BACILLARIOPHYTA. Diatoms. Division of the ALGAE. Microscopic unicellular plants, occurring singly or grouped in colonies. In additionto chlorophylls a and c, chloroplasts contain Cc, p and E carotenes,and xanthophylls, including fucoxanthi& Cells surrounded by rigid,siliceous and finely sculptured cell wall or two parts (valves).Asexual reproduction is by cell division; sexual repr-oduction iso gamous or aniso gamous, resulting in the characteristic auxospore. Abundant in marine and fresh waters, both plankton’and benthos. Past deposition of countless numbers of silicified cell walls has formed siliceous or diatomaceous earths, while oil stores of past diatoms have contributed to the petroleum supplies of today. ~Extremely important microfossils in palaeolimnology, enabling interpretationsof past lake histories. BACILLUS. General term for any rod-shaped bacterium. Also a genus of bacteria: Bacillus.
BACKBONE. SEE VERTEBRAL COLUMN.
BACKCROSS: Cross (mating) between a parent and one of its offspring. Employed in CHROMOSOME MAPPING, when the parent is homozygous and recessive for at least. two character traits, and to ascertain genotype of..offspring (i.e: whether homozygous or hetero? zygous for a character), the parent used being the homozygous recessive. Where the organism of known genotype in the cross is not a parent of the other, the term testcross is often used.
BACTERIA. Unicellular, filamentous and mydelial PROKARYOTES, of the Kingdom Monera. Among the simplest of all known organisms (see CELL for diagram). O$nions differ on whether to include the blue-green algae within bacteria (see CYANOBACTERIA). Work on ARCHAEBACTERIA Suggests that these form a distinctive side-branch. The description which follows relates to ‘true’ bacteria, or EUBACTERIALES, which vary greatly in shape, being rod-like (bacilli), spherical (cocci), more or less spiral (spirilli), filamentous and occasionally mycelial. Multiplication is by simple fission; other forms of asexual reproduction, e.g. production of aerially dispersed spores or figellated swarmers, occur in some i>acteria. As prokaryotes, they have no meiosis or syngamy, but genetic recombination occurs in many of them (see F FACTOR, PILI, PLASMID, RECOMBINATION). Bacteria are ubiquitous, occurring in a large variety of habitats. of soil may contain from a few thousand to several million; 1 cm3 of BACTERICIDAL sour milk, many millions. Most are saprotrophs or parasites; but a few are autotrophic, obtaining energy either by oxidation processes or from light (in the presence of bacteriochlorophyll). In soil, their activities are of the utmost importance in the decay of dead organic matter and return of minerals for higher plant growth (see DECOMPOSERS, CARBON CYCLE, NITROGEN CYCLE). Some are SOUrCeS Of ANTIBIOTICS (e.g. Streptomyces griseus produces streptomycin). As-agents of plant disease bacteria are less important , than fungi; but they cause many diseases in animals and humans (e.g. diphtheria, tuberculosis, typhoid, some forms of pneumonia). See ANTISEPTIC,, BACTERICIDAL, BACTERIORHODOPSIN, ESCHERICHIA COLI, GRAM’S STAIN, MYCOPLASMAS, SPIROCHAETES.
BACTERICIDAL. Substances which kill bacteria. Includes many ANTIBIOTIC, see PLAsMID. Compare BACTERIOSTATIC. BACTERIOPHAGE (PHAGE). a VIRUS parasitizing bacteria. The genetic material is always housed in the centre of the phage particle and may be DNA or RNA the former either double or single stranded, the latter double-stranded. RNA phages are very simple; the more complex T-even DNA phages have a head region, collar, tail and tail fibres; are entirely spherical; others are filamentous. contain a lipid bilayer between an outer and inner protein shell. The complex phage T4 has a polyhedral head about 70 nm across containing double-stranded DNA, enclosed by a coat (capsid) of protein subunits (capsomeres); a short collar or ‘neck’ region; a cylindrical tail (or tailsheath) region 1 and six tail fibres. The particle (viron) of T4 phage is quite large - about- 300 nm in length. It initiates infection by attachment of tail fibres to the bacterial cell wall at specific receptor sites. This is followed by localized lysis of the wall by previously inert viral enzymes in the tail and by contraction of the viral tail sheath, forcing the hollow tail core through the host cell wall to inject the phage DNA. Virulent phuges engage in a subsequentcy insidethe hoti~cell Their- circular genomes -are transcribed and translated, causing arrest of host macromolecule synthesis and production of virion DNA and coat proteins. Lysis of the cell releases the virions. Filamentous phages do not lyse the host cell, but even permit it to multiply. Eventually they get extruded through the cell wall. Some temperatephages can insert their genome into their host’s and be replicated with it; others replicate within a bacterial plasmid. This non-lethal infective relationship does not involve transcription of phage genome, and is termed lysogeny, the phage being termedprophuge. Lysogenic bacteria (those so infected) can produce infectious phage, but are immune to lytic infection by the same or closely related phage (superinfection immunity). Conversion from lysogenic state to lytic cycle (induction)
BACTERIO RHODOPSIN. Conjugated protein of the ‘purple membrane’ of the photosynthetic bacterium Halobacterium halobium, forming a proton channel whose prosthetic group (retinal) is light-absorbing and identical to that of vertebrate rod cells. Allosteric change on illumination of the pigment‘ results in proton ejection from the cell, the resulting proton gradient being used to power ATP synthesis. Its integration into LIPOSOMES along with mitochondrial AT P synthetase showed that the latter is also a proton channel, driven by a proton gradient. If chlorophyll-based photosynthesis evolved after a rhodopsin-based variety then absence of green-absorption in chlorophyll’s action, spectrum may be due to selection in favour of a pigment avoiding competition with abundant rhodopsin- based forms. See CELL MEMBRANES, CHLOROPLAST, ELECTRON TRANSPORT SYSTEM, MITOCHONDRION, ROD CELL.
BACTERIOSTATIC. Inhibiting growth of bacteria, but not killing them. See BACTERICIDAL.
BALANCED LETHAL SYSTEM. Genetic system operating when the two homozygotes at a locus represented by two alternative alleles each produces a lethal phenotype, yet the two alleles persist in the population through survival of heterozygotes, which thus effectively breed . Compare HETEROZYGOUS ADVANTAGE.
BALANCE OF NATURE. Phrase glossing observations that in natural ecosystems, communities and the biosphere at large, herbivores do not generally overgraze, predators do not generally over-predate nor parasites decimate host populations; that populations of all appear to be held roughly in equilibrium, and that drastic (sometimes trivial) disturbance of this harmony between organisms and the physical environment will have inevitable and generally unfavourable consequences for mankind. Causal processes involved in the complex systems with which ecologists deal are increasingly amenable to computer simulation. See DENSlTY DEPBNDENCE, NEGATIVE FEEDBACK.
BALANOGLOSSUS (GLOSSOBALANUS). Genus of acorn worms. Wormlik members of the HEMICHORDATA, with vertebrate affinities.
BALBIANI RING. See PUFF. ’ BALDWIN- EFFECT
BALBIANI RlNG may be enhanced by UV light and other mutagens. A replication cycle from adsorption to release of new phage takes /1: 15-20 mins.Transduction occurs when temperate phage from one lysogenic culture infects a second bacterial culture, taking with it a small amount of closely-linked DNA which remains as a stable feature of the recipient cell. Antibiotic resistance can be transferred this way (see ANTIBIOTIC RESISTANCE ELEMENT). See PHAGE CONVERSION, PHAGE RESTRICTION.
BALDWIN EFFECT. Reinforcement or replacement of individually aeguired adaptive responses to altered environmental conditions, through ,selection (artificial or natural) for, genetically determined characterswith similar functions. Not regarded as evidence of Lamarckism. See GENETIC ASSIMILATION,LAMARCK.
BALEEN. ‘Whalebone’; thin sheets of cornified skin hanging from the roof of the mouth of the largest (whalebone) whales, with which food is filtered. Such whales are toothless.
BARBS. Side-branches in a row on each side of the rachis of a contour FEATHER making up the expanded pennaceous part (vane), and giving off barbules.
BARBULES. Minute filaments lying in two rows, one proximal and one distal to each barb of a’contour FEATHER, those distal bearing hooks which slot into grooves on the proximal barbules a$ hnking barbs together, as occurs during preening.BARK. Protective corky tissue of dead cells, present on the outside of older stems and roots of woody plants, e:g. tree trunks. Produced by activity of cork cambium. Bark may consist of cork only or, when other layers of cork are formed at successively deeper levels, of alternating layers of cork and dead cortex’ or phloem tissue (when it is known BS rhytidome), Popularly regarded as everything outside the wood.
BARORECEPTOR (PRESSORECEPTOR). Receptor for hydrostatic pressure of blood, In man and most tetrapods, located in carotid sinuses, aortic arch and wall of the right atrium. Basically a kind of stretch receptor. When stimulated, those in the first two locations activate the cardio-inhibitory centre and inhibit the CARDIO ACCELERA TORY CENTRE, while those in the atrium stimulate the cardio-acceleratory centre, helping to regulate blood pressure. BARR BODY. Heterochromatic X-chromosome occurring in female placental mammals. Paternally- or maternally-derived- X-chromosomes may behave in this way, often a different X-chromosome in differentcelllineages. See HETEROCHROMATIN
BASAL BODY. Structure indistinguishable from centriole, acting as an organizing centre (nucleation site) for eukaryoticcilia and flagella, unlike which its ‘axoneme’ is a ring of nine triplet microtubules, each comprising one complete microtubule fused to two incomplete ones. It is a permanent feature at the base of each such flagellum or CILIUM. See CENTRIOLE fordetails.
BASAL LAMINA. Thin layer of several proteins, notably collagen and the glycoprotein laminin, about 50-80 nm thick, varying in composition from tissue to tissue. Underlying and secreted by sheets of animal epithelial cells and tubes they may farm (e.g. many glands, and endothelial linings of blood vessels). In kidney glomerulus and
lung al,veolus, ,may be an important selective filter of molecules betweencellsheets: See
BASEMENT MEMBRANE.
BASAL METABOLIC RATE (BMR). The respiratory rate of a resting animal, normally measured by oxygen demand. The ‘background’ respiration rate, as required for unavoidable muscle contractions (e.g. heart), growth, repair, temperature maintenance, etc. See THYR0ID HORMONES.
BASAL PLACENTATION. (Rot.) Condition in which ovules are attached to the bottom of the locule in the ovary.BASE. Either a substance releasing hydroxyl ions (OH-) upon dissociation, with a pH in solution greater than 7, or a substance capable of acting as a proton acceptor, In this latter sense, the nitrogenous cyclic or heterocyclic groups combined with ribose to form nucleosides are termed bases. See PURINE, PY RIMIDINE.
BASEMENT MEMBRANE. Combination of BASAL LAMINA with underlying reticular fibres and additional glycoproteins, situated between many animal epithelia and connective tissue.
BASE PAIRING. Hydrogen bonding between appropriate purine and pyrimidine bases of (antiparallel) nucleic acid sequences, as during DNA synthesis Mrna transcription and during translation(see-TEIN SYNTHESIS). If two DNA strands align then adenine in one strand pairs with a thymine and a guanine with a cytosine (i.e. A:T, G : C); but if a D N A strand aligns with an RN A strand, then adenines in the DNA pair align with uracils in the RNA (i.e. A:U, G:C). Without these ‘rules’ there could be no GENETIC CODE or heredity asweknowit. See DNA HYBRIDIZATION.
BASE RATIO. The (A + T):(G + C) ratio in double-stranded (duplex) DNA. It varies widely between different sources (i.e. from different species). The amount of adenine equals the amount of thymine; the ‘amount of, guanine equals the amount of cytosine. See BASE PAIRING.
BASIC DYES. Dyes consisting of a basic organic grouping of atoms (cation) which is the actively staining part, usually combined with an inorganic acid. Nucleic acids, hence nuclei, are stained by them and are consequently referred to as BASOPHILIC.
BASIDIOMYCOTINA. Subdivision of the fungi (EUMYCOTA), known informally as ‘basidiomycetes’. Contains a large variety of species (e.g. jelly fungi, bracket fungi, smuts, rusts, mushrooms, toadstools, puffballs). Most of these common names refer to the visible part of the fungus, the conspicuous reproductive or ‘fruiting’ body (basidiocarp) supported nutritionally by an extensive assimilative mycelium that penetrates the plant or soil and derives nutrients. Primarily
BASIDIOSPORE- terrestrial, with perforated septa (cross-walls) in their hyphae. Complete septa cut off the reproductive bodies. Chitin predominates in the hyphal walls; sexual reproduction involves formation of basidia. BASIDIOSPORE. Characteristic spore type of Basidiornycotina, produced within the BASIDIUM by meiosis.
BASIDIUM Specialized reproductive cell of Basidiomycotma; often club-shaped, cylindrical, or divided into four cells. Nuclear fusion and meiosis occur within it, resulting in formation of four basidiospores borne externally on minute stalks called strigmata.
BASIPETAL. (Bot.) (Of organs.) Developing in succession towards the base, oldest at the apex, youngest at the ’ base. Also used of the direction of transport of substances within a plant: away from -apex. Compare ACROPETAL.
BASOPHIL. Blood POLYMORPH. Very similar to MAST CELL in structure and’ probably function, but with peroxidase rather than acid and alkaline phosphatase activity in its granules.
BASOPHILIC. Staining strongly with basic dye, Expecially characteristic of nucleic acids, and hence of nucleus (during division of which the condensed chromosomes are strongly basophilic), and of cytoplasm when actively synthesizing proteins (due to rRN A and mRN A).
BATRACHIA. Rarely used term for AM'PHIBIA.
B-CELL (B LYMPHOCYTE). ALBUCOCYTE, derived from a LYMPROID TISSUE stem cell which has migrated fr”om foetal liver to bone marrow and has not entered the thymus but has settled either in a lymph node or in the spleen. B-cells express a specific immunoglobulin (Ig, or ANTIBODY) on their plasma membranes. This can bind appropriate antigen, when the cell becomes activated to divide repeatedly by mitosis and produce a clone’ of specific antibody-coated cells. This primary immune response (see IMMUNITY) is also characterized by their secretion of specific IgM antibody into the blood, J’the combined effect being to remove antigen. Some B-cells do not greatly participate in Ig production, but circulate in the body and may persist for years as memory cells, capable of clonal expansion and rapid Ig secretion (a secondary immune response) if activated by the initial antigen. This provides immunological memory. Still other B-cells mature into plasma cells (the major Ig-producer in a secondary response) after multiplication, and although these progeny cells may have Igs of more than one class, they all have the same antigen . specificity. A fully mature plasma cell will have little surface Ig but will be secreting an Ig of one class and of one antigen specificity. See IgA-IgM, MYELOID TISSUE,T-CELL.
B - C H R O M O S O M E . SEE SUPERNUMERARYCHROMOSOME.
BETA-GLOBULINS BELT D~~SMOSOME. See DEMOSOME.
BENEDICT’S TEST. A modification of Eehling’s test for sugar using just one solution. Contains sodium citrate, sodium carbonate and copper sulphate dissolved in water in the ratio, 1.7 : 1 .O : 0.17 g : 30 cm3 water. Five drops of test solution are added to 2 cm3 Benedict’s solution. If a REDUCINS SUGAR is present then a rust-brown cuprous oxide precipitate forms on boiling.
BENNETTTALES (see ADEOIDALES). ’ Extinct fossil gymnophytes, present from the Triassic to the CretaceousBesembled cyads (Cycadales), with leaves entire or pinnate. Epidermal cells differed from , cycads in having sinuose cell walls, the guard cells and subsidiary cells originating from the same mother cell. Further, in most forms, the cones were bisexual, with both micro- and megasporophylls.They may have self-pollinated.
BENTHOS. General term referring to those animals and plants living on the bottom of the sea, lake or river (crawling or burrowing there, or attached as with sea weeds and sessile animals), from high Water mark down to deepest levels. There have been many schemes for _ subdividing the benthos. With respect to aquatic higher plants and ’ algae the benthos can be subdivided as follows: (1) Rhizobenthos - vegetation rooted in the sediments, e.g. Chara, submersed and emergent aquatic higher plants. (2) Haptobenthos - plants attached to solid ~.surfaces. (3’) Herpobenthos - the community living on, or moving through,sediments, e.g. ENDOPELON, EPIPELON. (4) Endobenthos - the community living and often boring into solid substrata, e.g.
endolithon living inside rock. Organisms feeding primarily upon . the benthos are termed benthophagous. See PELAGIC, NEKTON, PLANKTON
BERGMANN’S RULE., States. that in geographically variable species of HOMOIO THERMI c animals, body size tends to be larger in cooler ir , :regions pf a species range. See ALLEN’ S RULE..
BERRY. Many-seeded - succulent fruit, in ‘which the wall (carp) consists of an outer skin (epicarp) enclosing a thick fleshy mesocarp and inner membranous endocarp, as in gooseberry, currant, tomato. Compare DRUPE
BETA-BLOCKER. Substance, such as the drug propanolol, selectively blocking BETA RECEPTORS. Clinical use is to slow heart rate and lower blood pressure. See OPIATES. BETACELLS. See PANCREAS.
BETA-GLOBULINS. A class of vertebrate plasma proteins including certain lipoproteins, TRAN SFERRIN and plasminogen (precursor of fibrinolysin, see FISRINOLYSIS).
BETA RECEPTOR ADRENERGIC receptor sites (associated with adenylate cyclase in appropriate postsynaptic membranes, but not identical with it) binding preferentially to adrenaline rathei than noradrenaline. Heart muscle has beta receptors predominantly which result in ‘increased heart rate and blood pressure when stimulated; other beta effects include dilation of arterioles supplying skeletal muscle, bronchial relaxation and relaxation of the uterus. All these effects are mimicked by the synthetic drug isoprenaiine. See ALPHA RECEPToR ,BETABLOCKER ,AUTONOMIC NERVOUS SYSTEM. BETA-SHEETS (BETA-CONFORMATION). On 6 type of protein secondary Structure.SeePROTEIN,AMINOACID
B-FORM HELIX. Paracrystalline form of DNA* (DNA-B) adopted in A-FORM and Z-FORM HELIX.
BICOID GENE. (bed). A pattern-specifying MATERNAL GENE in Drosophila, whose mRNA transcript passes From maternal ovary nurse cells to anterior poles of developing oocytes where it becomes ’ localized, apparently trapped by the oocyte cytoskeleton. Its eventual protein product is a transcription activator (with a DNA-binding homoeodomain), is located in nuclei of the syncytial blastoderm, and is translocated half-way along the embryo from the anterior pole.With a similar translocation of OsKAR GENE product in the opposite i ” direction, two opposing gradients of maternally encoded proteins provide quantitative POSITIONAL INFORMATION which the embryo genome converts into qualitative phenotypic differences. Genes involved in this conversion process, i.e. Kriippel (Kr), @nchback (hb) and knirps (kni), belong to the GAP GENE class, and all three encode proteins with DNA-binding FINGER DOMAINS.
Bicoid product is responsible for production of a normal head and thorax, structures absent in embryos from females lacking functional bed, and both it and the oskar product are examples of MORPHOGENS. By repressing transcription of Kr in anterior and posterior embryo regions, bicoid and o&r morphogens- only permif Kr expression in mid-embryo. High lev& of bcdprodvhb transcription, while low levels permit Kr transcription. However, their abilities to elicit such zygotic (embryo) gene expression depend upon the affinities of their respective DNA-binding sites for them, and these are known to vary. The hb and Kr products are mutual repressors of each others’ transcription, so that their ‘domains’ of prod&t expression become stable and restricted. These gap gene domains delimit the domains of HOMOEOTICGENE expression but also enable position-specific regulation of the PAIR-RULE GENES, whose expression leads to metameric segmentation of the embryo.
BICELLETERAL BUNDLE. See VASCULAR BUNDLE.
BIENNIAL. Plant requiring two years to complete life cycle, from seed BINOMIAL NOMENCLATURE germination to seed production and death. In the first season, biennials store up food which is used in the second season to produce flowers and seed. Examples include carrot and cabbage. Compare ANNUAL,EPHEMERAL.
BILATERALCLEAVAGE. Old term for radial cleavage. See CLEAVAGE.
BILATERAL SYMMETRY. Property of most metazoans, having just one plane in which they can be separated into two halves, approximately mirror images of one’ another. This is usually the antero-posterior and dorso-ventral plane, separating left and right halves. Major metazoan phyla excluded are coelenterates and echinoderms (having
radial symmetry as adults). In flowers, the condition is termed zygomorphy.BALE. Fluid produced by vertebrate liver cells (hepatocytes), containing both secretory and excretory products, and passed through bile duct to duodenum. Contains (i) BILE s A L TS (e.g. sodium taurocholate and glycocholate) which emulsify fat, increasing its surface area for lipolytic activity. Also form micelles for transport of sterols and unsaturated fatty acids towards intestinal villi (see CHY LOMI -CRON); (ii) bile pigments, breakdown products of haemoglobin such as bilirubin, which are true excretory wastes and colour faeces; (iii) LECITHIN and CHOLESTER0L as excretory products. Bile is aqueous and alkaline due to NaHCO3, providing a suitable pH ,for pancreatic and subsequent enzymes. Stored in gall bladder. See MICELLE.
BILE DUCT. Duct from liver to
BILE.& GALLBLADDER. duodenum of vertebrates, conveying
BILE SALTS. Conjugated compounds of- bile acids (derivatives of CHOLESTEROL and taurine -and glycine), forming up to two thirds of dry’mass of hepatic BILE (BILLUS).
BILHARZIA. Schistosomiasis. See SCHISTOSOMA.
BINARY FISSION. Asexual reproduction occurring when a single divides into two equal parts. Compare MULTIPLE FISSION. cell
BINOCULAR VISION. Type of vision occurring in primates and many other active, predatory vertebrates; eyeballs can be so directed that an image of an object falls on both retinas. Extent to which eyes converge to bring images on to the foveas of each retina gives proprioceptive information needed in judging distance of object.Stereoscopic vision (perception of shape in depth) depends on two slightly different images of an object being received in binocular vision, the eyes viewing from different angles.
BINOMIAL NOMENCLATURE. The existing method of naming organisms scientifically and the lasting contribution to taxonomy of LINNAEUS. Each newly described organism (usually in a paper published in a recognized scientific journal) is.placed in a genus, which gives it the, first of its two (italicized)- latin names - its generic name - and is always given a capital first letter. Thus the genus- Canis would probably be given to any new placental wolf discovered. Within this genus there- may or may not be other species already described; in any case, the new species will receive a second (specific) name’ to follow its generic name, this time with a lower case first letter.,“The wolf found in parts of Europe, Asia , North America belongs to the species Lupus; jackals, coyotes-and dogs (including the domesticated dog) also belong to- the genus , a but each species is ~ further identified by a different specific name: the domestic dog, in all its varieties, is C. familiaris (often only the first letter of the genus is given, if it is contextually clear what the genus is). If a previously described species is subsequently moved into another genus, it takesthe new generic name, but carries the old specific name. The author who ‘published the original description of a species often. Receives credit in “the form of an abbreviation of their name after the initial mention of the species in a paper or article. Thus one might see Canis lupus Linn.,.since Lirinaeus first described this species. The specimen upon which the initial published description was based is termed the - type specimen, or ‘type’, and is probably housed in a useum for comparison with other specimens. If sufficient variation exists within a species range for SUB SPECIES or varieties to ,be recognized, then a trinomial is’ emfiloyed to identify the subspecies or variety. The British wren rejoices under the, trinomial Troglodytes troglodytes .
BIOASSAY. Quantitative estimation of biologically active substances by measurement of their. activity in standardized, conditions on living organisms or their parts. A standard curve’ is first produced, relating the response of the tissue or organism to known quantities of the substance. From this the amount giving a particular experimental response can be read.
BIOCHEMICAL OXYGEN DEMAND (BOD). Amount of dissolved oxygen (in mg/dm3 water) which disappears from a’ tvater sample in a given time at a certain temperature, through decomposition of organic m,atter by microorganisms. Used as an index of organic’ pohution, especially sewage
BICHEMSSTRY. The study of the chemical changes within, and produced by, living organisms. Iucludes molecular biology, or molecular genetics.
BIOGENESIS. The theory that all living organisms arise from”preexisting life forms. The works of Redi (1688) for macroorganisms, and of Spallanzani (1765) and PASTEUR (1860) in particular for microorganisms, stand as landmarks in the overthrow of the theory of SPONTANEOUS GENERATION. Since their time it has become ’* generally accepted that every individual organism has a genetic ancestry involving prior organisms. The first appearance of living systems on the Earth (see ORIGIN OF LIFE) is still problematic, but there,is no reason in principle to dispense with faith in natural and terrestrial causation, and every reason to pursue that line of inquiry.
BIOGENETIC LAW. Notorious view propounded by Ernst HaeckeI in about 1860 (a more explicit formulation of his mentor Muller’s view) that during an animal’s development it passes through ancestral adult stages (‘ontogenesis is a brief and rapid recapitulation of phylogenesis’). Much of the evidence for this derived from the work of embryologist Karl von Baer. It is now accepted that embryos often pass through stages resembling related embryonic, rather than adult, forms. Such comparative embryology provides important evidencefor EVOLUTION. See PAEDOMORPHOSIS.
BIOGEOGRAPHY. The study and interpretation of geographical distributions of organisms, both living and extinct. One approach (dispersal biogeography) stresses the role of dispersal of organisms from a point of origin across pre-existing barriers; another approach (vieariance biogeography) takes barriers which occur within a pre-existing continuous distribution to be more important. See GONDWANALAND, LAURASIA,ZOOGEOGRAPHY.
BIOLOGICAL CLASSIFICATION. See CLASSIFICATION.
BIOLOGICAL CLOCK. In its widest sense, any form of biological timekeeping, such as heart beat or ventilatory movements; more often used in contexts of PHOTOPERIODISM, DORMANCY and DIAPAUSE; but most usually associated with physiological, behavioural, etc., rhythms relating to environmental cycles, notably CIRC ADIAN, tidal, lunar-monthly and annual. Mechanisms ‘counting’ the number of cell divisions that have elapsed since some signal would also be included. See PINEAL GLAND.
BIOLOGICAL CONTROL. Artificial control of PESTS and parasites by use of organisms or their products; e.g. of mosquitoes by fish and aquatic insectivorous plants which feed on their larvae; or of the prickly pear (Opuntia) in Australia by the moth Cactoblastis cactorum. There is increasing use of PHEROMONES in attracting pest insects, which may then be killed or occasionally sterilized and released. Sometimes attempts are made to encourage spread of genetically harmful factors in the pest population’s gene pool, although this needs great care. Success depends on a thorough grasp of relevant
ecology.
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