Yesterday, a commenter asked why I said this and what I meant by it:
All species at a given time have exactly the same evolutionary duration, and on average, probably the same number of ancestral species, as their nearest relatives.
Consider this diagram:
This is an evolutionary tree, or rather part of it since we probably do not know all the branches and taxa that actually occurred in it, for four extant species. The concestor c for A and D is shown by the lowest circle, for A and B and C at the next highest circle, and for B and C at the top circle. The number of previous species to any arbitrary earlier point on the tree shared by all of this clade is denoted by the letter M.
Now suppose you ask "how many ancestors does A have compared to any of the other species in this clade?" The answer is, of course M + c(A, x) + n where x is the other species and n is the difference in the number of branching points from c(A,x). Given that for any decent length of M, the number of ancestors is going to be very large, and the subsequent number of shared and unshared ancestors is going to be relatively small for a constrained clade, the latter makes almost no difference in the final number. The existence of extinct taxa at any point doesn't affect this total number much. So A and D have M + 3±1 ancestors, A and B or C have M + 2 or 3, and of course B and C have M + 2 ancestors. So if M is, say, 100,000, the difference isn't all that great across the entire clade.
Why does this matter at all? Well, a lot of nonsense is said about this or that taxon being "more evolved" that another. Most recently we heard that chimps are "more evolved" than humans, in this case because they had more selected gene changes. But often the confusion here is that a species is "more evolved" because there has been more taxonomic change over time. However, no matter what the speciation rates in unit time for a given lineage, it is on average going to be close to the rest of its clade, unless the concestor of that clade is so far back down the evolutionary tree that it includes bacteria, which have a generation time sometimes of hours, and elephants that generate every 40 years or so, in which case the number is largely meaningless.
Of course, if the clade includes, as it must, past species, then it includes members within M, and the number can vary wildly. But for extant species, the difference won't be all that great.
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Thank you. I was afraid that you'd just dropped the subject.
OK, again just trying to see if I understand:
You are talking about total number of ancestors back to the
cenancestor rather than only back to the concestor. And then, although the numbers may be very different back to the concestor - say 40 times as great for frogs as for elephants - those numbers are dwarfed by the number of ancestors from there back to the cenancestor.
And, thinking about it, the relative number of ancestors of two species is not necessarily the same as the number of generations. So even though frogs reproduce 40 times as fast as elephants, doesn't mean that they have 40 times as many ancestors back to the concestor.
And further, I see on re-examination of your statement above, you are talking about speciation rate rather than just rate of reproduction.
Having learned all that (I think I have learned something), I still don't see the significance of it. It seems like you are quibbling about the meaning of "more evolved". What else can "more evolved" mean other than "has more ancestors"?
If this is all incredibly stupid and missing the point, let me know and I'll stop bothering you with questions.
Well back to some arbitrary concestor (like, the concestor of eukaryotes), yes. The rate of reproduction is relevant only if you think that speciation occurs at some more or less constant rate relative to that measure. I have heard people argue that the true "metric" of evolution is number of generations.
"More evolved" can mean many things. This is only one of the misunderstandings that phrase has accrued. Others include more genes, more selection, more "complexity" (a term that means all things to all people), and "higher grade", a term that has no meaning whatsoever.
None of this is besides the point. As the President for Life of the Campaign to Eliminate Detrimental Expressions in Biology and Evolution (CEDE-BE), all of this is grist for the mill.
This is where your background in dialectics (I presume) would come in handy (at least up to a point) as each of the different senses of "more evolved" is useful and even correct within the appropriate context and inasmuch as, from within that context, it is a perspective on things focuses on certain aspects but not on others where all of the aspects are aspects of the same thing. "That a thing is" and "what a thing is" are the same thing in terms of its existence, but "what a thing is" is the thing considered in a relationship of "being different" from other things, and the identification of a thing is done in terms of its relationships with other things where they form the context for its identification.
Of course, in the case of "more evolved," whenever someone uses that particular phrase, it might be useful to always insist on having them state the sense in which they mean "more evolved." For example, humans like to think of themselves as more evolved than bacteria, and there are no doubt many ways in which this is true, but in terms of the number of ancestors or generations since our common ancestor, clearly bacteria have us beat. Likewise, since their populations are a great deal larger, they have been and are subject to a great deal more selection than we are. But as there are so many senses and widely varying meanings in which one might use the term "more evolved," it might be better to do away with the use of the phrase altogether in favor of terms and phrases which have more specific meaning.