directional selection The type of selection that moves a character toward one of its extremes. This process could lead to taller and taller redwood trees.
disruptive selection The type of selection that moves a character toward both of its extremes, operating against individuals with the average for that character. This type of selection seems to be less common in nature than either stabilizing or directional selection.
distal tubule In the nephron tubule of the kidney, the convoluted portion that lies between the loop of Henle and the collecting duct. Some reabsorption of salt and water occurs here.
DNA polymerase An enzyme that is active in DNA replication, separating strands of DNA, bringing bases to the parental strands, and correcting errors by removing and replacing incorrect base pairs.
domain A major taxonomic grouping of organisms. There are only three domains: Archaea, Bacteria, and Eukarya.
dominance hierarchy A persistent power ranking in an animal population that gives those of higher rank the ability to control some aspect of the behavior of those of lower rank. This control often results in higher-ranking individuals gaining better access to such resources as food and mates.
dominant Expressed in the heterozygous condition. When an organism possesses two different alleles of the same gene, but only one allele of the two is expressed, that allele is said to be dominant.
dominant disorder Genetic conditions in which a single faulty allele can cause damage, even when a second, functional allele exists.
dormancy A state in which growth is suspended and there is a prolonged low level of metabolic activity. Dormancy allows organisms to conserve energy during times of unfavorable environmental conditions.
dorsalnerve cord A rod-shaped dorsal structure consisting of nerve cells, running from the chordate animal's head to its tail.
dorsal root ganglion The collection of cell bodies of the sensory neurons that send their axons into the spinal cord through the dorsal root, carrying sensory information into the central nervous system.
double bond Type of covalent bond formed when two pairs of electrons are shared between atoms.
double fertilization In plants, the fusion of sperm with the egg and with the central cell at the same time, one resulting in the zygote, the other in endosperm to provide food for the embryo. Double fertilization occurs almost exclusively in flowering plants.
Down syndrome A disorder in humans in which affected individuals usually have three copies of chromosome 21. Individuals with this syndrome have short stature, shortened life span, and low IQ.
duodenum The portion of the small intestine in vertebrates that is closest to the stomach.
