[Taxacom] Evolutionary misconceptions (mother-daughter pairs)

[Curtis Clark]

On 3/12/2010 12:18 PM, Richard Pyle wrote:

> Scenario 1:
> Just as Ken described it -- after great^10 generations, we've got two extant
> species -- the one on the island, and the original ("ancestral") population
> on the mainland.  Are we in agreement that this is a clear-cut example of a
> linage split resulting in speciation?

Yes. Except that islands aren't required, this is a common mode of 
speciation in plants.

> Scenario 2:
> Soon after the pregnant female arrived at the island (before her descendants
> had diverged from the original population), there was a major environmental
> event that wiped out the original population, but left the pregnant female's
> population intact.  There is a rich fossil record for the original
> population.

You've gone right to the heart of the matter. This is more or less 
equivalent to an evolutionary bottleneck. The new species doesn't "know" 
genetically that the ancestral species is extinct; for all it "knows" 
(again, genetically), it just never happened to encounter its ancestor. 
In my view, this is lineage splitting, whether the ancestral species 
survives or not, since its survival is immaterial to the initial fate of 
the new species.

> Scenario 4:
> Same as scenario 2, except the extinction of the original population was
> caused by humans in recent times, well after the great^10 generations of the
> insular population had diverged from the parent population.

I don't see any difference between this and Scenario 1.

> My question: Did speciation happen on the island in all four scenarios?

Speciation happened; saying it happened "on the island" misses the point 
of cladogenesis, but certainly the divergence happened there.

> If you answer yes, then it seems to me that you can't escape the fact that
> the pregnant mother was a member of an ancestral species that totally
> transformed into a descendant species (on the island), without lineage
> splitting.  If you want to say that lineage splitting happened prior to the
> transformation process, and was therefore necessary for the speciation to
> occur, then what if the pregnant female was not geographically isolated from
> the original population, but was rather the only survivor of the extinction
> event? Does a population bottleneck count as a lineage split (with only one
> continuing branch)?

I think you missed my point. Cladogenesis can only be observed in 
retrospect. That pregnant female was the first member of the new 
species. Unless we accept that Schindewolfian birds from dinosaur eggs 
are the norm, speciation has to happen this way: more or less gradually, 
at the time span of individuals.

You may ask if I'm saying that speciation happens before there are any 
genetic or phenotypic changes, and I'll respond, yes, absolutely, but 
only in retrospect.

> If you don't like the extreme example of a single pregnant female, then
> replace all of the scenarios with a large population that was cut in half by
> some major vicariant event (e.g., closing of the isthmus of Panama), with
> the same four scenarios.

Vicariance is actually muddier, because it's harder to pinpoint the 
cladogenesis in time. But in restrospect, one can see that it must have 
happened.

> Maybe I completely misunderstand what you mean when you say "an ancestral
> species totally transforming into a descendant species without
> lineage-splitting ... never happens".

A famous example of this is the multi-regional hypothesis of the origin 
of Homo sapiens: Homo erectus populations all evolved into Homo sapiens 
in parallel, across the range of Homo erectus. It is this sort of 
anagenetic speciation that I reject.

 > But by my reckoning, *every* species
> is the product of a sequence of reproductive events in which an ancestral
> species transforms into a descendant species -- whether or not any of the
> siblings or cousins happened to persist long enough to exist today or be
> represented in the fossil record.

By my reckoning, species originate in speciation events. Effectively all 
speciation events involve lineage splitting, in one way or another, 
regardless of the persistence of the split lineages.

-- 
Curtis Clark                  http://www.csupomona.edu/~jcclark/
Director, I&IT Web Development                   +1 909 979 6371
University Web Coordinator, Cal Poly Pomona