The evolution of plant structure can be explored by looking at alternative growth strageties of related plants

Question

that occur in different environments. In this respect, Darwin was one of these earliest observers to note that many plant species with herbaceous growth forms on continental mainlands have woody-tree like relativeson remote oceanic islands. In the Hawaiian Islands, for example, one can find tree lobelias and tall, woody violets, groups that occur as small herbs in North America. What would be an good suggestion for an evolutionary hypothesis for this trend: Why is it so common for woody tree-like forms to evolve from the herbaceous ancestors that colonize isolated islands?

Answer 1

That first answer makes no sense. Woody plants are subject to less grazing than herbaceous plants, if there are less herbivores on the island the plants would all be herbaceous. What makes you think there are less herbivores on islands anyway?

Let's try to come up with an alternative hypothesis. The environmental effects probably have a lot more to do with it. Tropical islands are often prime growing sites (plenty of sun and rain), with lots of plants in a dense environment. In a crowded environment growing taller gives you an advantage in competing for sunlight and spreading your seeds, for example. Developing a woody body might also increase your lifespan, and increase your chances for pollination and reproduction.

Answer 2

You answered your own question. Isolated islands typically mean fewer herbivores or insects that would attack the plant. The genes that would allow for the long term growth would have a chance to survive. Also, climate has a great deal to do with it. Take a look at the smaller forms you mentioned versus the larger woody species. Not as many selection pressures on the woody ones.