by Piter Kehoma Boll
I finally decided to go on and write the second part of my article about species concepts. You can see the first part here, where I talked about horizontal species concepts. Today I’m going to talk about the other perspective, the vertical species concepts, which are based on lineages, i.e., how species happen through time.
Vertical concepts are hardly used to actually define a species, since they do not represent the current situation of living beings and it’s hard to know the real history of a population to determine its status through all his existence. However these concepts are useful in phylogenetic reconstructions and to understand how new species arrive from others through time.
Well, let’s see the two main vertical species concepts.
1. Cladistic species concept
Proposed by Ridley in 1989, it defines a species as a set of organisms between two speciation events, or between one speciation event and one extinction event. According to this, a species comes to exist when a lineage of organisms is split in two. There are no paraphyletic species in this concept, since when a speciation event occur, the ancestral species becomes extinct, giving rise to two new species.
2. Evolutionary species concept
An evolutionary species is defined as a set of organisms from a single lineage that has its own evolutionary tendencies and historical fate. Differently from the cladistic species, the evolutionary species does not necessarily become extinct when another lineage split from it, so being able to be paraphyletic, i.e., if a population is divided in two, the one that continues to have the same general features and the same evolutionary path is considered the same species as the ancestral one.
Since there is no record of the evolutionary history of organisms, there is no way to determine it for any species. Some ideas may be proposed and highly supported by genetic analyses, but we can never know for sure how things really happened, so that vertical concepts cannot be applied practically and are more useful to infer genetic relationships between different populations and so guide their correct management in conservation efforts.
Another point is that by vertical concepts, two organisms are considered separate species as soon as they move on in different lineages, in different populations that will not come in touch again, so that even two cousins would be different species, even though genetically, morphologically and ecologically very similar.
So vertical concepts are more useful to determine phylogeny and help in population genetics, but not to actually define species in any ecosystem, since in this case the situation is characterized by the present status of organisms and so better supported by horizontal approaches.
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Mayden, R. L. 1997. A hierarchy of species concepts: the denoument in the saga of the species problem, in M. F. Claridge, H. A. Dawah and M. R. Wilson (eds.), Species: The units of diversity, London: Chapman and Hall, 381-423
Ridley, M. 2004. Evolution. Blackwell Publishing. ISBN 1-4051-0345-0.
Stamos, D. N. 2002. Species, languages, and the horizontal/vertical distinction. Biology and Phylosophy, 17, 171-198.