[Taxacom] Diagnosing species

[Ken Kinman]

Richard,
     I agree to an extent.  SUBspeciation in particular is probably usually 
a long gradual process (the early stages that can eventually lead to full 
reproductive isolation).  However, I also suspect that full speciation is 
*fairly often* a relatively short-term (even violent) occurrence due to 
rapid climatic change, geological disasters, disease, or other events that 
separate or even bottleneck two subspecies into geographically isolated 
remnants.  The founder effect can thrust one or both remnants into a 
fast-track toward reproductive isolation from each other.  Just think how 
much isolation and speciation must have occurred during various Ice Ages in 
North America and Eurasia.  Climate change and bottlenecking of populations 
can cut off gene flow relatively quickly.  Not instantaneously as with an 
asteroid strike, but even a few decades is just a blink in geological time.  
If the recovering populations are able to come back together fast enough, 
gene flow might easily resume (speciation hasn't occurred), and it might 
take a second Ice Age to finish the job of speciation for good.  I think 
animals tend to speciate easier and more cleanly.  Plants tend to 
reestablish gene flow more easily, thus making speciation a "messier" 
fuzzier process.  But in the end, a major extinction wipes away the messy 
reticulations and the survivors are pretty much ALL distinct species until 
new reticulations can evolve.

    Anyway, the younger of the two former subspecies (farther from where the 
species originated) is probably more likely to change more, and it can be 
regarded as a daughter species, just like a population that colonizes an 
island.  True dichotomies producing "sister" species are probably rare 
events.  That's one reason I maintain that speciation and evolution in 
general is mainly a paraphyletic process.  In summary, there is a very wide 
variation in the fuzziness of species, depending on their internal 
cohesiveness (very strong in some plants) and the vagaries of external 
forces.  But Darwinian gradualism (population expansion and subspeciation) 
is a longer process that operates during the equilibrium phases of 
punctuated-equilibria.  Speciations tend to mainly occur in bunches after 
punctuations, even among the fuzziest of plant species.  Therefore, those 
punctuations would be the singular events we can mark on a historical 
timeline, whether that event was instantaneous (massive asteroid impact) or 
climate changes that take decades or centuries to cut a species into two 
remnants.  Even the latter is geologically fast.
    -----Ken Kinman
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Richard Pyle wrote:
   This makes it sound like "reproductive isolation" is a singular event 
that we could mark on a historical timeline. In most cases, I suspect, it's 
probably a long gradual transition; perhaps often marked by cross-branches 
of hybridization/introgression/reticulation.

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