XIANG, (JENNY) QIU-YUN* AND DOUGLAS E. SOLTIS. Department of Biological Sciences, Idaho State University, Pocatello, ID 83209-8007. - Phylogenetic Patterns and Divergent Times of Disjunct Taxa: Insights into Historical Biogeography of Angiosperms in the Northern Hemisphere
.
One of the most fascinating aspects of angiosperm biogeography in the
Northern Hemisphere is the intercontinental disjunct distribution of
closely related species. Analyses of phylogenetic relationships and
divergence times are crucial in seeking possible explanations for the
origins of various disjunct patterns. Molecular phylogenetic analyses
of diverse plant taxa with clades disjunctly distributed in eastern
Asia, eastern North America, western North America (e.g.,
Trillium, Cornus , Boykinia , Tiarella
, Trautvetteria , Thermopsis , Aralia sect.
Aralia , Calycanthus , Asarum , Staphylea
), and taxa also found in one or more of the following areas:
Europe, western Asia, and South America in addition to eastern Asia
and North America (e.g., Aesculus , Chrysosplenium ,
Gleditsia , Styrax , Astragalus , Nuphar
, and Rubus ) revealed similar phylogenetic patterns: the
North American species form a monophyletic group sister to the eastern
Asian or the Eurasian species, with the old world species basal and
the South American species appearing in derived positions.
Vicariance-Dispersal analyses suggested that 1) many taxa diversified
in eastern Asia and then spread from the old-world to the new-world;
2) The modern disjunctions are mostly the result of vicariance
following dispersal and geographical isolation; 3) vicariance events
within North American or Eurasian continent occurred subsequent to
that between the two continents; and 4) disjunct distributions in
South America are due to long-dispersal from North America or from
eastern Asia. Analyses of divergence times using molecular clocks
indicated that disjunct species from different genera diverged at
different geological times (Oligocene to recent) with most of the
eastern Asian-eastern North American species analyzed diverged from
the late Miocene to the Quaternary. These data suggested that the
congruent geographic distributions and phylogenetic pattern exhibited
by these disjunct taxa represent "pseudocongruence".
Key words: angiosperms, disjunct taxa, divergence time, historical biogeography, Northern Hemisphere, phylogenetic pattern