Spores are asexual reproductive units used by plants, algae, fungi, and protozoa. They evolved at least 1,600 million years ago and are most commonly used by r-selected species. Spores do not need to combine with anything to grow into an adult organism. They are usually unicellular and can increase in mass and volume by a factor of billions or trillions. Spores are associated with fungi and molds, and the first land plants reproduced exclusively through spores.
A spore is an asexual reproducing unit adapted to spend an extended period of time in unfavorable conditions before developing into progeny of the plant, algae, fungus or protozoan that created it. They are a bit like seeds, but contain fewer nutrients and are used by groups other than plants. Spores are most frequently used by r-selected species, i.e. species whose evolutionary strategy is rapid reproduction and high reproductive rate rather than the quality of each individual. Such species do best when environments are unstable and change rapidly, although they also exist in stable environments.
Spores first evolved at least 1,600 million years ago, as they are used by some red algae. They can be contrasted to gametes; while gametes (such as human eggs and sperm) must combine with another gamete to create an individual, spores do not have to combine with anything and can grow independently into an adult organism provided conditions are right. Spores are usually unicellular and measure between 4 and 20 micrometres (microns). In the process of cell division between the spore stage and the adult stage, the mass and volume of the organism can increase by a factor of billions or trillions. A large fern can have tens of trillions of cells.
In everyday experience, spores are most closely associated with fungi, especially fungi and molds. A single mushroom can release millions or billions of spores, just as a single drop of human ejaculate contains millions of sperm. When it comes to breeding in the wild, redundancy is a good thing. Smaller organisms that release them, such as single-celled fungi, algae, and protozoa, will release fewer, one at a time. Many organisms that release spores and reproduce asexually also have the ability to reproduce sexually, which they do by releasing gametes. Gametes are similar but have only half a genome: they require another to produce a complete genome. In fungi, gametes are also sometimes referred to colloquially as spores.
The first land plants reproduced exclusively through spores, as their descendants do today. These are called seedless plants and include hornworts, liverworts, mosses, ferns, fern allies, clubmosses, and several extinct groups. Aside from the presence or absence of vascular tissue, another primary characteristic used to classify plants is whether or not they reproduce by this method. The first plants that used spores evolved about 470 million years ago, in the Ordovician. Their fossilized spores are the first concrete example of multicellular life on land.
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