The Extended Phenotype: The Long Reach of the Gene (Popular Science) (61 page)

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.

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.