Fossil Evidence of Allopatric Speciation
Seldom can we find fossil evidence to illustrate the beginnings of the speciation process, but the history of the cockle genus Bassina in the New Zealand Tertiary perhaps illustrates a species (B. speighti) which occupied most of New Zealand in the Oligocene and Miocene splitting to form two forms (B. parva and B. yatei) which were allopatric geographic races in the Pliocene, eventually becoming sympatric in about the Early Pleistocene before the extinction of B. parva. A marked size differentiation suggests the presence of ecological differences necessary for sympatric existence In view of the imperfection of the geological record, with Bassina as with other fossils, I must add, quoting Darwin115 that it is “no more likely that I should be right in nearly all points, than that I should toss up a penny and get heads twenty times running”.
Fig. 9.—Fossil evidence of geographic speciation in bivalve Mollusca. Bassina speighti (A, Mid-Tertiary) was succeeded in the Pliocene by two forms, C and B, at first strictly allopatric in the north and south of New Zealand, but later overlapping as sympatric species (B. parsa and B. yatei) in central districts.
[Footnote] 115 Life and Letters, II: 240.