A 'value' at a particular position on a genotype of a species. There are
usually many values for most positions (locuses or loci) in
a species or large population of organisms, which is
expressed in the phrase 'most species are polymorphic at
most loci'. An allele can cover one base on a gene, or more
usually, a sequence of bases on a gene.
Allopatry
The geographical isolation of two or more populations
or species. Also, speciation that
occurs in geographical isolation from the main population
of the parent species. It is thought to be the dominant
mode of speciation. A small population of the species is
isolated long enough for genetic drift and natural
selection to make it different enough in either
reproductive behaviour or mechanisms so that if and when it
rejoins the original species it is too distinct to
interbreed. There is debate in the evolutionary biological
discipline as to whether different ecological behaviour and
the adaptations needed to survive in them are the result of
speciating selection, or follow from selection between the
original species and the now reproductively distinct new
species which have become that way through drift. Most
think that selection is relevant only after allopatric
speciation, although recent work has revived the sympatric speciation model.
Archetype
An archetype is the transcendent 'original' that
numerous species that have an affinity of design
approximate, according to the transcendentalist philosophy
that preceded Darwin. The term literally means 'ruling
kind'. A term used to describe archetypes is the German
Bauplan, meaning 'blueprint' or 'building plan'.
Nowadays this German term has lost its essentialist
flavour, and instead is understood to represent ancestral
novelties shared by all or most of a group of species.
Catastrophism
The name coined by William Whewell in 1832 to describe
a view in geology championed by William Buckland that the rate and
mechanisms of past geological change are dramatically
different than those of today. More recently it has been
used to refer to 'episodic' or 'quantum' evolutionary
processes, and to scientific models that requires extremely
large scale change to account for past developments. It is
wrongly applied to the punctuated equilibrium theory of
Gould and Eldredge, which involves variable rates but not
dramatically distinct causes. See Uniformitarianism.
Evolution/evolutionism
This is in its bare form the thesis that species
transform. Some historical varieties involve an assumption
of inevitable progress. Others suppose that there is a
strong analogy between the development of an organism
through its life-cycle and the changes in a species. All
sorts of metaphors have been used to describe evolution,
including mechanical/physical metaphors of "inertia", and
"evolutionary force", organismic metaphors of "racial
senescence", "species vitality" and so forth. These have no
relation to scientific evolutionary theory, and have been
abandoned. Darwinism is a theory of outcomes, and does not
insist on progress. Species are seen as lineages that do
whatever they do, and are not subject to "racial decay" or
"devolution" or "drives to perfection". Initially the term
"evolution", which is a Latin term meaning to unscroll or
unfold and is used of reading a scroll, was applied to the
ontogenetic stage of the life cycle of a form of organism.
Since early views of evolution used an analogy with ontogeny, the term was passed over to
cover phylogenetic change as well,
and then became used exclusively for it.
Genotype
The general structure of a species' or population's
genetic makeup. A genotype is comprised of all the various
alleles that are available in the gene pool of the
species.
Homology
A similarity of structures due to their 'affinity'
through an ideal type, or archetype, according to Richard Owen.
In modern evolutionary theory, the similarity is homologous
if it evolved from the same structure in a common ancestor.
Otherwise, any similarity is convergent and therefore
'analogous'. See also transcendentalism.
A term introduced by Theodosius Dobzhansky in 1937,
referring to evolution at levels higher than the
populational. Macroevolution in his view was evolutionary
change at the
level of speciation and above.
Recently, the term has been used simply to refer to large
scale change, mostly at the superspecies level, eg, by
Niles Eldredge.
Macromutation
A term introduced early in the 20th century when the
term mutation no longer referred to
large scale genetic changes after the integration of
Mendelian genetics and Darwinian theory.
Microevolution
A term referring to evolutionary changes beneath the
level of the species. It includes, but is not limited to,
adaptation to local environments. See also macroevolution.
Mutation
A term introduced in the late-nineteenth century to
refer to large scale phenotypic change but which was
appropriated by modern genetics to mean any genetic change,
large or small. A 'point mutation' is the single
substitution of one base. A 'translocation' is the
reshuffling of a long sequence. An 'inversion' is the
inverting of a long sequence, and so on.
The development of the organism from the sex cells
('gametes') to birth ('partition' in animals). It includes
the 'zygote' and 'fetal' stages.
Parapatry/Peripatry
Existing in neighbouring and nonoverlapping regions.
Also, speciation that occurs at
the extremes of a species' range, either in environmental
terms or geographical location. It is a form of allopatricspeciation.
Phenotype
The organism-level traits and characters of a species
or individual organism. There are typically economically
important, if they function in natural selection, but it is
held by some that not all phenotypic traits are selectively
biassed. Phenotypic traits are the result of the expression
of the genes of the organism. Sometimes a trait is actually
a 'norm of reaction', for it may be expressed in different
ways in different environments (eg, height depending on
diet).
Phylogeny
The 'pedigree' of a species - the branches of the
ancestral tree of species from which the current species
derives. A related species is derived from a common
ancestor. There are two kinds of phylogenies -
branching, or divergent, phylogenies, and merging,
or reticulate, phylogenies. Hybridisation of two
species is a case of reticulation if the result is a viable
and reproductively persistent form.
Speciation
The creation of a species through the splitting of one
species into two or more, through descent.
Sympatry
Existing in overlapping regions. Also, a contentious
but now generally accepted, if rare, mode of speciation where the populations are
not isolated, but adopt distinct ecological behaviour and
are forced by selection to diverge. One form of sympatry -
stasipatry - is the result from genetic (mainly
chromosomal) reorganisation. Selection is prominent in
sympatric speciation.
Transcendentalism
A philosophy of nature that holds that everything is an
approximation to an ideal standard or type. It was popular
in the early decades of the nineteenth century, and derives
from Platonism and Goethe, the latter tradition known in
Germany as Naturphilosophie. On this account,
species are ideal types and have no variation that is not
degradation of the type. Variation is seen as monstrosity.
Some transcendentalists, like Owen, saw species as
implementing the types differently, and from this Owen
developed his idea of homology (same function from the same
part of the archetype) and analogy
(same function using different structures).
Uniformitarianism
The name coined by William Whewell in 1832 to describe
a view in geology championed by Charles Lyell that the rate
and mechanisms of geological change operating in the modern
era are sufficient to explain changes in the past. It was
contrasted with catastrophism.
More recently, uniformitarianism has been applied to any
'steady state' theory or model of historical change,
including evolution (cf Gould and Eldredge's Punctuated
Equilibrium theory).