Ever heard of a mer-cow? On page 48, Coyne pokes fun at such a transition that could occur between terrestrial and aquatic mammals.
What does Coyne give as an example of a creature that is dependent on both land and water without evolving? Can you research any other creatures that may create a similar scenario? Furthermore, in respect to this transition of animals moving from land to water, like the whale, why did such a phenomenon occur (disadvantage and advantages)? Comment on the transitions found between old and new evolutionary changes (page 53; ex. Wings of bird from the legs of dinosaurs, etc.)
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On page 48, Coyne talks about the hippopotamus amphibius. This particular species of hippo lives, procreates, and nurses in water, while feeding (grazing) on land.
ReplyDeleteDolphins are another such creature that had a common, terrestrial ancestor. Dolphins evolved from the Odontoceti, meaning "toothed whales." This sub-order evolved from the order Condylartha, which divulged from the family, Mesonychidae (mammals) (1)(2) .
Terrestrial condylarthrans were found to live by river beds and lagoons, most likely feeding on slow moving fish. As ecological niches in aquatic environments cleared out, these pre-cetaceans moved in to the newly re-opened environments, in prospect of food. Speculations exist that pre-cetaceans had were seal-like and came onto land solely for the purpose of procreation, much like turtles lay eggs on a beach, but live in the sea.
According to Coyne, some mammals may have returned to the water because of changes in the ecological niches. One of the many speculations is that many of the fish-eating dinosaurs, which would have provided competition along with even being predators of whales and other mammals living in water, disappeared. This promptly reopened a once inhabited ecological niche, and produced an environment with a large food supply, and a small (relative to the same niche populated by dinosaurs) risk for predation and competition (p. 51-52). This same theory exists for the pre-cetaceans (and eventually dolphins). (1)
There do not seem to be any disadvantages with moving back to an aquatic environment if the ecological niche is open for a new species to dominate it. It may take a "long time" for the an old species to re-adapt to an aquatic environment, and may be costly in terms of energy, but the prospect of having a large supply of food readily available is more than enough to justify the change. Also, the fact that predation and competition was reduced with the disappearance of dinosaurs only adds to the appeal or adapting to an aquatic environment. All in all, there seems to be no disadvantage in such a transition.
The ancestors of whales, when moving from water to a terrestrial environment, had developed features such as four legs and a neck. However, when moving back to the aquatic environment, such adaptations were no longer suitable for the environment. As a result, the hind legs of the Indohyus (the whales' ancient ancestor) had all but disappeared. This is appropriate due to the fact that hind legs are no longer necessary in water (aka, vestiges), if the spinal column extends through the entire back of organism, and becomes a tail fin. This would provide far more propulsion in water, especially for a creature as large as a whale (or blue whale) and would cost less energy to produce (since it is an extension of the spinal column, instead of an entirely new bone structure). Also, the neck had entirely disappeared from the Indohyus through the Balaena, which is very similar to how the physiology of fish looks. This adaptation most likely occurred because the ancient ancestors of mammals had developed necks when they came onto land, and when some mammals returned to the sea, this adaptation became useless to them. Additionally, the entire organism increased in size as it transition back into the sea. This adaptation most likely occurred for several reasons. First, water can support a much larger mass than land, because water partially counteracts the effect which gravity has on objects, and generally, larger organisms live longer (which means more time to reproduce and create offspring) (3). Second, a larger size would deter many predators because with larger sizes comes more muscle mass (and generally, more strength). - P.50
1: http://library.thinkquest.org/17963/evolution.html
2:http://en.wikipedia.org/wiki/Mesonychidae
3:http://en.wikipedia.org/wiki/Whale
As Krill previously mentioned, Coyne uses hippopotamuses as an example of an animal which lives on both land and in the water. Hippos spend much of their time lounging in the water and they mate and give birth in the water. This is why their babies learn to swim before they can walk. Yet, hippopotamuses venture out of the water at night to graze. Coyne explains that “if they could find enough food in the water, [hippos] might eventually evolve into totally aquatic, whale-like creatures” (48).
ReplyDeleteSince you mentioned “mer-cows,” why don’t we address manatees, which are considered to be “sea cows.” Manatees are the only herbivorous mammal that is completely aquatic and there is a variety of evidence which proves that its descendants were once land dwelling animals. First of all, manatees have toenails similar to those found on elephants. In the water manatees uses these toenails to grab things like vegetation, similar to what they would have been used for on land. In addition, manatees have a tetrapod forelimb structure in their flippers including an upper arm bone, two forearm bones, a cluster of wrist bones and five finger bones. This structure resembles that of modern terrestrial animals like humans. To continue, manatees have a rudimentary pelvic bone and vestigial hip sockets which implies that at one point in time they were tetrapods with hind limbs. All of this evidence allowed scientists to determine that the descendants of manatees were land dwelling animals.
The reverse migration of animals back into the water is a result to ecological pressures. Coyne states that one such ecological pressure could have been the extinction of aquatic dinosaurs. With a large chunk of competition gone, “the sea was ripe for invasion” (52). Therefore, the advantage of moving to the sea would be that there would be an abundance of fish to feed on and less predators, allowing them to successfully survive. However, they had to go continue to go through the process of evolution to become properly adapted to this new environment. Yet, there is evidence that this was not too daunting of a task. For example, “the evolution of whales from land animals [. . .] took place within only 10 million years” (51). Relatively speaking, this is not very much time considering how much they evolved.
I think that the “transitions between old and new evolution” are a great piece of evidence in favor of evolution as form fits function beautifully. The arms of reptiles developed into wings allowing animals to fly. Arms of other land dwelling creatures evolved into flippers allowing animals to more easily maneuver in water. For example, the flippers of hippos have the same basic structure on the insides (an upper arm bone, two lower arm bones, wrist bones, five finger bones), yet they are all connected together in a triangular shape that allows marine species to easily navigate through the water. These species started with arm bones, and over time they evolved to fit the needs of the animal in this new environment.
SOURCES:
ReplyDeleteWhy Evolution is True by Jerry A. Coyne
http://en.wikipedia.org/wiki/Evolution_of_sirenians
http://www.manatee-world.com/manatee-evolution.html
http://www.youtube.com/watch?v=co2kpLp3Q50