Reductive evolution in parasites

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It is common for parasites, especially endoparasites, to evolve by simplification, and the more the parasite relies on it's host for the things it needs, the more it can simplify. This ‘reductive evolution’ (which used to be called ‘degeneration’, or sometimes ‘devolution’, thus the name of the inexplicably popular band of the early 80's, ‘Devo’) allows the parasite to focus it's resources on reproduction. For example, many parasitic worms lack digestive systems (they absorb their nutrients directly through their cuticle), circulatory system, &c, &c, and in extreme cases are little more than genitals than hook onto the GI tract (or elsewhere) of their host. Even ectoparasites often become simplified; there are a slew of ectoparasites that are little more than stomachs, sucking mouths, and reproductive organs (i.e. leeches).

Even bacterial parasites can evolve by simplification, good examples being the Chlamydiae, Rickettsiae, and Mycoplasmas. Perhaps more extreme examples would be plastids and mitochondria, and some viruses may have originated by simplification of cellular intrcellular parasites. In Bacteria, this simplification is most easily seen in their genomes; the sizes of the genomes of these obligate intracellular parasites has been drastically reduced. Any gene that can be done without is eliminated. The genomes of the human pathogenic Chlamydia are only 1Mbp; about a thousand genes, only 1/4th as big as wild E. coli and about 1/2 that of the smallest genomes of free-living Bacteria. The reasons for simplification are many; in addition to the usual reasons for simplification in parasites generally, the smaller the genome, the faster you can replicate it, and the simpler the organism, the faster it can evolve. The latter is especially important for a bacterial parasite, which is in a continuous arms race with it's host.