Seems a fair request. I know I am just asking for trouble but I'll bite on
it. I too hear about cases of observed speciation all the time but have
seen few people put the data in front of me. I am not really all that
surprised by the lack of examples for several reasons:
1) We often have confusion (for good reason) because of you definitions of
species and thus what is entailed in speciation
2) I personally am not surprised NOT to see many cases of speciation since
I think it takes longer than most realized. I am saying that even though I
am an advocate of more of a punctuated equilibrium type model.
3) It is rather difficult to substantiate actual cases of observed
Expanding on the last point: Even many significant speciation events may
easily be missed. Polyploidy, chromosomal fragmentation, etcc may occur
all the time in nature but how often have actually had a plant or animal
for which we already have observed the chromosome number and then we grow
up seeds and some of them produce plants with double the number? Its just
not worth looking for something like this. What we are left with are
examples in which we go out into a field and there we see two or three
species of a plant, some hybrids between them and some polyploids and we
INFER that originally the diploid species there originally. Such examples
are very common in ferns. One site in the Smokey Mountains has three well
defined species living in the same open area and there are several well
documented hybrid plants representing all the possible combinations of
In _Isoetes_, which I have worked on, there are many many polyploid taxa.
In some cases polyploids have apparently undergone polyploidy. The
parental genomes can be fairly readily idenfied by genetic analyses. So
for example one might have:
I. lacustris (2n=110)
I. x hickeyi (2n=66) I.heterospora (I. x harveyi)
/ I. tuckermannii (2n=44)
/ / \
/ / \
/ I. echin x tuck. I. x faveolata
/ / (2n=33) (2n=33)
/ / \
/ / \
I. echinospora ---------->I. x eatonii <---- I. engelmannii
(2n=22) (2n=22) (2n=22)
This is just a hypothetic pedigree based on morphology and cytological
characters. (sorry I am not as proficient at these ascii diagrams as Glenn
Morton) There are many othere cases of presumed polyploidy including many
populations of species that include diploid and polyploid members. From my
work on the cpDNA genome I found that in almost all cases the polyploid
species had cpDNA genomes that were identical or nearly identical to
another diploid species while the diplid species all differed from one
another. So I can't say that I have observed speciation in this genes. At
best I can infer that in many cases speciation is an ongoing process. I
don't believe there are any cases of actual observations of allopolyploidy.
I do not think though it is unreasonable to assume that allopolyploidy has
occurred within a relatively short period of time.
Another example I just picked up is speciation by polyploidy in treefrogs.
What cought my eye there was the fact that genetic analyis showed that the
sampled tetrapoid treefrog _Hyla versicolor_ came out positioned in three
different positions in a phylogeny of the dipolid (progenitor?) _Hyla
chrysoscelis_. The implication was that this species has THREE origins
(distinct allopolyploidy events). Samples from each "origin" did fall into
very neat geographical groupings.
Those are some ramblings from the top of my head. I am not talking about
macroevolution at all. I will think on it some more. I've listed a couple
of refs below of some interesting articles I just picked up in the past
Bull, J. J., Cunningham, Molineux, Badgett, and Hillis. 1993. Experimental
molecular evolution of bacteriophage T7. Evolution 47: 993-1007
Ptacek, M., Herhardt, and Sage. 1994. Speciation by polyploidy in
treefrogs: multiple origins of the tetraplid, Hyla versicolor. Evolution
Rice, W. R., and E. E. Hostert. Laboratory experiments on speciation:
What have we learned in 40 years. Evolution 47: 1637-1653.
This last one is very interesting. I'll just type the last the abstract
and last paragraph for those interested:
We integrate experimental studies attempting to duplicate all or part of
the speciation process under controlled laboratory conditions and ask what
general conclusions can be made conderning the major models of speciation.
Strong support is found for the evolution of reproductive isolation via
pleiotropy and/or genetic hitchhiking with or without allopatry. Little or
no support is found for the bottleneck and reinforcement models of
speciation. We conclude that the role of geographical separation in
generating allopatry (i.e. zero gene flow induced by spatial isolation) has
been overemphasized in the past, whereas its role in generating diminished
gene flow in combination with strong, discontinuous, and multifarious
divergent selection, has been largely unappreciated.
Because we will probably never have "time machines," the relative
importance of various speciation mechanisms will never be known with
certainty. We can, however, experimentally determine the genetic
feasibility of alternative speciation mechansisms and the requisite
ecological conditions for those deemed genetically plausible. Only the
process of speciation via pleiotropy/hitchhiking is strongly supported by
extant experimental evidence. This may be driven by divergent selection,
sampling drift, and sexual/sexually antagonistic selection. It is now
clear from direct experimentation that divergence-with-gene-flow speciation
is genetically feasible. Paleaontological evidence suggests that the
requisite Restricted-Strong-Discontinuous-Multifarious conditions may have
been common in the past. The common assertion that only the allopatry
model is important in animal speciation is no longer tenable.
Joel and Dawn Duff / | ' \ Spell Check?
Carbondale IL 62901 ( ) 0
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