corridor A to B habitats dispersal e.g. desert barrier wide variety easy A B limited array (oases) difficult; only certain organisms make it A B none occasional migrants
Land bridges as terrestrial corridors Real: e.g. Beringia (later lecture) or Panama Hypothetical: e.g. Lemuria (proposed by Sclater and others in 19thC to explain presence of fossil lemurs in India and Pakistan Lemuria?
Panama land bridge: the corridor as filter Northern ancestry Stopped by filter Now stopped in filter Crossing filter Southern ancestry Shrews Pocket mice Pocket gophers Beavers Bobcats Pronghorns Bison Sheep Rabbits Squirrels Field mice Raccoons Weasels Otters Skunks Pumas Deer Coatis Kinkajous many Cats Tapirs Peccaries Camels Porcupines most Armadillos Anteaters Sloths Guinea pigs capybaras Seaway closed ~ 4.5 Ma BP
Vicariance biogeography Based on the precept that regional biotas are remnants of more extensive grouping that have been fragmented by vicariant events. Vicariant events include such processes as continental fragmentation, mountain building, desertification. Emphasizes the role of allopatric speciation in populations isolated by vicariant barriers.
“the breakup of a large landmass into smaller units would necessarily lead to the extinction or local extermination of one or more species and the differential preservation of others” Alphonse de Candolle, 1855 Continental fragmentation and biogeography Biogeographic realms are continental rafts; their boundaries reflect tectonic divergences and convergences
Vicariant events: ratites and the Gondwana breakup
Bird evolution in mid-Jurassic (from theropod dinosaurs?) T K J T R Dinosaurs Birds Struthiomimus
Living ratites and tinamous (large, mainly flightless birds) Tinamous: SAm 2. Kiwis: NZ 3. (Moas: NZ) 4. (Elephant birds: Africa) 5. Cassowaries: Aus-NG 6. Emus: Aus 7. Ostrich: Af-EurA 8. Rheas: SAm (extinct groups)
Cladistics of the ratites (and their close relatives, the tinamous) Taxon: Area: S.Am NZ NZ Ma Au,NG Au Af S.Am
Distribution of the ratites [and close relations] prior to human-induced extinctions ,3 4
Distribution of the ratites [and close relations] prior to human-induced extinctions ,3 4 Inter-continental dispersal?
Vicariance hypothesis: fragmentation of Gondwana ,3 5,6 8 Gondwana
Similar area cladograms = similar vicariant histories South America Australia New Guinea South America Australia New Guinea South America Australia Europe North America South America Australia Europe North America New Guinea Ratite birds New Zealand Madagascar Chelid turtles Hylid frogs Galliform birds
Extinctions and radiations why is evolutionary history marked by catastrophes? what causes mass extinctions? what controls survival? what controls post-catastrophe success? are there biogeographic lessons?
Biotic/evolutionary metaphors “Great Chain of Being” “The cone of diversity” “The ultimate lottery” “The ladder”
The ladder of progress: evolution of horses
The ladder of progress
The tree of life or cone of diversity Haeckel, 1866
The cone of diversity: arthropod evolution (1986)
Punctuated equilibrium Proposed by Niles Eldredge and Stephen Jay Gould (in 1972) Evolution tends to be characterized by long periods of virtual standstill punctuated by episodes of rapid diversification (mass extinction events are simply megascale examples of this), rather than the “gradualism” proposed by Darwin.
“prior life” “visible life” recent middle ancient
The geological time scale is subdivided by events of mass extinction which are followed by phases of rapid evolution. Were these extinction events a product of catastrophes?
Time Diversity PD after Phillips (1860) The evolutionary roller-coaster Palaeozoic Mesozoic Cenozoic EE Radiation
Marine invertebrates: extinctions and radiations
Survivals and radiations
Extinction events potential catastrophic causes: Glaciation Continental collision Mountain building and climate change Volcanism Asteroid/comet impacts Gamma-ray outbursts
Terrestrial animals through time The K/T boundary
The Cretaceous/Tertiary (K/T) extinction Time: 65 Ma Magnitude: 11% of marine families Victims: dinosaurs Inferred cause: asteroid impact, (and associated vulcanism?) Beneficiaries: mammals, birds, etc.
K/T “smoking guns”
The K/T impactor theory Helen Frank Walter Luis Michaels Asaro Alvarez Alvarez
Cretaceous - Tertiary boundary rocks, Los Brazos, SW Texas sandstone broken sandstone =tsunami deposit? v. dark clays: soot & rare elements (e.g. Ir)
Post-K/T radiation Diversification in aftermath of extinction event (highly “punctuated”) Diversification within orders after extinction, but orders predate event Diversification primarily prior to extinction event (“gradualism”) Source: Springer et al., PNAS 100:
Bird radiation (chronology based on molecular clock) Source: Slack, K.E Mol. Bol. & Evol. 23, K-T boundary
Source: Springer et al., PNAS 100: Mammal radiation
Biogeographic implications Surviving a mass extinction or major vicariant event is a product of luck, genes and geography. Organisms that opportunistically disperse into devastated habitats win the lottery. In the aftermath of the catastrophe distributions may be fragmented, leading to rapid adaptive radiation.
Plant diversity through time: radiation in the Tertiary resulting from continental fragmentation?