Tuesday, March 29, 2011
Hand Evolution
In the vertebrate unit, we learned about phylogenetic trees. There are two ways to make a phylogenetic tree. One way is to separate animals based on physical characteristics. The other way is to base the tree of molecular characteristics. There is an important distinction between the two. For example, we could make a tree that separated humans from birds because we have hands and birds have wings. However, we can't separate the two groups based on ZPA and Sonic hedgehog because both birds and humans possess them. Explain the importance between the distinction of the two types of trees and the evolutionary significance.
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The two trees in question are morphological trees and molecular trees. These man-made trees are major tools biologists use to trace the origins of life and map out the direction of evolution. They are an evolutionary family tree, if you will. They are organized as a way to find the origins of any species because clades, or branches of the tree “are inferred from shared derived characters that are unique to members of the clade and their common ancestor” (Campbell 662). However, though there is one common ancestor to all organisms on earth (Campbell 662), there has yet to be made a tree that would encompass all of them, due to missing (yet unfound) information about organisms and the outrageous cost that would be a side effect.
ReplyDeleteMorphological trees relate to an organism’s structure and function. For an example of a morphological tree , check out this excellent diagram (which features humans!) http://www.ekcsk12.org/faculty/jbuckley/lelab/phylogenetictreeactivity.htm or just look at page 662 of Campbell. This type of phylogenic tree is constructed mainly based on how an organism looks , as in structures resembling limbs or bilateral symmetry or whatnot, how an organism appears to use these structures, and type of development. Morphological trees are precursors to molecular ones and have been created and used by zoologists, paleontologists, and evolution scientists for centuries.
Molecular trees are constructed based on similarities between organisms’ continuity and change in genetic code over very long periods of time. An example of a molecular tree can be found here http://www.bioone.org/doi/abs/10.1663/0006-8101%282002%29068%5B0488:UFSAIE%5D2.0.CO%3B2 or page 663 of Campbell. These trees require the use of technology to map out gene sequences, thus they are more expensive and were not created until about the 1980s. Pre-existing morphological trees often are not consistent with molecular trees. This leads to insight on convergent—the evolution of similar features in independent evolutionary lineages—and divergent—“the development of different characteristics in animals that were closely related in response to being placed in different environments”—evolution (Campbell G-9, http://medical-dictionary.thefreedictionary.com/Divergent+evolution). Though as stated, there is no one all-encompassing phylogenic tree, the experiment comparing genetic similarity, Evolutionary Analysis by Whole-Genome Comparisons by Arvind K Bansal and Terrence E. Meyer, which appeared in the April 2002 edition of Journal of Bacteriology, provides an excellent example of a meticulous 37 specie molecular phylogenic tree.
There are two types of phylogenetic trees. One is based off molecular characteristics, while the other is based off physical characteristics. Although the tree based off physical characteristics is much less correct, it allowed for an earlier theory on the origins of each type of organism. This allowed scientists to start to piece together what caused the evolution of the organisms. Many of these causes were proven wrong when DNA of each organism was compared, which led to the creation of the molecular phylogenetic tree. This is more accurate because genes can show identical characteristics, unlike physical which only shows similarities between the organisms.
ReplyDeleteA reason to why that these two different types of phylogenetic trees are very similar in their theories is because some characteristics formed because of natural selection due to some pressure from the nature. Many of these characteristics were obvious from a third party perspective, while some were not. Also, at the beginning of time, organisms were evolving splitting in much more obvious points, therefore creating a large difference in physical differences between organisms not closely related (Porifera is one example of an organism that is much different than others in the phylogenetic tree).
One reason that some organisms share physical characteristics that are not truly related based on molecular characteristics is because organisms distantly related were often still faced with the same environmental pressures that made them evolve in the same way. Organisms that aren't closely related still can live in the same or similar environments. Therefore, when some external pressure occurs, all organisms in that environment must evolve or die. The ones still around today are the ones that evolved and survived. Therefore, the characteristics that were formed that allowed their survival must have been similar, which confused scientists later on down the road. Even the present day phylogenetic tree has its problem because, “the analysis can be confounded by horizontal gene transfer,[9] hybridisation between species that were not nearest neighbors on the tree” (http://en.wikipedia.org/wiki/Phylogenetic_tree).
Jackie James (Jackie.james@comcast.net)
Like Jackie mentioned, biologists use two main types of trees to classify organism based on common descent. Their goal is to place organisms into groups that correspond to clades, "each of which includes an ancestral species and all of its descendants. Based on cladistic methods, a phylogenetic tree takes shape as a hierarchy of clades nested within larger clades" (Campbell 662). The morphological system is considered the "traditional" while the molecular is "new," but the overall goal of constructing the evolutionary history of life is the same. The morphological system is based on an animal's anatomical structure, while a molecular system is based on finding molecular similarities between animals. In his article, David Hillis acknowledges that there can be conflict between the two approaches, since molecular systematics grew mostly out of population genetics, while morphological systematics stemmed from comparative anatomy. However, he argues for the "increased combination of molecular and morphological data in order to maximize phylogenetic information" (http://www.jstor.org/stable/2097123?seq=2). Thus, both trees, with their similarities and differences, must be studied carefully in order to gain a full appreciation for evolutionary history and where us humans came from.
ReplyDeleteAlthough Jackie explained the differences between the two trees, it is important to note the similarities also. There are a few points of agreement between the morphological and molecular trees. First, they both show that all animals share a common ancestor, and that the animal kingdom is monophyletic, representing a clade called Metazoa. Sponges are the basal animals in both trees, Eumetazoa is a clade of animals with true tissues in both trees, most animals belong to the clade Bilateria, and chordates and some other phyla belong to the clade Deuterostomia. These similarities can be seen on page 662 in your Campbell textbook for a clear comparison.
The main difference that scientists have found through molecular systematics is that the bilaterally symmetrical animals are split into three clades instead of two: Deuterostomia, Lophotrochozoa, and Ecdysozoa. In a molecular tree, there are two taxa in Deuterostomia rather than one: the ecdysozoans and the lophotrochozoans. This details in the tree are constructed by observing the continuity as well as the change in genetic code over very long periods of time, as Jackie correctly mentioned. However, there are still many details to straighten out; animal systematics is a work in progress. Most scientists think that the molecular tree is more strongly supported than the morphological tree, but this opinion may change (Campbell 664). Researchers will continue to analyze the genes of many animal phyla to improve the tree that they already have to increase our knowledge of our evolutionary history.
Hannah Kay (hgkay@aol.com)