[Taxacom] more science nonsense

[Stephen Thorpe]

 David,It may not be as simple as a clade having a particular place as origin. Suppose that a group is widespread (in the temperate N Hemisphere, say) and that a particular mutation occurs all the time, but fails to be selected for, due to some environmental factors like temperature too low. If the average global temp increases sufficiently (interglacial warming, say), then the frequent mutation might start to be selected for, with numerous simultaneous "origins", spread over a wide area.Stephen
    On Thursday, 3 October 2019, 03:35:38 pm UTC, David Campbell via Taxacom <taxacom at mailman.nhm.ku.edu> wrote:  
 
 The fossil indicates that the clade was present in the southern hemisphere
at the time.  Lacking a corresponding northern hemisphere find, it very
weakly supports a southern origin.  But this depends a lot on the dispersal
abilities, the level of knowledge about the group, and other factors.
Large seabirds can easily travel almost anywhere, so finding one in a
particular place is not particularly informative, and the avian fossil
record is rather patchy.  Thus, it is extremely weak evidence.  In
contrast, extensive records of a shallow marine invertebrate clade with
hard shells and high abundance in one region but not another would be much
better evidence of a region of origin.  Most taxa will originate in a
particular region, simply because that's where a particular mutation first
occurred (although gradual replacement with interbreeding with other groups
is possible, making the identification of a specific point in time or space
of origin difficult.)


> --
Dr. David Campbell
Associate Professor, Geology
Department of Natural Sciences
Box 7270
Gardner-Webb University
Boiling Springs NC 28017
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Nurturing nuance while assaulting ambiguity for 32 some years, 1987-2019.