Glossary of Major Terms and Concepts The following terms and concepts form the core of much of this book. Generally, they appear in boldface type in the text. The chapters in which they are more fully discussed are also given. Acceleration: Faster rate of developmenral evenrs (at any level: cell, organ, individual) in the descendanr; produces peramorphie traits when expressed in the adult pheno­ type. (Chapters 1 and 2) Allometric heterochrony: Change in a trair as a function of size (as opposed to a function of age in " true" heterochrony); it is useful as adescriptor and can be inrerpretive considering that body size is often a beuer metric of " inrrinsic" age than chronologi­ cal age. The same categories apply as in " true" heterochrony, only the adjective "allometric" is prefixed: e.g. , " allometric progenesis" is when the descendant ter­ minates growth at a smaller size (as opposed to age) than the ancestor. (Chapter 2) Allometry: The study of size and shape, usually using biometrie data; the change in size and shape observed. Basic kinds of allometric change include the following. Com­ plex allometry: occurs when the ratio of the specific growth rates of the traits compared are not constant, a log-log plot comparing the traits will not yield a straight line (i.e., " k" in the allometric formula is inconstanr). Isometry: occurs when there is no change in shape with size increase; that is, when the traits being compared on a log-log plot yield a straight line with a slope (k in the allometric formula) that is effectively equal to 1. Negative and positive allometry: occur where there is significant change in shape with size increase; in negative allometry, trait y on a log-log plot is increasing more slowly than trait x (slope, k < 1) ; in positive allometry, the opposite is true (slope, k > 1). Vertical allometry: the " noise" around the best-fit line estimating size and shape change: such deviations represent importanr information about " local" effects (e.g. , local environmental influence on growth rates, dietary differences). Basic kinds of allometric comparisons include the following. Cross-sectional: size and shape change when comparing different indi­ viduals of differenr ages. Longitudinal: size and shape change in one individual as it grows through time. Static: size and shape change when comparing individuals of 383 384 GLOSSARY roughly the same age or stage (usually adults). All three kinds of comparisons can compare intraspecific or interspecific change. Of the three comparisons, longitu­ dinal is usually preferred, but the most difficult to make. (Chapter 2) Astogeny: Growth and development of a colony (e.g., of bryozoans, corals), as opposed to the individual; like individuals, colonial development can show heterochronies. (Chapter 6) Atavism: Appearance of an ancestral structure that had formerly been "suppressed" or unexpressed; in other words, a " throwback," e.g., horses born with lateral " toes" or snakes with rudimentary legs. (Chapter 5) Bauplan: The basic architecture ("body plan") of a major group (usually phylum) , estab­ lished early in the group's hisrory, usually in the late Precambrian to early Paleozoic metazoan radiation. (Chapters 6 and 8) Bimaturism: Dimorphie size and shape differences caused by different timing of matura­ tion in males versus females; common in primates, including humans. (Chapters 4 and 7) Biogenetie law: Haeckel's famous " ontogeny recapitulates phylogeny," although that is not his exact formulation; in a very coarse way, there is some validity to this since evolutionary eomplexity must usually build on previous contingencies. See reeapitu­ lation. (Chapters 1 and 8) Breadth: The number of eells or traits affected by an ontogenetic change; contrast with depth. (Chapters 3 and 8) Canalization: Buffering of developmental " program" against perturbations. (Chapter 4) Cladogenetie asymmetry: The "nowhere but up" process whereby a group (clade) originates at a character state (e.g., size) that is physically restricted to expansion mainly in one direction (e .g., a mammal can only get so small before encountering prohibitive metabolie problems); the descendant radiation is therefore asymmetrie in that charaeter state (e.g., "Iarger size"), the resulting "trend" is an inerease in variance or maximum value of state. (Chapters 6 and 8) Condensation: The shortening of ontogeny by " teleseoping" stages; strict recapitulation­ ists thought it necessary, to keep omogenies from becoming too lengthy as terminal additions accumulated. (Chapter 1) Cope's rule: Evolutionary tendeney of many fossillineages to inerease in body size, and of many clades to increase in maximum body size of largest species in clade ("in­ crease in variance," see cladogenetie asymmetry) . (Chapters 6 and 8) Critieal period: A limited time when an outside stimulus is required for development of a behavior. (Chapter 7) C-value: The mass of DNA in an unreplicated genome, also known as genome size; affects cell size and developmental rates. (Chapter 6) Depth: The degree to whieh cells or traits are changed, i.e., the amount of aceeleration, or early onset (for example), relative to ancestral cells or traits; contrast with breadth. (Chapters 3 and 8) Developmental constraint: Role of ontogenetic " rules" in determining the course of GLOSSARY 385 evolution; the degree of "intrinsic" control over evolutionary direction (in contrast to "extrinsic" natural selection). The importance of constraint, relative to selection, is a matter of much current debate, awaiting further study of developmental sys­ tems, especially with the aid of quantitative genetics. (Chapters 4-6 and 8) Developmental eonversion: Specific environmental cues activate one of a number of alternative genetic programs controlling development. (Chapter 4) Developmental gene: A gene active only during development; half of an oversimplified dichotomy with maintenance gene. (Chapter 3) D-gene: A "developmental" gene; this includes both the genes that directly produce enzymes (e.g., morphogen) participating in development (structural D-gene), and the genes that regulate them (regulator D-gene); contrast with R-gene. (Chapter 3) Differentiative heterochrony: Change in rate or timing of development before final differentiation of tissue/organ occurs; therefore, "novel" changes can occur, other than simply size/shape change (as occurs in growth heterochronies). There are two kinds of differentiative heterochronies. Novel differentiative heterochronies result in significant tissue alterations (cell juxtapositions; "disjunctions" in ancestral morphospace) by creating new cell interactions (e.g., inductions by changing time of migration of some cells) and/or preventing old ones; in regulative developmental programs, this can be very "creative" due to cell pluripotency. Size differentiative heterochronies result in only size/shape ("allometric") change; even though they act before differentiation, such heterochronies (unlike novel differentiative heter­ ochronies) do not change kinds of cell interactions, only the end result of old ones (e.g., number of cells allocated to organs). All types ean act wirh varying degrees of extent, or breadth. (Chapter 3) Differentiative phase: The second phase of development (after neofertilization and before the growth phase); the phase when the zygotic genome begins to take over from maternal contro!, and culminating in organogenesis and tissue differentiation; includes a complex series of mitosis, cytodifferentiation and cell migration. (Chapter 3) Diffuse coevolution: The tendency for so me large groups (e.g., higher taxa such as mammalian predators and ungulates) to affect each other's evolution in a general, loosely integrated way (see Futuyma, 1986a). (Chapter 8) Direet size seleetion: Natural selection acting directly on body size; contrast with indi­ reet size selection, where selection is on life history events and body size change is a by-product. (Chapter 6) Dissociated heteroehrony: An ontogenetic change in rate or timing of a trait Ce.g., organ, or even bioehemical trait) that does not oeeur in some other traits; a change that occurs only in a local growth field; contrast with global heterochrony, see heter­ ochronocline (dissociated). (Chapter 2) Dwarfism: Evolutionary size decrease with no change in shape; the smaller descendant is the same in all proportions (isometrie); opposite is giantism. Both dwarfism and giantism are probably rare, as significant size change requires allometric (dispro­ portionai) scaling of body parts. (Chapters 2 and 6) Ecological and evolutionary cascades: The sometimes " rapid" cascading effects on an ecosystem (and hence its selective regime and hence its evolution) caused by the 386 GLOSSARY introduction of some " new" biotic element (Gray, 1988); humans do this artifi­ cially: to what extent does development introduce such " new" inputs? (Chapter 8) Ecophenotypic plasticity: The degree tO which a single genotype may show phenotypic variation under differing environmental conditions (contrast to "phenotypic varia­ tion"). (Chapter 4) Evolutionary ratchet: As organisms evolve, especially those that become more complex, there is an accumulation of contingencies: an increasing interdependence among its components, making radical change more difficult, i.e., a " hardening" occurs. Levinton (1988) suggests three kinds of evolutionary ratchets. The genetic ratchet refers to genetic contingencies: genes in the genome become more interactive and integrated through time. The epigenetic ratchet