include proteins (and protein parts) coded in the nucleus. Such changes in mtDNA
must be matched by changes in nuclear genes. Mismatches are a significant cause of
mating failures in experimental crosses between distant populations (e.g. Burton et al.
2006). Thus, rapidity of mitochondrial evolution is likely a major component in
genetic divergence of allopatric populations. Better phylogeographical insight will
come with study of more genes, particularly developmental and other nuclear genes.
Eventually, molecular sequences are going to give us a blazing spotlight on timing
issues. It’s just not fully powered up, yet.
Coastal Distributions and the Indicator Species
Concept
(^) The strong El Niño events of recent years along the west coast of the Americas
include extensive poleward flow along the Americas that affects the distributions of
plankton. El Niño events are lapses in the trade winds, allowing the east to west
upslope of the tropical Pacific to collapse eastward, moving the boundaries of
distributions rapidly, and changing the composition of the plankton at a given point
along the route. El Niño provides an excellent application of the notion of “indicator
species.”
(^) The idea that planktonic species composition can be informative about water
movements has a long, honorable history. In global perspective, distribution patterns
appear to correspond with the spatial extent of the major gyral circulations of the
oceans. That is established. However, at most coastal sites there is considerable
fluctuation in the pelagic fauna between sampling times, and these fluctuations often
have a rhythm or regular sequence. Unlike a site such as 35°N, 150°W, where a
standard central Pacific gyre fauna is always present, the fauna alongshore varies with
season and with changes in the flow. This was first studied in detail in the vicinity of
the British Isles, for which Russell (1939) suggested the notion that particular forms
were indicative of the sources of the water that happened at any given time to be
lapping the shore.
(^) Russell dealt primarily with species of chaetognath, and he named different water
types after the dominant species of Sagitta (sensu lato) they contained:
“serratodentata water”, “setosa water”, and “elegans water”. These chaetognaths
appear in waters surrounding the British Isles under different conditions of wind and
flow, so he called them “indicator species”. The indicator-species notion must, to be
fully effective, be calibrated by a survey of the distributions on a very large scale.
That has been done, and the following sources have been identified:
(^1) S. serratodentata: a form of the waters well offshore to the west of
Britain, waters of the North Atlantic West Wind Drift, or north branch of the