terphase and condenses during mitosis, reaching a maximum density at metaphase. In polytene chro- mosomes, the banded segments contain euchroma- tin. See heterochromatin. eugenics the improvement of humanity by alter- ing its genetic composition by encouraging breeding of those presumed to have desirable genes (positive eugenics), and discouraging breeding of those pre- sumed to have undesirable genes (negative eugen- ics). The term was coined by Francis Galton. Euglena gracilis a flagellated protoctist belonging to the Euglenida (See Appendix A). It was in this spe- cies that circular molecules of DNA from chloro- plasts were first observed. Later when the complete nucleotide sequence of chDNA was determined, genes were identified with introns and twintrons (in- trons within introns). See Appendix C, 1961, Man- ning et al.; 1972, Pigott and Carr; 1993, Hallick et al. eukaryon the highly organized nucleus of an eu- karyote. eukaryote a member of the superkingdom Eukary- otes (q.v.). Eukaryotes the superkingdom containing all or- ganisms that are, or consist of, cells with true nuclei bounded by nuclear envelopes and that undergo meiosis. Cell division occurs by mitosis. Oxidative enzymes are packaged within mitochondria.
The su- perkingdom contains four kingdoms: the Protoctista, the Fungi, the Animalia, and the Plantae. See Appen- dix A, Eukaryotes; Appendix C, 1937, Chatton; TATA box-binding protein. Contrast with Prokaryotes. eumelanin one of the pigment molecules found in the coat and pigmented retinal epithelium of mam- mals. It is derived from the metabolism of tyrosine and is normally black in color.
Its precise coloration is affected by a large number of mutant genes. See agouti, MC1R gene, melanin. Eumetazoa the subdivision of the animal kingdom containing organisms possessing organ systems, a mouth, and digestive cavity. See Appendix A. euphenics the amelioration of genotypic malad- justments brought about by efficacious treatment of the genetically defective individuals at some time in their life cycles. euploid a polyploid cell or organism whose chro- mosome number is an exact multiple of the basic number of the species from which it originated. See polyploidy. eupyrene sperm See sperm polymorphism. eusocial a social system in which certain individu- als incur obligate sterility, but enhance their fitness by aiding their collateral kin to rear their offspring. For example, sterile female worker bees may rear the offspring of their fertile sister queens. Eusocial animals exhibit (1) cooperative care of the young; (2) a reproductive division of labor castes are sterile or less fecund); and (3) an overlap of at least two generations (i.e., the offspring assist the parents during some period of their lives). Many eusocial species, such as bees and ants, are haplodi- ploid (q.v.).
However, in eusocial naked mole rats and snapping shrimp (Synalpheus regalis) both males and females develop from fertilized (diploid) eggs. See inclusive fitness. euthenics the control of the physical, biological, and social environments for the improvement of hu- manity. eV electron volt. evagination an outpocketing. eversporting referring to a strain characterized by individuals that, instead of breeding true, produce variations of a specific sort in succeeding genera- tions. Such strains generally contain mutable genes. evocation the morphogenetic effect produced by an evocator (q.v.).
evocator the morphogenically active chemical emitted by an organizer. evolution those physical and biological changes that take place in the environment and the organ- isms occupying it during geological time. The organ- isms undergo hereditary transformations in their form, functioning, and behavior making their de- scendants different from their ancestors in adaptive ways. Therefore lineages accumulate successive ge- netic adaptations to the way of life that characterizes the species. Potentially reversible changes in the gene pool of a population constitute microevolution. Irreversible changes result in either anagenesis (q.v.) or cladogenesis (q.v.).
The production of novel adap- tive forms worthy of recognition as new taxa (e.g., the appearance of feathers on a reptilian ancestor) is considered to define a new taxon—Aves (birds). Such changes are classified as macroevolutionary. See adaptive radiation, Archaeopteryx, cladogram, concerted evolution, Darwin’s finches, Dollo law, founder effect, genetic drift, gradualism, intron ori- gins, in vivo evolution, Linnean Society of London, mtDNA lineages, natural selection, Origin of Species,
orthogenesis, parasite theory of sex, punctuated equi- limbrium, sexual selection, speciation. evolutionarily derived character See phylogenetic classification. evolutionarily primitive character See phyloge- netic classification. evolutionary clock See DNA clock hypothesis, pro- tein clock hypothesis. evolutionary rate rapid evolution is called tachy- telic, slow evolution is called bradytelic, and evolu- tion at an average rate is called horotelic.
evolutionary studies: milestones See Appendix C, 1858, Darwin and Wallace; 1859, Darwin; 1868, Huxley; 1872, Gulick; 1876, Wallace; 1891, Tutt; 1911, Robertson; 1917, Winge; 1930, Fisher; 1931, Wright; 1932, Haldane; 1936, Sturtevant and Dobz- hansky; 1937, Dobzhansky, Chatton; 1945, White; 1947, Mourant; 1948, Clausen, Keck, and Hiesey; 1952, Patterson and Stone, Bradshaw; 1955, Flor; 1956, Brown and Wilson; 1958, Kettlewell; 1961, von Eherenstein and Lipman; 1962, Zukerkandl and Pauling; 1963, Margoliash, Mayr; 1964, Hamilton; 1967, Spiegelman, Mills, and Peterson, Fitch and Margoliash; 1968, Kimura, Wright; 1972, Kohne, Chisson, and Hoger; 1974, Stebbins; 1977, Woese and Fox; 1978, Schwartz and Dayhoff; 1981, Mar- gulis; 1983, Kimura and Ohta; 1985, Carson; 1987, Cann, Stoneking and Wilson; 1988, Kazazian et al.; 1990, Maliki, Schughart and McGinnis; 1991, Bal- dwin et al., Sogin, Ijdo et al.; 1992, Rivera and Lake, Haig; 1993, Baldauf and Palmer; 1994, Morral et al.; 1995, Nilsson and Pelger, Wilson and Szostak, Horai et al.; 1996, Burgher et al.; 1997, Krings et al.; 1999, Petren, Grant, and Grant; 2000, Lemieux, Otis and Thurmel, Singh and Kulathinal; 2001, Masden et al., Nachman, Hoehstra and D’Agostino; 2004, Rice et al. ewe a female sheep. See ram.
exaptation a character that provides a selective advantage under current conditions but had a differ- ent original function. For example, the metabolic enzymes aldehyde dehydrogenase, glutathione trans- ferase, and transketolase have been appropriated (have undergone exaptation) as lens crystalins. Parts of the old reptilian jaw became the ear bones of mammals. The term predaption (q.v.) is also used for molecules or more complex structures that were present earlier and then switched function in re- sponse to changed selective pressures. Exaptation is the preferred term, since it does not suggest fore- sight or preplanning for the ultimate use of the structure. See aptation.
exchange pairing the type of pairing of homolo- gous chromosomes that allows genetic crossing over to take place. Synaptonemal complexes (q.v.) play a critical role in exchange pairing. excision the enzymatic removal of a polynucleo- tide segment from a nucleic acid molecule. See im- perfect excision. excisionase an enzyme required (in cooperation with an integrase) for deintegration of prophage from the chromosome of its bacterial host.
excision repair See cut-and-patch repair, repair synthesis. excitation the reception of a quantum of energy by an atomic electron: an altered arrangement of planetary electrons in orbit about an atom resulting from absorption of electromagnetic energy. exclusion principle the principle according to which two species cannot coexist in the same local- ity if they have identical ecological requirements. exclusion reaction the healing reaction of a phage-infected bacterium that strengthens its enve- lope and prevents entry of additional phages. exconjugant 1. ciliates (e.g., paramecia) that were partners in conjugation and therefore have ex- changed genetic material. 2. a female (F−) recipient bacterial cell that has separated from a male (Hfr) donor partner after conjugation and therefore con- tains some of the donor’s DNA. exergonic reaction a reaction proceeding sponta- neously and releasing energy to its surroundings. exocrine referring to endocrine glands that release their secretory products into ducts that open on an epithelial surface. Examples would be the sweat glands and glands secreting mucus.
Compare with endocrine system. exocytosis the discharge from a cell of materials by reverse endocytosis (q.v.). exogamy the tendency of an individual to mate se- lectively with nonrelatives. Contrast with endogamy. exogenic heredity transmission from generation to generation of information in the form of knowl- edge and various products of the human mind (i.e., books, laws, inventions, etc.).