this relationship further in Chapter 16.
Does Speciation Really Occur in This Way?
(^) Do disjunct populations, or possibly even the distant ends of very extensive conjunct
populations, of animals that obviously are closely related on the basis of morphology
show genetic divergence? If so, how great is the divergence? Some of the cases that
would provide the best answers have yet to be examined by molecular geneticists. For
example, sequences for a suite of genes from the genus Pontellina, discussed above
(Fig. 10.16), and particularly for P. plumata collected in the three oceans, would show
the range of genetic divergence installed by recent evolution in distant, possibly
isolated stocks. However, other cases providing indications of amounts of divergence
have been examined. The study of M. norvegica by Papetti et al. (2005), mentioned
above, is one.
(^) A sequence of studies by Erica Goetze shows that strong divergence occurs. She
began by sequencing several genes from copepods identified initially as Eucalanus
hyalinus (Claus 1866), which is bi-antitropical in the Pacific, subtropical in the
southern Indian Ocean, and lives in all warm-water areas of the Atlantic. She found
that gene differences indicated two quite distinct subsets and initiated a close
morphological inspection of many specimens from across the range. She found two
groups that were separable by a set of subtle morphological distinctions: consistently
different head shape, different asymmetries between the two antennules, different
lengths of the terminal tail segments, and a size difference in adults. The smaller form
was described from the Gulf of Guinea by T. Scott (1894), who gave it the name
Eucalanus spinifer. However, E. spinifer had long since been called a synonym of E.
hyalinus. More extended genetic study confirmed that the two species differed
consistently in the bases of two mitochondrial genes, genes for 16S rRNA and
cytochrome oxidase I (COI), and of nuclear ITS2 (a spacer in the gene for ribosome
structure that does not code rRNA). For COI, the worldwide average within-species
differences were 2.7 of 348 base pairs (bp) for E. hyalinus (N = 450 individuals) and
0.8 of 348 bp for E.spinifer (N = 383), whereas the between-species difference was
36.1 of 348 bp. That level of within vs. between differences is typical, or relatively
large, compared to other pairs of closely related but distinct species. On the basis of
both the morphological and genetic differences, Goetze and Bradford-Grieve (2005)
removed E. spinifer from the limbo of synonymy, declaring it to be a real population
entity living in the ocean.
(^) Goetze (2005) followed that discovery with identification and sequencing studies of
both species from sites across their global range. She found that while they live in
general sympatry, they do have distinct habitat preferences, with E. spinifer more