(^) Do more contiguous populations spread across wide ocean basins also diverge? The
general within-gyre similarity of Goetze’s data for subtropical Eucalanus suggest not,
that circulation generates enough genetic mixing across up to 12,000 km to sustain
homogeneity. While a number of studies have addressed this issue, none yet proves
the generality of this conclusion. Among more coastally bound species, it is common
to find statistically significant differences in proportions of haplotypes or alleles in
different parts of the distribution. These occur within the sequence-variation range
usual for morphologically recognized species, within ∼1–4% in mtDNA. An example
is the variation in cytochrome oxidase II (mtDNA) among stocks of the chaetognath
Sagitta setosa around the British Isles and in several of the basins of the
Mediterranean (Peijnenburg et al. 2004, 2006). Haplotypes among 551 base pairs for
148 specimens from scattered sites (Fig. 10.23) showed strong divergence between
North Atlantic, Mediterranean, and Black Sea sites, and there were significant
regional differences within the Mediterranean. At first glance, the NE Atlantic vs.
Mediterranean difference is not surprising, since S. setosa has a distribution gap from
45°N in the Atlantic to the Spanish coast off Barcelona, apparently not at present
passing the Straits of Gibraltar. This study has an added advantage: the authors tell us
that, while almost every single specimen had an at least slightly unique haplotype,
only seven specimens showed a coding difference for the COII protein and in those
cases the same difference. Oddly, those specimens were from both the Black Sea and
the NE Atlantic, at the maximum separation in distance and minimum likelihood of
transfers. An attempt by Peijnenburg et al. to assign separation times to even the most
obvious divisions among subsets of haplotypes fell short of providing close dating,
primarily because the evolutionary rates for genes (the “molecular clock”), even for
presumably selectively neutral, synonymous mutations, remain radically uncertain.
The strongest arguments suggest that a NE Atlantic–Mediterranean split occurred in
S. setosa in early to mid-Pleistocene, forced by northern glaciation. The ancestral
stock transferred south and into the Mediterranean due to cold, then a separation was
forced by drying of the straits. On glacial retreat, the southern stock moved back
toward Britain, and the Mediterranean stock remained east of the straits, likely subject
to subsequent isolation.
Fig. 10.23 Species distribution map and COII (mtDNA) cladogram for Sagitta setosa,
an eastern North Atlantic and Mediterranean/Black Sea chaetognath. Map dots are
sampling sites: A , Adriatic Sea; B, Black Sea; G, Gulf of Gabès; L, Ligurian Sea; N,
Northeast Atlantic; T, Tyrrhenian Sea. Dots in cladogram represent refined branching
to many specimens; 1 represents a distinctive individual.
(^) (After Peijnenburg et al. 2004.)
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