of allopatry vanish. Step 3 is critical and is often forgotten in discussion of allopatric
speciation. Division of a species into several not only requires genetic divergence but
subsequent selection for mating barriers in renewed sympatry.
(^) Edward Brinton (1962) suggested several mechanisms by which allopatric
speciation could have operated in the sea. These were theoretical ideas in 1962, but
subsequent developments have made possible some very direct tests, genetic
comparisons of closely related species. The problem of accurate calibration of
molecular clocks may eventually be sufficiently solved to show the timings of
divergences. The mechanisms all depend upon the warming and cooling effects of
global glaciation upon ocean temperature patterns during the Pleistocene. Before that,
the dominant mechanisms must have been different, and they were probably fewer.
Brinton’s patterns were reconstructed from the array of different patterns observed
among modern pelagic species, much as Darwin explained the formation of coral
atolls from the array of variations he observed on the Beagle expedition. Brinton used
euphausiid patterns based on his own work and global records.
(^) Thysanoessa gregaria occupies the subpolar–subtropical transition zones in all of
the world’s oceans (it is bi-antitropical), and it would have been able to survive at
much lower latitudes during a cooler interval, particularly across the eastern boundary
of the oceans. This former continuity of the pattern is an explanation for the bi-
antitropicality of the species at present. The present situation is conducive to division,
and speciation is presumably in process at present. The Nematoscelis difficilis–N.
megalops pair (Fig. 10.5) is a further example. In 1962 there were no species known
to have a subpolar and eastern-boundary-current distribution continuous across the
equator, but as we have seen, Oithona similis (Fig. 10.10) provides an example.
(^) Central species possibly would be oppositely affected by general cooling. At
present, the warmest waters, except for those at the very surface, are in the central
gyres, not along the equator (excepting the Indian Ocean). For a species like
Euphausia brevis (Fig. 10.6) with a vertical range of hundreds of meters, warming
could cause trans-equatorial coalescence of ranges, while cooling could intensify their
separation. Euphausia brevis is presumably undergoing speciation at present. The
range of an all-warm-water form like Clausocalanus parapergens (Fig. 10.2a) might
be expected to divide during a colder interval and speciation could occur.
(^) Global warming could break up the present Indo-Pacific or circumglobal
distributions of equatorial species like Euphausia diomediae, leaving isolated
populations in the cooler equatorial regions like the present distribution of Euphausia
distinguenda in the eastern tropical Pacific and the Arabian Sea (Fig. 10.15).
Sebastian (1966) has shown that the Arabian Sea population of E. distinguenda has
small but consistent morphological differences, naming it Euphausia sibogae, which
implies that the development of two separate species is already well advanced.