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S

scientific method

The process of scientific inquiry for investigating phenomena, acquiring new knowledge, and correcting and integrating previous knowledge. To be called scientific, a method must be based on observable, measurable evidence collected by observation and/or experimentation.

sexual dimorphism

The two sexes of a species having different shapes, sizes, etc., from each other.

sexual selection

A type of selection in which the forces determined by mate choice act to cause one genotype to mate more frequently than another genotype.

siphons

Posterior extensions (usually two) of the mantle through which water is directed in and out of the body, along with waste products and gametes of bivalves. (adj. siphonal)

speciation

The process in which one species evolves over time into a different species (anagenesis) or in which one species diverges to become two or more species (cladogenesis); see also allopatric speciation, parapatric speciation, peripatric speciation, sympatric speciation.

species

A group of organisms formally recognized as distinct from other groups; the taxon rank in the hierarchy of biological classification below genus; the basic unit of biological classification, defined by the reproductive isolation of the group from all other groups of organisms.

species richness

The total number of species, or biodiversity, in a given geographical area.

stabilizing selection

A type of selection that removes individuals from both ends of a phenotypic distribution, thus maintaining the same distribution mean.

suspension feeding

Feeding type of most bivalves during which organic particles are harvested from the water column.

symbiosis

A long relationship between two different species; see also mutualism, commensalism, parasitism.

sympatric speciation

Speciation in which the evolution of reproductive isolating mechanisms occurs within the range and habitat of the parent species.