Biological Oceanography

(^) Apart from distribution maps for foraminiferan and other microfossils sedimented
in glacial eras, is there evidence for glacial impacts on pelagic populations? Bucklin
and Wiebe (1998) made the claim that the low diversity of mtDNA in subpolar
zooplankters is in fact an indicator of glacial impact. Among 216 sequences of
mitochondrial SSU ribosomal DNA from Calanus finmarchicus, they found that 79%
were just one haplotype, much lower genetic diversity than expected for a
reproductive population on the order of 10^15 females. Diversity was much lower than
that of subtropical species of the same family with stocks of ∼ 108 females. They
propose that during at least the most recent glacial era the subpolar habitat was both
modified and reduced in area by 75% (based on CLIMAP results), forcing the C.
finmarchicus stock through a population-genetic “bottleneck” Of course, this glacial-
era bottleneck effect could have operated repeatedly through the Pleistocene. Provan
et al. (2010), in contrast found no DNA-sequence evidence for genetically significant
glacial population restriction in Calanus finmarchicus. All of these issues will remain
unsettled; we cannot go back to sample the past. The likelihoods will become clearer
with larger samples, more sequences, and more secure methods in bioinformatics.
(^) It might seem that divergence in haploid mitochondrial genes would not imply
general divergence, since mitochondria are inherited from mothers only and are
strictly clonal organelles. Perhaps having any functional mitochondrion would serve
for oxidative metabolism. That is likely true for the substantial majority of haplotype
differences that do not code for different amino acids; those are simply an indicator of
genetic drift and possibly time of stock separation (evidently substantial between
oceans and northern vs. southern hemispheres). However, mtDNA does acquire non-
synonymous mutations, and likely more rapidly than nuclear DNA. Mitochondria
(and chloroplasts) are not constructed solely from proteins coded in mtDNA, but