increasing nor decreasing, then the genotype with a rel-
ative fitness of more than one will increase in frequen-
cy, whereas if the genotype has a relative fitness of less
than one, it will decrease.
relaxation If a system is disturbed from its state of
equilibrium, it returns to that state, and the process is
referred to as relaxation.
releaser Asignal stimulus that functions as a com-
munication signal between individuals of the same
species and initiates a fixed action pattern (FAP), a
stereotyped species-to-species behavior.
reniform Kidney shaped, such as a kidney bean.
repetitive DNA Repeated DNA sequences that may
occur in the thousands of copies in the chromosomes of
eukaryotes; represents much of the human genome.
These sequences of variable length can be repeated up
to 100,000 (middle repetitive) or over 100,000 (highly
repetitive) copies per genome. Much of the DNA in
eukaryotes is repetitive.
replication fork The Y-shaped portion of the repli-
cating DNA where new strands are growing.
repressible enzyme An enzyme whose synthesis is
inhibited or regulated by a regulatory molecule, a spe-
cific metabolite.
repressor A protein that prevents gene transcription;
prevents RNA polymerase from commencing mRNA
synthesis.
Reptilia The class of vertebrate animals that includes
snakes, turtles, lizards, tuatara, crocodilians, and
extinct fossil species. Reptiles have scales or modified
scales, breathe air, are cold blooded, and usually lay
eggs. The reptiles evolved from amphibians during the
late Carboniferous or early Permian periods. Today
there are about 2,500 species of snakes, 3,000 of
lizards, 250 of turtles and tortoises, and 21 species of
crocodilians and distributed worldwide throughout
temperate and tropical regions.reservoir host The host, a vertebrate, that harbors a
particular parasite and acts as a long-termsource of
infection of other vertebrates or vectors.resistance The ability of an organism to resist
microorganisms or toxins produced in disease.resolving power A property of instruments, like
microscopes and telescopes, that distinguish objects
that are close to each other; the smaller the minimum
distance at which two objects can be distinguished, the
greater the resolving power.A closeup of the head of a ribbonsnake, a member of the Reptilia
class.(Courtesy of Tim McCabe)290 relaxation