A

Allele: A single variant or "version" of a gene.

Anterior angle: The "front" half of the frontal plane. Antonymous with posterior.

Apomorphy: A derived trait. Antonymous with plesiomorphy.

Autapomorphy: A derived trait present in and unique to an entire single clade and used to identify members of a clade.

B

Beneficial (allele/trait): Increasing the fitness of its host. Antonymous with deleterious.

C

Clade: Any group containing an ancestor and all of its descendants.

Codon: A base pair triplet that encodes for an α-amino acid.

Convergence/Convergent evolution: Multiple descendants displaying a trait absent in their last common ancestor, usually as a result of similar selection pressures. Produces homoplasy.

D

Deleterious (allele/trait): Decreasing the fitness of the host. Antonymous with beneficial.

Derived: Evolutionary novel or new to the clade of its origin, either in degree or in kind. Antonymous with ancestral.

Directional selection: Selection in favor of one extreme of the variation in a trait. Produces a skewed unimodal distribution.

Disruptive selection: Selection in favor of two extremes of the variation in a trait. Produces a bimodal distribution.

Dorsal angle: The "back" half of the transverse plane. Antonymous with ventral.

E

Evolution: Any change in the heritable traits of a population across multiple generations.

Exon: A nucleotide sequence in a gene that is present in the RNA product during transcription. Compare with intron.

F

Fitness: The likelihood that an organism will be selected for, i.e. enjoy higher reproductive success.

Frontal plane: The plane that divides a body or organ into the "front" (anterior) and the "back" (posterior).

G

Gene: Any locus that encodes wholly or partially for a particular trait.

Genetic drift: Evolution as a result of sampling bias during mating.

H

Hardy-Weinberg Equilibrium (HWE): The state of stasis in the heritable traits of a population across generations in the absence of evolutionary influences such as genetic drift, selection, mutation, gene flow, and bottleneck.

Heterozygous (allele): A Mendelian gene with one dominant and one recessive allele (Aa). Antonymous with homozygous.

Holotype: A specimen or illustration used to describe a taxon. Synonymous with type specimen.

Homology: A trait present in an entire clade due to inheritence. May be a symplesiomorphy or a synapomorphy.

Homoplasy: A trait absent in an ancestor but present in multiple descendants. The product of convergence.

Homozygous: A Mendelian gene with two dominant or two recessive alleles (AA or aa). Antonymous with heterozygous.

Homozygous dominant: A Mendelian gene with two dominant alleles (AA).

Homozygous recessive: A Mendelian gene with two recessive alleles (aa).

I

Intron: A nucleotide sequence in a gene that is spliced from the RNA product during transcription. Compare with exon.

L

Last common ancestor (LCA): The most recent organism that two descendants are directly related to.

Lateral angle: Either side of the sagittal plane.

Linkage: The likelihood of two loci being on the same chromosome after recombination.

Locus (pl. loci): Any point on a chromosome at least one base pair long.

M

Medial: The center of the sagittal plane.

Monotypic (taxon): A taxon with only one immediately subordinate taxon. Compare with polytypic.

N

Negative selection: Selection against a particular trait, i.e. a trait decreasing the host's fitness. Compare with positive selection.

Neutral (allele/trait): Having no effect on the fitness of its host.

P

Plesiomorphy: An ancestral trait. Antonymous with apomorphy.

Polytypic (taxon): A taxon with multiple immediately subordinate taxa. Compare with monotypic.

Positive selection: Selection in favor of a particular trait, i.e. a particular trait increasing the host's fitness. Compare with negative selection.

Posterior angle: The "back" half of the frontal plane. Antonymous with anterior.

S

Sagittal plane: The plane that divides a body or organ into "left" or "right" (lateral) and "middle" (medial).

Selection: Any difference in the reproductive success of organisms caused by variation in traits.

Selection pressure: The degree of difference between the reproductive success of organisms caused by variation in traits.

Senescence: The gradual deterioration of functional traits; aging.

Sexual selection: Selection caused by nonrandom mating.

Stabilizing selection: Selection in favor of the moderate value of the variation in a trait. Produces a narrower normal distribution.

Symplesiomorphy: A derived trait present in but not unique to an entire clade.

Synapomorphy: A derived trait present in and unique to an entire clade split into multiple lower clades.

T

Taxon (pl. taxa): A taxonomic group. Ideally, but not always, representative of a clade.

Transcription: The process of converting a DNA sequence into an RNA sequence.

Translation: The process of converting an RNA sequence into a protein.

Transverse plane: The plane that divides a body or organ into "back" (dorsal) and "belly" (ventral).

Type specimen: A specimen or illustration used to describe a taxon. Synonymous with holotype.

V

Ventral angle: The "belly" half of the transverse plane. Antonymous with dorsal.