Speciation, Again
(^) The most instructive CLIMAP comparison for those interested in pelagic speciation
processes is that of the North Atlantic subpolar assemblage (Fig. 10.21). In the core
tops it extends south to about 15°N in the east, while at 18,000 YBP it crosses the
equator. Thus, the patterns of former sympatry suggested by Brinton (1962) for
species like Thysanoessa gregaria and the Nematoscelis difficilis–N. megalops pair
clearly did exist for species preserved in the fossil record, like Globorotalia
truncatulinoides dominant in the subpolar assemblage. Brinton’s theoretical
suggestion has been borne out: allopatry of bi-antitropical species has developed in
the post-glacial era exactly as he proposed.
Fig. 10.21 Maps comparing the extent of sediment deposits dominated by a subpolar
foraminiferan faunal group between the present (a, after Kipp 1976) and at 18,000
YBP
(b, after McIntyre et al. 1976),
when these cold-water forms constituted 25% of the foraminiferan fauna right across
the equator. The ticked dashed line in (b) shows the core of distribution from (a).