Home :: Glossary

Glossary

Click on a letter below to jump to it in the glossary. Letters that are not linked have no terms starting with that letter.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

A

absolute fitness: The per capita growth rate of a genotype.
adaptation: A process of genetic change in a population whereby, as a result of natural selection, the average state of a character becomes improved with reference to a specific function, or whereby a population is thought to have become better suited to some feature of its environment. Also, an adaptation: a feature that has become prevalent in a population because of a selective advantage conveyed by that feature in the improvement in some function. A complex concept; see Chapter 11.
adaptive landscape: A plot of mean fitness against allele frequency, representing a “surface” or “hillside.”
adaptive peak: That allele frequency, or combination of allele frequencies at two or more loci, at which the mean fitness of a population has a (local) maximum. Also, the mean phenotype (for one or more characters) that maximizes mean fitness. An adaptive valley is a set of allele frequencies at which mean fitness has a minimum.
adaptive radiation: Evolutionary divergence of members of a single phylogenetic lineage into a variety of different adaptive forms; usually the taxa differ in the use of resources or habitats, and have diverged over a relatively short interval of geological time. The term evolutionary radiation describes a pattern of rapid diversification without assuming that the differences are adaptive.
adaptive zone: A set of similar ecological niches occupied by a group of (usually) related species, often constituting a higher taxon.
additive effect: The magnitude of the effect of an allele on a character, measured as half the phenotypic difference between homozygotes for that allele compared with homozygotes for a different allele.
additive genetic variance: That component of the genetic variance in a character that is attributable to additive effects of alleles.
additive inheritance: Inheritance in which the heterozygote’s phenotype is precisely intermediate between those of the homozygotes.
allele: One of several forms of the same gene, presumably differing by mutation of the DNA sequence. Alleles are usually recognized by their phenotypic effects; DNA sequence variants, which may differ at several or many sites, are usually called haplotypes.
allele frequency: The proportion of gene copies in a population that are a given allele; i.e., the probability of finding this allele when a gene is taken randomly from the population; also called gene frequency.
allochthonous: In reference to a taxon of a geographical region, meaning it originated elsewhere. Cf. autochthonous.
allometric growth: Growth of a feature during ontogeny at a rate different from that of another feature with which it is compared.
allopatric speciation: The evolution of reproductive barriers in populations that are prevented by a geographic barrier from exchanging genes at more than a negligible rate.
allopatric: Of a population or species, occupying a geographic region different from that of another population or species. Cf. parapatric, sympatric.
allopolyploid: A polyploid in which the several chromosome sets are derived from more than one species.
allozyme: One of several forms of an enzyme encoded by different alleles at a locus.
alternative splicing: Splicing of different sets of exons from mRNA to form mature transcripts that are translated into different proteins (thus allowing the same gene to encode different proteins).
altruism: Conferral of a benefit on other individuals at an apparent cost to the donor.
anagenesis: Evolution of a feature within a lineage over an arbitrary period of time.
ancestral character state: The character state originally present in the ancestor of a given lineage. It may be retained or changed in descendants of that ancestor.
aneuploid: Of a cell or organism, possessing too many or too few of one or more chromosomes.
antagonistic selection: A source of natural selection that opposes another source of selection on a trait.
apomixis: Parthenogenetic reproduction in which an individual develops from one or more mitotically produced cells that have not experienced recombination or syngamy.
apomorphic: Having a derived character or state, with reference to another character or state. See synapomorphy.
aposematic: Coloration or other features that advertise noxious properties; warning coloration.
arithmetic mean: See mean.
artificial selection: Selection by humans of a deliberately chosen trait or combination of traits in a (usually captive) population; differing from natural selection in that the criterion for survival and reproduction is the trait chosen, rather than fitness as determined by the entire genotype.
asexual: Pertaining to reproduction that does not entail meiosis and syngamy.
associative overdominance: The apparent but erroneous superiority of a heterozygote at an observed locus resulting from masking of a recessive deleterious allele. See overdominance.
assortative mating: Nonrandom mating on the basis of phenotype; usually refers to positive assortative mating, the propensity to mate with others of like phenotype.
autapomorphy: A derived character that is restricted to a single lineage.
autochthonous: In reference to the taxon of a geographical region, meaning it originated within the region. Cf. allochthonous.
autopolyploid: A polyploid in which the several chromosome sets are derived from the same species.
autosome: A chromosome other than a sex chromosome.

B

back mutation: Mutation of a “mutant” allele back to the allele (usually the wild type) from which it arose. Usually detected by its phenotypic effect.
background extinction: A long-prevailing rate at which taxa become extinct, in contrast to the highly elevated rates that characterize mass extinction.
background selection: Elimination of deleterious mutations in a region of the genome; may explain low levels of neutral sequence variation.
balancing selection: A form of natural selection that maintains polymorphism at a locus within a population.
base pair substitution: See substitution (base pair or amino acid).
base pairs (bp): In nucleic acids, the pairs of purines (adenine, A, or guanine, G) and pyrimidines (thymine, T, or cytosine, C) that make up a genome’s genetic material.
Bauplan (pl., Baupläne): The body plan of an animal.
behavioral isolation: Reduction in the frequency of successful matings between individuals in separate populations of a species as a result of behavioral differences. Also called sexual isolation or ethological isolation.
benthic: Inhabiting the bottom, or substrate, of a body of water. Cf. planktonic.
biodiversity: Biological diversity, typically measured by species richness, or the number of species found in a given area.
biogeographic realm: One of several defined, continental-scale regions of Earth, each of which has a biota distinct from that of the others.
biogeography: The study of the geographic distribution of organisms.
biological homology concept: An evolutionary concept based on the realization that homology must take into account information on the developmental genetics of morphological traits.
biological species: A population or group of populations within which genes are actually or potentially exchanged by interbreeding, and which are reproductively isolated from other such groups.
bottleneck: A severe, temporary reduction in population size.

C

Cambrian explosion: The rapid diversification of animal phyla that took place during the Cambrian period.
canalization: The evolution of internal factors during development that reduce the effect of perturbing environmental and genetic influences, thereby constraining variation and consistently producing a particular (usually wild-type) phenotype.
candidate gene: A gene thought to be involved in the evolution of a particular trait based on its mutant phenotype or the function of the protein it encodes.
carrying capacity: The population density that can be sustained by limiting resources.
category: In taxonomy, one of the ranks of classification (e.g., genus, family). Cf. taxon.
character: A feature, or trait. Cf. character state.
character displacement: Usually refers to a pattern of geographic variation in which a character differs more greatly between sympatric than between allopatric populations of two species; sometimes used for the evolutionary process of accentuation of differences between sympatric populations of two species as a result of the reproductive or ecological interactions between them.
character state: One of the variant conditions of a character (e.g., yellow versus brown as state of the character “color of snail shell”).
chimeric gene: A gene that consists of pieces derived from two or more different ancestral genes.
chronospecies: A segment of an evolving lineage preserved in the fossil record that differs enough from earlier or later members of the lineage to be given a different binomial (name). Not equivalent to biological species.
cis-regulatory element: A noncoding DNA sequence in or near a gene required for proper spatiotemporal expression of that gene, often containing binding sites for transcription factors. Often used interchangeably with enhancer.
clade: The set of species descended from a particular ancestral species.
cladistic: Pertaining to branching patterns; a cladistic classification classifies organisms on the basis of the historical sequences by which they have diverged from common ancestors.
cladogenesis: Branching of lineages during phylogeny.
cladogram: A branching diagram depicting relationships among taxa; i.e., an estimated history of the relative sequence in which they have evolved from common ancestors. Used by some authors to mean a branching diagram that displays the hierarchical distribution of derived character states among taxa.
cline: A gradual change in an allele frequency or in the mean of a character over a geographic transect.
clone: A lineage of individuals reproduced asexually, by mitotic division.
coadapted gene pool: A population or set of populations in which prevalent genotypes are composed of alleles at two or more loci that confer high fitness in combination with each other, but not with alleles that are prevalent in other such populations.
coalescence: Derivation of the gene copies in one or more populations from a single ancestral copy, viewed retrospectively (from the present back into the past).
codon: A triplet of nucleotides in mRNA that direct the placement of a particular amino acid into a polypeptide chain.
codon bias: Non-random usage of alternative codons that encode the same amino acid.
coefficient of relationship: In altruism, the fraction of the donor’s genes that are identical by descent to any of the recipient’s genes.
coefficient of selection: The amount by which the fitness of a genotype differs from that of a reference genotype.
coevolution: Strictly, the joint evolution of two (or more) ecologically interacting species, each of which evolves in response to selection imposed by the other. Sometimes used loosely to refer to evolution of one species caused by its interaction with another, or simply to a history of joint divergence of ecologically associated species.
commensalism: An ecological relationship between species in which one is benefited but the other is little affected.
common ancestor: An ancestor shared by two or more descendants or groups of descendants.
common garden: A place in which (usually conspecific) organisms, perhaps from different geographic populations, are reared together, enabling the investigator to ascribe variation among them to genetic rather than environmental differences. Originally applied to plants, but now more generally used to describe any experiment of this design.
comparative genomics: The comparative study of whole genomes.
comparative method: A procedure for inferring the adaptive function of a character by correlating its states in various taxa with one or more variables, such as ecological factors hypothesized to affect its evolution.
compartment: A contiguous group of cells, descended from the same progenitor cell, that form a spatially discrete part of a developing organ or structure and often act as a discrete developmental unit. Cells from one compartment typically do not intermix with cells from other compartments.
competition: An interaction between individuals of the same species or different species whereby resources used by one are made unavailable to others.
competitive exclusion: Extinction of a population due to competition with another species.
components of fitness: Any of the factors that contribute to a genotype’s fitness.
concealed genetic variation: Genetic variation not revealed in an organism’s phenotype; for example, genetic variation represented by a rare recessive allele.
concerted evolution: Maintenance of a homogeneous nucleotide sequence among the members of a gene family, which evolves over time.
condition-dependent indicator: A trait correlated with genetic quality and dependent on an organism’s overall vigor or condition.
conservative characters: Characters retained with little or no change over long periods among the many descendants of an ancestor; for example, the pentadactyl limb, which first evolved in early amphibians and is retained in humans.
conspecific: Belonging to the same species.
constraints: Factors that limit evolution; they may be physical, functional, genetic, or developmental.
control regions (enhancers and repressors): Areas on a protein-encoding gene that regulate transcription of one strand of the gene into RNA.
cooperation: Activity that benefits both the actor and other individuals.
convergent evolution (convergence): Evolution of similar features independently in different evolutionary lineages, usually from different antecedent features or by different developmental pathways.
co-option: The evolution of a function for a gene, tissue, or structure other than the one it was originally adapted for. At the gene level, used interchangeably with recruitment and, occasionally, exaptation.
Cope’s rule: The tendency observed in some taxa (e.g., horses) for body size to increase over evolutionary time.
copy number variants: Variations in the copy number of chromosomal segments.
correlated selection: Selection that favors some combination of genetically independent character states over others, usually because the characters are functionally related.
correlation: A statistical relationship that quantifies the degree to which two variables are associated. For phenotypic correlation, genetic correlation, environmental correlation as applied to the relationship between two traits, see Chapter 13.
cost: A reduction in fitness caused by a correlated effect of a feature that provides an increment in fitness (i.e., a benefit).
cost of reproduction: A reduction in fitness (e.g., decreased survival or growth) caused by reproductive effort.
cost of sex: A reference to the fact that, all else being equal, the rate of increase of an asexual genotype is approximately twice as great as that of a sexual genotype.
creationism: The doctrine that each species (or perhaps higher taxon) was created separately, essentially in its present form, by a supernatural Creator.
creationist movement: A movement in the United States and a few other countries that opposes the teaching of evolution in public schools, or at least demands equal time for teaching creationism.
creationists: Those who believe in creationism.
crown group: A group of species with distinctive derived characters that is descended from a stem group; for example, the class Aves, or birds.
C-value paradox: The lack of correlation between the DNA content of eukaryotic genomes and a given organism’s phenotypic complexity (i.e., the genome of a less complex eukaryotic organism, such as a plant, may contain far more DNA than that of a more complex organism, such as a human being). The paradox is explained by the amount of noncoding repetitive DNA sequences in a genome.

D

deme: A local population; usually, a small, panmictic population.
demographic: Pertaining to processes that change the size of a population (i.e., birth, death, dispersal).
density-dependent: Affected by population density.
derived character (state): A character (or character state) that has evolved from an antecedent (ancestral) character or state.
descent with modification: The Darwinian theory that all species, living and extinct, have descended, without interruption, from one or a few original forms of life. One of the two major theses of Darwin’s Origin of Species.
deterministic: Causing a fixed outcome, given initial conditions. Cf. stochastic.
developmental circuits (= developmental pathways): The hierarchies, or networks, in which genes that regulate morphogenesis function.
differential gene expression: Differences in the time, location, and/or quantitative level at which a gene expresses the protein it encodes. Differential gene expression involves differences between species, developmental stages, or physiological states in the specific cells, tissues, structures, or body segments that express a given gene; it is believed to be a significant agent of morphological change over evolutionary time.
dioecious: Referring to organisms in which eggs and sperm are not produced in the same individuals.
diploid: Of a cell or organism, possessing two chromosome complements. See also haploid, polyploid.
direct development: A life history in which the intermediate larval stage is omitted and development proceeds directly from an embryonic form to an adult-like form. Cf. indirect development.
directional selection: Selection for a value of a character that is higher or lower than its current mean value.
disjunct distribution: A geographical distribution that has a gap in it.
disparity
dispersal: In population biology, movement of individual organisms to different localities; in biogeography, extension of the geographic range of a species by movement of individuals.
disruptive selection: Selection in favor of two or more modal phenotypes and against those intermediate between them; also called diversifying selection.
divergence: The evolution of increasing difference between lineages in one or more characters.
diversification: An evolutionary increase in the number of species in a clade, usually accompanied by divergence in phenotypic characters.
diversification rate: In mathematical modeling of the rate of change in taxonomic diversity, the number of new taxa that originate minus the number of taxa that become extinct during a given interval.
diversifying selection: See disruptive selection.
diversity-dependent factor: A factor that affects changes in the number of species or higher taxa by reducing origination rates or increasing extinction rates as the number of species increases.
Dobzhansky-Muller incompatibility: Epistatic incompatibility between loci.
domain: A relatively small protein segment or module (100 amino acids or less) that can fold into a specific three-dimensional structure independently of other domains.
domain accretion: The production of new genes by the addition of domains to the beginnings or ends of ancestral genes.
dominance: Of an allele, the extent to which it produces when heterozygous the same phenotype as when homozygous. Of a species, the extent to which it is numerically (or otherwise) predominant in a community.
driven trend (= active trend): See trend.
duplication: The production of another copy of a locus (or other sequence) that is inherited as an addition to the genome.

ecological biogeography: The study of the geographic distributions of organisms by focusing on ecological factors. Complements historical biogeography.

F

F₂ breakdown: The manifestation of hybrid sterility and inviability in F₂ hybrids among different geographic populations of the same species.
fact: A hypothesis that has become so well supported by evidence that it is accepted as true.
fecundity: The quantity of gametes (usually eggs) produced by an individual.
fission (of chromosomes): The process whereby a chromosome splits, resulting in an additional chromosome in the genome. One of the mutational foundations for the evolution of chromosome number. Cf. fusion (of chromosomes).
fitness: The success of an entity in reproducing; hence, the average contribution of an allele or genotype to the next generation or to succeeding generations. See also relative fitness.
fixation: Attainment of a frequency of 1 (i.e., 100 percent) by an allele in a population, which thereby becomes monomorphic for the allele.
founder effect: The principle that the founders of a new population carry only a fraction of the total genetic variation in the source population.
founder effect speciation: See peripatric speciation.
frameshift mutation: An insertion or deletion of base pairs in a translated DNA sequence that alters the reading frame, resulting in multiple downstream changes in the gene product.
frequency: In this book, usually used to mean proportion (e.g., the frequency of an allele is the proportion of gene copies having that allelic state).
frequency-dependent selection: A mode of natural selection in which the fitness of each genotype varies as a function of its frequency in the population.
function: The way in which a character contributes to the fitness of an organism.
functional constraint: In reference to a gene, an interaction that reduces the rate of a protein’s evolution.
fusion (of chromosomes): The process whereby two chromosome fuse, resulting in one less chromosome in the genome. One of the mutational foundations for the evolution of chromosome number. Cf. fission (of chromosomes).

G

G matrix: A table of values showing additive genetic variance and covariance between characters. Also called the genetic variance-covariance matrix, or simply G.
gene: The functional unit of heredity. A complex concept.
gene complex: A group of two or more genes that are members of the same family and in most cases are located in close proximity to one another in the genome, often in tandem separated by various amounts of intergenic, noncoding DNA.
gene conversion: A process involving the unidirectional transfer of DNA information from one gene to another. In a typical conversion event, a gene or part of a gene acquires the same sequence as the other allele at that locus (intralocus or intra-allelic conversion), or the same sequences as a different, usually paralogous, locus (interlocus conversion). One consequence of gene conversion may be the homogenization of sequences among members of a gene family.
gene dispensability: The relationship between the elimination of a particular gene and its genome’s fitness.
gene duplication: When new genes arise as copies of preexisting gene sequences. The result can be a gene family.
gene family: Two or more loci with similar nucleotide sequences that have been derived from a common ancestral sequence.
gene flow: The incorporation of genes into the gene pool of one population from one or more other populations.
gene-for-gene interactions: A mechanism for resistance to pathogens, in which a host’s resistance is triggered by the interaction of genes in the host and pathogen.
gene frequency: See allele frequency.
gene pool: The totality of the genes of a given sexual population.
genetic assimilation: The process of a character state that initially developed in response to the environment becoming genetically determined.
genetic code: The set of instructions, in the form of nucleotide triplets, that translate a linear sequence of nucleotides in mRNA into a linear sequence of amino acids in a protein.
gene tree: A diagram representing the history by which gene copies have been derived from ancestral gene copies in previous generations.
genetic conflict: Antagonistic fitness relationships between alleles at different loci in a genome.
genetic correlation: Correlated differences among genotypes in two or more phenotypic characters, due to pleiotropy or linkage disequilibrium.
genetic distance: Any of several measures of the degree of genetic difference between populations, based on differences in allele frequencies.
genetic drift: Random changes in the frequencies of two or more alleles or genotypes within a population.
genetic load: Any reduction of the mean fitness of a population resulting from the existence of genotypes with a fitness lower than that of the most fit genotype.
genetic variance: Variation in a trait within a population, as measured by the variance that is due to genetic differences among individuals.
genic selection: A form of selection in which the single gene is the unit of selection, such that the outcome is determined by fitness values assigned to different alleles. See individual selection, kin selection, natural selection.
geological time scale: A chronological model showing the major events of Earth’s history. Times on the scale are approximate, and subject to revision as more information accumulates.
genome: The entire complement of DNA sequences in a cell or organism. A distinction may be made between the nuclear genome and organelle genomes, such as those of mitochondria and plastids.
genotype: The set of genes possessed by an individual organism; often, its genetic composition at a specific locus or set of loci singled out for discussion.
genotype × environment interaction: Phenotypic variation arising from the difference in the effect of the environment on the expression of different genotypes.
genotype frequency: The proportion of a population that has a certain genotype.
geographic variation: Differences among spatially distributed populations of a species.
Gondwana: The Southern Hemisphere supercontinent that separated from Pangaea during the Jurassic. Present-day remnants are South America, Africa, India, Australia, New Zealand, and Antarctica.
grade: A group of species that have evolved the same state in one or more characters and typically constitute a paraphyletic group relative to other species that have evolved further in the same direction.
gradualism: The proposition that large differences in phenotypic characters have evolved through many slightly different intermediate states.
group selection: The differential rate of origination or extinction of whole populations (or species, if the term is used broadly) on the basis of differences among them in one or more characteristics. See also interdemic selection, species selection.
guild coevolution (= diffuse coevolution): Evolution of several species whose effects on one another are not independent; for example, evolution of a prey species and two or more of its predators.

H

habitat selection: The capacity of an organism (usually an animal) to choose a habitat in which to perform its activities. Habitat selection is not a form of natural selection.
habitat tracking: Shifting of the geographic distribution of a species in concert with the distribution of its typical habitat.
Haldane’s rule: The generalization that when there is reduced fitness of hybrids and only one sex is affected, it is usually the heterogametic sex.
Hamilton’s rule: The principal that an altruistic trait can increase in frequency if the benefit received by the donor’s relatives, weighted by their relationship to the donor, exceeds the cost of the trait to the donor’s fitness.
haploid: Of a cell or organism, possessing a single chromosome complement, hence a single gene copy at each locus.
haplotype: A DNA sequence that differs from homologous sequences at one or more base pair sites.
Hardy-Weinberg: Pertaining to the genotype frequencies expected at a locus under ideal equilibrium conditions in a randomly mating population.
heritability: The proportion of the variance in a trait among individuals that is attributable to differences in genotype. Heritability in the narrow sense is the ratio of additive genetic variance to phenotypic variance.
hermaphroditic: Referring to individuals that can produce both male and female gametes.
heterochrony: An evolutionary change in phenotype caused by an alteration of timing of developmental events.
heterokaryotype: A genome or individual that is heterozygous for a chromosomal rearrangement such as an inversion. Cf. homokaryotype.
heterotopy: Evolutionary change in the position within an organism at which a phenotypic character is expressed.
heterozygosity: In a population, the proportion of loci at which a randomly chosen individual is heterozygous, on average.
heterozygote: An individual organism that possesses different alleles at a locus.
heterozygous advantage: The manifestation of higher fitness by heterozygotes than by homozygotes at a specific locus.
higher taxon: See taxon.
historical biogeography: The study of the geographic distribution of organisms by focusing on historical circumstances. Complements ecological biogeography.
hitchhiking: Change in the frequency of an allele due to linkage with a selected allele at another locus.
homeobox genes: A large family of eukaryotic genes that contain a DNA sequence known as the homeobox. The homeobox sequence encodes a protein homeodomain about 60 amino acids in length that binds DNA. Most homeobox genes are transcriptional regulators. Cf. domain; Hox genes.
homeodomain: See homeobox genes.
homeostasis: Maintenance of an equilibrium state by some self-regulating capacity of an individual.
homeotic mutation: A mutation that causes a transformation of one structure into another of the organism’s structures.
homokaryotype: A genome or individual that is homozygous for a chromosomal rearrangement such as an inversion. Cf. heterokaryotype.
homology: Possession by two or more species of a character state derived, with or without modification, from their common ancestor. Homologous chromosomes are those members of a chromosome complement that bear the same genes.
homonymous: Pertaining to biological structures that occur repeatedly within one segment of the organism, such as teeth or bristles.
homoplasy: Possession by two or more species of a similar or identical character state that has not been derived by both species from their common ancestor; embraces convergence, parallel evolution, and evolutionary reversal.
homozygote: An individual organism that has the same allele at each of its copies of a genetic locus.
horizontal (lateral) gene transfer: See horizontal transmission.
horizontal transmission: Movement of genes or symbionts (such as parasites) between individual organisms other than by transmission from parents to their offspring (which is vertical transmission). Horizontal transmission of genes is also called lateral gene transfer.
Hox genes: A subfamily of homeobox genes, conserved in all metazoan animals, that controls anterior-posterior segment identity by regulating the transcription of many genes during development.
hybrid: An individual formed by mating between unlike forms, usually genetically differentiated populations or species.
hybrid zone: A region in which genetically distinct populations come into contact and produce at least some offspring of mixed ancestry.
hypermorphosis: An evolutionary increase in the duration of ontogenetic development, resulting in features that are exaggerated compared to those of the ancestor.
hypothesis: An informed conjecture or statement of what might be true. A hypothesis may be poorly supported at first but can gain support to the point that it is effectively a fact. Cf. scientific theory.
hypothetico-deductive method: The method of predicting what one will find by deducing it from hypotheses.

I

identical by descent

Of two or more gene copies, being derived from a single gene copy in a specified common ancestor of the organisms that carry the copies.

inbreeding

Mating between relatives that occurs more frequently than if mates were chosen at random from a population.

inbreeding coefficient

The probability that a random pair of gene copies are identical by descent.

inbreeding depression

Reduction, in inbred individuals, of the mean value of a character (usually one correlated with fitness).

inclusive fitness

The fitness of a gene or genotype as measured by its effect on the survival or reproduction of both the organism bearing it and the genes, identical by descent, borne by the organism’s relatives.

incumbent replacement

An evolutionary process whereby the extinction of one taxon allows an ecologically similar taxon to diversity.

indirect development

A life history consisting of a larval stage between embryo and adult stages. Cf. direct development.

individual selection

A form of natural selection consisting of nonrandom differences among different genotypes (or phenotypes) within a population in their contribution to subsequent generations. See also genic selection, natural selection.

individualization

The acquisition of distinct identities by certain modules in the bodies of organisms; for example, the differentiation of teeth into incisors, canines, premolars, and molars during the evolution of mammals. An important basis for mosaic evolution.

ingroup

In phylogenetic analysis, a group of species whose relationship is reconstructed.

inheritance of acquired characteristics

See Lamarckism.

intelligent design (ID)

The doctrine that many biological phenomena are too complicated to have arisen by natural processes, and can only be explained by an intelligent designer (e.g., God).

intensity of selection

Magnitude of fitness differences among genotypes/phyenotypes.

inter-, intra-

Prefixes meaning, respectively, “between” and “within.” For example, “interspecific” differences are differences between species and “intraspecific” differences are differences among individuals within a species.

interaction

Strictly, the dependence of an outcome on a combination of causal factors, such that the outcome is not predictable from the average effects of the factors taken separately. More loosely, an interplay between entities that affects one or more of them (as in interactions between species). See also genotype × environment interaction.

interdemic selection

Group selection of populations within a species.

intragenic recombination

Recombination that takes place within a gene (as opposed to between genes).

intrinsic rate of natural increase

The potential per capita rate of increase of a population with a stable age distribution whose growth is not depressed by the negative effects of density.

introgression

Movement of genes from one species or population into another by hybridization and backcrossing; carries the implication that some genes in a genome undergo such movement, but others do not.

intron

A part of a gene that is not translated into a polypeptide. Cf. exon.

inversion

A 180° reversal of the orientation of a part of a chromosome, relative to some standard chromosome.

isolating barrier, isolating mechanism

A genetically determined difference between populations that restricts or prevents gene flow between them. The term does not include spatial segregation by extrinsic geographic or topographic barriers.

isolation by distance

In population genetics, a model for studying gene flow in a continuously distributed population in which each individual is the center of a “neighborhood” and the probability of mating declines with distance from the center.

iteroparous

Pertaining to a life history in which individuals reproduce more than once. Cf. semelparous.

K

karyotype

The chromosome complement of an individual.

key adaptation

An adaptation that provides the basis for using a new, substantially different habitat or resource.

kin selection

A form of selection whereby alleles differ in their rate of propagation by influencing the impact of their bearers on the reproductive success of individuals (kin) who carry the same alleles by common descent.

L

Lamarckism

The theory that evolution is caused by inheritance of character changes acquired during the life of an individual due to its behavior or to environmental influences.

lateral gene transfer

See horizontal transmission.

Laurasia

The Northern Hemisphere supercontinent that separated from Pangaea during the Jurassic.

lethal allele

An allele (usually recessive) that causes virtually complete mortality, usually early in development.

levels of selection

A reference to the fact that differences in survival and reproduction exist not only among individual organisms but also among genes and among populations and species. Thus, different kinds of biological entities may vary in fitness, resulting in different levels of selection.

life history

The stages an individual goes through during its life.

life history trait

A trait that characterizes an individual’s life history and is a component of fitness; for example, probability of survival, age at which adult size is reached, age at first reproductive event.

lineage

A series of ancestral and descendant populations through time; usually refers to a single evolving species, but may include several species descended from a common ancestor.

lineage sorting

The process by which each of several descendant species, carrying several gene lineages inherited from a common ancestral species, acquires a single gene lineage; hence, the derivation of a monophyletic gene tree, in each species, from the paraphyletic gene tree inherited from their common ancestor.

linkage

Occurrence of two loci on the same chromosome: the loci are functionally linked only if they are so close together that they do not segregate independently in meiosis.

linkage disequilibrium

The association of two alleles at two or more loci more frequently (or less frequently) than predicted by their individual frequencies.

linkage equilibrium

The association of two alleles at two or more loci at the frequency predicted by their individual frequencies.

locus (plural: loci)

A site on a chromosome occupied by a specific gene; more loosely, the gene itself, in all its allelic states.

logistic equation

An equation describing the idealized growth of a population subject to a density-dependent limiting factor. As density increases, the rate of growth gradually declines until population growth stops.

M

macroevolution

A vague term, usually meaning the evolution of substantial phenotypic changes, usually great enough to place the changed lineage and its descendants in a distinct genus or higher taxon. Cf. microevolution.

mass extinction

A highly elevated rate of extinction of species, extending over an interval that is relatively short on a geological time scale (although still very long on a human time scale).

maternal effect

A nongenetic effect of a mother on the phenotype of her offspring, stemming from factors such as cytoplasmic inheritance, transmission of symbionts from mother to offspring, or nutritional conditions.

maximum parsimony

See parsimony.

McDonald-Kreitman test for selection

In molecular evolution, a statistical test used to compare between-species divergence and within-species polymorphism at non-synonymous and synonymous sites.

mean

Usually the arithmetic mean or average; the sum of n values, divided by n. The mean value of x, symbolized as x, equals (x₁ + x₂ + … + x_n)/n.

mean fitness

The arithmetic average fitness of all individuals in a population, usually relative to some standard.

meiotic drive

Used broadly to denote a preponderance (> 50 percent) of one allele among the gametes produced by a heterozygote; results in genic selection.

metapopulation

A set of local populations, among which there may be gene flow and patterns of extinction and recolonization.

microevolution

A vague term, usually referring to slight, short-term evolutionary changes within species. Cf. macroevolution.

microRNAs

Short RNA sequences (about 22 base pairs long) that bind to RNA transcripts and repress their translation into proteins.

microsatellite

A short, highly repeated, untranslated DNA sequence.

migration

Used in theoretical population genetics as a synonym for gene flow among populations; in other contexts, refers to directed large-scale movements of organisms that do not necessarily result in gene flow.

mimicry

Similarity of certain characters in two or more species due to convergent evolution when there is an advantage conferred by the resemblance. Common types include Batesian mimicry, in which a palatable mimic experiences lower predation because of its resemblance to an unpalatable model; and Müllerian mimicry, in which two or more unpalatable species enjoy reduced predation due to their similarity.

modern synthesis

See evolutionary synthesis.

modularity

The ability of individual parts of an organism, such as segments or organs, to develop or evolve independently from one another; the ability of developmental regulatory genes and pathways to be regulated independently in different tissues and developmental stages.

molecular clock

The concept of a steady rate of change in DNA sequences over time, providing a basis for dating the time of divergence of lineages if the rate of change can be estimated.

monomorphic

Having one form; refers to a population in which virtually all individuals have the same genotype at a locus. Cf. polymorphism.

monophyletic

Refers to a taxon, phylogenetic tree, or gene tree whose members are all derived from a common ancestral taxon. In cladistic taxonomy, the term describes a taxon consisting of all the known species descended from a single ancestral species. Cf. paraphyletic, polyphyletic.

mosaic evolution

Evolution of different characters within a lineage or clade at different rates, hence more or less independently of one another.

multigene family

A large group of genes related to one another by clear ancestry and descent, and often having diverse functions that have a common theme.

multilevel selection

Selection at multiple levels, i.e., gene, organism, population.

multiple-niche polymorphism

A phenomenon in which different homozygotes in a single population are best adapted to different microhabitats or resources.

multiple stable equilibria

See equilibrium.

mutation

An error in the replication of a nucleotide sequence, or any other alteration of the genome that is not manifested as reciprocal recombination.

mutational variance

The increment in the genetic variance of a phenotypic character caused by new mutations in each generation.

mutualism

A symbiotic relation in which each of two species benefits by their interaction.

N

naturalistic fallacy

The supposition that what is “natural” is necessarily “good.”

natural laws

Statements that certain patterns of events will always occur if certain conditions hold.

natural selection

The differential survival and/or reproduction of classes of entities that differ in one or more characteristics. To constitute natural selection, the difference in survival and/or reproduction cannot be due to chance, and it must have the potential consequence of altering the proportions of the different entities. Thus natural selection is also definable as a deterministic difference in the contribution of different classes of entities to subsequent generations. Usually the differences are inherited. The entities may be alleles, genotypes or subsets of genotypes, populations, or, in the broadest sense, species. A complex concept; see Chapter 11. See also genic selection, individual selection, kin selection, group selection.

neo-Darwinism

The modern belief that natural selection, acting on randomly generated genetic variation, is a major, but not the sole, cause of evolution.

neofunctionalization

Divergence of duplicate genes whereby one acquires a new function. Cf. subfunctionalization.

neoteny

Heterochronic evolution whereby development of some or all somatic features is retarded relative to sexual maturation, resulting in sexually mature individuals with juvenile features. See also paedomorphosis, progenesis.

neutral alleles

Alleles that do not differ measurably in their effect on fitness.

neutral mutation rate

The mutation rate to alleles that do not affect fitness.

neutral theory of molecular evolution

A view of molecular evolution that postulates that although many mutations are deleterious, and a few are advantageous, most molecular variation within and among species is selectively neutral.

niche conservatism

Long-continued dependence of related species on much the same resources and environmental conditions.
node

On a phylogenetic tree, any point at which the tree “branches.”

nonadaptive evolution

Evolution that does not equip organisms for reproduction and survival.

nonsynonymous mutation

See nonsynonyomous substitution.

nonsynonymous substitution

A base pair substitution in DNA that results in an amino acid substitution in the protein product; also called replacement substitution. Cf. synonymous substitution.

norm of reaction

The set of phenotypic expressions of a genotype under different environmental conditions. See also phenotypic plasticity.

normal distribution

A bell-shaped frequency distribution of a variable; the expected distribution if many factors with independent, small effects determine the value of a variable; the basis for many statistical formulations.

nucleotide substitution

The complete replacement of one nucleotide base pair by another within a lineage over evolutionary time.

O

ontogeny

The development of an individual organism, from fertilized zygote until death.

optimality theory

An approach to understanding particular life history trait adaptations by specifying, often on the basis of mathematical models, which state of some character, among a specified set of plausible states, would maximize individual fitness, subject to specified constraints. Also called optimization theory.

organism

Usually used in this book to refer to an individual member of a species.

orthologous

Refers to corresponding (homologous) members of a gene family in two or more species. Cf. paralogous.

outcrossing

Mating with another genetic individual. Cf. selfing.

outgroup

A taxon that diverged from a group of other taxa before they diverged from one another.

overdominance

The expression by two alleles in heterozygous condition of a phenotypic value for some character that lies outside the range of the two corresponding homozygotes.

P

paedomorphosis

Possession in the adult stage of features typical of the juvenile stage of the organism’s ancestor.

Pangaea

The single land mass formed when all of the continents came together during the Permian period.

panmixia

Random mating among members of a population.

parallel evolution (parallelism)

The evolution of similar or identical features independently in related lineages, thought usually to be based on similar modifications of the same developmental pathways.

paralogous

Refers to the homologous relationship between two different members of a gene family, within a species or in a comparison of different species. Cf. orthologous.

parapatric

Of two species or populations, having contiguous but non-overlapping geographic distributions.

parapatric speciation

The evolution of reproductive barriers in spatially distinct populations between which there is some gene flow.

paraphyletic

Refers to a taxon, phylogenetic tree, or gene tree whose members are all derived from a single ancestor, but which does not include all the descendants of that ancestor. Cf. monophyletic.

parental investment

Parental activities or processes that enhance the survival of existing offspring but whose costs reduce the parent’s subsequent reproductive success.

parent-offspring conflict

The conflict that arises when offspring try to obtain more resources from a parent than it is optimal for the parent to give.

parsimony

Economy in the use of means to an end (Webster’s New Collegiate Dictionary); the principle of accounting for observations by that hypothesis requiring the fewest or simplest assumptions that lack evidence; in systematics, the principle of invoking the minimal number of evolutionary changes to infer phylogenetic relationships.

parthenogenesis

Virgin birth; development from an egg to which there has been no paternal contribution of genes.

passive trend

See trend.

patterns of evolution

Common themes observed in the history of evolutionary changes in the characteristics of organisms.

PCR (polymerase chain reaction)

A laboratory technique by which the number of copies of a DNA sequence is increased by replication in vitro.

peak shift

Change in allele frequencies within a population from one to another local maximum of mean fitness by passage through states of lower mean fitness.

peramorphosis

Evolutionary change that results in delayed maturity and reproduction at a larger size, associated with the extended development of “hyper-adult” features. Cf. paedomorphosis.

period

A division on the geological time scale. Each of the three eras of Phanerozoic time (marked by the first appearance of diverse animals) is divided into periods.

peripatric

Of a population, peripheral to most of the other populations of a species.

peripatric speciation

Speciation by evolution of reproductive isolation in peripatric populations as a consequence of a combination of genetic drift and natural selection.

phenetic

Pertaining to phenotypic similarity, as in a phenetic classification.

phenogram

A diagram portraying the relative genetic difference (or similarity) among populations (or species).

phenotype

The morphological, physiological, biochemical, behavioral, and other properties of an organism manifested throughout its life; or any subset of such properties, especially those affected by a particular allele or other portion of the genotype.

phenotypic correlation

The relationship shown between two phenotypic characters (e.g., body size and fecundity).

phenotypic integration

A hypothesis that functionally related characteristics should be genetically correlated with one another.

phenotypic plasticity

The capacity of an organism to develop any of several phenotypic states, depending on the environment; usually this capacity is assumed to be adaptive.

phenotypic variance

In a phenotypic trait, the sum of genetic variance and environmental variance.

phyletic gradualism

Slow, incremental evolutionary change. Cf. punctuated equilibria.

phylogenetic niche conservatism

A pattern in which related species often have similar ecological requirements, presumably derived from their common ancestor.

phylogenetic species concept

Any of several related concepts of species as sets of populations that are diagnosably different from other populations.

phylogenetic tree

A graphic representation of lines of descent among organisms or their genes.

phylogeny

The history of descent of a group of taxa such as species from their common ancestors, including the order of branching and sometimes the absolute times of divergence; also applied to the genealogy of genes derived from a common ancestral gene.

phylogeography

The description and analysis of the processes that govern the geographic distribution of lineages of genes, especially within species and among closely related species.

planktonic

Living in open water. Cf. benthic.

plate tectonics

The geologic theory that the lithosphere, the solid outer layer of Earth bearing both the continents and the crust below the oceans, consists of eight major and several minor plates that move independently over the denser asthenosphere below.

pleiotropy

A phenotypic effect of a gene on more than one character.

ploidy

The number of chromosome complements in an organism.

point mutation

In classic genetics, a mutation that maps to a single gene locus. In modern usage, often restricted to single base pair substitutions.

polygenic character

A character whose variation is based wholly or in part on allelic variation at more than a few loci.

polymorphism

The existence within a population of two or more genotypes, the rarest of which exceeds some arbitrarily low frequency (say, 1 percent); more rarely, the existence of phenotypic variation within a population, whether or not genetically based. Cf. monomorphic.

polyphenism

The capacity of a species or genotype to develop two or more forms, with the specific form depending on specific environmental conditions or cues, such as temperature or day length. A polyphenism is distinct from a polymorphism in that the former is the property of a single genotype, whereas the latter refers to multiple forms encoded by two or more different genotypes.

polyphyletic

Refers to a taxon, phylogenetic tree, or gene tree composed of members derived by evolution from ancestors in more than one ancestral taxon; hence, composed of members that do not share a unique common ancestor. Cf. monophyletic.

polyploid

Of a cell or organism, possessing more than two chromosome complements.

population

A group of conspecific organisms that occupy a more or less well defined geographic region and exhibit reproductive continuity from generation to generation; ecological and reproductive interactions are more frequent among these individuals than with members of other populations of the same species.

positive selection

Selection for an allele that increases fitness. Cf. purifying selection.

postzygotic

Occurring after union of the nuclei of uniting gametes; usually refers to inviability or sterility that confers reproductive isolation.

postzygotic barriers

Barriers to gene flow that occur after the union of the nuclei of uniting gametes; for example, reduced survival or reproductive rates.

preadaptation

Possession of the necessary properties to permit a shift to a new niche, habitat, or function. A structure is preadapted for a new function if it can assume that function without evolutionary modification.

Precambrian time

The geological time period that includes both the Archean (prior to 2.5 billion years ago) and Proterozoic (2.5 billion–542 million years ago) eras.

premating barriers

Barriers to gene flow that prevent (or reduce the likelihood of) transfer of gametes to members of other species.

prezygotic

Occurring before union of the nuclei of uniting gametes; usually refers to events in the reproductive process that cause reproductive isolation.

prezygotic barriers

Barriers to gene flow that occur before the union of the nuclei of uniting gametes; for example, the failure of gametes of different species to unite.

primordium

A group of embryonic or larval cells destined to give rise to a particular adult structure.

processed pseudogene

A pseudogene that has arisen via the retrotransposition of mRNA into cDNA.

progenesis

A decrease during evolution of the duration of ontogenetic development, resulting in retention of juvenile features in the sexually mature adult. See also neoteny, paedomorphosis.

progress

A word that usually implies movement toward a goal, improvement, and betterment. In this sense, there can be no “progress” in evolution. Characters improve in their capacity to serve certain functions, but these functions are specific to the ecological context of each species.

promoter

Usually refers to the DNA sequences immediately 5′ to (upstream of) a gene that are bound by the RNA polymerase and its cofactors and/or are required in order to transcribe the gene. Sometimes used interchangeably with enhancer.

provinciality

The degree to which the taxonomic composition of a biota is differentiated among major geographic regions.

pseudogene

A nonfunctional member of a gene family that has been derived from a functional gene. Cf. processed pseudogene.

pull of the Recent

In paleobiology, the appearance that species diversity seems to increase as we approach the present. This bias arises because the more recently a taxon arose, the more likely it is to still be extant.

punctuated equilibria

A pattern of rapid evolutionary change in the phenotype of a lineage separated by long periods of little change; also, a hypothesis intended to explain such a pattern, whereby phenotypic change transpires rapidly in small populations, in concert with the evolution of reproductive isolation.

punctuated anagenesis

Evolution of characters occurring between long-stable states in populations that do not undergo speciation.

punctuated gradualism

See punctuated anagenesis.

purifying selection

Elimination of deleterious alleles from a population. Cf. positive selection.

pyrosequencing

A new method of DNA sequencing that isolates each molecule in an individual picoliter well.

Q

QTL

quantitative genetics

The field of genetics that studies quantitative (continuously varying) characters.

Quantitative trait locus (or loci); a chromosome region containing at least one gene that contributes to variation in a quantitative trait. QTL mapping is a procedure for determining the map positions of QTL on chromosomes.

quantitative trait

A phenotypic character that varies continuously rather than as discretely different character states.

R

race

A poorly defined term for a set of populations occupying a particular region that differ in one or more characteristics from populations elsewhere; equivalent to subspecies. In some writings, a distinctive phenotype, whether or not allopatric from others.

radiation

See adaptive radiation.

radiometric dating

A process that determines the “absolute” ages of geological events by measuring the decay of certain radioactive elements in minerals that form in igneous rock.

random genetic drift

See genetic drift.

random walk

Random fluctuation in allele frequency.

randomness

In science, the inability to predict a particular outcome, because physical causes can result in any of several outcomes.

realized heritability

Heritability that can be estimated by a response to experimental selection.

recessive

In genetics, an allele associated with a phenotypic effect only when in the homozygous state.

reciprocal translocation

The exchange of segments, by breakage and reunion, between two nonhomologous chromosomes.

reciprocity

Cooperation based on repeated interactions.

recombinational speciation

Speciation that results when the recombinant offspring of F₁ hybrids between two species include genotypes that are fertile but reproductively isolated from the parent species. Also called hybrid speciation.

recruitment

(1) In evolutionary genetics, the evolution of a new function for a gene other than the function for which that gene was originally adapted. (2) In population biology, refers to the addition of new adult (breeding) individuals to a population via reproduction (i.e., individuals born into the population that reach reproductive age).

recurrent mutation

Repeated origin of mutations of a particular kind within a species.

refugia

Locations in which species have persisted while becoming extinct elsewhere.

regression

In geology, withdrawal of sea from land, accompanying lowering of sea level; in statistics, a function that best predicts a dependent from an independent variable.

regulatory modularity

See modularity.

reinforcement

Evolution of enhanced reproductive isolation between populations due to natural selection for greater isolation.

relative fitness

The fitness of a genotype relative to (as a proportion of) the fitness of a reference genotype, which is often set at 1.0.

relict

A species that has been “left behind”; for example, the last survivor of an otherwise extinct group. Sometimes, a species or population left in a locality after extinction throughout most of the region.

repeated sequence

The occurrence of repeated base pairs in a gene.
replacement substitution

See nonsynonymous substitution.

reporter construct

A DNA segment in which a putative cis-regulatory sequence is spliced upstream of a gene whose expression can be easily assayed, such as β-galactosidase or green fluorescent protein.

reproductive effort

The proportion of energy or materials that an organism allocates to reproduction rather than to growth and maintenance.

reproductive success

See fitness.

response to selection

The change in the mean value of a character over one or more generations due to selection.

restriction enzyme

An enzyme that cuts double-stranded DNA at specific short nucleotide sequences. Genetic variation within a population results in variation in DNA sequence lengths after treatment with a restriction enzyme, or restriction fragment length polymorphism (RFLP).

reticulate evolution

Union of different lineages of a clade by hybridization.

retrosequence

A cDNA copy of RNA. Most retrosequences are processed pseudogenes.

RFLP

See restriction enzyme.

S

saltation

A jump; a discontinuous mutational change in one or more phenotypic traits, usually of considerable magnitude.

sampling error

Random variation, meaning the likelihood that proportions of different kinds of items (e.g., alleles) in a sample are likely to differ, by chance, from the proportions in the set of items from which the sample is drawn.

scala naturae

The “scale of nature, ” or Great Chain of Being: the pre-evolutionary concept that all living things were created in an orderly series of forms, from lower to higher.

scientific creationism (= creation science)

A body of teaching that consists of attacks on, and supposedly scientific disproofs of, evolution.

scientific theory

A body of interconnected statements, based on reasoning and strongly supported by evidence, that explain some aspect—or often many aspects—of nature. Cf. hypothesis.

secondary contact

Interbreeding of two formerly allopatric populations that have become genetically differentiated.

selection

Nonrandom differential survival or reproduction of classes of phenotypically different entities. See natural selection, artificial selection.

selection coefficient

The difference between the mean relative fitness of individuals of a given genotype and that of a reference genotype.

selection differential

The difference (S) in mean trait between the population and the selected parents of the next generation.

selection gradient

The slope of the relation between phenotype values and the fitnesses of those phenotypes. A measure of the strength of natural selection.

selection plateau

The point at which response to selection (e.g., for a particular trait) ceases. Commonly observed in selection experiments.

selective advantage

The increment in fitness (survival and/or reproduction) provided by an allele or a character state.

selective sweep

Reduction or elimination of DNA sequence variation in the vicinity of a mutation that has been fixed by natural selection relatively recently.

selfing

Self-fertilization; union of female and male gametes produced by the same genetic individual. Cf. outcrossing.

“selfish DNA”

A DNA sequence that has the capacity for its own replication, or replication via other self-replicating elements, but has no immediate function (or is deleterious) for the organism in which it resides.

selfish genetic elements

See “selfish” DNA

semelparous

Pertaining to a life history in which individuals (especially females) reproduce only once. Cf. iteroparous.

semispecies

One of several groups of populations that are partially but not entirely isolated from one another by biological factors (isolating mechanisms).

senescence

Aging; deteriorative changes with aging; the increased probability of dying with increasing age.

sensory bias

The tendency in an organism for certain traits to be intrinsically stimulating and evoke a greater response simply because of the organization of the sensory system.

serial homology

A relationship among repeated, often differentiated, structures of a single organism, defined by their similarity of developmental origin; for example, the several legs and other appendages of an arthropod.

sex

The union of two genomes, usually carried by gametes.

sex-linked

Of a gene, being carried by one of the sex chromosomes; it may be expressed phenotypically in both sexes.

sex ratio

The proportion of males in a population.

sexual reproduction

Production of offspring whose genetic constitution is a mixture of those of two potentially genetically different gametes.

sexual selection

Differential reproduction as a result of variation in the ability to obtain mates.

sibling species

special creation

The doctrine that each species, living and extinct, was created independently by God, essentially in its present form.

Species that are difficult or impossible to distinguish by morphological characters, but may be discerned by differences in ecology, behavior, chromosomes, or other such characters.

silent substitution

See synonymous substitution.

single nucleotide polymorphism (SNP)

Variation in the identity of a nucleotide base pair at a single position in a DNA sequence, within or among populations of a species.

sister taxa

Two species or higher taxa that are derived from an immediate common ancestor, and are therefore each other’s closest relatives.

speciation

Evolution of reproductive isolation within an ancestral species, resulting in two or more descendant species.

species

In the sense of biological species, the members of a group of populations that interbreed or potentially interbreed with one another under natural conditions; a complex concept (see Chapter 15). Also, a fundamental taxonomic category to which individual specimens are assigned, which often but not always corresponds to the biological species. See also biological species, phylogenetic species concept.

species hitchhiking

The phylogenetic association of a character with other characters that affect speciation or extinction rates.

species selection

A form of group selection in which species with different characteristics increase (by speciation) or decrease (by extinction) in number at different rates because of a difference in their characteristics.

specific coevolution

The evolution of two species in response to each other.

specific mate recognition system

Within a species, the signals and responses used by potential mates. One sex (often the female) will not respond to inappropriate signals.

sperm competition

The opportunity for sperm of two or more males to fertilize one female’s eggs.

spliceosomal intron

An intron that requires a complex group of proteins called a spliceosome to be removed from the mRNA.

stability

Often used to mean constancy; more often in this book, the propensity to return to a condition (a stable equilibrium) after displacement from that condition.

stabilizing selection

Selection against phenotypes that deviate in either direction from an optimal value of a character.

standard deviation

The square root of the variance.

stasis

Absence of evolutionary change in one or more characters for some period of evolutionary time.

stem group

An extinct group of species early in evolutionary time; for example, the dinosaur order Theropoda. Cf. crown group.

stochastic

Random. Cf. deterministic.

strata

Layers of sedimentary rock that were deposited at different times.

subfunctionalization

Divergence of duplicate genes whereby each retains only a subset of the several functions of the ancestral gene. Cf. neofunctionalization.

subspecies

A named geographic race; a set of populations of a species that share one or more distinctive features and occupy a different geographic area from other subspecies.

substitution

The complete replacement of one allele by another within a population or species over evolutionary time. Cf. fixation.

substitution (base pair or amino acid)

The replacement of one nucleotide base pair (e.g., A–T) by another (e.g., G–C), or of one amino acid by another, in an entire population or species. Sometimes, but not always, changes the amino acid specified by the genetic code.

superspecies

A group of semispecies.

symbiosis

An intimate, usually physical, association between two or more species.

sympatric

Of two species or populations, occupying the same geographic locality so that the opportunity to interbreed is presented.

sympatric speciation

The evolution of reproductive barriers within a single, initially randomly mating (panmictic) population.

synapomorphy

A derived character state that is shared by two or more taxa and is postulated to have evolved in their common ancestor.

synonymous substitution

Fixation of a base pair change that does not alter the amino acid in the protein product of a gene; also called silent substitution. Cf. nonsynonymous substitution.

T

tandem repeat

In DNA, a pattern of repeated sequences of base pairs (e.g., ABBC).

target gene

In developmental genetics, a gene regulated by a transcription factor of interest. This regulation may be direct or indirect.

taxon (plural: taxa)

The named taxonomic unit (e.g., Homo sapiens, Hominidae, or Mammalia) to which individuals, or sets of species, are assigned. Higher taxa are those above the species level. Cf. category.

taxonomic category

See category and taxon.

teleology

The belief that natural events and objects have purposes and can be explained by their purposes.

tension zone

A hybrid zone in which the hybrids between populations have low intrinsic fitness, because of heterozygote disadvantage or breakdown of coadapted gene complexes.

territory

An area or volume of habitat defended by an organism or a group of organisms against other individuals, usually of the same species; territorial behavior is the behavior by which the territory is defended.

theistic evolution

The doctrine that God established natural laws (e.g., natural selection) and then let the universe run on its own, without further supernatural intervention.

theory

See scientific theory.

threshold trait

A trait controlled by polygenic variation rather than by single loci.

trade-off

The existence of both a fitness benefit and a fitness cost of a mutation or character state, relative to another.

transcription factor

A protein that interacts with a regulatory DNA sequence and affects the transcription of the associated gene.

transition

A mutation that changes a nucleotide to another nucleotide in the same class (purine or pyrimidine). Cf. transversion.

translational robustness

The ability of an amino acid sequence to maintain proper protein folding in the face of ongoing mutations.

translocation

The transfer of a segment of a chromosome to another, nonhomologous, chromosome; the chromosome formed by the addition of such a segment.

transposable element

A DNA sequence, copies of which become inserted into various sites in the genome.

trans-regulation

Effects on gene expression controlled by factors (often transcription factors) not tightly linked to the gene.

trans-regulatory element

A nucleotide sequence, usually encoding a regulatory protein, that is not closely linked to the structural gene whose expression it regulates. Cf. cis-regulatory element.

transposition

The production of copies of genetic material that become inserted into new positions in the genome.

transversion

A mutation that changes a nucleotide to another nucleotide in the opposite class (purine or pyrimidine). Cf. transition.

trend

A persistent, directional change in the average value of a character or lineage over the course of time. In a passive trend, a character (or lineage) shifts in both directions with equal probability. In a driven (= active) trend, change is more likely in one direction than in the other.

U

underdominance

The manifestation of lower fitness by heterozygotes than by homozygotes.

unequal crossing over

In recombination, the unequal exchange of genetic material. Occurs most commonly when two repeated genes or sequences mispair with their homologues.

uniformitarianism

The principal that the same geological processes that operated in the past operate in the present, and that the observations of geology can therefore be explained by causes we can now observe.

unstable equilibrium

An equilibrium to which a system does not return if disturbed.

V

variability

The ability, or potential, to vary.

variance (σ², s², V)

The average squared deviation of an observation from the arithmetic mean; hence, a measure of variation. s² = [Σ(x_i − x)²]/(n − 1), where x is the mean and n the number of observations.

vegetative propagation

A form of asexual reproduction in which offspring arise from a group of cells, as in plants that spread by runners or stolons.
vertical transmission

See horizontal transmission.

vestigial

Occurring in a rudimentary condition as a result of evolutionary reduction from a more elaborated, functional character state in an ancestor.

viability

Capacity for survival; often refers to the fraction of individuals surviving to a given age, and is contrasted with inviability due to deleterious genes.

vicariance

Separation of a continuously distributed ancestral population or species into separate populations due to the development of a geographic or ecological barrier.

virulence

Usually, the damage inflicted on a host by a pathogen or parasite; sometimes, the capacity of a pathogen or parasite to infect and develop in a host.

W

wild-type

The allele, genotype, or phenotype that is most prevalent (if there is one) in wild populations; with reference to the wild-type allele, other alleles are often termed mutations.

Z

zygote

A single-celled individual formed by the union of gametes. Occasionally used more loosely to refer to an offspring produced by sexual reproduction.

© 2009 Sinauer Associates, Inc.