Chapter 10
Biogeography of pelagic habitats
Throughout the history of biology, ideas of broad scope have been derived from
studies of the distribution patterns of organisms upon the Earth. Darwin and Wallace
were biogeographers, and many evolutionary mechanisms have been discovered
through comparison of the distributions of closely related species and of subspecies.
Because of this record of success for biogeography, biological oceanographers have
determined distributional patterns for a modest fraction of the oceanic biota,
particularly epipelagic zooplankton. As expected, the patterns suggest hypotheses
about the history and ecology of the sea. Some hypotheses have been tested by
examination of ancient patterns of distribution preserved in ocean sediments.
What is a “Species”?
(^) The basic unit in the study of distribution is the species. The notion of species has a
long – and sometimes contentious – history that cannot be reviewed here. At present,
several species concepts are in use. There is the “biological species” concept: a
species is an interbreeding, or potentially interbreeding (if individuals could be moved
over some barrier to mate), population of organisms. They are sufficiently similar to
be inter-fertile. In the cases of Homo sapiens and other domestic animals, we have a
great deal of direct experimentation with the question of inter-fertility. We have tried
most of the possible crosses, and we know which work and which do not. However,
for the typical marine animal or alga this experience is missing. We don’t know which
deep-sea fishes would be successful at mating, if they tried it. We have no operational
way to decide which variation is intraspecific and which interspecific according to the
biological species definition. The typical plankton or benthos sample is a jar of dead
bodies. It contains no information about inter-fertility. In this case, a species,
sometimes called a “morpheme”, is a group of organisms that share a great many
characteristics, usually aspects of body form. Morphemes are separated by gaps in a
continuum of degree of overall similarity. The degree of alikeness can be evaluated by
sophisticated numerical techniques or by the common sense of experienced
systematists. It is actually “species” described in the latter, somewhat subjective,
mode that make up the vast majority of those we recognize. As an old saw goes, “a
species is whatever a competent, recognized systematist says it is”.