fluidistic said:
I'm trying to understand the tree-graph on the right side of
http://en.wikipedia.org/wiki/Arachnid#Systematics.
If I understand well, Opiliones evolved from scorpions, ambyplygi evolved from spiders and schizomida evolved from ambyplygi. Is this right?
How do they determine which order evolved from which order? Genetic I guess but how exactly?
Thanks in advance.
Be very careful. Your statements may be colloquially correct but technically ambiguous. You can say something like "this extant species was descended from another extant species. However, the descendent may differ from the ancestor in significant ways. So you may be dinged later. The most common ancestor of two groups of animals doesn't necessarily "belong" to either group of animal. The common ancestor may be very different from both. Further, colloquial names almost surely have more ambiguities that scientific names. For instance, some ambyplygi species are colloquially referred to as the "whip scorpions". Many species in the group "aranea" are colloquially referred to as "spiders". There is no way from the diagram to tell what the common ancestor of aranea and ambyplygi were like. The common ancestor may have resembled a modern day "scorpion" more than a spider. In fact, most of the species on that part of the diagram resemble "scorpions" rather than spiders.
So it is rather dangerous to say that the common ancestor of aranea and ambyplygi was a "spider". Paleontologists may someday find fossil remains the common ancestor of arenae and ambypligi. Or maybe a geneticist can reconstruct the common ancestor from junk DNA. Or maybe we will never have a clue what the common ancestor was like.
Now, maybe the common ancestor of all ambyplygi had a morphology more like a extant spider instead of an extant scorpion. Maybe this is an example of convergent evolution (two groups converging on a similar form). Maybe.
However, a naive person may be surprised to hear you say that "whip scorpions are really spiders, not scorpions". He may not be so surprised if you stick to the scientific words. Maybe in some discussions, scientific words are easier than colloquial words. Or use the colloquial words with appropriate qualifiers.
Scientific words are not absolutely free of ambiguity. Check out the thread where everybody is trying to distinguish between "Fermi level" and "chemical potential". However, scientific words are less ambiguous than colloquial words.
One thing that you could do is be precise in your questions. Instead of saying "are scorpions descended from spiders" say "what do you suppose the common ancestor of scorpions and spiders looked like?" Or even, "what do you suppose the common ancestor of aranea and ambypligi looked like".
I was looking at the Wikipedia descriptions of other animals sharing the same node with aranea and ambypligi. What I found is that most of the extant species sharing the same most-recent-common-ancestor (MRCA) superficially resembled scorpions rather than spiders. Therefore, my educated guess is that the MRCA also resembled scorpions rather than spiders.
I would be very careful about saying that "spiders are descended from from scorpions." I would say something like, "My educated guess is that the most recent common ancestor of most modern scorpions and most modern spiders had an anatomy more like that of the modern scorpions than the modern spider.
That way, I give other scientists a reasonable chance of proving me wrong. Its called falsifiability. It is the greatest gift a scientist can give other scientists !-)
