Meiosis thus ensures genetic variability in the gametes, and therefore in the offspring, resulting from their subsequent fusion. Sexual reproduction, unlike  asexual reproduction, therefore generates variability within a species. However, it depends on there being reliable means of bringing together male and female gametes, and many elaborate mechanisms have evolved to ensure this.

Sexual selection. The means by which it is assumed that certain secondary sexual characteristics, particularly of male animals, have evolved. Females presumably choose to mate with the male that gives the best courtship display and therefore has the brightest coloration, etc. These features would be inherited by its male offspring and would thus tend to become exaggerated down the generations.

Shoot. That part of a vascular plant which is above the ground consisting of stem and leaves.

Shoulder girdle (Pectoral girdle). Skeletal support in body wall for attachment of front fins or limbs of vertebrate. Consists primitively of a curved bar of cartilage or bone.

Sigma factor. A sub-unit of RNA polymerase that determines the site where transcription begins.

Sinuate. Having a wavy outline.

Sinus. (Bot). A space. Recess between two lobes of a leaf or other expanded organ. (Zool.). Blood-sinus, enormously expanded vein, found particularly in elasmobranches. Applied also to haemocoel cavity of certain invertebrates.

Nasal sinus, in mammals, air-filled space within certain bones of face, lined by mucous membrane, communicating with nasal cavity.