[Taxacom] Invisible evolution

[Richard Zander]

Did you every wonder why there was such insistence in cladistics years
ago on ancestral species dying out when generating two new species?
Nowadays this idea has pretty much been abandoned, and two sister
lineages are largely seen as more-closely-related than more basal
branches with little emphasis on the reality or importance of ancestors
reflected in phylogenetic hierarchical classifications (where each level
was supposed to be a different level of classification). The damage has
been done, however, in that ancestors are seldom seen as surviving. Only
those species with no morphological autapomorphies at all (in a
cladistic analysis of morphology) are even considered as possible
surviving ancestors, and molecular analysis will almost certainly find
base changes that are considered autapomorphies. But how many surviving
ancestors are possible in any one group? None? One? Maybe half? All but
one? I think the last, where (potentially) only one has not pupped off a
surviving sister group, according to any analysis.
 
With haplotypes and asexually reproducing individuals, every line of
descent may accumulate non-coding base changes that track nothing if no
evolution of expressed traits occurs. Any line of descent is a potential
fully cryptic species. With loci that are subject to recombination,
populations may undergo bottlenecks that result in fixation of
particular non-coding base changes, but if through stabilizing selection
or the like the original expressed traits are retained, non-coding
traits track nothing. In any case, there is the possibility of a clear
mismatch between expressed evolution and the neutral base changes that
are supposed to track it (with base changes generated randomly, thus
exposing convergence in expressed traits). 
 
There are clear examples of taxa presently extant but found in fossils.
The present coelacanth doubtless has non-coding DNA quite different from
that of its million-year old ancestors that had apparently identical
expressed traits. Suppose ancestors of the present-day coelacanth
generated various modern genera, one by one over time? The coelacanth
must then be divided into several species or higher taxa to preserve
monophyly. Shall we treat the surviving ancestor as a genus, family or
order different from the million-year old identical organism? I suggest
that massive homoplasy in many groups now uncovered by study of
non-coding base changes could be due to base changes tracking nothing
within a long-surviving taxon, coupled with generation of many
derivative branches from the long-surviving paraphyletic taxon over
time. 
 
I note here that evolution is presently pretty much defined as any
hereditable changes fixed in a population. I doubt Darwin would agree
with our present focus on invisible changes, especially the total focus.

 
 
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Richard H. Zander 
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Missouri Botanical Garden
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