 |
Business | Industries | Finance | Tax |
| Index: > A B C D E F G H I J K L M N O P Q R S T U V W X Y Z |
|
|||||
| First Prev [ 1 2 ] Next Last |
Based on a wide variety of information, which includes genetic analysis, biochemical analysis, and analysis of morphology, treelike relationship-diagrams called "cladograms" are drawn up to show different possibilities.
A vertical orientation yields a cladogram reminiscent of a tree.
In a cladogram, all organisms lie at the leaves, and each inner node is ideally binary (two-way). The two taxa on either side of a split are called sister taxa or sister groups. Each subtree, whether it only contains one item or a hundred thousand, is called a clade. A correct cladogram should have all the organisms contained in any one clade share a unique ancestor for that clade, one which they do not share with any other organisms on the diagram. Each clade should be set off by a series of characteristics that appear in its members but not in the other forms it diverged from. These identifying characteristics of a clade are called synapomorphies (shared, derived characters). For instance, hardened front wings are a synapomorphy of beetles, while circinate vernation, or the unrolling of new fronds, is a synapomorphy of ferns.
Typically, an analysis begins by collecting information on certain features of all the organisms in question, and then deciding which versions were present in their common ancestor (plesiomorphies) and which have been derived since (apomorphies). Usually this is done by considering some outgroup of organisms we know are not too closely related to any of the organisms in question. Only apomorphies are of any use in characterising cladistic divisions.
Next, different possible cladograms are drawn up and evaluated. Clades are typically drawn so that they can have as many synapomorphies as possible. The idea is that a sufficiently large number of characteristics should be large enough to overwhelm any examples of convergent evolution. In other words, there are many ways in which plants and animals, etc., may evolve features that resemble each other because of environmental conditions. A well-known example of convergent evolution is wings. Though the wings of birds and insects may superficially resemble one another and serve the same function, both evolved independently.
In practice, neutral features like exact ultrastructure (a term for extremely fine structure, microscopic or molecular composition of cellular structure) tend to do just that, to provide evidence for real relationships even when the appearance of organisms makes it otherwise difficult. When equivalent possibilities turn up, one is usually chosen based on the principle of parsimony: the most compact arrangement is likely the best (a variation of Occam's razor). Another approach, particularly useful in molecular evolution, is maximum likelihood, which selects the optimal cladogram that has the highest likelihood based on a specific probability model of changes.
Cladistics has taken a while to settle in, and there is still wide debate over how to apply Hennig's ideas in the real world. In particular, apomorphies are not always easy to distinguish and data are often unavailable due to a sparsity of available forms or a lack of knowledge of characters, and these may invalidate cladograms. There is also concern that use of widely different data sets, for instance structural versus genetic characteristics, may produce widely different trees. However, by and large the phylogenetic approach to systematics has proven useful and coherent and has gained general support.
As DNA sequencing has become easier, phylogenies are increasingly often constructed with the aid of molecular data. Computational systematics allows the use of these large data sets to construct objective phylogenies. These can more accurately filter out true synapomorphy from parallel evolution.
Cladistics does not assume any particular theory of evolution, only the background knowledge of descent with modification. Thus, cladistic methods can be, and recently have been, usefully applied to non-biological systems, including determining language families in historical linguisticsHistorical linguistics (also diachronic linguistics or comparative linguistics is primarily the study of languages which are recognizably related through similarities such as vocabulary, word formation, and syntax. Historical linguistics aims to classify and the filiation of manuscripts in textual criticismTextual criticism is a branch of philology that examines the extant manuscript copies of an ancient or medieval literary work to produce a text that is as close as possible to the original. The original is called the autograph. Before the invention of pri.
| Politics | Business | Finance |
 |
 |
 |
|