Evolutionary biology journals of the 1950s and 60s were filled with
musings on the improbable. Disjunct distributions of related organisms on
continents separated by hundreds of miles of open ocean or in seas divided
by mountainous barriers were a historical challenge. Pregnant porcupines
were rafted across the South Atlantic on mangrove trunks to explain the
presence of Palaeogene hystricomorph rodents in Africa and South
America. Limpets were lofted across the Isthmus of Panama in the talons
of birds to connect latter day Caribbean descendants with their Miocene
Pacific ancestors. By the 70s continental drift lumbered into the paradigms
of palaeontology and vicariance inserted itself as the null hypothesis – the
evolutionary hegira was redefined as abduction by subduction. Dramatic
finds like the Triassic vertebrate Lystrosaurus on the Antarctic mountains
that once lay within the genus’s now disjunct African and Indian range
confirmed the predictive power of vicariance. In some instances, the
vicariant sword grew so sharp that scenarios of circuitous continental
conveyers became even wilder than earlier tales of drifting diaspora. By
the end of the century, however, ideological battles between migrationists
and vicariants had given way to more dialectic mixed model of
biogeographic history.
In the past decade, molecular phylogeography and geological isotope
geochemistry have renewed interest in migration on much smaller
temporal scales. Surveys of molecular diversity within species revealed
that much geographic variation was phylogenetically structured. Pictures
of species as panmictic gene pools have been redrawn so that speciation is
no longer an event, but a never-ending narrative of the gradual breakdown
in relationship. Rivers and hills, rather than oceans and mountains,
separate subspecific clades. At the same time, geochemical evidence has
revealed fine-scale wobbles in the Earth’s climate. The Pleistocene, for
example, is no longer the time when regal glaciers made stately advances
and retreats across the continent, but a jumble of transient alternations of
warm and cold overlain by more regular glacial and interglacial cycles.
Warm spikes only hundreds of years long saw Saiga antelope sweep
احمد رجب 
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