[Taxacom] Invisible evolution

[Richard Jensen]

As I recall, the "extinction" of taxa in a cladistic scheme was based on 
Hennig's assertion (argument; supposition; opinion?) that, in a 
sister-group relationship, no extant taxon could be the ancestor of its 
sister group. Thus, if a population of species X becomes isolated and 
evolves into a new species, Y, then X goes extinct (at least 
nomenclaturally) and is replaced by, say Z, the remaining populations of 
X. The fact that Z has no apomorphy by which it could be distinguished 
from X is immaterial.

I've never been convinced of the logic of this approach. I once 
discussed this with David Hull, whose reply suggested that he, too, 
found this puzzling. But, he argued that if a species is an individual, 
then when one or more populations becomes isolated, it is no longer the 
same individual. I have wondered why we do not treat the natural 
extinction of populations the same as the "evolutionary" extinction of 
populations. For example, wide-ranging species X becomes divided by a 
geological event into two subsets, one of which evolves into a new 
species, Y. Species X is now extinct. However, I have heard no one make 
the claim that, as a consequence of a geological event causing the 
demise of the same populations of species X, that species X is now 
extinct. Instead, we become concerned that species X is endangered or 
threatened!. But at bottom, the change to species X was the same. The 
same subset of populations "disappeared".

Cheers,

Dick J

Richard Jensen, Professor
Department of Biology
Saint Mary’s College
Notre Dame, IN 46556
Tel: 574-284-4674



Richard Zander wrote:
> 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.
>
>  
>  
> ******************************
> Richard H. Zander 
> Voice: 314-577-0276
> Missouri Botanical Garden
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