Friday, April 1, 2011
In chapter 4 of "Your inner fish," it mentions and explains the evolution of teeth to bones. Ostracoderms were the first animals found with bony structures that were made entirely from fused teeth which served as a defense from predators. Explain why it would be beneficial for bones to occur after teeth. Include at least one other example in which a similar evolutionary pathway occurs.
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For a population to evolve a certain characteristic there needs to be some selective advantage due to an environmental pressure. The development of teeth before bones fits this pattern. Shubin describes this process himself. He claims, “Hard bones arose not to protect animals, but to eat them,” (Shubin 76). He furthers this claim by explaining that fish began to eat other fish, and as their diets changed, the population of carnivorous fish developed sharper teeth. For example, in carnivorous or predatory fish, teeth may be present on the jaws, tongue and inner mouth. These teeth do not bite or crush food but simply hold it and prevent prey from escaping (http://www.aqualex.org/elearning/fish_feeding/english/digestion/structure.html).
ReplyDeleteThus, smaller fish were more likely to be consumed by larger fish. In turn, these smaller fish needed to develop some defense system against their predators. Following the process of evolution, the presence of bones gave individuals a selective advantage over the individuals who did not possess bones, “armor” as Shubin calls it (Shubin 76). Then, individuals with bones were able to reproduce and pass on those traits to the next generation; over time, the entire population would possess bones. But without the presence of hard teeth, there would be no selective advantage to having a bone structure.
Another example that follows this pattern is the concept of mimicry in animals. If a predator eats a distasteful animal, then in the future it will refrain from hunting that specific animal. The predator, to identify distasteful prey, will associate characteristics of the prey to the bad-taste or often the toxicity of the prey. An example of this relationship, which also demonstrates interdependence in nature, is the Batesian mimicry hoverflies to wasps and bees. Hoverflies are not toxic or distasteful and they don’t invest any energy in maintaining this expensive defense mechanism. Instead, many species have evolved yellow and black bands so they mimic wasps and bees. Their predators will associate the hoverfly color pattern with the horrible taste of truly toxic species and so the predators will avoid them, even though they are perfectly edible (http://www.christs.cam.ac.uk/darwin200/pages/index.php?page_id=g6).
The ability to develop yellow and black bands presents the hoverfly with a selective advantage over hoverflies who do not have yellow and black bands. However, this selective advantage is only present because of the pressures posed by predation, just as bones only provided a selective advantage against predators with teeth. The relationship between pressure and selective advantage in a sense defines the process of evolution. Without an environmental pressure, evolution cannot take place, because it truly would serve no purpose.
Vickram Pradhan 1/2A vickram.pradhan@yahoo.com
Like Vickram pointed out, bones occurred after teeth because it was a selective advantage for fish to have bones and enabled them to survive moreso than those fish without bones. Shubin said that "hard bones arose not to protect animals, but to eat them" (Shubin 76). He describes the process of fish developing teeth and bones as an "arms race...Little fish developed armor, big fish obtained bigger jaws to crack the armor, and so on" (Shubin 76). Fish had already developed sharp teeth of all different shapes and in different locations to more efficiently digest their prey; some teeth of fish are located in the throat while others are on the roof of their mouths (http://media.wiley.com/product_data/excerpt/54/04714499/0471449954.pdf). However, bones were extremely beneficial to smaller fish who were being preyed upon because it would be harder to crack through strong armor, or bones. Because of the environmental pressure of predators with sharp teeth, many fish, through numerous mutations and generations, developed the selective advantage of bones. This gets at the overall theme of evolution: the development of selective advantages over time in response to environmental pressure.
ReplyDeleteAnother example of a selective advantage that has developed because of environmental pressure is pollen. When plants began to move from water to land, flagellated sperm was only useful for reproduction in moist areas. In addition, these swimming sperm could only travel short distances, perhaps a few centimeters. Thus, seed plants developed a sperm-producing male gametophyte inside a pollen grain. This was a huge selective advantage because the sperm of seed plants "do not require motility because sperm are carried directly to the eggs by pollen tubes," which also eliminated the dependence on water for sperm transport, a response to the movement of plants onto land (Campbell 620). Although the selective advantage of pollen grains is not directly related to predation, it still follows the same evolutionary pathway, in which an environmental pressure causes a selective advantage to develop over many generations in order to better survive and reproduce.
http://media.wiley.com/product_data/excerpt/54/04714499/0471449954.pdf
Hannah Kay (hgkay@aol.com)
Just as Vickram and Hannah said, the reason why teeth developed before bones was because there was no use of bones as protection. Teeth developed to eat their prey easier and faster. This also allowed them to kill their prey faster and be able to digest it mechanically faster. This prey then started to grow its own armor to protect it from the hunters of the ocean. This was developed through the theme of evolution. A random mutation which allowed bone to be formed became selectively advantageous because the teeth weren't able to break them. All the other prey didn't survive or reproduce, so the genes to not have bones were signifigantly less. And over time, more and more fish started to get a bone skeleton. But then as Hannah said, "arms race...Little fish developed armor, big fish obtained bigger jaws to crack the armor, and so on" (Shubin 76). Bigger fish began to develop stronger jaws and larger ones, strong enough to break those bones. This continued for a long time to get what we have right now in the oceans, but this is just part of the series, its not the final product. These populations are constantly changing over time.
ReplyDeleteAnother example of selective advantage is pathogens and antibodies. Pathogens are constantly evolving producing different antigens. B cells divide and rearrange their DNA to create different receptors, hopefully creating one that recognizes the antigen in time before it changes or becomes fatal. Some problems that are arising right now is that some of these antigens are changing at such a rapid rate that B cells cant be produced at the same rate (HIV). That is why HIV is currently incurable because scientists can't produce the right strand of antibodies that kill those specific cells because it constantly changes. http://www.medicinenet.com/human_immunodeficiency_virus_hiv_aids/article.htm#tocb. Another problem that is arising is the potential cause of 'superbugs' where the pathogens don't even have any antigens, therefore there can't be any antibodies that can bind to any of these. “It was literally resistant to every meaningful antibiotic that we had,” http://www.newyorker.com/reporting/2008/08/11/080811fa_fact_groopman#ixzz1JH51SIVm . This is an example of an evolutionary arms race because there are two organisms that are competing for the same resources (the human body).