The Extended Phenotype

by Richard Dawkins

  Baldwin/Waddington Effect First proposed by Spalding in 1873. A largely hypothetical evolutionary process (also called genetic assimilation) whereby natural selection can create an illusion of the inheritance of acquired characteristics. Selection in favour of a genetic tendency to acquire a characteristic in response to environmental stimuli leads to the evolution of increased sensitivity to the same environmental stimuli, and eventual emancipation from the need for them. On page 44 I suggest that we might breed a race of spontaneously milk-producing male mammals by treating successive generations of males with female hormones and selecting for increased sensitivity to female hormones. The role of the hormones, or other environmental treatment, is to bring out into the open genetic variation which would otherwise lie dormant.

  central dogma In molecular biology the dogma that nucleic acids act as templates for the synthesis of proteins, but never the reverse. More generally, the dogma that genes exert an influence over the form of a body, but the form of a body is never translated back into genetic code: acquired characteristics are not inherited.

  chromosome One of the chains of genes found in cells. In addition to DNA itself, there is usually a complicated supporting structure of protein. Chromosomes become visible under the light microscope only at certain times in the cell cycle, but their number and linearity may be inferred by statistical reasoning from the facts of inheritance alone (see linkage). The chromosomes are usually present in all cells in the body, even though only a minority of them will be active in any one cell. There are usually two sex chromosomes in every diploid cell as well as a number of autosomes (44 in humans).

  cistron One way of defining a gene. In molecular genetics the cistron has a precise definition in terms of a specific experimental test. More loosely it is used to refer to a length of chromosome responsible for the encoding of one chain of amino acids in a protein.

  codon A triplet of units (nucleotides) in the genetic code, specifying the synthesis of a single unit (amino acid) in a protein chain.

  clone In cell biology, a set of genetically identical cells, all derived from the same ancestral cell. A human body is a gigantic clone of some 1015 cells. The word is also used of a set of organisms all of whose cells are members of the same clone. Thus a pair of identical twins may be said to be members of the same clone.

  Cope’s Rule An empirical generalization that evolutionary trends towards larger body size are common.

  crossing-over A complicated process whereby chromosomes, while engaged in meiosis, exchange portions of genetic material. The result is the permutation of an almost infinite variety of gametes.

  D’Arcy Thompson’s transformations A graphical technique demonstrating that an animal shape can be transformed into the shape of a related animal by a mathematically specifiable distortion. D’Arcy Thompson would draw one of the two shapes on ordinary graph paper, then show that it was transformed approximately into the other shape if the coordinate system were distorted in some particular way.

  diploid A cell is said to be diploid if it has chromosomes in pairs, in sexual cases one from each parent. An organism is said to be diploid if its body cells are diploid. Most sexually reproducing organisms are diploid.

  dominance A gene is said to be dominant to one of its alleles if it suppresses the phenotypic effect of that (recessive) allele when the two are together. For example, if brown eyes are dominant to blue, only individuals with two blue-eyed genes (homozygous recessive) would actually have blue eyes; those with one blue and one brown gene (heterozygotes) would be indistinguishable from those with two brown genes (homozygous dominant). Dominance may be incomplete, in which case heterozygotes have an intermediate phenotype. The opposite of dominant is recessive. Dominance/recessiveness is a property of a phenotypic effect, not of a gene as such: thus a gene may be dominant in one of its phenotypic effects and recessive in another (see pleiotropy).

  epigenesis A word with a long history of controversy in embryology. As opposed to preformationism (q.v.) it is the doctrine that bodily complexity emerges by a developmental process of gene/environment interaction from a relatively simple zygote, rather than being totally mapped out in the egg. In this book it is used for the idea, which I favour, that the genetic code is more like a recipe than a blueprint. It is sometimes said that the epigenesis/preformationism distinction has been made irrelevant by modern molecular biology. I disagree, and have made much of the distinction in Chapter 9, where I claim that epigenesis, but not preformationism, implies that embryonic development is fundamentally, and in principle, irreversible (see central dogma).

  epistasis A class of interactions between pairs of genes in their phenotypic effects. Technically the interactions are non-additive which means, roughly, that the combined effect of the two genes is not the same as the sum of their separate effects. For instance, one gene might mask the effects of the other. The word is mostly used of genes at different loci, but some authors use it to include interactions between genes at the same locus, in which case dominance/recessiveness is a special case. See also dominance.

  eukaryotes One of the two major groups of organisms on Earth, including all animals, plants, protozoa and fungi. Characterized by the possession of a cell nucleus, and other membrane-bounded cell organelles (analogue of ‘organ’ within the cell) such as mitochondria. Contrast with prokaryotes (q.v.). The prokaryote/eukaryote distinction is much more fundamental than the animal/plant distinction (not to mention the relatively negligible human/‘animal’ distinction!).

  eusociality The most advanced of the grades of sociality recognized by éntomologists. Characterized by a complex of features, the most important of which is the presence of a caste of sterile ‘workers’ who assist the reproduction of their long-lived mother, the queen. It is usually considered to be confined to wasps, bees, ants and termites, but various other kind of animals approach eusociality in interesting ways.

  evolutionarily stable strategy (ESS) [Note ‘evolutionarily’ not ‘evolutionary’. The latter is a common grammatical error in this context.] A strategy that does well in a population dominated by the same strategy. This definition captures the intuitive essence of the idea (see Chapter 7), but is somewhat imprecise; for a mathematical definition, see Maynard Smith, 1974.

  extended phenotype All effects of a gene upon the world. As always, ‘effect’ of a gene is understood as meaning in comparison with its alleles. The conventional phenotype is the special case in which the effects are regarded as being confined to the individual body in which the gene sits. In practice it is convenient to limit ‘extended phenotype’ to cases where the effects influence the survival chances of the gene, positively or negatively.

  fitness A technical term with so many confusing meanings that I have devoted a whole chapter to discussing them (Chapter 10).

  game theory A mathematical theory originally developed for human games, and generalized to human economics and military strategy, and to evolution in the theory of evolutionarily stable strategy (q.v.). Game theory comes into its own wherever the optimum policy is not fixed, but depends upon the policy which is statistically most likely to be adopted by opponents.

  gamete One of the sex cells which fuse in sexual reproduction. Sperms and eggs are both gametes.

  gemmule A discredited concept espoused by Darwin in his ‘pangenesis’ theory of the inheritance of acquired characteristics—probably the only serious scientific error he ever made, and an example of the ‘pluralism’ for which he has recently been praised. Gemmules were supposed to be small hereditary particles which carried information from all parts of the body into the germ cells.

  gene A unit of heredity. May be defined in different ways for different purposes (see page 85). Molecular biologists usually employ it in the sense of cistron (q.v.). Population biologists sometimes use it in a more abstract sense. Following Williams (1966, p. 24), I sometimes use the term gene to mean ‘that which segregates and recombines with appreciable frequency’, and (p. 25) as ‘any hereditary in
formation for which there is a favorable or unfavorable selection bias equal to several or many times its rate of endogenous change’.

  gene-pool The whole set of genes in a breeding population. The metaphor on which the term is based is a happy one for this book, for it de-emphasizes the undeniable fact that genes actually go about in discrete bodies, and emphasizes the idea of genes flowing about the world like a liquid.

  genetic drift Changes in gene frequencies over generations, resulting from chance rather than selection.

  genome The entire collection of genes possessed by one organism.

  genotype The genetic constitution of an organism at a particular locus or set of loci. Sometimes used more loosely as the whole genetic counterpart to phenotype (q.v.).

  gens (pl. gentes) ‘Race’ of female cuckoos all parasitizing one host species. There must be genetic differences between gentes, and these are presumed to be on the Y chromosome. Males have no Y chromosomes, and do not belong to gentes. The word is poorly chosen, since in the Latin it refers to a clan tracing descent through the male line.

  germ-line That part of bodies which is potentially immortal in the form of reproductive copies: the genetic contents of gametes and of cells that give rise to gametes. Contrast with soma, the parts which are mortal and which work for the preservation of genes in the germ-line.

  gradualism The doctrine that evolutionary change is gradual and does not go in jumps. In modern palaeontology it is the subject of an interesting controversy over whether the gaps in the fossil record are artefactual or real (see Chapter 6). Journalists have blown this up into a pseudo-controversy over the validity of Darwinism, which they say is a gradualist theory. It is true that all sane Darwinians are gradualists in the extreme sense that they do not believe in the de novo creation of very complex and therefore statistically improbable new adaptations like eyes. This is surely what Darwin understood by the aphorism ‘Nature does not make leaps’. But within the spectrum of gradualism in this sense, there is room for disagreement about whether evolutionary change occurs smoothly or in small jerks punctuating long periods of stasis. It is this that is the subject of the modern controversy, and it does not remotely bear, one way or the other, on the validity of Darwinism.

  group selection A hypothetical process of natural selection among groups of organisms. Often invoked to explain the evolution of altruism (q.v.). Sometimes confused with kin selection (q.v.). In Chapter 6 I use the replicator/vehicle distinction to distinguish group selection of altruistic traits from species selection (q.v.) resulting in macroevolutionary trends.

  haplodiploid A genetic system in which males grow from unfertilized eggs and are haploid, while females grow from fertilized eggs and are diploid. Therefore males have no father and no sons. Males pass all their genes on to their daughters, while females receive only half their genes from their fathers. Haplodiploidy occurs in nearly all social and non-social Hymenoptera (ants, bees, wasps, etc.), and also a few bugs, beetles, mites, ticks and rotifers. The complications which haplodiploidy introduces into closeness of genetic kinship have been ingeniously invoked in theories of the evolution of eusociality (q.v.) in Hymenoptera.

  haploid A cell is said to be haploid if it has a single set of chromosomes. Gametes are haploid, and when they fuse in fertilization a diploid cell (q.v.) is produced. Some organisms (e.g. fungi and male bees) are haploid in all their cells, and are referred to as haploid organisms.

  heterozygous The condition of having nonidentical alleles at a chromosomal locus. Is usually applied to an individual organism, in which case it refers to two alleles at a given locus. More loosely it may refer to the overall statistical within-locus heterogeneity of alleles averaged over all loci in an individual or in a population.

  homeotic mutation A mutation causing one part of a body to develop in a manner appropriate to another part. For example, the homeotic mutation ‘antennopedia’ in Drosophila causes a leg to grow where an antenna normally does. This is interesting, as it shows the power of a single mutation to have elaborate and complex effects, but only when there is elaborate complexity already there to be altered.

  homozygous The condition of having identical alleles at a chromosomal locus. Is usually applied to an individual organism, in which case it indicates that the individual has two identical alleles at the locus. More loosely it may refer to the overall statistical within-locus homogeneity of alleles averaged over all loci in an individual or in a population.

  K-selection Selection for the qualities needed to succeed in stable, predictable environments where there is likely to be heavy competition for limited resources between individuals well-equipped to compete, at population sizes close to the maximum that the habitat can bear. A variety of qualities are thought to be favoured by K-selection, including large size, long life, and small numbers of intensively cared-for offspring. Contrast with r-selection (q.v.). The ‘K’ and ‘r’ are symbols in the conventional algebra of population biologists.

  kin selection Selection of genes causing individuals to favour close kin, owing to the high probability that kin share those genes. Strictly speaking ‘kin’ includes immediate offspring, but it is unfortunately undeniable that many biologists use the phrase ‘kin selection’ specifically when talking about kin other than offspring. Kin selection is also sometimes confused with group selection (q.v.), from which it is logically distinct, although where species happen to go around in discrete kin groups the two may incidentally amount to the same thing—‘kin group selection’.

  Lamarckism Regardless of what Lamarck actually said, Lamarckism is nowadays the name given to the theory of evolution that relies on the assumption that acquired characteristics can be inherited. From the point of view of this book, the significant feature of the Lamarckian theory is the idea that new genetic variation tends to be adaptively directed, rather than ‘random’ (i.e. non-directed) as in the Darwinian theory. The orthodox view today is that the Lamarckian theory is completely wrong.

  linkage The presence on the same chromosome of a pair (or a set) of loci. Linkage is normally recognized by the statistical tendency for alleles at linked loci to be inherited together. For example, if hair colour and eye colour are linked, a child that inherits your eye colour is likely to inherit your hair colour too, while a child that fails to inherit your eye colour is also likely to fail to inherit your hair colour. Children are relatively unlikely to inherit one but not the other, though this can come about due to crossing-over (q.v.), the probability being related to the distance apart of the loci on the chromosome. This is the basis for the technique of chromosome mapping.

  linkage disequilibrium The statistical tendency for alleles to occur together, in the bodies or gametes of a population, with particular alleles at other loci. For example, if we observed a tendency for fair-haired individuals to be blue-eyed, this might indicate linkage disequilibrium. Recognized as any tendency for the frequency of combinations of alleles at different loci to depart from the frequencies that would be expected from the overall frequencies of the alleles themselves in the population.

  locus The position on a chromosome occupied by a gene (or a set of alternative alleles). For instance, there might be an eye-colour locus, at which the alternative alleles code for green, brown and red. Usually applied at the level of the cistron (q.v.), the concept of the locus can be generalized to smaller or larger lengths of chromosome.

  macroevolution The study of evolutionary changes that take place over a very large time-scale. Contrast with microevolution, the study of evolutionary changes within populations. Microevolutionary change is change in gene frequencies in populations. Macroevolutionary change is usually recognized as change in gross morphology in a series of fossils. There is some controversy over whether macroevolutionary change is fundamentally just cumulated microevolutionary change, or whether the two are ‘decoupled’ and driven by fundamentally different kinds of process. The name macroevolutionist is sometimes misleadingly restricted to partisans on one side of this
controversy. It should be a neutral label for anybody studying evolution on the grand time-scale.

  meiosis The kind of cell division in which a cell (usually diploid) gives rise to daughter cells (usually haploid) with half as many chromosomes. Meiosis is an essential part of normal sexual reproduction. It gives rise to the gametes which subsequently fuse to restore the original chromosome number.

  meiotic drive The phenomenon whereby alleles affect meiosis so that they secure for themselves a greater than 50 per cent chance of finding themselves in a successful gamete. Such genes are said to be ‘driving’ because they tend to spread through the population in spite of any deleterious effects they may have on organisms. See also segregation distorter.

  meme A unit of cultural inheritance, hypothesized as analogous to the particulate gene, and as naturally selected by virtue of its ‘phenotypic’ consequences on its own survival and replication in the cultural environment.

  Mendelian inheritance Non-blending inheritance by means of pairs of discrete hereditary factors (now identified with genes), one member of each pair coming from each parent. The main theoretical alternative is ‘blending inheritance’. In Mendelian inheritance genes may blend in their effects on a body, but they themselves do not blend, and they are passed on intact to future generations.

  microevolution See macroevolution.

  mitochondria Small complex organelles within eukaryotic cells, made of membranes, the site of most of the energy-releasing biochemistry of the cells. Mitochondria have their own DNA and reproduce autonomously within cells, and, according to one theory, they originated in evolution as symbiotic prokaryotes (q.v.).

  mitosis The kind of cell division in which a cell gives rise to daughter cells having a complete set of all its chromosomes. Mitosis is the ordinary cell division of bodily growth. Contrast with meiosis.

  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.