Tuesday, March 30, 2010

Fossil Confirmation of Evolution

On page 96, Mr. Coyne notes that we can usually tell how an organism evolved over time by looking at the fossils in that area. Two of the examples he gives are the fossils of kangaroos that resemble modern-day kangaroos in Australia, and the armadillos (unique in that they have a carapace of bony armor) that match very closely the glyptodonts, ancient armored mammals. What are some other examples of ancient fossilized organisms that resemble fairly closely their modern-day counterparts (their descendants)? What factors might contribute to the similarities that the descendants of the organisms have to their ancestors? What themes of biology can you connect these factors and reasons to?

4 comments:

  1. One example of a fossilized organism that resembles its modern counterpart is the Megalodon. The Megalodon was a gigantic carnivorous fish that reached lengths of 45 to 90 ft. This organism closely resembles species in the class Chondricthyes and the subclass Elasmobronchii. (www.kqed.org). As we learned in class, the class Chondricthyes refers to organisms with a skeloton composed predominantly of cartilage, often impregnated with calcium (Campbell 706). This class includes sharks, rays, and guitarfish.
    Another example of an ancient fossilized organism is actually not an animal. The giant fossil Prototaxites closely resembles modern trees, yet they are from a time before trees existed (www.dailygalaxy.com). It's clear that ancient plants and animals both resemble their descendants fairly closely. Take the ancient primate Gigantopithecus blacki (italicized, this ancient organism is a clear ancestor of the modern ape (www.uiowa.edu).
    The ancestors and the descendants are similar primarily because of evolution; in fact, this partly proves that evolution is true. If we think about it, as organisms disperse and colonize, they evolve. Over time, the species evolves into a new species through mutations in genes during sexual reproduction or through the influences of natural selection. Certain traits that are selective advantages persevere and others that are not so useful disappear; a new species emerges. However, because this species is, in a way, a mutation of the former, it still retains certain similarities with its ancestor. Thus, as Coyne says, by digging in a given area, "we should find fossils that resemble the organisms treading that ground today" (96). Moreover, this clearly relates to the theme of continuity and change. As the species evolves, gene mutations occur and natural selection forces change continuously into more selectively advantageous adaptations. The changes in gene pool and appearance can be attributed partly by natural selection for the fittest geneotypes.

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  2. An example of an ancestor that looks like its present day counterpart are wooly mammoths. Coyne explains that thousands of years ago, wooly mammoths had to build up their hair after generations of modification to adapt to the cold climate (11). One could infer that these were the years of the ice age. We have learned in class that hair helps an organism thermoregulate. Because of the cold environment, one could assume that a high metabolism and hair helped a wooly mammoth survive the frigid cold, as well as its insulating layer of adipose tissue (fat) (Prehistoric World Images).

    However, as time went on, the ice began to melt, and hair was not needed anymore. Generations of modification led to a change in the wooly mammoth genome, and less hair was expressed. This brings us to our modern day elephant that we see today.

    Though the modern day elephant doesn’t have a thick layer of fur, it retains its tusks that its ancestral wooly mammoth had. The modern day tusks of the elephant most likely serves one of the same purposes as it did thousands of years ago: defending their bearer. While wooly mammoths in the past had to fight off saber tooth tigers that were living at the time, our modern day elephants have to defend themselves from prides of lions and other animals in the African savanna (Eliaid). Both wooly mammoths and their decedents use tusks to defend themselves from predators.

    Tusks make a good connection to the theme structure and function. Even after thousands of years of modification, natural selection has not wiped elephants with defective tusk genes off the face of the Earth. The length of a tusk and its pointed tip allows the tusk to perform its function of allowing elephants and wooly mammoths to burrow and attack predators.

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  3. Adding to Connie's response about the Carcharodon Megalodon, the reason that the shark, after generations of modification, has been reduced is because of the waning food supply in the ocean (Elasmodiver.com). Though the low gravity in the ocean facilitated the grown of large animals, as these animals began populating the water, food supplies dropped. Next generations of C. Megalodon lived if they were smaller - they ate less food. The ones that were still big or even bigger died. That is an example of natural selection at work.

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  4. Another example of an organism that very closely resembles living counterparts is the prehistoric crocodile Sacrosuchus imperator and modern gharials. Sacrosuchus imperator lived in the Cretaceous Period, and resembles the modern gharial in body shape, especially in relation to the shape of the skull. The major difference between the Sacrosuchus imperator and the modern gharial is size. While modern gharials average 16.5 ft in length, Sacrosuchus imperator was 36 ft in length. Though Sacrosuchus imperator is not directly related to modern gharials, their similarity in appearance suggests that the two species evolved in response to similar environmental stresses. (http://chronicle.uchicago.edu/011101/supercroc.shtml)

    Both Sacrosuchus imperator and modern gharials hunted for prey in aquatic environments. The modern gharial is highly specialized for this task. When floating in the water, the modern gharial's skull is shaped so that only the eyes and the nostrils are visible above water, with the remainder of the head and body hidden beneath the water's surface. This adaptation aids the gharial on sneaking up on its prey. Sacrosuchus imperator's skull is very similar to the modern gharial's, and likely served the same function. Because both the modern gharial and Sacrosuchus imperator were under similar environmental stresses to survive, the modern gharial and Sacrosuchus imperator evolved similar adaptations in response to the stresses.

    All of this relates to the biological theme of the relationship of between structure and function. The function of both the modern gharial skull and the Sacrosuchus imperator skull is to hide the predator from their prey. Because the skulls needed to serve a similar function, they developed similar structures, visible in the similarity in skull shape of the two organisms.

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