|MadSci Network: General Biology|
I've got two questions here, that I will try to address in turn. First, you ask for the reasons that "the more committed to an aquatic existence an organism is the more highly specialized its forelimbs are." In some ways, your perspective is backwards. The best available evidence suggests that animals started out in the water, which means that the forelimb arrangement you see in most fish (pectoral fins) has a better claim to being the "normal" situation. It is land animals that then show the specializations in their forelimbs. The forelimbs really became more specialized for terrestrial locomotion (walking on dry land) as the animals evolved a greater commitment to life away from the water. However, I suspect that you were really asking about animals that demonstrate a secondary adaptation to aquatic existence. These are species that can trace their ancestry to land animals, but which are now more suited for life in the water. These are animals such as whales, seals, sea lions, otters, polar bears, and brown bears. If you look carefully, you will notice that I listed these animals in order from most aquatic (whales) to least aquatic (bears). What you are seeing here is a list of animals that are adapted to their environments. Each one shows a series of specializations (not just in their forelimbs, but in their entire body) suited to their lifestyle, and we could continue this continuum of animal types to move up into the tress, or out into the plains, or even into the desert (as unaquatic as you can get). The point that I am trying to make here is that each species is committed to exploiting its part of the environment (what ecologists refer to as the "niche"). Evolutionary forces shape the animal over time, so that the animal that is better at exploiting its environment will produce more offspring that will be equally equipped to live in that environment. No particular environmental choice is more "specialized" than any other, in the grand sense of things. A horse is as specialized for living in the open plains as a whale is specialized for living in the open ocean. The animals that live on the margins of an environment (taking advantage of both) are the animals that might be called generalized. Thus, future evolutionary pressures might force the otter into a more aquatic environment more easily than you would expect the horse lineage to adapt to the aquatic environment. But, as the otter lineage adapted a greater commitment to the aquatic environment the "evolved otter" would face new ecological competition. To succeed in that competition, the "evolved otter" population might choose a strategy of becoming better swimmers - and thereby may overtime start to look more like a sea lion. Or, it might take the route of becoming larger and more predatory, and end up looking something like a polar bear. Both the polar bear and the sea lion are specialized aquatic predators. But the sea lion's evolutionary history chose a path that made its forelimbs adopt a more fin-like shape, while the polar bears followed a path so that their forelimb was able to maintain a more "paw"-like shape. Neither animal is more, or less, specialized in it environment or in its forelimb, than the other. The second question is: What features of a forelimb indicate it is suited to high speed?" Here I am assuming that you mean high speed on the land, and not in the water or in the air. Animals that are specifically adapted to running are known as "cursorial." The modifications of the forelimb that are generally associated with cursorial lifestyles are: reduction in the number of digits (fingers or toes), adoption of the digitigrade posture (where the animal stands on the tips of the toes/fingers), elongation of the carpal (wrist) and antebrachial (forearm) bones, with a typical reduction of the brachium (upper arm bone). These modifications increase the length of the leverage associated with the limb so that it moves quickly rather than powerfully. However, these are just general trends - you really need to look at the modifications in the rest of the body. In most cursorial animals, the forelimbs are more structured to bearing the body weight, and it is the hindlimbs that move the animal forward. So, adaptations for speed would be better seen in the hindlimb then in the forelimb. References: Alexander, R. McNeill (1968) Animal Mechanics. University of Washington Press: Seattle Hildebrand, Milton (1982) Analysis of Vertebrate Structure. John Wiley & Sons: New York. Radinsky, Leonard B. (1987) The Evolution of Vertebrate Design. University of Chicago Press: Chicago
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