sábado, 1 de septiembre de 2007

Evolutionary species and their delimitation.

Ana Marcela Florez Rueda



This document attempts to describe the evolutionary species concept and to provide criteria to delimit evolutionary species. The evolutionary species concept was first stated by Simpson in 1961 and restated by Wiley in 1981 as “An evolutionary species is a single lineage of ancestor-descendant populations which maintains its identity from other such lineages and which has its own evolutionary tendencies and historical fate”. Following Hening1 evolutionary species are “logical individuals bounded by speciation events with origins, existence and ends” 1 , they have an origin by cladogenesis, undergo evolution by anagenesis, and disappear by extinction. There has to be a “correlation with the species and the number of cladogenetic events that have occurred during the course of descent within a clade”1, this definition ties species to speciation and seeks for process, “divorcing species from cladogenesis destroys de distinction between tokogeny (nonhierarquical descent) and phylogeny ( hierarquical descent)”1 and this distinction is one of the premises of phylogenetic analysis.

The species problem involves different questions one is the species concept per se (an idea of what kind of entity species are) and the other is the operational question (a methodological approach to recognizing species in a particular case)2. Several authors1,2,3 have argued for the distinction of these two questions considering it the root of the “species problem”. Operational concepts (that try to answer both questions at once) are not universal because all operational species criteria will fail in some cases3 due to missing data, or simply inapplicability, consider the biological criteria of reproductive isolation, it is not applicable to asexual species, and sometimes it is impossible to determine. But the operational question is indeed the most important and accurate operational methods are a necessity. The ESC does not provide a method for the delimitation of species, but based on it one can choose the right method to delimit evolutionary species, there is plenty of empirical data and methods that can be brought to the question of whether a group of specimens is worthy of being hypothesized as parts of an evolutionary species, and when we consider this we can see the links between this concept and other species concepts.

The operational concepts must be reinterpreted as delimitation criteria. There are more than 20 species concepts which may have a delimitation criteria, each one of them should be evaluated to see if the species that delimits are evolutionary species, here I will only evaluate four species concepts/delimitation criteria. Biological delimitation agrees with the ESC because it proves lineage independence and identity, in the few cases when this kind of data is available it should be the primary criteria, but this is as previously stated a non universal delimitation criteria, and probably the less applicable, discerning potential reproductive barriers can be difficult, time consuming, expensive, and fraught with error. Homology and genealogical concordance are delimitation criteria that agree with ESC these are embodied in the PSC (sensu Mishler and Theriot), and GSC (sensu Baum and Shaw), they take into account the process of cladogenesis, and prove lineage independence, contrary to biological delimitation this methodology is widely applicable to all the taxa and there is a lot of data available. Delimitation criteria based on PSC (sensu Wheeler and Platnick) might result in every diagnosable subpopulation be called a species ignoring its genealogy, with terminals that may have tokogenetic relationships, making the delimited species unuseful for phylogenetic analysis, this approach is at odds whit the ESC, and thus prohibited.

The ESC provides an universal idea of what The Species is, but accurate operational methods are a necessity, hopefully biologist are now in a position to shift its attention away from the endless debate about the definition of The Species and “focus instead on estimating accurately the boundaries and numbers of species and studying the diverse processes involved in their origin and maintenance”. 2




Bibliography

1. Wheeler, Q. D. and Meier, R. (Editors). 2000. Species Concepts and Phylogenetic Theory: A Debate. New York: Columbia University Press.

2. de Queiroz, K. 2005. Different species problems and their resolution. Bioessays 27:1263-1269.

3. Wiens, J. J., and M. R. Servedio. 2000. Species delimitation in systematics: inferring diagnostic differences between species. Proceedings of the Royal Society of London, Series B 267:631–636.

5 comentarios:

Salva dijo...

Bueno, este es el uniko de los 5 ensayos ke no estaba a favor del PSC... como en yo soy pro-PSC, po's creo ke en este ensayo va a verse una critica mayor :P...

Mi primer desacurdo es en decir ke los conceptos operativos se preoucpan tando del 'ke es', como de la operacion... En realidad, los conceptos operativos (como el PSC de Nixon, Wheeler, Platncik y ellos) solo se interesan en el como se distiguen, sin importar ke es cada especie (ke creo depende de los distintos grupo de organismos)... El BSC si trata de cubrir ambos campos, pero no es 'operativo', de hehco tiene una idea muy especifica de lo ke es una especie y por eso es ke no es aplicable a muchos tipos de organismos!

No veo komo el BSC puede ser el criterio principal, eso creo solo vale para los algunos vertebrados y otros grupos donde el aislamiento reproductivo es severo, en otros organismos, como las plantas, los mecanismos de aislamieno son muy laxos y eso implicaria agrupar dentro de una especie muchas cosas ke de otro lado serian especies muy validas!

Tambien problematico es el concepto 'monofiletico' de Mishler y ellos, ke sin dua desemboca en no-especies, pero aun asi, estos autores deben recurir a una 'unidad taxonomica inclusiva minima' (LITU), donde, como en las especies el concepto de monofilia es ambiguo, y ademas implica ke cuando se fija un caracter... la especie hermana (por algun medio ke no me puedo imaginar) tambien fija un caracter!

En cuanto a la critica de la PSC, me parece ke es, como escribi en el ensayode Julian, una pregunta que todo defensor del PSC debe responder (sin recurrir a la retorica :P), y que depende en gran medida del concoimiento ke se tenga del grupo estudiado, y de como e la variabilidad en ese grupo... Por cierto, en caso de ke uno se excedira en la cantidad de 'especies' usando cada subpoblacion diagnosticable no pasaria nada en el analisis filogenetico (salvo ke se demora mas), pues si las especies aun forman parte del cluster 'tokogenetico' entonces kedaria bien fuera formando un grupo monofiletico o uno parafiletico, pero bastante vecinos... es decir, pasaria lo mismo ke usando los LITU de los de epecies 'monofileticas', sin el inconveniente de tener ke buscar monofilia reciproca!

Como cosa final, de mi critica general a la ESC es ke se trata de un concepto superambiguo, sin ningun apegio a la realidad: usa a los imperseptibles linajes, distingue 'cladogenesis' de 'anagenesis' desligando la especiacion de la fijacion de caracteres, y con ello creando una 'especiacion sin modificacion' (cladogeneisis) cuando ocurre una especiacion (Se fija un caracter en una especie hija), y se 'extingue' la especie padre dando lugar a una especie hija exactamrnte identica a la especie ancestral... y al mismo tiempo permite modificacion sin especiacion (anagenesis)... Eso hace ke el termino de linaje y un linaje independiente o nuevo se vea aun mas elusivo!

Saludos pues, y ojla la critica sirva de algo (Reocrdar ke yo soy pro-PSC y anti-anti-EC xD)

Salva

iGoR dijo...

Bueno el ESC...lo primero que se me vino a la cabeza fue: quien usara eso?..dado que el BSC y PSC son las dos posiciones mas comunes, hubiera sido cheveron haber nombrado un par de ejemplos donde se haya usado el ESC..

Ana dice "The ESC does not provide a method for the delimitation of species, but based on it one can choose the right method to delimit evolutionary species"
cual podria ser ese "right" method..algun hint?

Por otro lado, el PSC no niega la genealogia :o (si Ana se refiere a la teoria), de hecho la idea es conseguir las mejores unidades pa descifrar el arbol.

El PSC de cierta forma requiere que nos relajemos un poco acerca de la realidad/verdad de las especies.

yo la verdad prefiero una buena convencion a un concepto cargado de terminos como realidad, species fate y indenpendencia de linajes que no me ayuda en nada a la hora de poder reconocer las especies.

Creo que bajo el BSC o el ESC no hay relato evolutivo falso sino mal contado ;)

shaylito dijo...

concepto sin operación en realidad en que ayuda? y la plétora de operaciones sin jerarquía generará conflictos cuando se usen dos operaciones distintas y si aislamiento reproductivo o distancia génica son los objetivos entonces volvemos a los problemas de BSC, ergo en que avanzamos? o por lo menosque sabemos que antes no sabíamos?. Suponer que ESC es un meta-criterio limpio y puro es un tanto naive y nos deja en peor sitio que BSC (que eso es ya mucho decir!)

Christian Julian dijo...

"The Operational concepts (that try to answer both questions at once) are not universal because all operational species criteria will fail in some cases", In which cases the PSC sensu Wheeler and platnick cannot be applied?, because when u attacked it saying that "it might result in every diagnosable subpopulation be called a species" looks that u r worried about the number of species! Is a problem?

And How I identify the "right method" to delimit evolutionary species if I've got all the available data...? a percent rule? if the 75% of the available data groups this taxa, r they a species?

anamar dijo...

For July:

I'm answering your question here because in the rebutal, I'm not going to dicuss the issue of when PSC is not applicable. I wrote "operational species criteria will fail in some cases due to missing data, or simply inapplicability" in this case it will be just missing data, unless many characters are sampled, the number of species present may be underestimated because characters with fixed states may not be observed and unless many individuals are sampled, the number of species may be overstimated by considering traits which ate actually polymorphic to be fixed, so it's not a problem of the method but availability of data.

saludos!