Adaptationism is one of two major competing theories in evolutionary biology. According to adaptationism, organisms inherit genes (and therefore) traits through random genetic modification, and then pass them on to their offspring. The traits that best suit the organisms overall success are passed on. Therefore, all traits of an organism are optimized for survival.
A pinnately compound leaf is a compound leaf in the general shape of a feather. A compound leaf is a leaf made up of many separate leaflets, all of which grow from a single leaf bud.
To return to the above, the definition of adaptationism might sound very familiar to some people, because it sounds to most people like the definition of evolution. The entire history and evidence for adaptationism, and where it stands in general in evolutionary theory, is beyond me, but I am going to attempt to explain the theory by comparing one trait, the pinnately compound leaf, across many species, and see how it fits in with adaptationist theory.
While there are many different plants that have pinnately compound leaves, I am just going to survey a few of the more obvious ones that are common in temperate areas.
- Juglans, or the Walnut: is a large, tall tree bearing nuts, that tends to grow in open woodland, and likes climates that are temperate to Mediterranean.
- Fraxinus, commonly known as Ash, is a family of medium sized trees, with a range from the subtropics to the northern temperate latitudes. Some varieties grow in dry locations, while some are riparian.
- Sorbus, sometimes called the Mountain Ash (by the ignorant) or Rowan (by the poetical), is a genus of small shrubs to medium sized trees in the Rose family, with a range from the temperate to the sub-arctic, and preferring montane locations.
- Rosa, or the Rose, while most famous as an ornamental plant, is a naturally occurring shrub that grows wild widely, from temperate latitudes northwards.
- Ailanthus, a genus of tall, rapidly growing trees that are native to tropical and sub-tropical Asia, and which are widely introduced and invasive in North America and parts of Europe.
- Mahonia, the Oregon Grape, is a small shrub native to the Pacific Northwest, usually growing as an understory plant in shady forests.
There are many ways to quantify and describe what trees need, and while quite a bit of science can go into it, common sense might be as good of a guide. Trees (and their leaves) need light, air water and minerals. They also need to protect themselves, from dehydration, cold, heat and predation. So a tree should, through evolution, adapt itself to make the most of its resources and protect itself from these challenges.
If we look at why a tree (or shrub) might evolve to have a pinnately compound leaf, we can make various guesses based on the above mentioned varities of plants. In the six examples given, we have large trees that grow in open woodland, and small shrubs that grow as an understory. We have trees that grow in the tropics, and trees that grow in the Yukon. We have trees that grow in damp locales, and some that grow in Mediterranean climates. Some bear edible fruit and others don't. Some grow fast and some grow slow. In other words, there is not a single defining feature that could easily be identifiable as to why these plants would all independently evolve similar leaf shapes.
Along with that, there is the question of why similar trees didn't evolve such shapes. For example, Crataegus and Pyrus (Hawthorne and Apple, for the vulgar) are closely related to Sorbus, share similar habitats and are of similar size, and yet do not have pinnately compound leaves.
In other words, at least in this one, rather limited, hastily researched example, there seems to be no unifying environmental pressure for the various plants to adopt this trait. And, if there was, it would seem to make sense that related plants with similar environmental pressures would also adopt the trait. And while there could be hypothesis put forward about what hidden environmental factor (Scandium concentration in the soil?) these trees adapted pinnately compound leaves to take advantage of, it will be after the fact, and might not take into account counter examples.
If I don't accept adaptationism as the best example of why these trees evolved similar leaves, what do I believe? Do I believe that Global warming is a hoax and Jesus rode a dinosaur? I don't, because while adaptationist explanations are often taken by some to be synonymous with evolutionary theory, there is another way to look at evolution. Adaptationism makes the strong argument: anything in an organism is there because it has been selected for. But it is a somewhat more modest argument to say that evolution mostly works for selecting against. In other words, some fifty million years ago in the incipient Himalyas, a proto-Sorbus mutated to have pinnately compound leaves. The mutation neither helped nor hindered that plant, but for whatever reason, it was part of a lineage that was more successful. The gene that told the leaf to take that shape was copied from generation to generation, and the lineage was carried over the world. This wasn't because the shape of the leaf somehow made the tree more likely to survive. It is just that the shape of the leaf didn't make the tree less likely to survive.
Of course, this is just supposition on my part, based on a single trait in a limited amount of examples. It could be that there is a complete explanation for why this trait is present in some trees and not in others. However, the larger point is that observation of the world, even on a casual level, can often raise questions about scientific theories (or misunderstandings therefore) that bring the natural world into a new light.