Monday, March 15, 2010
Intermediate Stages
On page 39, Coyne mentions that the intermediate stages of the evolution of flight provided a selective advantage to its possessors by allowing them to glide or jump. Give another example of an intermediate stage and its possible selective advantages. Keep in mind the relationship between structure and function (Campbell 7), as well as the examples of derived traits on the chordate phylogeny tree (Campbell 699).
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ReplyDeleteEvolution of structure and function has occurred in the evolution of humans. When studying the fossils of Sahelanthropus, Coyne notes that scientists have found that this spinal chord that connects with the head is "farther forward than in knuckle-walking apes." He then notes that this evolutionary change "suggests that bipedal walking was one of the first evolutionary innovations to distinguish us from other apes" (199).
ReplyDeleteThe book discusses some advantages of being bipedal on page 209. Among the many speculations, I believe that being bipedal was important to be able to use hands to handle food. I'm sure many of us have seen apes eat berries from plants, and being bipedal would have helped an ape move while being able to scour for food. As we have learned, if this species could move while scouring food, then the species could cover more ground than it did before, allowing this primitive human to obtain more food in a shorter period of time.
Finally, along the evolutionary line, these primitive humans would have evolved into Homo habilis, being able to handle tools to hunt for food. Since the head sits more directly above the spinal chord, this species would be able to walk on two legs more easily than before. With the ability to walk well on two legs as well as use tools, Homo habilis would be able to hunt effectively to take down animals, letting these more recent humans survive and reproduce.
Henry provided an excellent example of an intermediate stage of bipedalism. Henry points out that as a result of being bipedal, apes had the selective advantage of being able to move while scavenging for food at the same time. I’d like to point out that being bipedal also contributes to a selective advantage of increased awareness of predators and other dangers. By having the head and body more upright an intermediate bipedal ape can be more aware of its surrounds (by seeing outward and forward), and thus avoid dangers such as predators more easily.
ReplyDeleteAnother intermediate species discussed in the book is Tiktaalik, an intermediate between the fish and amphibian (37-38). Tiktaalik had a neck, sturdy ribs, and rudimentary limbs with sturdier bones than those found in its ancestors the fish. These characteristics allowed Tiktaalik to crawl around the bottom of shallow waters and breathe air above the surface. The strength is the bone structure of the rudimentary limbs allowed Tiktaalik to move more swiftly on the bottom of shallow water in order to capture more prey. This selective advantage is a great example of how structure relates to function. The sturdy ribs which supported the transport of oxygen from the gills to the lungs allowed Tiktaalik to go above the surface of the water and to capture insects (which had already evolved) flying above. With more food than their non-evolved counterparts, natural selection favored Tiktaalik.
The decedents of Tiktaalik with stronger more bony limbs colonized the land and became the first amphibians. As Jamie suggested, you can refer to pg. 699 of Campbell to see that the presence of legs is what taxonomists use to separate fish from amphibians. All forms of amphibians are in themselves still living intermediate species. Amphibians (amphi meaning both) can live on land or in water (for at least part of their lifecycles). A reminant of amphibians aquatic past is seen in their reproductive methods. According to Campbell, many amphibians rely on water for fertilization. External fertilization occurs in water and the eggs develop here (1000).
PS. I'd like to italicize Tiktaalik because it is the scientific name, but this comment box won't let me even if I copy-paste from a word document.