Selection coefficient
The difference between two fitness values, representing a relative measure of selection
against an inferior genotype.
Selective permeability
A property of biological membranes that allows some substances to cross more easily than
others.
Selective pressure
An environmental factor that favors the survival and reproduction of those genetic variants
within a population that are better adapted to the environment.
Self-incompatibility
The capability of certain flowers to block fertilization by pollen from the same or a closely
related plant.
Self-fertilization
The union of egg and sperm produced by a single hermaphroditic organism.
Self-pollination
The transfer of pollen from anther to stigma in the same flower or to another flower of the
same plant, leading to self-fertilization.
Semen
The fluid that is ejaculated by the male during orgasm; contains sperm and secretions from
several glands of the male reproductive tract.
Semicircular canals
A three-part chamber of the inner ear that functions in maintaining equilibrium.
Semilunar valve
A valve located at the two exits of the heart, where the aorta leaves the left ventricle and
the pulmonary artery leaves the right ventricle.
Seminal vesicle
A gland in males that secretes a fluid (a component of semen) that lubricates and nourishes
sperm.
Seminiferous tubules
Highly coiled tubes in the testes in which sperm are produced.
Sensation
An impulse sent to the brain from activated receptors and sensory neurons.
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