GLOSSARY
Acoelomate An animal that lacks a coelom. The Platyhelminthes are acoelomates.
Adaptation Any property of an organism believed to add to its fitness.
Adaptationist program The investigation of the possible adaptive value of a structure or other attribute of a taxon.
Adaptive radiation Evolutionary divergence of members of a single phyletic line into different niches or adaptive zones.
Allele One of the alternate forms (nucleotide sequences) of a gene. Different alleles of the same gene usually produce different effects on the phenotype.
Allopatric Pertaining to populations or species the ranges of which do not overlap.
Allospecies Species that are members of a superspecies but that are geographically separated from the other allospecies of this superspecies.
Allozyme The particular amino acid sequence of an enzyme produced by one allele of a gene that also has other alleles producing enzymes with different amino acid sequences.
Alvarez event The impact of an asteroid on the Earth at the very end of the Cretaceous, 65 million years ago, causing the mass extinction of the dinosaurs and other fauna and flora, as postulated by the physicist Walter Alvarez.
Anagenesis So-called progressive (“upward”) evolution.
Anlage In development, the propensity of a tissue to give rise to a particular structure or organ.
Anoxia Deficiency or absence of oxygen.
Anthropomorphism An unjustified attribution of a human characteristic to other organisms or objects.
Australopithecines Early African hominids, living about 4.4 to 2.0 million years ago, who had a small brain (less than 500 cc), were bipedal, but were still largely arboreally living; they had no stone tools.
Background extinction The steady extinction at all geological periods of a certain number of individual species.
Baldwin effect The selection of genes that strengthen the genetic basis of a variant of the phenotype.
Bauplan (body plan) Structural type, as that of a vertebrate or arthropod.
Biological species Groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups.
Biota The combined fauna and flora of an area.
Budding The origin of a new side branch of a phyletic lineage by speciation and subsequent entry of this species and its descendants into a new niche or adaptive zone, resulting in a distinct new higher taxon.
Category A taxonomic category designates the rank of a taxon in a hierarchy of levels; a class whose members are all taxa assigned the same categorical rank.
Causation, proximate Causation due to currently acting biological, chemical, or physical factors.
Chromosomes Structural elements, usually rod-shaped, found in the nucleus of a cell and containing the major part of the hereditary material (the genes). Chromosomes are composed of DNA and proteins.
Clade Portion of a phylogenetic tree between two branching points or from a branching point to the end of the branch.
Cladogenesis The branching (divergence) component of evolution.
Cleavage One of the series of mitotic divisions of the fertilized egg (zygote) giving rise to the early embryonic tissues.
Cline Gradual variation of a character in a species, usually parallel to the variation of a climatic or other environmental gradient.
Clone Genetically identical individuals produced by any process of asexual (uniparental) reproduction; also monozygotic twins.
Coalescence method A method based on molecular clock determined rates of divergence to infer the time of the split of two related taxa from the lineage of their common ancestor.
Codon A nucleotide triplet in the genetic program (genome), designating a particular amino acid.
Coevolution The parallel evolution of two kinds of organisms that are interdependent, like flowers and their pollinators, or where at least one depends on the other, like predators on prey or parasites on their hosts, and where any change in one will result in an adaptive response in the other.
Competitive exclusion principle Two species cannot exist at the same locality if they have identical ecological requirements.
Continental drift The movement of continents in geological time owing to the drift of the plates of the Earth’s mantle caused by plate tectonics.
Contingency A nonpredictable occurrence.
Convergence Phenotypic similarity of two taxa that is independently acquired and is not produced by a genotype inherited from a common ancestor.
Copying error Failure of a gene to replicate itself precisely during mitosis or meiosis, resulting in a mutation.
Creationism Belief in the literal truth of Creation as recorded in the Book of Genesis.
Crossing-over The exchange of corresponding segments between maternal and paternal chromosomes. It occurs when maternal and paternal homologous chromosomes are paired during prophase of the first meiotic division.
Cynodonts An extinct group of reptiles, ancestral to the mammals.
Daphnia A planktonic crustacean of the order Cladocera.
Darwinism Darwin’s concepts and theories on which his followers base the explanation of evolution.
Darwinism, social A political theory postulating that ruthless egotism is the most successful policy.
Deme A local population of potentially interbreeding individuals.
Dendrogram A diagram in the form of a branching tree designed to indicate degrees of relationship among taxa.
Diploid Possessing a double set of chromosomes, one set derived from the mother, the other from the father.
Discontinuity, phenetic A discontinuity (gap) in the range of variation of the phenotypes in a population.
Discontinuity, taxic A discontinuity (gap) in the range of variation among related taxa, such as species of a genus or genera of a family.
Dispersal The movement of individuals from their birthplace; more broadly, the spread of individuals of a species beyond the current species range.
Ecological role The contribution made by a characteristic of an organism to its survival.
Elimination, nonrandom The elimination of the less fit individuals of a population during the process of so-called natural selection.
Entropy The degradation of matter and energy in the universe to an ultimate state of inert uniformity. Entropy can be reached only in a closed system.
Epistasis Interactions between two or more genes.
Essentialism A belief that the variation of nature can be reduced to a limited number of basic classes, representing constant, sharply delimited types; typological thinking.
Evolution The gradual process by which the living world has been developing following the origin of life.
Evolutionary synthesis The achievement of consensus among previously feuding schools of evolutionists, such as experimental geneticists, naturalists, and paleontologists, taking place particularly in the period 1937–1947; the unification of various branches of evolutionary biology, such as those studying anagenesis and those studying cladogenesis.
Fauna The species of animals living in a given geographical area at a given time.
Fertilization Fusion between the male gamete (spermatozoon) and the female gamete (ovum). It results in the joining of a haploid set of maternal chromosomes with a haploid set of paternal chromosomes in the newly formed zygote, which thereby becomes diploid.
Finalism Belief in an inherent trend in the natural world toward some preordained final goal or purpose, such as the attainment of perfection. See teleology.
Flora The species of plants living in a given geographical area at a given time.
Founder population A population beyond the previous species range founded by a single female (or a small number of conspecifics).
Gaia hypothesis The hypothesis that the interactions, particularly chemical ones, between organisms and the inorganic world in which they live (including the atmosphere) are regulated by a control program, called Gaia.
Gamete A male or female reproductive cell; spermatozoon or ovum (egg).
Gene A genetic unit (set of base pairs) situated on a particular locus on a chromosome.
Gene flow The movement of genes in a species from population to population.
Genetic drift The occurrence of changes in gene frequency brought about not by selection but by chance. It occurs especially in small populations.
Genetic program The information coded in an organism’s DNA.
Genotype The set of genes of an individual.
Group selection, theory of The theory that a social group can be the object of selection if the cooperative interaction among the members of the group enhances the fitness of the group.
Haploid Possessing a single set of chromosomes, like the gametes.
Heliocentricity The theory that the sun is in the center of the solar system and that the planets circle around the sun.
Heterozygous Possessing two different alleles of a particular gene on a pair of homologous chromosomes.
Homeostasis, genetic The capacity of the genotype to compensate for disturbing environmental influences.
Homologous Referring to the structure, behavior, or other character of two taxa that is derived from the same or equivalent feature of their nearest common ancestor.
Homoplasy Similarity of characters in two taxa not due to derivation from the same characters in the nearest common ancestor. See parallelophyly and convergence.
Homozygous Possessing identical alleles of a particular gene or a pair of homologous chromosomes.
Infusorian Obsolete term for small aquatic organisms (mostly protozoans, crustaceans, rotifers, and one-celled algae).
Isolating mechanism Genetic (including behavioral) properties of individuals that prevent populations of different species from interbreeding where they coexist in the same area.
Kin selection Selective advantage due to the altruistic interaction of individuals sharing part of the same genotype, such as siblings.
Linnaean Named for the Swedish naturalist Carolus Linnaeus (1707–1778), who invented the binomial classification system.
Living fossil A living species surviving after all of its relatives have become extinct more than 50–100 million years ago.
Locus The position of a particular gene on a chromosome.
Macroevolution Evolution above the species level; the evolution of higher taxa and the production of evolutionary novelties, such as new structures.
Mass extinction The extermination of a large proportion of the biota on Earth by a climatic, geological, cosmic, or other environmental event.
Meiosis A special form of nuclear division that occurs during the formation of the gametes (spermatozoa and eggs) in sexually reproducing organisms. Crossing-over and the reduction division of the chromosomes take place during meiosis.
Microevolution Evolution at or below the species level.
Mimicry, Batesian Resemblance of a palatable species to an unpalatable or toxic one.
Mimicry, Müllerian Resemblance of an unpalatable or toxic species to another likewise unpalatable one.
Missing link A fossil bridging the large gap between an ancestral and a derived group of organisms, such as Archaeopteryx, between reptiles and birds.
Mitosis A form of cell division in which each chromosome “splits” lengthwise (it replicates itself), each daughter cell receiving one daughter chromosome. This is the typical division of somatic cells.
Molecular clock The clocklike regularity of the change of a molecule (gene) or a whole genotype over geological time.
Mosaic evolution Evolutionary change that occurs in a taxon at different rates for different structures, organs, or other components of the phenotype.
Mutation Any inheritable alteration in the genetic material, most commonly an error of replication during cell division, resulting in the replacement of an allele by a different one. In addition to such gene mutations, there are also chromosomal mutations, i.e., major chromosomal changes, including polyploidy.
Natural selection The process by which in every generation individuals of lower fitness are removed from the population.
Necessity The inevitable force of circumstances.
Niche A constellation of properties of the environment making it suitable for occupation by a species.
Normalizing (stabilizing) selection The elimination by selection of variants beyond the normal range of variation of a population.
Open reading frame DNA sequence that potentially can be translated into a protein.
Organizer A tissue capable of inducing a specific type of development in other undifferentiated tissues.
Orthogenesis The refuted hypothesis that rectilinear trends in evolution are caused by an intrinsic finalistic principle.
Orthologous genes Genes in different species that are sufficiently similar in their nucleotide sequences to indicate that they were derived from a common ancestor.
Panmictic Pertaining to populations and species of such great dispersal capacity that there is complete interbreeding of populations from all parts of their range.
Parallelophyly Multiple independent occurrence of the same character in different species derived from the nearest common ancestor that has the genetic disposition for this character but did not show it in its own phenotype.
Parapatric Pertaining to contiguously living but nonoverlapping populations or species.
Phenotype The total of all observable features of a developing or developed individual (including its anatomical, physiological , biochemical, and behavioral characteristics). The phenotype is the result of interaction between the genotype and the environment.
Philopatry The drive (tendency) of an individual to return to (or stay in) its home area (birthplace or another adopted area).
Phyletic evolution The evolutionary change of a phyletic lineage in the time dimension.
Phyletic lineage A branch of the phylogenetic tree; all the linear descendants of an ancestral species.
Phylogeny The inferred lines of descent of a group of organisms, including a reconstruction of the common ancestor and the amount of divergence of the various branches.
Plate A piece of the Earth’s crust that moves owing to plate tectonics.
Plate tectonics The theory that the crust of the Earth consists of movable plates that may join or separate in different geological periods.
Pleiotropic Pertaining to how a gene may affect several aspects of the phenotype.
Polygenic inheritance Inheritance of a trait (e.g., height) governed by several genes (polygenes or multiple factors). Their effect is cumulative.
Polymorphism The simultaneous occurrence of several different alleles or discontinuous phenotypes in a population, with the frequency of even the rarest type higher than can be maintained by recurrent mutation.
Polymorphism, balanced The condition in which two different alleles coexisting in the same population produce a heterozygote of greater fitness than either homozygote.
Polyphyly Derivation of a taxon from two or more different ancestral sources.
Preadapted Pertaining to a character capable of adopting a new function or ecological role without loss of fitness; the possession of the required properties to permit a shift into a new niche or habitat, without interference with the original functions.
Protists A convenient collective name for the vast variety of unicellular eukaryotes.
Punctuated equilibria Alternation of extremely rapid and normal or slow evolutionary change in a phyletic lineage, as a result of speciational evolution.
Recapitulation The appearance of a structure or other attribute of a larval or immature individual of a species that resembles a similar attribute of the adults of an ancestral species; it is interpreted as evidence for descent from that ancestor.
Recessive gene A gene that is unable to express its effect when it is present in the heterozygous state (single dose). It must be present in the homozygous state (double dose) to express its effect.
Recombination A reshuffling of the genes in a new zygote as a result of crossing-over and reassortment of the chromosomes during meiosis. A new set of genotypes is thus produced in each generation.
Reductionism The belief that the higher levels of integration of a complex system can be fully explained through a knowledge of the smallest components.
Saltation A sudden event, resulting in a discontinuity (gap), such as the sudden production of a new species or higher taxon.
Saltationism The belief that evolutionary change is the result of the sudden origin of a new kind of individual that becomes the progenitor of a new kind of organism.
Scala naturae A linear arrangement of all forms of life from the lowest, nearly inanimate to the most perfect; the Great Chain of Being.
Scientific Revolution The period in the sixteenth and seventeenth centuries in which scientists, including Galileo and Newton, laid the foundation of modern science.
Sex-linkage The type of linkage produced when a gene is located on the X or the Y chromosome.
Sexual selection Selection for attributes that enhance reproductive success.
Sickle cell disease A genetic disease of the red blood corpuscles. Homozygosity for the sickle cell gene results in early death, while heterozygotes have superior fitness in malarial regions.
Somatic mutation The occurrence of a mutation in a somatic cell.
Somatic program In development, the information contained in neighboring tissues that may influence or control the further development of an embryonic structure or tissue.
Speciation, allopatric The origin of a new species through the acquisition of effective isolating mechanisms by a geographically isolated portion of the parental species.
Speciation, dichopatric The origin of a new species through the division of a parental species by a geographical, vegetational, or other extrinsic barrier.
Speciation, peripatric The origin of new species through the modification of peripherally isolated founder populations. See budding.
Speciation, sympatric Speciation without geographical isolation; the origin of a new set of isolating mechanisms within a deme.
Speciational evolution Accelerated evolutionary change toward species status in a founder or relict population, sometimes leading to the origin of a new higher taxon.
Species concept The biological meaning or definition of the word “species”; the criteria on the basis of which a species taxon is delimited.
Species taxon A taxon qualifying as a species according to the accepted species concept.
Spontaneous generation A refuted early concept that complex organisms can be produced spontaneously from inanimate material.
Stasis A period in the history of a taxon during which evolution seemed to have been at a standstill.
Symbiosis The usually mutually beneficial interaction of individuals of two different species.
Sympatric Pertaining to species the ranges of which overlap; species coexisting in the same area.
Taxon A monophyletic group of organisms (or lower taxa) that can be recognized by sharing a definite set of characters.
Teleology The study of final causes; the belief in the existence of direction-giving forces.
Therapsida An order of fossil synapsid reptiles that gave rise to the mammals.
Transformationism The refuted theories that attributed evolution to a change of the essence of a species either by inheritance of acquired characters, or by direct influence of the environment, or by final causes.
Transmutationism The theory that evolutionary change is caused by sudden new mutations or saltations, producing instantaneously a new species. See saltationism.
Typological species concept The recognition of species on the basis of their degree of phenotypic difference.
Typologist One who disregards variation and considers the members of a population to be replicas of the type; an essentialist.
Uniformitarianism The theory of some pre-Darwinian geologists, particularly Charles Lyell, that all changes in the Earth’s history are gradual, rather than occurring in saltations or jumps. Being gradual, these changes cannot be considered acts of special creation.
Vestigial character A deconstructed, nonfunctional characteristic that had been fully functional in a species’ ancestor, like the eyes in cave animals and the human appendix.
Wallace’s Line In biogeography, a line through the Indo-Malayan archipelago that indicates the eastern edge of the continental Sunda Shelf, serving as the eastern limit of the range of much of the tropical Asian mainland fauna, particularly in mammals.
Zygote A fertilized egg; the individual that results from the union of two gametes and their nuclei.