While describing a new species (in any types of organisms), how much amount of DNA variation should be ascertained so as to describe it as a new species?
Million dollar question. I’m sure this thread won’t devolve into a splitter vs lumper debate.
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as @egordon88 hinted at, the answer is hotly debated between scientists, and between taxonomists in particular.
in ye olden times, genetics were not a factor at all, and many of those older classifications are preserved today despite having tenuous genetic reasons.
also, you must consider that life is very good at hybridizing. the line between species blurs quickly.
As far as I know, species see delimitation is done on whatever basis it’s possible; it could include multiple axes for the purpose of delimitation: molecular divergence, morphology, behaviour, species geographical distribution, reproductive isolation, and so on
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And yeah, we consider species as separate, distinct evolutionary lineages, right (atleast therotically) ?
It’s important to remember that all species/species concepts are arguments over delineations of a continuum, and that all species descriptions are hypotheses proposed by the authors, with different lines of evidence – molecular divergence, morphology, behavior, ecological niche, geographic distribution, etc. – contributing to supporting that hypothesis. See the famous de Queiroz (2007)'s Figure 1 - each “SC” represents a line of evidence (species concept) being used to support the delimitation of two lineages, but you can argue anywhere within that gray zone that those two lineages are two separate species - it’s just that the amount of evidence (or lack there of) will impact whether the scientific community will agree with your assessment. Just because a paper gets published doesn’t mean the scientific community is required to follow your taxonomic hypothesis if they don’t believe it is well-supported.
These days, molecular distinctiveness is essentially a requirement to publish a species description, but unlike 10-15 years ago where you could get away with sequencing just say one or two mitochondrial loci of a couple hundred basepairs, you need multiple genes (thousands/tens of thousands of basepairs) or preferably genomic-level data (AHE, RADseq, UCE’s, etc.) as a starting point for most groups, with rigorous statistical analyses as supporting evidence for most descriptions. The amount of molecular differentiation used in species descriptions varies widely by taxa and results should be consistent across multiple analyses. Each group has a loosely agreed upon metric that is the consensus of the literature, but those change as genetic/genomic sequencing and analyses have become more readily available. The best way to determine that is to keep up with the literature and see how recent (< 7 years) delimitations are carried out compared to older delimitations, both in your taxa of choice and others. If molecular distinctiveness holds up, then statistical analyses of other potential lines of evidence like morphology can be conducted to build your case.
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The original definer between species was if they appeared physically different to the naked eye. Considering some subspecies are more physically different between each other than entire genera, this informal rule has more or less fallen out of the expectation.
I think this is in part why a lot of widespread species have a bunch of subspecies, varieties, or localities.
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