Systematics

From Palaeos

Hadean  Archean  Proterozoic  Phanerozoic 
     LUCA
       |--Eubacteria ---------------------
       `--Archaea  -----------------------
            `---- Eukarya ----------------
                     |--Plantae  ---------
                     `-----+--Fungi ------
                           `--Metazoa ----

[edit] Systematics - a definition

As defined by Simpson 1961, Systematics is the scientific study of the kinds and diversity of organisms and of any and all relationships among them.

Systematics is the branch of biology that deals with the diversity and interrelationships of living beings, both current organisms ("neontology") and prehistoric ones ("palaeontology"). It can be divided into three parts.

Taxonomy - the describing and naming new taxa (a taxon is any specifically defined group of organisms). Taxonomic groups are used to categorize similar taxa for identification-like field guides that do not necessarily represent evolutionary trends. Smaller taxonomic groups are used to relate organisms at greater levels of similarities.

Classification - the organization of information about diversity arranging them into a convenient, formal classification into a hierarchical system, and providing means of identifying them (e.g., diagnostic keys). [see e.g. the Linnean system]

Phylogeny - determination of the ancestral relationships of organisms, and the group's evolutionary history through time. Phylogenetics is the field of biology concerned with identifying and understanding the evolutionary relationships between the many different kinds of life on earth.

[edit] Types of systematics

(this section adapted from Wikipedia):

Systematics uses taxonomy as a primary tool in understanding organisms, as nothing about an organism's relationships with other living things can be understood without it first being properly studied and described in sufficient detail to identify and classify it correctly. The systematist, a scientist whose specialty is systematics, must therefore, be able to use existing classification systems, or at least know them well enough to skillfully justify not using them.

Phenetic systematics (and more informerly, "evolutionary systematics") involves clarifying the biodiversity of life through time by using the morphology and physiology of the organisms.

Phylogenetic systematics, also called cladistics, uses apomorphies, or evolutionarily novel characteristics, to group earth's various organisms and their relationships through time. This may include molecular genetics, calculating cladograms on the basis of shared or unique characteristics, and so on.

[edit] Methodology

(this section from EvoWiki):

Methodological approaches for systematic studies vary widely, depending in part on the type of data being examined, as well as (to some extent) on the philosophical tastes of the researcher. The overall goal of any method in systematics is to find the dendrogram that best accounts for the data and that presumably reflects the pattern of descent within a clade.

Generally, systematics methods can be classified as either algorithm-based or criterion-based. Algorithm-based methods are those that apply a procedure (usually iteratively) to a dataset to build up a dendrogram. An example of an algorithmic method is neighbor-joining, in which the most similar taxa are successively joined.

Criterion-based methods are those that calculate a statistic for each given dendrogram, and then attempt to find that dendrogram with the most extreme value of that statistic. In the case of parsimony-based methods, the criterion is the number of 'steps', or character-state changes, and the objective is to minimize this value, i.e., to find the tree with the fewest number of steps. An example is cladistics, which seeks the most parsimonious distribution of synapomorphies.

Another class of criterion-based methods are those that depend on a model of evolution. The best example of this is the method of maximum likelihood. With this method, the objective criterion is the statistical function likelihood. Very generally, the likelihood of a tree under a particular model is proportional to the probability that that tree generated the data, given the model.

If the number of taxa in the dataset is small enough, finding the maximum likelihood or maximum parsimony tree can be accomplished by exhaustion: every possible tree is examined, and the best (by the criterion) is kept. (This assumes, of course, that there is a single "best" tree, which is rarely true.) For larger datasets, the entirety of "tree-space" cannot be examined, so some sort of heuristic searching algorithm must be employed.

[edit] References

• Hillis, D.M., Moritz, C. & Mable, B. Molecular Systematics. Sinauer.
• Hull, D.L. 1988. Science as a Process. University of Chicago Press.
• Simpson G.G., 1961. Principles of animal taxonomy. Columbia University Press, New York.

[edit] Credits