Hacker News Reader

Comment by antattack | Hacker News Reader

"Why does symmetry reign supreme? Biologists aren’t sure — there’s no reason based in natural selection for symmetry’s prevalence in such varied forms of life and their building blocks."

Of course there is a natural selection reason - symmetrical structures are stronger.

View on HN

They're also info-compressed. You can re-use the same DNA for both sides. This means you have less meaningful information to maintain against random mutation pressure over time.

Symmetry is also challenging to maintain and very visible; any malformation is obvious. This is useful in showing health and genetic fitness to mates.

Many tasks are also inherently symmetrical, like locomotion. Evolving the two legs totally separately would be very error-prone and you'd never quite get it right. Having them both just be copies of the same structure is much cleaner and always efficiently completes the task.

Same for things like binocular vision, having ears that hear equally and so can easily echolocate, etc.

"Some owls have ears located directly across from each other, in symmetrical placement. Others have asymmetrical placement, where an ear on one side of the head is located above the one on the other side of the head, which increases their ability to locate sound on a vertical axis. These owls use their uneven ears to judge exactly where sound is coming from."

https://www.owlresearchinstitute.org/adaptations

Interesting to see the confirmation - Horizontal alignment (symmetry) is best for most animals because they really need to echolocate on the horizontal plane. For the few that don't, some have adapted!

"Entropically more favorable to construct" was my first thought. If you have the option of a structure to do X with 8 copies of one molecule in a radially symmetric arrangement, and a structure to do X with 5 different molecules that have only a single possible arrangement, having a random assortment of those molecules in solution (or cytoplasm), you're a lot more likely to get working copies of the symmetric structure, and non-working copies of the particular structure. Think viral nucleocapsid self-assembly in cells.

The decreased coding required is a nice bonus, which probably contributes.

"symmetrical structures are stronger" - citation needed.

Stronger under what condition? For the same amount of material? For the same amount of information needed to describe how to build the structure?

Do we see evidence of structures which don't require strength being more likely to evolve to be asymmetric?

Or do you just mean 'stronger' in a survival-of-the-fittest sense - symmetrical structures are fitter?

It's not at all obvious to me that this should be the case. Indeed, some very strong natural structures like shells are not symmetric at all.

Mobile Cambrian seabed dwellers with asymmetrical legs probably didn't last very long.

Among the most successful seabed-dwelling form factors are crabs and lobsters, which frequently have distinctively asymmetrical legs, and gaits.

Crabs are such a successful shape, crustaceans have evolved into it five separate times.

I don't understand. Crabs walk sideways but they are basically symmetrical aren't they?

Fiddler and hermit crabs, plus many lobsters, have distinctly asymmetric claws, with one specialized for crushing and another for manipulation or cutting.

Notable for the exception to their otherwise symmetrical bodies, no?

Yes, that’s what asymmetry is.

You pointed at crabs and lobsters as proof of the frequency of asymmetry by citing two of the hundreds of species having two of their eight appendages being asymmetrical. I would suggest this might be confirmation bias in action.

Reply on news.ycombinator.com