lunes, 10 de septiembre de 2007

The Amadon's rule: is applicable really?
ergio David Bolívar Leguizamón
Escuela Biología


Infraspecific taxa are important in discussions of biodiversity because they represent evolutionary potential within a species (Haig et al, 2006). So, the Linnaean rank of subspecies became prevalent during the mid-twentieth century with the emergence of the biological species concept (Zink, 2004). Subspecies have functioned as units in at least three roles, namely in classifications, evolutionary theories and, more recently, conservation plans, without strong tests of how well they function in these roles (Zink, 2004). There are several statistical methods to delimit subspecies. Among them, the Amadon's rule or 75% rule claims that two populations belong to the different subspecies if the 75% of individuals in a population A is separable by same character from all members (+99%) of a overlapping population B (Amadon, 1949). 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 (Courtney et al, 2004). Several studies has used the Amadon's rule, but they is based in morphological characters (Patton & Unitt, 2002). A study on the feasibility of Amadon's rule in molecular sequences from three species of Pyrrhura (Psittacidae: Aves) will be made. A analysis of genetic distances and grouping will be used to deduce if the Amadon's rule is applicable to molecular characteres.


The mitochondrial sequences (mtDNA) of three species (24 populations) within of genera Pyrrhura were collected from GenBank (accessions number see Table 1), the populations are found in Peru, Brazil, and Bolivia. Populations from Pyrrhura picta (7), Pyrrhura roseifrons (7), and Pyrrhura snethlageae (10) were sampled (Table 1). The sequences were aligned in Bioedit (multiple alignment in ClustalW) version (2005). The genetic analiysis were made in Phylip version 3.67 (Felsenstein, 2007). The software dnadist and Neighbor (Phylip v. 3.67) were used to calculate genetic distances between sequences and grouping of populations, respectively. The Neighbor-Joinning and UPGMA methods were used to grouping the distance's matrix generated by dnadist.

Table 1.


Within of Pyrrhura picta are main two groups: the "Baramita + Iwokrama Reserve" group (Guyana) and the Guyanan "Acari + Baramita" group. The Brazilian population of Vila Sumuru forms a outgroup within of the P. Picta. However, the genetic variation in not significant among the two groups and the Brazilian population (0.004795 with population Baramita) see Table 2 and Fig. 1.

Pyrrhura roseifrons forms three groups. First, the Madre de Dios group with a variation of 0.017780 and 0.017381 with two populations from Contamana, respectively. Nevertheless, one sample fron Contamana posses a little divergence with population from Madre de Dios (0.008630). the Pucallpa's populations forms a defined group without variation between the samples (see Table 3).

The Pyrrhura snethlageae species is not forms sepatated groups, two groups divides the species: the "Alta Floresta + Nazaré" group (Brazil) and the "Santa Cruz + Porto Velho + Jacarecanga" group. The genetic distances between the populations from Alta Floresta and the populations from Santa Cruz is high - 0.011401 - (see Table 4).

Generally, the variation between populations from the three species (P. picta, P. roseifrons, and P. snethlageae) in not significant, and they are lower that 75%. So, the Amadon rule is not applicable to separated population (allopatric). In sympatric populations (samples from Alta Floresta) there is not variation.


When a species becomes divided into more or less isolated subpopulations, the latter initially vary genetically because of differences in the original founding individuals
(founder principle, Mayr, 1954). Subsequently, differentiation will continue at varying rates so long as these populations remain more or less isolated geographicaIIy (Amadon, 1976).
The clinal variation and the polimorphism are consequences of this differentiation. In Aves, the morphological variation is complex because the confused coloration patterns (Ribas et al, 2006; Burmfield, 2005). Statistical approaches as the Amadon's rule (Amadon, 1949) were stablished to delimit subspecies. However, the genetic diversification within populations of Pyrrhura is not significant, and the differentiation is very lower that 75%. The geographic distances between the populations may be the main causes, so, the relation among the variation and the geographic distances is showed in the genetic distances among the populations of P. roseifrons (Madre de Dios, Peru) and the others populations of P. roseifrons. The population from Madre de Dios (Peru) is more distant and its differentiation is major.
The 75% rule (Amadon, 1949) was used in morphological studies (Patton & Unitt, 2002; Cicero & Johnson, 2006; Groves & Meijaard, 2005). The Amadon's rule is useful in morphological data because the characteristics of these data. In morphological data, the given values are not attached to the variations ocurred in molecular sequences (there are not transversions for example). So, the Amadon's rule is not feasible to apply it to molecular characters. Other reason to reject the application of the Amadon's rule is that the sympatric populations within a species are not posses significant molecular variation, because events of interbreeding or short genetic distances. The complex patterns of plumaje and the morphological characters could be the results of a little variation in the genome of individuals, so, the variation among populations is not greater that 75%.
The limits of the sympatric subspecies (mainly endemics) must be analyzed carefully, the study of the morphological and genetic variation within them is the first step to delimit it. The most appropriate approach is the recognition of yours characters. The statistical methods similar to the Amadon's rule are good as a primary approach, but the phylogenetic analysis of characters is essential. The conservation of subspecies must be based in a efficient delimitation of the populations based in the recognition of yours characters.


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1 comentario:

shaylito dijo...

inicialmente una topología con las 24 poblaciones seria interesante, al menos como guía para ver la configuración de grupos y las distribuciones al lado nos ayudarían aún mas.