UCMP Glossary: Life history
ammonite -- n. One of a group of extinct, shelled, marine cephalopods related to squids; cephalopods are within the larger group Mollusca, which includes clams and snails. Ammonite shells were generally coiled, though some non-coiled forms did exist. The group went extinct 65 million years ago at the end of the Cretaceous Period.
asexual reproduction -- A type of reproduction involving only one parent that usually produces genetically identical offspring. Asexual reproduction occurs without meiosis or syngamy, and may happen though budding, by the division of a single cell, or the breakup of an entire organism into two or more parts.
diploid life cycle -- Occurs when the only multicellular stage in an organism's life cycle is diploid.
dispersal -- The scattering of organisms of a species, often following a major reproductive event. Spores and larvae are commonly dispersed into the environment. Pollen or gametes may also be dispersed, but in this case the intent is to target another individual so that reproduction may occur. Organisms may disperse as spores, seeds, eggs, larvae, or adults.
fertilization -- The process by which an egg is made capable of generating offspring. It is often synonymous with syngamy.
gametophyte -- The haploid stage in the life cycle of an organism undergoing alternations of generations. The gametophyte is multicellular and mitotically produces gametes. In plants, the gametophyte nourishes the zygote and young sporophyte.
haploid life cycle -- Occurs when the only multicellular stage in an organism's life cycle is haploid.
haploid-diploid life cycle -- Occurs when a multicellular diploid phase, or sporophyte, alternates with a multicellular haploid phase, or gametophyte. Only plants and certain algae possess this kind of life cycle, which is also called "alternation of generations".
larva -- Among invertebrates, an immature stage in the life cycle which usually is much smaller than, and morphologically different from, the adult. In insects with metamorphosis, the larva must become a pupa before reaching adulthood.
meiosis -- A two-stage type of cell division in sexually reproducing organisms. In meiosis, a diploid cell divides to produce four haploid cells, each with half the original chromosome content. For this reason, meiosis is often called a "reduction division". In organisms with a diploid life cycles, the products of meiosis are usually called gametes. In organisms with an alternation of generations, the products of meiosis are caled spores.
metamorphosis -- n. A process of developmental change whereby a larva reaches adulthood only after a drastic change in morphology; occurs in most amphibians and insects, for some insects, this change may include another stage (pupa) before the adult stage; metamorphose- v.
sedentary -- Living in a fixed location, as with most plants, tunicates, sponges, etc. Contrast with motile.
seed -- A structure produced by seed plants which encapsulates the embryo. The seed often provides nourishment during germination, but may lie dormant for many years first.
sexual reproduction -- A type of reproduction in which two parents give rise to offspring that have unique combinations of genes inherited through the gametes of the two parents. Sexual reproduction involves meiosis and syngamy.
spore -- n. A single cell that is dispersed as a reproductive unit, or that encapsulates a cell during unfavorable environmental conditions; in organisms with an alternation of generations; the products of meiosis are spores.
sporophyte -- The diploid stage in the life cycle of an organism undergoing an alternation of generations. The sporophyte is multicellular and develops from a zygote. The mature sporophyte meiotically produces haploid spores that later generate the gametophyte generation.
zygote -- The product of gamete fusion. In organisms with a haploid life cycle, the zygote immediately undergoes meiosis, but in organisms with a multicellular diploid stage, the zygote is merely the first stage in the diploid portion of the life cycle.