Biology Now, 2e

Glossary ■ (^) G5
dominant allele An allele that prevents a second allele from
affecting the phenotype when the two alleles are paired together.
Compare recessive allele.
dominant genetic disorder A genetic disorder that is inherited
as a dominant trait on an autosome. Compare recessive genetic
disorder.
double helix The spiral formed by two complementary strands of
nucleotides that is the backbone of DNA.
down-regulation The slowing down of gene expression. Compare
up-regulation.
E
Earth equivalent The number of planet Earths needed to provide
the resources we use and absorb the wastes we produce.
ecological community An association of species that live in the
same area.
ecological footprint The area of biologically productive land
and water that an individual or a population requires to
produce the resources it consumes and to absorb the waste it
produces.
ecological isolation The condition in which closely related
species in the same territory are reproductively isolated by slight
differences in habitat. Compare geographic isolation and repro-
ductive isolation.
ecological niche The set of conditions and resources that
a population needs in order to survive and reproduce in its
habitat.
ecology The scientific study of interactions between organisms
and their environment, where the environment of an organism
includes both biotic factors (other living organisms) and abiotic
(nonliving) factors.
ecosystem A particular physical environment and all the commu-
nities in it.
ecosystem process Any of four processes—nutrient cycling,
energy flow, water cycling, and succession—that link the biotic
and abiotic worlds in an ecosystem.
electron A negatively charged particle found outside the nucleus
of an atom. Compare neutron and proton.
electron transport chain An elaborate chain of chemical events
in which electrons and protons (H+) are handed over to other
molecules that ultimately generates ATP and NADPH.
element A pure substance that has distinctive physical and chem-
ical properties, and that cannot be broken down into other
substances by ordinary chemical methods.
embryonic development The process by which an embryo
develops. Common patterns of embryonic development across
species provide evidence of evolution.
endocytosis The process by which materials are transported into a
cell via vesicles. Compare exocytosis.
endoplasmic reticulum (ER) An extensive and interconnected
network of sacs made of a single membrane that is continu-
ous with the outer membrane of the nuclear envelope. See also
rough ER and smooth ER.
energy The capacity of any object to do work, which is the capacity
to bring about a change in a defined system.
energy capture The trapping and storing of solar energy by the
producers at the base of an ecosystem’s energy pyramid.
energy carrier A molecule that can store and deliver usable
energy.
energy pyramid A pyramid-shaped representation of the amount
of energy available to organisms in a food chain.
enzyme Any of a class of small molecules that speed up chemical
reactions.
epistasis A form of inheritance in which the phenotypic effect
of the alleles of one gene depends on the presence of alleles for
another, independently inherited gene.
ER See endoplasmic reticulum.
estuary An aquatic biome characterized by shallow depth and high
productivity. It is the tidal ecosystem where a river flows into the
ocean and consequently has a constant ebb and flow of fresh and
salt water. It is dominated by grasses and sedges.
Eukarya One of the three domains of life, including all the living
organisms that do not fit into the domains Archaea or Bacteria,
from amoebas to plants to fungi to animals.
eukaryote An organism that belongs to the Eukarya. Animals, plants,
fungi, and protists are all eukaryotes. Compare prokaryote.
eutherians One of three main groups of mammals, whose
members have a placenta and produce offspring that are born in
a well-developed state. Compare marsupials and monotremes.
evaporation The transition from liquid to gas. Compare conden-
sation.
evolution A change in the overall inherited characteristics of a
group of organisms over multiple generations.
evolutionary tree A model of evolutionary relationships among
groups of organisms that is based on similarities and differences
in their DNA, physical features, biochemical characteristics, or
some combination of these. It maps the relationships between
ancestral groups and their descendants, and it clusters the most
closely related groups on neighboring branches.
exocytosis The process by which materials are exported out of a
cell via vesicles. Compare endocytosis.
exon A stretch of DNA that carries instructions for building a
protein. Compare intron.
experiment A repeatable manipulation of one or more aspects of
the natural world.
experimental group See treatment group.
exploitative competition Competition between species in which
the two species indirectly compete for shared resources, such as
food. Compare interference competition.
exponential growth A pattern of population growth in which the
population increases by a constant proportion over a constant
time interval, such as one year. Exponential growth occurs when
there are no constraints on resources and is represented by a
J-shaped curve. Compare logistic growth.