Evolution__3rd_Edition

688 Glossary
random drift Synonym of genetic drift.
random mating A mating pattern where the probability of
mating with another individual of a particular genotype (or
phenotype) equals the frequency of that genotype (or phenotype)
in the population.
recapitulation Partly or wholly erroneous theory that an
individual, during its development, passes through a series of
stages corresponding to its successive evolutionary ancestors.
An individual thus develops by “climbing up its family tree.”
recessive An allele (A) is recessive if the phenotype of the
heterozygote Aa is the same as the homozygote (aa) for the
alternative allele a and different from the homozygote for
the recessive (AA). The allele a controls the heterozygote’s
phenotype and is called dominant. An allele may be partly,
rather than fully, recessive: then the heterozygous phenotype is
nearer to, rather than identical with, the homozygote for the
dominant allele.
recognition species concept A concept of species, according
to which a species is a set of organisms that recognize one
another as potential mates; they have a shared mate recognition
system. Compare biological species concept, ecological species
concept, phenetic species concept.
recombination An event, occurring by the crossing-over of
chromosomes during meiosis, in which DNA is exchanged
between a pair of chromosomes. Thus two genes that were previously
unlinked, being on separate chromosomes, can become
linked because of recombination, and vice versa. Linked genes
may become unlinked.
refuge (plural sometimes refugia) The contracted biogeographic
range of species during ice ages or other adverse times.
reinforcement An increase in reproductive isolation between
incipient species by natural selection. Natural selection can only
directly favor an increase in prezygotic isolation; reinforcement
therefore amounts to selection for assortative mating between
the incipiently speciating forms.
reproductive character displacement An increased reproductive
isolation between two closely related species where they
live in the same geographic region (sympatry) as compared with
where they live in separate geographic regions. It involves a kind
of character displacement, in which the character concerned
influences reproductive isolation, not ecological competition.
reproductive isolation Two populations, or individuals of
opposite sexes, are reproductively isolated from each other if
they cannot together produce fertile offspring.
ribosomal RNA (rRNA) The kind of RNA that constitutes the
ribosomes and provides the site for translation.
ribosome The site of protein synthesis (or translation) in the
cell, mainly consisting of ribosomal RNA.
ring species A situation in which two reproductively isolated
populations (see reproductive isolation), living in the same
region, are connected by a geographic ring of populations that
can interbreed.
RNA Ribonucleic acid. Messenger RNA, ribosomal RNA, and
transfer RNA are its three main forms. They act as the intermediaries
by which the hereditary code of DNA is converted into
proteins. In some viruses, RNA is itself the hereditary molecule.
secondary Such expressions as “secondary contact” or “secondary
reinforcement” mean that two species, or near species, have
been geographically separated in the past and have met up
again. The term usually alludes to the theory of allopatric speciation
and implies that sympatric speciation is not at work.
selection A short-hand synonym of natural selection.
selectionism The theory that some class of evolutionary
events, such as molecular or phenotypic changes, have mainly
been caused by natural selection.
selective sweep An increase in homozygosity (that is, genetic
uniformity) at neighboring nucleotide sites, when natural selection
fixes a favored nucleotide variant. The increased homozygosity
is due to hitch-hiking, because there is little recombination
between neighboring nucleotide sites. It can be used to test for
the recent action of selection in genomic sequences.
separate creation The theory that species have separate
origins and never change after their origin. Most versions of the
theory of separate creation are religiously inspired and suggest
that the origin of species is by supernatural action.
sex chromosome A chromosome that influences sex determination.
In mammals, including humans, the X and Y chromosomes
are the sex chromosomes (females are XX, males XY).
Compare autosome.
sexual selection The selection on mating behavior, either
through competition among members of one sex (usually
males) for access to members of the other sex or through choice
by members of one sex (usually females) for certain members of
the other sex. In sexual selection, individuals are favored by
their fitness relative to other members of the same sex, whereas
natural selection works on the fitness of a genotype relative to the
whole population.
soma (and somatic cells) All the cells in the body except the
reproductive cells (or germ plasm): that is, the skin, bone, blood,
nerve cells and so on.
spacer region A sequence of nucleotides in the DNA between
coding genes.
species An important classificatory category, which can be
variously defined by the biological species concept, ecological
species concept, phenetic species concept, and recognition species
concept. The biological species concept, according to which a
species is a set of interbreeding organisms, is the most widely
used definition, at least by biologists who study vertebrates. A
particular species is referred to by a Linnaean binomial, such as
Homo sapiens for human beings.
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