The island of Sri Lanka’s exceptional biodiversity and enigmatic biogeography begs
investigation, as the island is key in understanding the evolution of the Asian tropical
flora. Since the Jurassic, Sri Lanka has been subjected to remarkable tectonic changes,
thus its flora could have been influenced by that of a number of nearby landmasses, as
well giving Sri Lanka the potential to have played a wider role in the assemblage of
floras elsewhere. Firstly, as Sri Lanka originated as a fragment of the supercontinent
Gondwana, part of its flora may contain Gondwanan relict lineages. There is also the
potential for immigration from Laurasia after the Deccan Plate collided with it 45-50
Mya. Further, Sri Lanka may harbour floristic elements from nearby land masses such
as Africa and Southeast Asia as a result of long distance dispersals, and in situ
speciation has the potential to have played an important role in enhancing the endemic
Sri Lankan flora.
I tested the relative contributions of the above hypotheses for the possible origins of the
Sri Lankan flora using three representative families, Begoniaceae, Sapotaceae and
Zingiberaceae. These families represent both herbaceous and woody elements, and have
high diversity across the tropics. Dated molecular phylogenies were constructed for
each family. I used recent analytical developments in geographic range evolution
modelling and ancestral area reconstruction, incorporating a parameter J to test for
founder event speciation. A fine scale area coding was used in order to obtain a better
picture of the biogeography of continental Asia.
Amongst all the models compared, a dispersal-extinction cladogenesis model
incorporating founder event speciation proved to be the best fit for the data for all three
families. The dates of origin for Sri Lankan lineages considerably post-date the
Gondwanan break up, instead suggesting a geologically more recent entry followed by
diversification of endemics within the island. The majority of Sri Lankan lineages have
an origin in the Sunda Shelf (53%). Persistence of warm temperate and perhumid
climate conditions in southwestern Sri Lanka resembling those of Peninsular Malaysia
and Sumatra could have facilitated suitable habitats for these massive dispersals from
the Sunda Shelf region. Some trans-oceanic long distance dispersals from Africa (11%)
are also evidenced, again these are too young to accept a hypothesis of dispersal during
the Deccan Plate’s migration close to the African coast during the late Cretaceous, but
occurred later during the Miocene. Further, some lineages of Laurasian origin (20%) are
evidenced in the Zingiberaceae with ancestral areas of China and Indochina, which is
congruent with a post collision invasion. Among the families tested, dispersals have
occurred stochastically, one during the Eocene, six during the Oligocene, seven during
the Miocene, two during the Pliocene and one during the Pleistocene. The highest
number of dispersals occurred during the Miocene when a warm climate was prevailing
during the Miocene thermal maximum.
My results confirm that in situ speciation is an important contributor to the Sri Lankan
flora. More rapid radiation of endemics has occurred during Pliocene-Pleistocene; two
endemics in Begoniaceae, ten endemics in Sapotaceae and ten endemics in
Zingiberaceae have evolved in situ during this period. Sri Lanka will have been
subjected to expansion and contraction of climatic and vegetation zones within the
island during glacial and interglacial periods, potentially resulting in allopatric
speciation. As a conclusion, long distance dispersals have played a prominent role in the
evolution of the Sri Lankan flora. The young ages challenge the vicariant paradigm for
the origin and current disjunct distributions of the world’s tropical lineages and provide
strong evidence for a youthful tropics at the species level. The thesis contains six
chapters; first two are introductory chapters, then there are three analytical chapters, one
for each family, and finally a summary chapter is provided. Each analytical chapter is
written as a stand-alone scientific publication, thus there is some repetition of relevant
content in each