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Crabs Have Evolved Five Separate Times–Why Do the Same Forms Keep Appearing?
- ggm 4y agoSame thing happens in flowers, fish. D'Arcy Thompson writes about this kind of thing. The minimally advantageous outcomes here tend to lie in the same space. Putting to one side major claw asymmetry most morphology has bilateral symmetry at the root, as a function of cell division and what you might call the first "axis" of symmetry in the neoblast. Up or down, hotter or colder, lighter or darker, something like gravity or light or heat is detectable with a bias between two poles by the cell mass, or an entirely arbitrary point of bisection emerges. The question better put is why exoskeletons are the optimal choice. Having to divest the armour every moulting season feels like a poor choice but really, the risk window in a moult is less than opportunists munching on soft tissue all the rest of the time hence moulting en masse. It's not like there haven't been very large exoskeletons either. That said, crustaceans tend to smaller these days.
- eesmith 4y agoI liked the PBS Eons video on the topic, at https://www.youtube.com/watch?v=wvfR3XLXPvw https://www.youtube.com/watch?v=wvfR3XLXPvw . 7.5 minutes.