Friday, March 29, 2013

CH10: The One True Tree of Life (2/2)


Parsimony Principle

          Parsimonious means stingy. But in the sense of fitting species to a taxonomical tree, it means the ‘best fit’. Again, we know that there exists a single unique tree which can be obtained by cladistic taxonomy. It is not easy to find the cladistic tree. So, finding a tree which is very similar to the cladistic tree would be much easier. The parsimony principle here is in the sense that the difference from the cladistic tree must be as little/stingy as possible.           
          Taxonomy is a science of classification. Library classifies books for convenience. There can be several ways for classification such as subject-based or alphabetical. All these classifications equally makes sense, and make the access to the books more convenient.
        Biological taxonomy can also be based on various criteria. But, the single way which stands out is based on evolution relation. Biologists call this method ‘cladistic taxonomy.’


“The most parsimonious tree is the tree which is ‘economically meanest’ with its assumptions, in the sense that it assumes the minimum number of word changes in evolution, and the minimum amount of convergence.”

Minimum Amount of Convergence

        Molecular classification is more reliable than trait-based classification. Behavioral similarity can cause convergence (i.e., similarity) in traits. We can be fooled by trait-based classification. 
Molecular classification does not depend on behavior. It is the pure nature embedded in each cell of organism. So, we don’t have to be worried about accidental convergence at the molecular level. 

The Difficulty in Find All the Trees

        To find a tree, you can start at the bottom of the hierarchy. Here, you first identify all possible combinations of animal categorizations. Then, you pick the one that fits the best. Next, you repeat the process for the next level up the hierarchy. This time you do not categorize the animal. You categorize the categories you’ve got from the first step.  You can repeat this process until you reach the top,  and you will get a taxonomy tree. 
        The task for finding all possible trees is really hard. There are 15 trees for 4 animals. When you have 20 animals, there are more than 8 million million million trees. The fastest computer in this world would take 10,000 million years to find identify the most parsimonious tree.

Now What?

        We can’t just do all categorization by force. We need to use our intuition to rule out combinations which make no sense. We use trait-based categorization to fasten taxonomy process. 

Types of Taxonomists

        Taxonomists can be classified into
  1. Phyleticists: They try to find a ‘family tree’ based on evolutionary relationships. The belief is that there is only one unique family tree, because there is one true history. The family which differs from the history would be the wrong one. The objective of phyleticists is to find that one unique family tree. There are two sub-groups of phyleticists which pursue the same purpose in different ways:
    1. Cladists: These people are obsessed with branching, i.e., how species branch out. But they don’t care how much a descendant differ from its ancestor. They approach is to find all possible tree sand pick the best one. By ‘the best’ one, they mean to find the tree where animals in each group has most features in common. During the classification, they give more weight to more recently developed features. 
    2. Traditionalists: These people are similar to cladists but they also care about the amount of change at each branching point. 
  2. Pure-resemblance measurers: They try to find a ‘resemblance tree’ based sole only on similarity in (molecular or traits) pattern. Since resemblance trees are not based on history, there can be many trees. The upshot of pure-resemblance measurers is that their approach work not only for living things, but also for non-living things:
    1. Pheneticists, or Numerical taxonomists, or average-distance measurers: These people are more like mathematicians than biologists. They look at everything they can, and try to create resemblance index. They then plot each species in a space (e.g, X-Y Coordinate for a two dimensional space). Similar species are placed close to each other. Finally, they apply some kind of fitting model to define clusters of similar living thing. 
    2. Transformed cladists: These people use the same technique as what the cladists do. But they do not care about ancestor. Some of them even assume that there is something wrong with evolution, and taxonomy would be better without evolution. 


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Book or Audiobooks?

Personally, I prefer audiobooks. It's fun, and I can listen when I'm doing something else. It also makes other activities (e.g., jogging) a lot more fun. For more detail about audiobooks, please read [this post].

There is one more reason that may encourage you to go for the audiobook version. You can get it now for FREE. Audible offers you a free trial for 14 days. Even if you get the book and cancel the subscription right away (so that you don't have to pay), you can keep the book. And, don't worry if you lost the audiobook file. Just log into audible.com. You can keep downloading the over and over again.
About the summary: It takes time to finish up a book. And, when you do, sometimes, you want to review what you learn from the book. If you do not make  notes as you read, you might have to go through the book once again. This can be time-consuming when you are dealing with a book. But you can still flip through the book and locate what you are looking for.

However, when the material is an audiobook, it is extremely hard to locate a specific part of content. Most likely you will have to listen to the entire audiobook once again.

This book summary will help solve the pain of having to go through the book all over again.

I am leaving out the details of the books. Most books have interesting examples and case studies, not included here. Reading the original book would be much more entertaining and enlightening. If you like the summary, you may want to get the original from the source below.

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