abundant in the centers of oligotrophic gyres, and E. hyalinus dominant at the gyre
edges and in adjacent upwelling zones. Among 450 sequences for a 349 base-pair
region of COI from individuals of E. hyalinus collected worldwide, she found 239
haplotypes (recall that those differ on average by only 2.7 bases). Those fell into
seven distinct haplotype groups, two of which were strongly dominated by single
haplotypes (coded H161 and H2). The proportions of individuals in each group
differed dramatically among sampling areas, and differed with strong statistical
significance (large values of a proportional distinction measure, Φst, among three
regional clusters (Fig. 10.22): North Pacific, Atlantic, and South Indian–South
Pacific. A sample of 337 E. spinifer sequences showed similarly distinct groups,
except that (i) all sampled areas had substantial numbers of the globally dominant
haplotype (H1), and (ii) haplotype proportions in Atlantic and South Indian stocks
were similar, while North and South Pacific stocks were distinct from each other and
from the Atlantic–Indian group. All of that leaves no doubt that continental and
ocean-circulation barriers restrict gene flow in planktonic populations, and that
genetic distinction does emerge in isolated populations. It can be taken as likely that
the speciation process for both E. hyalinus and E. spinifer is in progress through the
genetic drift and selective differences between ranges – a key step in speciation.
Fig. 10.22 Frequencies of haplotypes of COI in Eucalanus hyalinus. Overall regional
differences are represented by the Φst values (significant values with *) alongside the
interchange arrows, which are darkened in proportion to the likely gene flow.
(After Goetze 2005.)