imposed on high-latitude systems by glacial–interglacial cycling. Plankton community
structure in higher latitudes also tends to be more strongly dominated by only a few
species, although the dominant forms may shift with season. Aspects of regional
distinctions in community processes are covered in the next chapter.
Speciation in Pelagic Habitats
(^) Charles Darwin’s seminal book, The Origin of Species, considered how a species
would evolve into a substantially different form (a new species) over time, with little
attention to how one species evolves into two or more. Later work and thinking have
shown that the majority of subdivisions into new species occur by an allopatric
process. The term derives from Latin for “different country”, and the process is also
referred to as geographical speciation. There are a number of sympatric (“same
country”) speciation mechanisms, but they are harder to demonstrate and certainly
operate less often. Allopatric speciation has three steps:
(^1) The geographical range of a species, throughout which there is consistent
exchange of mating individuals and thus “gene flow”, becomes divided into
two or more smaller ranges by a new, strong barrier to transit by the
organism. For example, a distribution on two mountain ridges and
contiguous across the valley between can divide onto the two ridges by
climatic change making the valley uninhabitably warm or dry.
2 Both genetic drift and differences among the new ranges in selective
advantages cause the genetics of the populations to diverge. This must reach
sufficient levels of difference that matings between individuals from
different ranges are disadvantaged, producing fewer or less fit offspring.
3 The barrier breaks down, the ranges again become contiguous, and
opportunities for breeding with individuals from other stocks become
frequent. However, because these outcrosses produce fewer or less fit
offspring, there will be a new selective advantage to any trait that allows
mate selection according to origin, causing like to favor like. Such traits are
often differences in mating ritual or in form of the copulatory apparatus,
particularly the latter in arthropods.
(^) The last fact explains why crustacean (and insect) taxonomists distinguishing
species spend concentrated effort examining and describing sex organs. An alternative
outcome of rejoining previously disjunct distributions is that matings between
members of the different populations work well or even benefit from a version of
“hybrid vigor”, the young having high proportions of heterozygous genes. After
recombination and selection over multiple generations, a markedly adaptive suite of
genes may spread through both original ranges. In these cases, effects from the period