An equation proposed by Dr. Frank Drake to estimate the number of intelligent civilizations in our Galaxy. The equation multiplies together factors such as the rate of star formation in the Galaxy, the fraction of stars with planets, and the fraction of planets that are habitable. The values of many of the parameters are currently highly uncertain, but some (such as the fraction of stars with planets) are increasingly well known.

Gene version with an effect that is observed even when paired with a non-identical gene version in the same individual.

In biology, a measure of the variety of the Earth’s animal, plant, and microbial lineages. Different measures of biological diversity (biodiversity) include number of species, number of lineages, variation in morphology, or variation in genetic characteristics.

The process of lineage splitting in which one ancestral lineage becomes two or more descendent lineages. In this process, unique traits are likely to evolve in each of the descendent lineages. These traits help us identify the descendent lineages as distinct.

A process in which a species’ range changes because some or all individuals move to a new location. Dispersal is usually contrasted with vicariance as a biogeographic mechanism.

The hypothesis that mutations that are useful under particular circumstances are more likely to happen if the organism is actually in those circumstances. In other words, the idea that mutation is directed by what the organism needs. There is little evidence to support this hypothesis.

Individual or cell that carries two sets of its chromosomes. Humans are diploid: we carry two copies of each of our 22 regular chromosomes, plus two sex chromosomes (either two Xs or an X and a Y).

(1848-1935)
Dutch botanist famous for his contributions to genetics. He rediscovered the results first obtained by Mendel and described genetic changes in his plants. Based on his observations, DeVries argued that individual mutations had wide-ranging effects and could cause speciation in a single step; however, T. H. Morgan later discovered that many mutations seemed to have rather small effects. DeVries, it turns out, had observed changes in chromosome number, not the minor change in base pair sequence that are typical of mutation.

Change in an organism over the course of its lifetime; the processes through which a zygote becomes an adult organism and eventually dies.

A term borrowed from engineering indicating failure to realize an “ideal” design because improving performance in one area entails decreasing performance in another area or because of another constraint on the design. Sometimes this term is applied to biological features even though these traits evolved and were not designed. In terms of evolution, a “design” compromise indicates a constraint or trade-off during a feature’s evolution. For more details, read about why evolved traits may not be perfectly “engineered” in Evolution 101.

See evolution.

Form of a trait that evolved from an ancestral form. For example, if a light-colored species of pocket mouse begins living on the dark-colored rocks of an ancient lava flow and evolves a darker coat color, we would consider dark fur to be the derived form of the trait and light fur to be the ancestral form.

The character state present in a lineage immediately after a character state change.

A version of a gene that, on average, decreases the fitness of the organism carrying it.

In phylogenetics, a data set that consists of a list of taxa and a set of those taxa’s attributes (called characters and character states). Data matrices are used to build evolutionary trees. For example, a data matrix for a beetle phylogeny might consist of a list of beetle species and would specify many traits of those species—how many antennal segments each has, which have spotted wings and which have solid wings, etc. Data matrices can also include the genetic sequence for a particular gene or part of the genome.

Process in which the random movement of molecules causes different types of molecules to mix, moving from regions of higher concentration to regions of lower concentration and eventually becoming evenly distributed.

A situation in which some individuals leave more offspring in the next generation than do others, often due to traits that confer advantages in survival and/or reproduction.

Deoxyribonucleic acid, the molecule that carries genetic information from generation to generation. For a more detailed explanation, see our resource on DNA in Evolution 101.