eg, certain things, like anti-freeze in fish, are evolutionarily possible, others, like creating a flagellum, are not
The evolution of the flagellum works because proteins used in it are useful in other context:
The best studied flagellum, of the E. coli bacterium, contains around 40 different kinds of proteins. Only 23 of these proteins, however, are common to all the other bacterial flagella studied so far. Either a “designer” created thousands of variants on the flagellum or, contrary to creationist claims, it is possible to make considerable changes to the machinery without mucking it up.
What’s more, of these 23 proteins, it turns out that just two are unique to flagella. The others all closely resemble proteins that carry out other functions in the cell. This means that the vast majority of the components needed to make a flagellum might already have been present in bacteria before this structure appeared.
Of the millions of proteins on the planet it is unremarkable most exist elsewhere, just as it's likely most parts in a car can be found in other machines. Further, these aren't identical proteins, merely 'homologous' ones, where there's a stretch of, say, a 70% match over 40% of the protein, so that makes this finding not surprising (lug nut in engine A like fastener in engine B). A Type 3 Secretory System has about 1/3 of the proteins in a flagellum (depends which T3SS, which flagellum), but to get from one to the other needs probably ten thousand new nu...
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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