Monday, March 8, 2010
Vertebrate Evolution
On page 8, Coyne breaks down how the common ancestor of vertebrates evolved into the organisms we see today. Describe how the different traits developed in these organisms and how these traits were passed on through mutations in genes from the common ancestor to the organisms we see today in the vertebrate phylogeny. Use key words such as vertebrate, jaws, digits, amniotic egg, hair, placenta, and opposable thumbs. What were the names of the different species that arose from the splitting of species from the common ancestor due to these traits? How are these species similar and dissimilar? Explain how these traits lead to a "natural" classification of these species and how "natural" classification is strong evidence for evolution.
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Traits in the different groups of phylum Chordata developed over millions of years as a result of natural selection, which entails the survival and reproduction of individuals which have a DNA mutation that gives them a slight advantage in their environment. Over time, a beneficial mutation (such as the amniotic sac) may become the norm for a certain group of animals, as individuals with the trait survive at a much higher rate than individuals without the trait (Wikipedia). In phylum Chordata, most of the species have vertebrae, with some exceptions such as lancelets, tunicates, and hagfishes (Campbell 700-702). Over time, natural selection selected for the presence of jaws in some individuals, while other vertebrates remained jawless because jaws were not an evolutionary advantage for them (these are the lampreys). In a similar fashion, all species of the phylum Chordata deviated from a common ancestor due to the selection for a certain trait: fish were not selected for digits, amphibians were not selected for amniotic eggs, reptiles and birds were not selected for hair, marsupials and monotremes were not selected for a placenta, and all other mammals except for primates were not selected for opposable thumbs. At the point of a split in the evolutionary tree, all species share the characteristics that were selected in their common ancestors. This branching tree helped scientists form a "natural" classification, because the nested arrangement of the tree shows how closely related different species are on the evolutionary timescale and how long ago they deviated from their common ancestor. This allows for only one logical way to classify species. Coyne uses the metaphor of matchbook classification to describe how a creationist would view speciation, and then shows that it contradicts with the nested arrangement of species because one can classify the matchbooks in myriad ways (pg. 9). Therefore, the "natural" classification shows that each progressive branch of the evolutionary tree split off from a common ancestor via natural selection, an incontrovertible piece of evidence for evolution.
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