[eli5] Why do insects have lots of legs while larger animals have 4 or 2? Also, why aren’t there any animals (that I know of) with 3 legs?
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ELI 5 on biology in general: it’s not entirely possible to answer the deep down why of many questions, sometimes there is no real reason that things happen other than: that body plan or evolutionary step happened to work out for that species and they didn’t die off before spawning.
We can talk about things we’ve found in nature, but we can’t really speak to why things worked out that way other than if there was any possibility for trilateral symmetry it was killed off so early that it wasn’t able to evolve.
The coolest bits of evolution are probably cephalopods where they have 9 brains enabling them to be very clever with their tentacles. Just imagine what we would have looked like if they’d evolved bones and made it to land before our ancestors.
Keep in mind “Survival of the fittest” doesn’t mean “in the best shape” it’s more: whatever fits their niche the best. Plankton, for example can’t do much of anything, but they’re still around because they breed in such big numbers that they can’t all get eaten. Insects tend to be incredibly stupid by our definitions, yet they survive with such diversity because of how they breed. Cicadas, for instance are massive and don’t even act like they want to live, but because they all pop out once every couple decades and lay eggs like crazy, they continue to exist. Evolution occurs through breeding and random mutations along with trait combinations over hundreds of generations…it can also happen quicker through artificial selection, like how people have trained dogs from wolves (and even then they aren’t separate species, the general definition of which being two groups of animals that can’t breed…and even for that rule there are exceptions like horses and donkeys and zebras).
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