The Extended Phenotype: The Long Reach of the Gene (Popular Science) (60 page)

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 10
¹⁵
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 information 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’.