GEOG2202: The Biosphere

Dispersal and Vicariance

Robert Kinlocke Department of Geography and Geology,UWI, Mona

Darwin’s point of viewDispersal comprises 5 steps: • Centre of origin• As numbers increased, there was movement of

species outwards• Moving away meant encountering inhospitable

terrain which existed before they evolved• Dispersed across gaps by jump dispersal and were

therefore separated into smaller groups • Acquired new characteristics different from

ancestors.

• Historical biogeographers argued that the gaps were too wide

• In response to the criticisms proponents of jump dispersal began to concoct ad hoc scenarios such as: – Birds carrying propagules on their feet – Pregnant females carried to distant regions

At the end of this session you should be able to: • Define vicariance and describe two

methodologies used for reconstructing the distribution of past biota

• Explain the evidence for the rival mechanisms of distribution

• Describe the evidence which supports the existence of the two mechanisms

Vicariance biogeography• Alternative to jump dispersal

• Assumes that biota of an area was once continuous

• Some barrier to genetic exchange causes the separation of the related taxa.

• Relationship between continental drift and biogeography

• Distribution of taxonomic groups are determined by splits (vicariance events) in the ranges of ancestral species.

Testing Biogeographic Hypotheses• Panbiogeography

– Leon Croizat (1952-1964)– Tracks on map connecting known distribution of related

taxa.– Areas of unrelated taxa coincided : fragments of ancestral

geologic and biotic world converged. – He further argued that a single taxon may have managed

to negotiate a sweepstake route but it was difficult to argue that taxa wih different means of dispersal and different ecologies would have been able to do so

The "tracks" are lines connecting areas which contain many disjunct species

Panbiogeography of Rhytidae: Carnivorous Landsnails

Testing Biogeographic Hypotheses• Cladistic Biogeography:

– Involves the reconstruction of evolutionary history of endemic species

– Cladistics is a method of biological classification that attempts to find phylogenetic relationships by constructing branching diagrams based on shared derived characters (synapomorphies)

– Cladograms (common ancestry trees) allow the identification of those groups which share a common ancestor.

(Phylogeny: the evolutionary relationships between an ancestor and all of its known decendants)

– Relation between the groups are established by the analysis of two types of characters:

• Plesiomorphic (primitive): characteristics that are common to a group inherited from a common ancestor. e.g.: four legs of mammals.

• Apomorphic (derived): characteristics that appear later in some groups. e.g.: disappearance of tails in some mammals.

– Every time a derived character is identified which only one group possesses, a branching node is generated

– Groups on either side are considered sister groups.

• Cladistic trees can be used to show similarity between areas.

• The more taxa two localities share the more similar they are.

• Former connections inferred

Evidence for Vicariance• Presence of disjunct distribution of species.• Mesosaurus in South America and Africa- fresh water

reptiles found in Brazil and in Africa. (likelihood of active dispersal? low … very low … It’s 3000+ miles of sea so that’s far … very far)

• Greater diversity in mammals compared to reptiles: – Reptiles: evolved during Laurasia and Gondwana; 7-14

orders.– Mammals: evolved when sea level were high, continental

drift was more rapid; 30 orders. • Flightless birds: Kiwi, Ostirch, Emu, Rhea (order

ratites)

Dispersal or Vicariance?

• Sea turtles in Ascension Island

• Feeding grounds are coastal South America.

Dispersal or Vicariance?

• Dispersal Hypothesis: they migrate long distances (5000 km) between feeding and nesting areas and dispersed from South America to Ascension Island.

• Vicariance Hypothesis: Ancestors of these Ascension Island turtles nested on beaches adjacent to S. American coast throughout the late Cretaceous (135-65 MY).