sábado, 1 de septiembre de 2007

THE SPECIES CONCEPTS IN AVES
Sergio David Bolívar Leguizamón
Biology
UIS

The number of species in Aves is 9000 to 10000 approximately. Within the group, there is clinal variation and populations intergradation. The Aves were identified by ornithologists using general morphology and patterns of coloration in the feathers. This
Tipological Species Concept (TSC) were used to delimit and to recognize species in many groups. However, the TSC was used to identify and delimited species in Aves indiscriminately, but the TSC is not useful to identify sibling species and groups with sympatric populations.
Since Dobzhansky (1937) and Mayr (1952), the
Biological Species Concept (BSC) has been used by ornithologists, the BSC claims that a species are groups of interbreeding natural populations that are reproductively isolated from other such groups. The geographic and reproductive isolation is the criterion of delimitation in th BSC. The sympatric species are defined as subspecies by BSC. The Aves posees great quantity of subspecies, because your high polymorphism within a same species. Studies claims that the sympatric species o subspecies must be considered as “emergent species” (Mayr, 1963). Nevertheless, BSC is not applicable in the field camp, the reproductive isolation is not applicable the Aves. In laboratory conditions the breeding of much birds is not successful, and the hybrid species have the uncertainty of if they are on or two species, futhermore, the sympatric species are not easy to identify because the variation in the populations could be insignificant. In fact, the ornithologists used coloration patterns to recognize species of birds. So, genera and species are difficult to determine. However, the BSC is proclamed as the principal species concept in Ornithology.
Nevertheless, other alternatives were emerged, as The
Phylogenetics Species Concept (Cracraft, 1983; Nixon & Wheeler, 1990), offers a new vision about species and the asumptions about them. PSC (sensu Cracraft, 1983) is based in recognition of characters and monophyly (ancestry-descent), the PSC (sensu Cracraft,1983) claims that the species must be reproductively cohesive and diagnosable. For Cracraft, the species concept has three key elements: Reproductive cohesion, diagnosability, and a criterion for ranking populations at the species-level. The PSC (sensu Cracraft, 1983) offers greater theorical ans empirical support that BSC.
In the Ornithology, the delimitation of subspecies is bounded to great debate and discusion, some researchs claims that the populational variation must be treated as polimorphisms within of a species, on the other hand, researchs states that the
“subspecies” hierarchy is very important in the classification and use of taxa.
In field camp, there are several statistical methods to delimit species, the Amadon's rule or 75% rule claims that two populations belong to the same species if the 75% of individuals in a population A posses the same character (or character state) that the 99% of a population B (Amadon, 1949). This statistical method was used by Patton & Unitt (2002), Meijaard & Groves (2004), and Cicero & Johnson (2006). The 75% rule is defined to mean that before a population is given subspecific status at least 75% of the individuals comprising it must be separable from +99% of the individuals of all other populations of the same species which may overlap with it as regards the geographically variable characters (Amadon, 1949).
The Methodology is based in statistical analysis, where the presence of trait of interest is plotted in the population and this is calculated for the others populations, and the comparisons are made. The standard deviation and media are calculated for the traits or characters of the population, if the standard deviation is not enough among the characters of populations (75%), then the population are the same species. But if the differentiation among the two populations in a character is > that 75%, the character separates the two populations in two subspecies.
For characters that occur as separate states, such as presence or absence of a plumage pattern or mtDNA haplotype or clade, the test involves a simple contingency table analysis. For continuously varying, normally distributed traits, such as measurements of body size, the rule involves comparison of the two distributions via their means, standard deviations and the expectation of 75% non-overlap from a t-distribution.
It rule provides a quantitative method to evaluate distinctiveness and in which to make subspecies designations (Patten and Unitt 2002). Nevertheless, the 75% rule is not strict, because sample effects or data's nature. Other approach similar to Amadon's rule are the 99% rule that is based on the same argument.
At least in Aves and for morphological data (Patton & Unitt, 2002; Meijaard & Groves, 2004; Cicero & johnson, 2006), the Amadon's rule is a way to delimit species and subspecies in sympatric populations. However, molecular analysis apply this method are non-existence, or they are few.
The PSC (sensu Cracraft, 1983) is more applicable and useful that the BSC (Mayr, 1952). The subspecies must be vievew as polimorphic populations, where the populational variation must be significative to postule new subspecies. The Amadon's rule (75%) is ambiguous because is usseful with morpholgical traits, but it is not quite applicable in molecular caracteres.

Bibliography

  • Amadon, D. (1949) The seventy-five per cent rule for subspecies. The Condor, 51, 250-158.
  • Cicero, C. & Johnson, N.K. (2006) Diagnosibility of subspecies: Lessons from sage sparrows (Amphispiza belli) for analysis of geographic variation in birds. The Auk, 123, 266-274.
  • Courtney, S.P., Blakesley, J.A., Bigley, R.E., Cody, M.L., Dumbacher, J.P., Fleischer, R.C., Franklin, A.B., Franklin, J.F., Gutiérrez, R.J., Marzluff, J.M., & Sztukowski, L. (2004) Scientific evaluation of the status of the Northern Spotted Owl. Scientific evaluation of the status of the northern spotted owl. Sustainable Ecosystems Institute, Portland, Oregon. 508 pp.
  • Cracraft, J. (1997) Species concepts in systematics and conservation biology – an ornithological viewpoint. Species The units of biodiversity. Ed. Chapman & Hall, 325-339.
  • Groves, C.P., Meijaard, E. (2005) Interspecific variation in Moschiola, the Indian chevrotain. The Raffles Bulletin of Zoology, 12, 413-421.
  • Mayr, E. (2000) The biological species concept. Species concepts and phylogenetics theory, Ed. Columbia University Press, pp 17-29.
  • Patten, M.A., & Unitt, P. (2002) Diagnosibility versus mean differences of sage sparrow subspecies. The Auk, 119, 26-35.

4 comentarios:

Salva dijo...

Bueno el ensayo me parecio un poko confuso en algunos puntos!

Hay algo bien importante cuando se habla del PSC y es desligarlo de la idea de las 'especies monofileticas'... lo de ancestria/descendencia lo dijo Cracraft? no sep, no he leido el paper de el :P, si lo dijo en el sentido de 'monofilia' (como esta en el ensayo), po's no hay duda ke mucha gente creyo ke se trataba de un concepto monfiletico de especie!

Por cierto, cual es la psocion final de Sergio con respecto a las subs? no me kedo claro el asunto, y no se si esta a favor o en cotra! (y mcuho menos los motivos)

Creo ke de los metodos estadisticos existen puntos mas debiles ke los ke menciona Sergio, en este ensayo como en el de Julian, creo ke elementos mas criticos ke invalidan ese tipo de analisis, desde el punto de vista del PSC (y del analisis filogenetico!)...

Y al final kede un poko deconcertado, pues al parecer Sergio prefiere el PSC, pero tiene una posicion ke parece comoda con el metodo de Amadon (en mi vision ambos metodos son mutuamente excluyentes!)

Saludos, y ojala la critica sirva :)
Salva

shaylito dijo...

en primer lugar cual es el asidero "lógico" de una regla como xx%? no importa como la maneje siempre tendré casos raros por lo que intuitivamente debería ser rechazada!
en cuanto a posiciones y las subespecies donde quedamos?

Eri dijo...

En la vida práctica existe alguna diferencia entre la aplicación del concepto tipológico y biológico de especie para los ornitólogos??

Christian Julian dijo...

A ranking decision? Does it mean that the species do not exist as real entities?
:( :( :(

what can I do if the 69.9, 75 and 75.1 percentages present a different number of species... do I continue using 75%? a percentage range?