altruism A costly or risky behavior carried out by one animal for the benefit of another animal.

alveoli (singular, alveolus) The many tiny air sacs that form the terminal branches in the lungs. Gases are exchanged with the capillaries through the walls of the alveoli.

amniocentesis A medical procedure in which amniotic fluid is withdrawn from a mother's abdomen with a needle and the fetal epidermal cells that are in the fluid are tested for genetic abnormalities.

amniotic egg An egg with a hard outer casing and an inner series of membranes and fluids that form a padding around the growing embryo. The evolution of the amniotic egg, in reptiles, freed reptiles from the constraint of having to reproduce near water.

amniotic fluid A protective and nutritive fluid that surrounds the fetus of mammals, including filling the lungs.

anagenesis A form of evolution in which a species changes over time, eventually having changed enough that it would be reproductively isolated from its ancestral species. In this nonbranching evolution, the original species changed into the new species and itself disappeared.

analogy A structure found in different organisms that shares a function and a superficial appearance, but is not due to inheritance from a common ancestor. Analogies must be distinguished from homologies to get a true picture of evolutionary relationships.

ancestral character A character that existed in the common ancestor of a group of organisms. Cladistics distinguishes ancestral from derived characters and uses these characters to determine evolutionary relationships.

aneuploidy A condition in which either more or fewer chromosomes occur than normal for that organism, so the individual has the wrong number of chromosomes in its diploid set. A different genetic disorder results depending on which chromosome is missing or extra.

angiosperm A flowering seed plant whose seeds are enclosed within the tissue called fruit. Angiosperms are the most dominant and diverse of the four principal types of plants. Examples include roses, cacti, corn, and deciduous trees.

animal pole The end of a zygote with relatively less yolk and lying closer to the cell's nucleus. The location of the egg's poles define the orientation in which the embryo develops.

annual A type of plant that goes through its entire life cycle, from germination of the seed, through growth, flowering, and death, in one year or less. Many crop plants, such as tomatoes and grains, are annuals.

annual ring The ring formed in wood, showing the abrupt change between the secondary xylem of late summer, when cells get less water and thus are smaller and have thicker walls, and the secondary xylem of early spring, when the cells get more water and are much larger.

anterior pituitary An endocrine gland that releases two hormones that work directly on target cells and four other hormones that regulate the production of hormones by other endocrine glands. .

anther The part of a flower that produces pollen grains. The anther is on top of a filament, and together they are called the stamen.

antibiotic A substance produced by one organism that is toxic to another. When we speak of antibiotics, we usually mean medicines directed against disease-causing bacteria.

antibody A protein of the immune system, found on the surface of a B cell or circulating free in the plasma, that is formed in response to a particular antigen and reacts with it specifically, deactivating it.

antibody-mediated immunity An immune system capability that works through the production of proteins called antibodies.

anticodon The end of the transfer RNA molecule that can form a base pair with a particular codon on the mRNA transcript.

antidiuretic hormone (ADH) Hormone, released by the posterior pituitary, that acts to decrease the amount of water lost by the kidney, helping to conserve water.

antigen Any foreign substance that elicits a response by the immune system. Certain proteins on the surface of an invading bacterial cell, for instance, act as antigens that trigger an immune response.

antigen-presenting cell (APC) A macrophage that has ingested an infected cell and displays fragments of proteins from the invader on its surface. The APCs bind with helper T cells and activate them, stimulating secretion of interleukin-2, which has multiple functions in mounting further attacks against the invader.

aorta The major artery in the circulatory system. The aorta has branches that send blood to all of the body's other tissues except the lungs.