|MadSci Network: General Biology|
Speciation is generally defined (in Biology and Evolutionary Biology textbooks, such as those authored by Futuyma and Ridley) as the result of two populations of an organism becoming isolated from one another for a sufficient period of time that they become distinguishable entities. The many approaches used to define species derive from the methods used to distinguish the separate populations. One of the older definitions is the Biological Species Concept, which defines species as closely related groups of organisms which cannot produce viable offspring. Unfortunately, reproductive isolation doesn’t necessarily hold for groups which have traditionally been defined as species, especially when one looks outside the realm of vertebrates. Recognition of limitations of the Biological Species Concept lead to various attempts to create a more robust definition of species. With the advent of methods for sampling genetic information from individuals in populations came attempts to use these data as the basis of a realistic and robust species definition. Even before genetic data were available evolutionary biologists spent a great deal of time and effort placing all of life into groups. Systematics is the subdiscipline of Biology dedicated to the partitioning of life into groups, generally on the basis of a hierarchical branching pattern called a phylogeny. Taxonomy is the subdiscipline of Biology dedicated to naming these groups and assigning them places in an organized system of classification. Many biologists feel that any robust and realistic definition of species must have an underlying phylogenetic component. The difficulty of most phylogenetic species concepts came from determining the level in the branching hierarchy where one would say “Below this Node lie Species”. The Genealogical Species Concept is one type of phylogenetic species concept. As defined in a recent paper investigating the mathematical implications of the Genealogical Species Concept (Hudson and Coyne, 2002, Evolution: Vol. 56, No. 8, pp. 1557–1565), a genealogical species is “a basal group of organisms whose members are all more closely related to each other than they are to any organisms outside the group ("exclusivity"), and which contains no exclusive group within it”. This idea comes from the idea (and the wealth of data that support it) that, when two populations become isolated from one another, they begin to accumulate differences in their DNA independently of one another. Differences in the DNA sequences for the same genes (or locations in the DNA that aren’t necessarily considered genes) are called alleles. Organisms that breed with one another will exchange genetic material and share alleles at some of their loci. The longer two groups are isolated from one another the more unique differences they will accumulate. Currently relationships among groups of organisms are measured by the similarity of genetic sequence information. The genetic data are used to construct phylogenies of the individuals sampled. Phylogenies are subtly different from genealogies, as each individual in a phylogeny will connect with only one ancestor, which will also have only one other descendant. In genealogies each individual connects to two ancestors, who in turn are connected to two ancestors, and so on. Genealogical ancestors can connect to many more than two descendants. Similarity and exclusivity are determined by the agreement of phylogenies derived from multiple independent loci. If the data support two independent groups then the phylogenies are described as being in a state of reciprocal monophyly, where one state of the data defines one group and another data state defines the second. Generally, when multiple independent loci are surveyed only a fraction of them will be reciprocally monophyletic. As more loci are investigated and the reciprocally monophyletic fraction increases the two groups are considered better genealogical species. Also, the use of multiple independent loci should help to eliminate potential exclusive groups within the hypothetical species due to the presence of rare alleles in local populations. Any recently identified species will have been identified using genealogical means. The recently discovered wild mouse on Cyprus (http://msnbc.msn.com/id/15233416/?GT1=8618), the yellow and black Yariguies Brush-Finch of Colombia (http://colombia.logtar.com/?p=76) and the Indonesian coelacanth (http://en.wikipedia.org/wiki/Coelacanth) are examples of recently discovered species defined using molecular phylogenetic methods, which would make them genealogical species.
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