[Taxacom] Dispersal... again
jfmate at hotmail.com
Tue Oct 25 13:04:13 CDT 2011
Sorry for the delay. Monday, monday
My reply below:
Although Mayr can be considered one of the founding fathers of the Modern Synthesis, his ideas are hardly stone-carved rules. Just look at how many species´concepts we operate with nowadays! So St Mayr is hardly the yard-stick to measure the field´s perception of vicariance. In any case, in your selected quote, Mayr is actually refering to the origin and spread of a species and it is in no way contrary to vicariance. In his mind species originate from a single ´point´ in "genome" space (i.e. an individual or panmictic population) and from this point the species expands its range. Of course the actual process is more complex but the approximation is fairly accurate. By extreme endemics he actually meant species that live within a single pond, rock or tree! probaly species that never increased their range or failed to do so(it could also be argued that they are contracting). We may quibble regarding the details of speciation but the starting range is surely smaller than the final distribution. Later, a species´ range can become split by physical barriers (allopatric) or just too large for geneflow to swamp diferentiaiton (ring species) , potentially resulting in later speciation if the divided populations survive. So vicariance is nothing more than a flavour of allopatric speciation: a growing ocean is a barrier like any other.
In connection with this I am not quite sure what you meant by developing vicariance and speciation. Mayr appreciated gene flow and incorporated it in his ideas so the concept of developing vicariance (allopatry in Mayr´s parlance) and concomitant speciation was very much understood and accepted by him. Maybe you mean something else?
As for vicariance being a bad word, I am too young to know about people´s perception in the 80´s but that was then. Nowadays vicariance is accepted and so is dispersal, so there is no point dragging in the past. Does vicariance deserve preferential treatment because of past wrongs?
"I wrote: "Why/how does the molecular variation show such unexpected, deep geographic structure? What does this tell us about evolution?"You replied: 'The wording tells me more about people than nature'. But forget the wording - what about the result?"
I quite liked Robert´s reply and would love to use it myself. However I would say that a barrier is relative and surely for many organisms the meerest obstacle can restrict gene-flow. This being the case, an expanding ocean or a growing mountain range will just present an ever worsening barrier. The result would be the same split between groups but with different timings. Alas genes can´t help us in distinguishing in this regard ;)
"Dispersal theory explains simple allopatry by chance. A founder disperses over a barrier, but then dispersal stops. Why? Chance. Not extrinsic change (geology, climate change) or intrinsic genetic change."
What do you mean that it stops? Do you mean why doesn´t the taxon jump back and forth? I would venture that dispersal is hard enough as it is and it is probably one way (i.e. facilitated by air or ocen currents going one way and not the other). Is that what you mean?
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