modifier gene A gene whose phenotypic effect is to modify the effect of another gene. Geneticists no longer make a distinction between two types of genes, ‘major genes’ and ‘modifiers’, but recognize that many (and perhaps most) genes modify the effects of many (and perhaps most) other genes.
monophyletic A group of organisms is said to be monophyletic if all are descended from a common ancestor which would also have been classified as a member of the group. For instance, the birds are probably a monophyletic group since the most recent common ancestor of all birds would probably have been classified as a bird. The reptiles, however, are probably polyphyletic, in that the most recent common ancestor of all reptiles would probably not have been classified as a reptile. Some would argue that polyphyletic groups do not deserve names, and that the Class Reptilia should not be acknowledged.
mutation An inherited change in the genetic material. In Darwinian theory mutations are said to be random. This does not mean that they are not lawfully caused, but only that there is no specific tendency for them to be directed towards improved adaptation. Improved adaptation comes about only through selection, but it needs mutation as the ultimate source of the variants among which it selects.
muton The minimum unit of mutational change. One of several alternative definitions of gene (with cistron and recon).
neo-Darwinism A term coined (actually re-coined, for the word was used in the 1880s for a very different group of evolutionists) in the middle part of this century. Its purpose was to emphasize (and in my opinion exaggerate) the distinctness of the modern synthesis of Darwinism and Mendelian genetics, achieved in the 1920s and 1930s, from Darwin’s own view of evolution. I think the need for the ‘neo’ is fading, and Darwin’s own approach to ‘the economy of nature’ now looks very modern.
neoteny An evolutionary slowing down of bodily development relative to the development of sexual maturity, with the result that reproduction comes to be practised by organisms which resemble the juvenile stages of ancestral forms. It is hypothesized that some major steps in evolution, for example the origin of the vertebrates, came about through neoteny.
neutral mutation A mutation that has no selective advantage or disadvantage in comparison with its allele. Theoretically, a neutral mutation may become ‘fixed’ (i.e. numerically predominant in the population at its locus) after a number of generations, and this would be a form of evolutionary change. There is legitimate controversy over the importance of such random fixations in evolution, but there should be no controversy over their importance in the direct production of adaptation: it is zero.
nucleotide A kind of biochemical molecule, notable as the basic building block of DNA and RNA. DNA and RNA are polynucleotides, consisting of long chains of nucleotides. The nucleotides are ‘read’ in triplets, each triplet being known as a codon.
ontogeny The process of individual development. In practice development is often taken to culminate in the production of the adult, but strictly it includes later stages such as senescence. The doctrine of the extended phenotype would lead us to generalize ‘ontogeny’ to include the ‘development’ of extracorporeal adaptations, for example artefacts like beaver dams.
optimon The unit of natural selection, in the sense of the unit for whose benefit adaptations may be said to exist. The thesis of this book is that the optimon is neither the individual nor the group of individuals but the gene or genetic replicator. But the dispute is in part a semantic one, whose resolution occupies portions of Chapters 5 and 6.
orthoselection Sustained selection on the members of a lineage over a long period, causing continued evolution in a given direction. Can create an appearance of ‘momentum’ or ‘inertia’ in evolutionary trends.
outlaw gene A gene which is favoured by selection at its own locus, in spite of its deleterious effects on the other genes in the organisms in which it finds itself. Meiotic drive (q.v.) provides a good example.
Paley’s watch A reference to the best known of William Paley’s (1743–1805) arguments for the existence of God. A watch is too complicated, and too functional, to have come about by accident: it carries its own evidence of having been purposefully designed. The argument seems to apply a fortiori to a living body, which is even more complicated than a watch. Darwin, as a young man, was deeply impressed by this. Although he later destroyed the God part of the argument, by showing that natural selection can play the role of watchmaker to living bodies, he did not destroy the fundamental point—still under-appreciated—that complicated design demands a very special kind of explanation. God apart, the natural selection of small inherited variations is probably the only agency capable of doing the job.
phenotype The manifested attributes of an organism, the joint product of its genes and their environment during ontogeny. A gene may be said to have phenotypic expression in, say, eye colour. In this book the concept of phenotype is extended to include functionally important consequences of gene differences, outside the bodies in which the genes sit.
pheromone A chemical substance secreted by an individual, and adapted to influence the nervous systems of other individuals. Pheromones are often thought of as chemical ‘signals’ or ‘messages’, and as the inter-body analogue of hormones. In this book they are more often treated as analogous to manipulative drugs.
phylogeny Ancestral history on the evolutionary time-scale.
plasmid One of a set of more or less synonymous words used for small, self-replicating fragments of genetic material, found in cells but outside chromosomes.
pleiotropy The phenomenon whereby a change at one genetic locus can bring about a variety of apparently unconnected phenotypic changes. For instance a particular mutation might at one and the same time affect eye colour, toe length, and milk yield. Pleiotropy is probably the rule rather than the exception, and is entirely to be expected from all that we understand about the complex way in which development happens.
pluralism In modern Darwinian jargon, the belief that evolution is driven by many agencies, not just natural selection. Enthusiasts sometimes overlook the distinction between evolution (any kind of change in gene frequencies, which may well be pluralistically caused) and adaptation (which only natural selection, as far as we know, can bring about).
polygene One of a set of genes each exerting a small, cumulative effect on a quantitative trait.
polymorphism The occurrence together in the same locality of two or more discontinuous forms of a species in such proportions that the rarest of them cannot be maintained merely by recurrent mutation. Polymorphism necessarily occurs during the transient course of an evolutionary change. Polymorphisms may also be maintained in stable balance by various special kinds of natural selection.
polyphyletic See monophyletic.
preformationism As opposed to epigenesis (q.v.) it is the doctrine that the form of the adult body is in some sense mapped in the zygote. One early partisan thought he could discern, with his microscope, a little man curled up in the head of a sperm. In Chapter 9 it is used for the idea that the genetic code is more like a blueprint than a recipe, implying that the processes of embryonic development are in principle reversible, in the same sense as, say, you may reconstruct its blueprint from a house.
prokaryotes One of the two major groups of organisms on Earth (contrast eukaryotes) including bacteria and blue-green algae. They have no nucleus and no membrane-bounded organelles such as mitochondria: indeed one theory has it that mitochondria and other such organelles in eukaryotic cells are, in origin, symbiotic prokaryotic cells.
propagule Any kind of reproductive particle. The word is used specifically when we wish not to commit ourselves over whether we are speaking about sexual or asexual reproduction, about gametes or spores, etc.
r-selection Selection for the qualities needed to succeed in unstable, unpredictable environments, where ability to reproduce rapidly and opportunistically is at a premium, and where there is little value in adaptations to succeed in competition. A variety of
qualities are thought to be favoured by r-selection, including high fecundity, small size, and adaptations for long-distance dispersal. Weeds, and their animal equivalents, are examples. Contrast with K-selection (q.v.). It is customary to emphasize that r-selection and K-selection are the extremes of a continuum, most real cases lying somewhere between. Ecologists enjoy a curious love/hate relationship with the r/K concept, often pretending to disapprove of it while finding it indispensable.
recessiveness Opposite of dominance (q.v.).
recon The minimum unit of recombination. One of several different definitions of gene, but, like muton, it has not yet received sufficient currency to be usable without simultaneous definition.
replicator Any entity in the universe of which copies are made. Chapter 5 contains an extended discussion of replicators, and a classification of active/passive, and germ-line/dead-end replicators.
reproductive value A demographic technical term, a measure of an individual’s expected number of future (female) children.
segregation distorter A gene whose phenotypic effect is to influence meiosis so that the gene has a greater than 50 per cent chance of ending up in a successful gamete. See also meiotic drive.
selfish See altruism.
sex chromosome A special chromosome concerned with the determination of sex. In mammals there are two sex chromosomes called X and Y. Males have the genotype XY, females XX. All eggs therefore bear one X chromosome, but sperms may bear either one X (in which case the sperm will give rise to a daughter) or one Y (in which case the sperm will give rise to a son). The male sex is therefore referred to as heterogametic, the female as homogametic. Birds have a very similar system, except that males are homogametic (the equivalent of XX) and females heterogametic (the equivalent of XY). Genes carried on sex chromosomes are called ‘sex-linked’. This is sometimes confused (e.g. page 10) with ‘sex-limited’, which means having expression in one sex or the other (not necessarily carried on sex chromosomes).
somatic Literally pertaining to the body. In biology it is used for the mortal part of the body, as opposed to the germ-line.
speciation The process of evolutionary divergence whereby two species are produced from one ancestral species.
species selection The theory that some evolutionary change takes place by a form of natural selection at the level of species or lineages. If species with certain qualities are more likely to go extinct than species with other qualities, large-scale evolutionary trends in the direction of the favoured qualities may result. These favoured qualities at the species level may in theory have nothing to do with the qualities that are favoured by selection within species. Chapter 6 argues that although species selection may account for some simple major trends, it cannot account for the evolution of complex adaptation (see Paley’s watch, and macroevolution). The theory of species selection in this sense comes from a different historical tradition from the theory of group selection (q.v.) of altruistic traits, and the two are distinguished in Chapter 6.
stasis In evolutionary theory, a period during which no evolutionary change takes place. See also gradualism.
strategy Like ‘altruism’, used by ethologists in a special sense, almost misleadingly distantly related to its common usage. It was imported from game theory into biology in the theory of evolutionarily stable strategies (q.v.), where it is essentially synonymous with ‘program’ in the computer sense, and means a preprogrammed rule that an animal obeys. This meaning is precise, but unfortunately strategy has become a much abused buzz-word, and is now bandied about as a trendy synonym for ‘behaviour pattern’. All individuals of a population might follow the strategy ‘If small flee, if large attack’; an observer would then observe two behaviour patterns, fleeing and attacking, but he would be wrong to call them two strategies: both behaviour patterns are manifestations of the same conditional strategy.
survival value The quality for which a characteristic was favoured by natural selection.
symbiosis The intimate living together (with mutual dependence) of members of different species. Some modern textbooks omit the mutual dependence proviso, and understand symbiosis to include parasitism (in parasitism, only one side, the parasite, is dependent on the other, the host, which would be better off alone). Such textbooks use mutualism in place of symbiosis as defined above.
symphylic substance Chemical substance used by social insect colony parasites (e.g. beetles) to influence the behaviour of their hosts.
teleonomy The science of adaptation. In effect, teleonomy is teleology made respectable by Darwin, but generations of biologists have been schooled to avoid ‘teleology’ as though it were an incorrect construction in Latin grammar, and many feel more comfortable with a euphemism. Not much thought has been given to what the science of teleonomy will consist of, but some of its major preoccupations will presumably be the questions of units of selection, and of costs and other constraints on perfection. This book is an essay in teleonomy.
tetraploid Having four of each chromosome type rather than the more usual two (diploid) or one (haploid). New species of plants are sometimes formed by a doubling of chromosomes to tetraploidy, but subsequently the species behaves like an ordinary diploid which happens to have twice as many chromosomes as a closely related species, and it is convenient to consider it diploid for most purposes. Chapter 11 suggests that although individual termites are diploid, the whole termite nest may be regarded as the extended phenotypic product of a tetraploid genotype.
vehicle Used in this book for any relatively discrete entity, such as an individual organism, which houses replicators (q.v.), and which can be regarded as a machine programmed to preserve and propagate the replicators that ride inside it.
Weismannism The doctrine of a rigid separation between an immortal germ-line and the succession of mortal bodies which house it. In particular the doctrine that the germ-line may influence the form of the body, but not the other way around. See also central dogma.
zygote The cell that is the immediate product of sexual fusion between two gametes.
Author Index
Adler, K., 150–151
Albon, S. D., 129, 183
Alcock, J., 56
Alexander, R. D., 36, 55, 56–58, 61, 74, 75, 81, 101, 110, 133–134, 138, 143–145, 181
Allee, W. C., 184
Axelrod, R., 155
Bacon, P. J., 220
Baerends, G. P., 49–50
Baldwin, J. M., 44, 169, 172
Barash, D. P., 5, 85, 185
Barlow, H. B., 32, 175
Bartz, S. H., 202
Bateson, P. P. G., 92, 98–99, 147, 150
Baudoin, M., 213–214, 216
Beatty, R. A., 136, 142
Bennet-Clark, H. C., 63
Benzer, S., 81
Bertram, B. C. R., 64, 185
Bethel, W. M., 213, 216–218
Bethell, T., 180
Bishop, D. T., 49
Blick, J., 57
Boden, M., 16, 17
Bodmer, W. F., 283
Bonner, J. T., 6, 176–177, 254, 256
Boorman, S. A., 115
Borgia, G., 81, 101, 133–134, 138, 143–145, 148
Boult, A., 59
Brenner, S., 39, 230
Brent, L., 165, 167
Brockmann, H. J., 46, 48–50, 78–80, 118, 121–131
Broda, P., 159
Brokaw, B., 89
Broom, D. M., 217
Brown, E. R., 193
Brown, J. L., 193–194
Bruinsma, O., 203
Buehr, M., 165
Bulmer, M. G., 77
Burnet, F. M., 166, 170
Bygott, J. D., 185
Cain, A. J., 30–31, 35, 40
Cairns, J., 164
Calloway, C. B., 45
Cannings, C., 49
Cannon, H. G., 116, 169
Carlisle, T. R., 48
Caryl, P. G., 58
Cassidy, J., 187
Cavalier-Smith, T., 157–158, 163
>
Cavalli-Sforza, L., 111, 283
Chandler, P., 165, 167
Chargaff, I., 91
Charlesworth, B., 137
Charlesworth, D., 22
Charnov, E. L., 75–76, 118, 140–141
Cheng, T. C., 210, 214–215
Clarke, B., 228
Clarke, B. C., 242
Clegg, M. T., 89
Cloak, F. T., 109
Clutton-Brock, T. H., 33, 129, 183
Cohen, J., 141, 157, 229
Cohen, S. N., 159
Cosmides, L. M., 140, 177–178, 224
Cracraft, J., 108
Craig, G. B., 138, 139
Craig, R., 75–76
Crick, F. H. C., 86, 156–157, 160–164, 168, 174
Croll, N. A., 216
Crow, J. F., 135–137, 141
Crowden, A. E., 217
Crozier, R. H., 92
Curio, E., 20, 35, 81
Currey, J. D., 39
Daly, M., 43, 57
Danielli, J. F., 159–160
Darwin, C. R., 5–6, 19, 31, 42, 167, 171–181
Davies, N. B., 57, 118, 129
Dawkins, M., 250
Dawkins, R., 9, 10, 14, 15, 46, 48–49, 57–59, 82, 90, 92, 98, 102–103, 109, 114, 116–131, 144, 152–155, 170, 187, 189, 194, 250, 251
Dilger, W. C., 207
Doolittle, W. F., 156–157, 160–164
Dover, G., 163
Dunford, C., 111
Dybas, H. S., 64–65
Eaton, R. L., 64
Eberhard, W. G., 177–178, 224
Eldredge, N., 101–108, 115
Emerson, A. E., 184, 193
Evans, C., 9, 16, 17
Ewald, P. W., 220
Falconer, D. S., 21, 182, 183
Feldman, M., 111
Fierz, W., 165, 167
Fisher, R. A., 2, 32–33, 34, 43, 51, 137, 151, 184, 185, 238–239, 242, 263
Ford, E. B., 32, 229, 241
Fraenkel, G., 36
Frisch, K. von, 31, 200, 205
Futuyma, D. J., 102
Ghiselin, M. T., 6, 55, 56, 100, 254
Gilliard, E. T., 199–200
The Extended Phenotype: The Long Reach of the Gene (Popular Science) Page 47