The building blocks of DNA. A chain of nucleotides forms DNA. Nucleotides are made of a sugar, a phosphate, and a base. See also base.

A flexible rod running the length of a chordate, providing structural support. The notochord is one of the inherited characteristics shared by all chordates.

The pattern of phenotypic plasticity for a particular genotype. A norm of reaction describes the way in which a genotype is expressed as a trait under different environmental circumstances. For example, for a particular plant genotype that affects height, the norm of reaction in relation to watering level might look like a bell curve: very small and very large amounts of watering result in shorter plants and normal amounts of watering result in taller plants. For plant with a different genotype, the ideal amount of water to grow a tall plant might be different because this genotype might have a different norm of reaction.

A mating system in which at least some individuals are more or less likely to mate with individuals of a particular genotype than with individuals of other genotypes.

In ecology, the part of the environment occupied by a particular species along with the resources it uses and produces. A species’ niche includes such factors as energy consumed, time of consumption, space occupied, temperature required, mode of reproduction, and behavior.

The idea that most of the molecular variation within populations is not being selected for or against — it is just neutral variation “drifting” around. The neutral theory de-emphasizes the role of natural selection in explaining molecular variation and emphasizes the importance of mutation and genetic drift. For a more detailed explanation, see our resource on neutral theory in Evolution 101.

(nebulae — pl.) A large cloud of gas and dust in space. Star formation takes place when clumps of matter in a nebula collapse to form stars. These new stars may be associated with protoplanetary disks, which go on to form solar systems.

A point on a phylogeny where a single ancestral lineage breaks into two or more descendent lineages.

Differential survival or reproduction of different genotypes in a population leading to changes in the gene frequencies of a population. The conditions required for the operation of evolution by natural selection include variation, a system of heredity, differential reproduction, and time. For a more detailed explanation, see our resource on natural selection in Evolution 101.