They are not definitions of the concept 'species', they are all definitions of entirely different concepts. We use the same word for all, which doesn't make things any easier of course. Understanding speciation will require you to use any of the concepts that regard species al lineages, ie the ones that incorporate change and ancestry.

I guess there is no 'most unifying' definition. It's all about who's talking about species and what his or her intentions are. To a museum taxonomist, there is little to say about possible interbreeding, for example, so the morphological species concept will often be his choice. An evolutionary biologist would be seriously hampered when using this particular concept.

@Arjan, I take it that the concept of "species" is not static; zoologist used which ever term is most applicable to the current set of experiments and organisms being studied.

However, I am not sure how favorable it is to use this line of logic and how well it will continue to work out. It is the equivalent to saying "a species is whatever a zoologist thinks/needs a species to be...."

In my case, I am arguing that "evolutionary specie concept" should be use as the true/universal definition of a specie.

If species do not share similar lineages how can they be classified together? In what cases would the evolutionary species concept not work?

In any case in which a lineage is impossible to determine, I guess?

It's not just a simple matter of the state of knowledge that dictates which of the various concepts will be used. An ecologist working with large amounts of material who is trying to figure out the difference between the biodiversity of two locations will usually be forced to use 'morphospecies'. Stuff that looks alike is lumped and called a species, whether correct or not according to taxonimists.

Indeed, a species is whatever one needs it to be. In fact, there is no such thing as 'a species' (plural same as singular, by the way). There are several very different concepts that can all be used at the same time.

@Arjan, I agree. Field research demands practical definitions of species.

My initial argument has many assumption about the nature of actually defining a species.

If you take practicality out of the situation, that an evolutionary species definition would be the best, or most accurate, to use.

So essentially, I am arguing that a "true species" is one indicated by evolutionary lineage, instead of other characteristics.

On a metaphysical level, the complexity of actually grouping living organism into categories called species does beg the question if species is a natural concept, and not just totally man made. Most things, at the level, in science could probably be regarded as mental constructions for human convenience.

I do agree for practical purposes in the field, having a variety of definitions is necessary.

For asexual species, I believe evolutionary species concept works best; BLS is out the window, and morphology may be impossible to use.

For HGT, BLS would probably be best; it would ultimately depend on the evolutionary tendencies of the offspring; certain selection factors could suggest that the offspring was actually in the species as one of the parents, despite having reproduction isolation. In those cases the classification of a species by BLS and evolutionary species concept would be different.

I agree with Michael that the ESC (Evolutionary Species Concept) is the most inclusive one and can embrace the majority of the others species concepts. Wiley (2011) exposes that opinion in his phylogenetics text-book and with good arguments favoring ESC.

The problem is that, by being so broad, sometimes ESC lets the practice of taxonomy subjective and vague, related to what would be a particular tendency of a lineage and where the cut between some lineages is.

I suspect that this is an intractable problem since some lineages really don't have clear cut delimitation if we are looking to process. But if, instead, we are worried on patters (eg. phylogenetics), ESC is more satisfactory them other concepts (BSC, ESC or PSC).

@Daniel, True Specie = Follows ESC, the rest of the concepts result in pseudospecie classification.

However, as you mentioned ESC does have some drawbacks that other definitions do not have. There are ultimately drawbacks and benefits to having some many definitions, and each has its own use.

The idea that species are best seen as lineages has become very popular and for good reasons. This is mainly due to three now-classic publications by Richard Mayden and Kevin de Queiroz (Mayden 1997, de Queiroz 1998, 1999). Their case rested on two important insights.

1. There is a fundamental distinction between the species concept (primary species concept, Mayden 1997), which tells us what species are, and species criteria (secondary species concepts, Mayden 1997), which describe how species taxa may be delimited in practice. Both Mayden and de Queiroz observed that most species concepts are actually little more than prescriptions of how species should be delimited (i.e. species criteria). Thus, many ’species concepts’ have confounded operational and theoretical aspects of species, and have defined the items to be discovered by the discovery process (Frost & Kluge 1994 Cladistics 10: 259-294).

2. Most views on species and species criteria are consistent with the idea that species are segments of population lineages (de Queiroz 1998, 1999), i.e. branches or parts of branches in the tree of life.

The lineage concept does not by itself dictate how species should be delimited but it provides a few important hints. First, because different species may have different properties, all traditional species criteria are potentially relevant for finding species. Importantly, different species criteria highlight different aspects of species. Thus, it makes sense to apply as many criteria as possible, not only to avoid overlooking species, but also to describe and better understand the nature of these species taxa (Sangster 2013 Biol. Rev. in press).

Second, there is no reason to presume that all the properties identified by traditional species concepts evolve in all lineages, let alone all at the same time. Thus, a species may exist even if it does not possess one’s preferred species criterion. The absence of a particular species property does not mean that the lineage itself does not exist. This is an important observation because from this it follows that no property can be considered ’defining’ for species. This explicitly rejects the equality of species; some may possess all properties, others may not; some may be highly distinctive, others are cryptic; some may be monophyletic in a gene tree whereas others are not; some may be reproductively isolated, but others may not.

Third, if species are lineages, then the species taxa delimited by taxonomists are hypotheses of lineages. Thus, species are not simply described (’facts’), but their existence is a testable hypothesis. Like any other scientific statement, these should be documented as thoroughly as possible.

@ George, due to the length of your response it is difficult and timely for me to navigate;

Are you simply arguing that due to biological complexity and diversity that it is necessary to have many criteria for species for trait identification? And that this complexity may bypass using lineages to define species?

Not to antagonize, if we have too many criteria for identifying species, does the definition not become to arbitrary to even debate? Many any 2 organism be grouped as species if the definition is encompassing enough?

In fields outside of zoology, having such arbitrary classifications would not be allowed.

From a theoretical standpoint, Why would the traits of the species not be encompassed in the lineages?