Sunday, April 5, 2009
Nonbelievers
According to many anti-evolution people, Charles Darwin did not believe that natural selection could account for the evolution of complex organisms, such as humans. These people argue that Darwin only thought evolution could occur in small populations with relatively simple organisms. What evidence does Carroll give to convince these people that evolution can account for complex organisms? Also, give an example/examples of evolution in a large population with complex organisms.
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Carroll states that "complex organisms" are "composed of trillions of cells", such as skin cells, that are continously being disgarded and then replenished. In order for new cells to be produced, DNA must be copied and during transcription or translation, a mistake may be made. This mistake may lead to a different genotype which may lead to a different phenotype. In other words, "some mutations can ignite" (182) the beginning of the evolution of a species. The example that Carroll uses is cancer in humans. Humans are complex organisms and a small mutation during the DNA copying process can lead to the spread of cancer or the growth of tumors. Carroll states that the only "ingredients" needed for the cancer or tumor are "chance mutations, selection, and time" (182). As long as a mutation is able to controls its ability to multiply, the mutation begins to spread rapidly, changing the organism that is being "attacked" over time.
ReplyDeleteWell David, the very fact of our existence is the proof of evolution in complex organisms. Carroll uses one example, the ice fish, as an example of complex evolution regarding the anti-freeze proteins and the hemoglobin-absent blood. Both of these adaptations evolved in the ice fish and were passed through generations of ice-fish, allowing them to occupy the very cold niche in which they currently reside. One other example is in human beings and many other verterbrates, the existence of eosinophils. These eosinophils specifically target large parasites, like blood flukes, to prevent them from sapping nutritive macromolecules from the blood stream. The existence of eosinophils is evidence of evolution in a complex organism, and begs the question: what really defines complex? I beleive that complexity is a relative term that has no bearing in biology. Instead, we should look at well-adapted species, like the cockroach and the tardigrade, two animals that have evolved such unique adaptations that have kept these species very unevolved for the past thousands of generations.
ReplyDelete"Complex" is a subjective and relative term that, as Sam said, shouldn't be used in biology.
ReplyDeleteI'll ignore the term "complex" and instead move onto the supposed "impossibility" of evolution in large populations. In this case, Carrol gave his example of the peppered moths (p52). The results of the experiments done to track the % of melanic moths are proof that large populations can evolve together. From the 1800s to 1900s, the frequency of melanic moths in BOTH England AND the US rose and dropped significantly. This shows that even very large populations of a species (even in different parts of the world) can evolve at the same time.
I like this example because proving that a "complex" organism such as humans evolved is difficult since we have little concrete evidence. On the other hand, this example of the rise and decline of melanic moths is absolute. It's definite evidence of natural selection's effect on a species' evolution.
Carroll uses the formation of eyes to prove that evolution can occur on a large scale. Eyes exist in many different animal phyla and serve the same purpose in each. However, the eyes in vertebrates, arthropods, and mollusks (to name a few) all have organisms with a variety of eyes. The types of eyes include “camera-type eyes”, “a mirror eye with a lens and a reflecting mirror”, or “compound eyes made up of from ten to eighty unit eyes”(194). Even though the eyes differ in many ways they all contain at least one photoreceptor and a pigment cell. Because of the similarities in the different eyes, it is likely that a simple two-celled eye was first developed in a common ancestor and has since evolved into the different types of eyes present in the different species. It is very unlikely for each of the species to have had to create an eye on their own since there is a small chance of its creation in the first place. This two-celled eye is seen in the larvae of the Platynereis dumerilii (ragworm). In the ragworm there is also an opsin protein that is used to detect the presence of light. At least one opsin is also present in all eyes and can therefore also be said to be present in the common ancestor. The more complex models of the other types of eyes are based upon the addition of photoreceptors, pigment cells, and opsins. The addition of more materials occurs through different mutations that occur by chance over time. The way that the addition materials form on the eye help to create a different eye structure that is more three dimensional. The change in structure creates a change in function in what the animal is able to see. The overall idea of a simple two celled eye that was created in a past common ancestor can be changed over time into the more complex organs that exist today.
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