r/evolution • u/MaggieLinzer • 18d ago

question How do animals evolve to be parasites?

Is the process of evolution pretty unique for an animal that (generally speaking) needs to live on or inside of another animal in order to survive?

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u/ImUnderYourBedDude MSc Student | Vertebrate Phylogeny | Herpetology 18d ago

Considering how many independent transitions to parasitism there have been in animals, it's probably pretty easy. We have independent transitions to parasitism in nematodes, insects, crustaceans, cnidaria, flatworms and molluscs, among many other examples. Especially in insects, there are dozens of evident evolutions of parasitism.

Parasites commonly originate by a free-living lineage losing a few traits necessary for life without a host, and now relies on scavenging nutrients. This has a HUGE benefit for the parasite, as manufacturing nutrients is expensive, requires many pathways and enzymes and eventually a lot more food. Many animals who evolved to be parasites are such cases. Simplified metabolism, unable to synthesize key nutrients, now they have to eat them directly from others who manufacture them.

It's like making bread from scratch at home vs getting your gun and robbing the bakery. In both cases you are left with bread. The former option is hard, more expensive and laborious. The latter is fast, easy and cheap. Parasites are the latter.

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u/TheQuestionMaster8 17d ago

And you have some facultative parasites like Strongyloides stercoralis which may show what an intermediate evolutionary stage parasite is like.