Wednesday, April 14, 2010
Whales!
From pages 47-52, Coyne talks about the evolution of whales. He notes how unique the evolution of whales was, since whales evolved from land mammals. There is striking evidence in support of this idea, mostly from fossils that clearly show whale fossils appearing no earlier than 30 million years ago. We know that whales evolved from land mammals, but my question is: why do you think that whales moved from land to aquatic/marine environments? What benefits did the whales gain from this transition? What new obstacles did/do whales face as a result of this transition? Be sure to use Coyne's book, as well as outside knowledge (look up information about whales!) when answering this question. Also, don't forget about themes!
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A possible explanation is that the whale's terrestrial environment quickly changed into a hostile environment. Many predators may have evolved. As we all know, Coyne points out that predator-prey interactions, very important to survival and reproduction, greatly influence evolution, specifically natural selection. Examples of this are different types of coloring of organism's coats/skins; Coyne's example is the wild mice (Peromyscus polionotus) (116). Additionally, it is probable that even on land, the whale's ancestors were large and not extremely mobile. Therefore, an increase in predators would be extremely disadvantageous for the creature.
ReplyDeleteAs our little saying goes, "Adapt, migrate, or DIE." I think the whale found a way to accomplish two out of three: adapt and migrate. The whale "migrated" in a way to the water, and began adapting. In the water, those pesky terrestrial consumers were not present. The whales could also have all the food (zooplankton) they wanted.
One obstacle was for the whales to sense stimuli in the environment, especially the ability to hear underwater. This change is described on a page dedicated to the evolution of whales on the website of PBS. The skull of an ancient organism called Pakicetus that was related to the ancestral whale demonstrated an intermediate form of the region of the ear between fully terrestrial and aquatic animals. In whales, the ear region has been modified for directional hearing underwater. This ability greatly helped the whale react to stimuli in the environment (http://www.pbs.org/wgbh/evolution/library/03/4/l_034_05.html). This adaptation once again shows the relationship between structure and function. The structure of the ear had to change in order to change the function.
Coyne briefly outlines the process on p51, and needless to say, there is a lot of fossil evidence showing the various adaptations of whales to adjust to life completely in water.
According to Coyne, one possible theory of why whales moved to sea was because that "dinosaurs along with their fierce marine cousins had just disappeared" (Coyne 51). With these creatures gone, there would be no more fear of competition or becoming prey. The sea was "an open niche, free from predators and loaded with food" (52). In fact, all of its benefits were only "a few mutations away" (52).
ReplyDeleteCoyne also describes the sequence of how whales took to the sea starting with a close relative to whales, an animal called Indohyus. There is evidence that this animal was at least partially aquatic because its bones were denser than fully terrestrial animals to keep the animal from bobbing in water. According to Coyne, "this part-time life in water probably put the ancestor of whales on the road to becoming fully aquatic" (49).
The next few evolutions have some missing link, but basically, over the next 10 million years, whales became fully aquatic, which is really fast considering that human and chimpanzees diverged in the same amount of time.
Although not mentioned anywhere, a few obstacles of moving completely to sea can be inferred. First off, since whales have be able to spend significant time underwater, a heightened mammalian diving reflex would be necessary so that the whale would not have to resurface often. Also, better motility in water would be required to be an effective hunter in the water. A different structure of limbs would have to evolve and a tail to push water more efficiently.
This connects to the theme of structure and function because the whale evolved certain structures to be more effective in water such as the fanned out tail and the arms with a large surface area to propel the whale through water. The Indohyus could only wade in water because its limbs were not suited for a life based in the sea. The whale's arms and tail makes sense for the function is has to perform for the whale, motility.
Sources:
http://www.pbs.org/wgbh/evolution/library/03/4/l_034_05.html
http://www-personal.umich.edu/~gingeric/PDGwhales/Whales.htm
For the first question, I would just like to add that most whales live in marine not aquatic environments. Anyway, I believe that the vestigial hind limbs, combined with fossil evidence of animals with similar structures as whales, provide sufficient evidence to support the theory that whales once walked on land. Perhaps, when whale ancestors first began to swim, they used their tails for propulsion and not their hind legs. So, over time, the essentially useless hind legs disappeared. The fossils of the creatures, shown on page 50, discovered in Pakistan show many similarities in the structure of the skull and other characteristics. “One of the most interesting was the ear region of the skull. In whales, it is extensively modified for directional hearing underwater. In Pakicetus, the ear region is intermediate between that of terrestrial and fully aquatic animals” (http://www.pbs.org/wgbh/evolution/library/03/4/l_034_05.html). Another piece of convincing evidence that whales evolved from earlier land mammals is that modern whale fossils don't seem to be older than 30 million years old, which is 30 million years younger than other mammal fossils discovered. I agree with the statement made by Coyne, which Frank mentions above, when he says that a probable reason for mammals migrating back to the water is that “with their reptilian competitors extinct, the ancestors of whales may have found an open niche, free from predators and loaded with food” (52). This migration to a new niche over time can, in a way, be compared to diffusion across concentration gradients. Since there were now fewer species inhabiting that niche, at least a fewer amount of predators, mammals could begin to move in. However, switching to a marine environment presented new challenges for whales to overcome. One of the most difficult obstacles to overcome, that I can think of, is finding a way to give birth without the offspring drowning. A fossil of a new species discovered in Pakastan, dubbed Maiacetus inuus, shows a fetus positioned for head-first delivery, “like land mammals but unlike modern whales, indicating that these whales still gave birth on land” (http://www.sciencedaily.com/releases/2009/02/090204085133.htm). This means that early whales couldn't travel far from the shore if the offspring were to survive. Considering the size of whales, I believe this would have caused a lot of competition for food and breeding grounds between them. So, the whales that could give birth further out into the ocean stood a better chance of survival.
ReplyDeleteI agree with Frank that one theme that this relates to is structure and function. The structure of the tiny hind legs served no function, and probably slowed the whales down due to increased drag. These legs then decreased in size over time, and can now be seen as vestigial structures inside modern whales.
Eric Hennings