QuestionEvolution of thorns on trees
      – George, 2010-10-05 at 12:10:47   (9 comments)

On 2010-10-05 at 12:11:59, George wrote...
Using two related African examples... with a giraffe, I can see how genetic mutation and natural selection could lead to the species developing a very long neck over a very long time period: a giraffe born with a slightly longer neck would have an advantage over others because it could graze on foliage that it's competitors could not reach. So over millennia, the length of the giraffe's neck could creep up and up thanks to natural selection, with an evolutionary advantage all the way along. Many acaia trees have big, nasty and sharp thorns which protect their leaves from grazing animals. But how did these evolve - surely a genetic mutation in one generation didn't mean that suddenly a previously thornless tree now had sharp thorns which gave it an evolutionary advantage?
On 2010-10-05 at 12:12:42, George wrote...
... If the thorns evolved gradually, where was the natural advantage for the (presumably thousands of) generations of the tree which had little bumps or stumps on the branches and twigs, which weren't nearly nasty enough to put off a grazing animal? In other words, until the point (no pun intended) at which a thorn was developed enough to function as a real deterrent to grazing animals, how and why would it successfully develop? (FWIW I'm not a creationist or intelligent design enthusiast :)
On 2010-10-07 at 07:34:31, Lee J Haywood wrote...
The first step on the road to creating a thorn has nothing to do with the thorns you end up with. The plant has no concept of a thorn, and no goal in mind. Instead, there would have been a chance mutation - usually in a single gene - which most likely created a large protrusion. Mutations aren't limited to the 'bumps' that you imagine, and they can have unexpected but large phenotypic effects. Surprisingly Wikipedia doesn't have much to say on the evolution of thorns, save to mention that they come in many forms, but it's more surprising that you'd pick on such a simple feature. There are vastly more complex things to explain in nature, yet of course even if no-one has an explanation that doesn't mean that magic is involved.
On 2010-10-07 at 18:05:43, George wrote...
I picked such a simple feature on purpose - although they're simple, the strong and sharp thorns on many Acacia trees provide a clear and straightforward evolutionary advantage. Your explanation makes the most sense - in one generation, a mutation appeared which had sufficiently 'thorny' thorns to provide an advantage in protecting the plant. Obviously they then developed further, getting stronger and sharper etc. But the initial change between a tree species with no thorns to one with useful, 'working' thorns must have been quick, unlike the gradual elongation of a giraffe's neck, for example.
On 2010-10-07 at 19:32:47, Thelevellers wrote...
@George: Also, don't forget that before thorns it may have been that mouths/tongues could have been much less tough, meaning that mere bumps could have been plenty painful enough to put of prospective munchers... Natural 'arms wars' have two sides - I'm sure there are now plenty of animals that have tough enough tongues/mouths to munch on thorny trees. In fact there is one on the tip of my brain, but I can't quite put my finger on...
On 2010-10-07 at 21:46:32, Lee J Haywood wrote...
Thorns are also only one type of defence, and not necessarily the best. A plant might also lubricate itself to make grasping/climbing difficult, or sprout a mesh of barbs to prevent the smallest attackers from climbing. And, going beyond my imagination... http://en.wikipedia.org/wiki/Plant_defense_against_herbivory
On 2010-10-11 at 21:17:00, TheRevolutionary wrote...
A thorn is not that complex of a structure. It wouldn't take a particularly unusual mutation to create a few small thorns that could deter some herbivores. It's then a matter of time until they become the big nasty thorns that can deter almost any herbivore.
On 2010-11-05 at 23:48:36, Lee J Haywood wrote...
Richard Dawkins examines the laryngeal nerve of a giraffe. http://www.youtube.com/watch?v=cO1a1Ek-HD0
On 2010-11-14 at 11:00:30, BorgClown wrote...
Of course you had to be there, but I guess it's also possible that thorns devolved from some other characteristic, for example: rugged bark, branches, leaves, etc. If a plant reached a point where it was safe from most herbivores, it could "focus" on deterring the specialized ones.