duga

I still hold my ground, but, you are right about plants. Even so, all plants diverge genetically until they can no longer form hybrids (without the help of humans, anyway). The same could be said about the Prokaryote kingdoms. Maybe there is a need to refine the system, but since even plant species head out on different evolutionary paths to the point where they can't form hybrids where they once might have tells me my "reproductive isolation" standard still holds. To me, anyway.

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Guybrush

I think a problem with cladograms and that representation of phylogeny at the moment is that when branches are resolved, they only allow for dichotomous divergence. There's only one evolutionary event in a cladogram and it's something splitting up to form two somethings.

I guess some might be confused about what I'm writing about so here's a cladogram :



It's essentially a hypothesis about the evolutionary history and relationships between taxa such as species. A problem with it is branches can only split, there's only divergence - and no convergence. In nature, plant species sometimes converge to form new species.

duga, you've got a good point about species eventually diverging anyways, but just like they will keep diverging, they will also keep converging in the future so the problems I pointed out with the sexual isolation definition still won't go away. Furthermore, it doesn't seem practical that the species definition will be valid eventually, possibly some million years in the future. It seems to me like a species definition should be applicable here and now.

You make a good point about evolutionary paths, though. I personally like the idea that a species is something on an evolutionary "path" which is different from that of other species, an idea that - as you also mention - is applicable even if there's a little flow of genetics with other groups.

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Neapolitan

That would only apply to Cladogenesis, but what about Anagenesis? If you take in consideration the working hypothesis of Evolution, a species on a "path" may be entirely difference from the beginning of it's evolutionary "path" to the end it's evolutionary "path."

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duga

It would apply to both. Though one results in a split while the other results in a replacement, they both have the same result by our current "species" definition: reproductive isolation. Though it may be harder to see on a cladogram, it still holds...so I fail to see your point.

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Guybrush

I agree with duga and would argue further that it actually holds up well for every monophyly, potentially every taxonomic level and some new ones we don't have yet. If you did a genetic analysis of different populations of a species and created a cladogram, you could find that each population forms a monophyletic group, even if there is gene flow. It's a sign that the populations are diverging, they are on different evolutionary paths - although that wouldn't mean they won't converge again some time in the future.

Similarly, you could do it on a larger scale and say that wasps have taken a different evolutionary path from beetles, which is basically what taxonomy does already.

Then you can have a new way of defining species not yet discussed in this thread based on genetic analysis. At the moment, results vary with markers used, but imagine a future where it's easy to do whole-genome sequencing and you had a sequencer where you could just put in some rat, cat and dog DNA. They should come out as separate monophylies for you to base species on which is actually a proper result and not just something theorized, so it has something going for it. It would require some equipment we don't have yet and a helluva lot of processing power if you are to compare whole genome sequences of many species, but it might still be just a matter of time.

It would still run into the problem of changes between many species being transitional though and where to draw the line is a problem. Rats, cats and dogs are easy, but what about the brassicas? Still sounds like it could be an effective way of defining species to me.

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Neapolitan

So are you agreeing with me by saying that the critea of defining a species should have two qualifiers:

1. hybridization barriers
2. genetic information

Whether there are other qualifiers, I can't think of any at the time, but I think at least both should be taken in consideration.

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duga

On what grounds are you making these arguments? Do you know what these things mean? Hybridization barriers result in reproductive isolation...and well...every living thing has genetic information. Which is exactly what tore and I have been talking about. And you have been disagreeing with. Maybe you can expand a bit because I am not getting your point.

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Guybrush

No to hybridization barriers. I've already written a few points now about why I don't think it's a very good criteria in previous posts.

Genetic information? I'm slightly confused as to what you mean.


In my latest post, I mentioned it might be possible to determine species based on sequencing data. It's not a new idea, it's already being done. However, I mentioned that in the future, it might be possible to do whole-genome sequencing quickly and if you store whole genomes of organisms in a database with some tremendous calculating powers, that could be used to infer phylogeny on a grand scale.

A bit like this (for ghost insects) :



I argued that species should be based on monophylies. That would be one criteria, but you would need more of course. A monophyly is a group in a cladogram like the one above which includes all descendents of an ancestor. For example the bottom two are a monophyly. So are the bottom 3 or the bottom 7, but if you then leave out the last (Haaniella dehaanii), you would have a paraphyletic group - one which does not include all descendents of an ancestor.

Monophylies can be said to have been on an evolutionary path which differs from other monophylies - which is why they form monophylies in the first place. It does not mean sexual isolation which promotes divergence, but rather divergence itself - coupled with common ancestry. Evolution takes them in a different direction from other monophylies. A different direction/divergence basically means their genes are becoming different.

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duga

This is perfect. This would probably be the end all for the species debate, but as you said, we simply lack the tools to sequence whole genomes as rapidly as would be needed for this level of bioinformatics. We are getting close, though! We have several organisms sequences (including humans, obviously) and can now use those existing genome sequences to "map" another individual's genome to determine if they have certain genetic disorders or predispositions to one. This level of computation was unheard of and unthinkable even 15 years ago...10 more years and I'm sure we will be at the level needed for mass genome sequencing.

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Guybrush

I agree. I keep feeling there's a huge snag somewhere, but all I can see at the moment are problems that look/sound like they can be overcome, but that's no different from and perhaps even slightly better than for the other species definitions.

Comparing whole genomes would need some smart algorithms and so on so you'd need some clever programmers and mathematicians. However, even if there's a problem with an algorithm used to define species, at least you can just improve the algorithm and run it again. Although one should definetly try and find one best way to do it, you could easily define species in a number of different ways with something like this. For example, one could use different algorithms or perhaps restrict analysis to only certain bits of DNA (f.ex mitochondrial DNA) and so on.

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