Scott Faust
Speciation
Hi, Wes. Have recently been posting a somewhat edited down version
of the speciation file on your BBS to good effect in several
forums. ("Good effect" thus far seems to mean that the
creationists try to ignore me .)
I also wrote my own post with some additional examples of
speciation that I have posted along with with your file in the
DEBATE echo. Thought I would share that here:
STEVE BEDARD to SIMON EWINS, 05-19-93, re: "EVOLUTION is wrong"
> Evolution is observable? I thought it was supposed to be so
> gradual and take so long that you could not observe it. Show me
> an example. I do not deny certain changes within a species. I do
> believe that humans and animals have been able to adapt to our
> environment. Where I have a problem is evolutions theory that
> there is a change from on species to another.
Well, you may have a problem with it, but there is MASSIVE evidence
indicating that new species do orginate from other species.
Following this response I am going to enter (another) long
multi-part message with the subject line "Speciation" which gives
some examples and many references. I will offer some of my own
first.
> If evolution is supposed to be as gradual as they say, there
> should be examples of animals that are in the state between in one
> species and another.
Actually there are numerous such examples. It must be understood
that species actually tend to vary more in space than they do in
time. What I mean by this is that the main, core, or central
population(s) of a species tends to remain fairly stable in its
form over extended periods of time; but a widespread species
typically develops a number of geographical subspecies or races
that show greater variety. (Keep this in mind when you read about
"ring-species" below.)
The fact is that equally well informed authorities will often
disagree about when a certain group of organisms should be ranked
as an independent species or as subspecies of an already existing
species. Also, an individual scientists may recognize cases were
such questions are not clearly resolvable and make what they fully
realize are arbitrary decisions. This attests indirectly that
there are indeed the "in between" states you deny.
There is even a technical term -- semispecies -- for certain
instances of an "in between" state. Below is a portion of the
discussion of semispecies from Dobzhansky, Ayala, Stebbins &
Valentine, 1977, _Evolution_, W. H. Freeman, p. 164. The term
"sibling species" used here refers to species that are fully
isolated in their reproduction (and are therefore good biological
species), but in which the individuals are completely or virtually
indistinguishable in terms of outward appearance or form:
Mayr (1970) defines semispecies as "showing some of the
characteristics of species and some of subspecies," and Grant (1971)
as "populations [that] are neither good species but are connected by
a reduced amount of interbreeding and gene flow." In other words,
semispecies are borderline situations between races and species.
Such borderline situations are bound to exist if the process of
species formation is gradual rather than instantaneous. _Drosophila
paulistorum_, one of the sibling species related to _D. willistoni_,
is a complex of six semispecies. No outwardly visible
characteristics by which one could distinguish the semispecies have
been found. In laboratory experiments they manifest strong
preferences for mating within their own semispecies. The ethological
[behavioral, i.e., mating behavior] isolation, however, is less
strong than between the sibling species.
In crosses between these semispecies all hybrids are viable, but
males are sterile. This incipient semispecies stage in the
formation of species may be followed by a second stage where
repoductive barriers are strengthened to the point where complete
reproductive isolation is finally achieved. This is seen in
process in this particular complex of semispecies. The
behaviorally based isolating mechanisms are significantly stronger
and more complex between those semispecies that inhabit the same
geographical regions -- natural selection then actively favoring
the development of such barriers due to the sterility of male
hybrids -- and much weaker among those which inhabiting different
regions. (Ayala and Valentine, 1979, _Evolving_,
Benjamin/Cummings, p209.)
This semispecies stage is, indeed, but one comprising a range of
the genetic isolation which at one extreme defines fully distinct
species. From Dobzhansky, et al, again, p192:
The most detailed studies of allozyme variation between closely
related species have been made in the _Drosophila willistoni_ group
of sibling species. Five levels of evolutionary divergence are
represented among these flies:
A. Local populations of a species, which intercross easily and
yeild fertile hybrids.
B. Geograpically seperated subspecies, showing only a trace or no
ethological isolation, but producing sterile male hybrids in at least
one of the reciprocal crosses.
C. Semispecies, geographically allopatric or sympatric (not
inhabiting vs. inhabiting the same region), showing strong but not
complete ethological isolation, and producing in laboratory
experiments fertile female but completely sterile male hybrids.
D. Sibling species, similar or identicle in external visible
traits, yet giving rise to no viable hybrids in nature, and
exhibiting strong ethological isolation.
E. Reproductively fully isolated and morphologically
distinguishable species.
Dobzhansky et al give many other example of semispecies. The following
page, for example, mentions two types of New Guinea birds of paradise
which are so different in appearance that they had been classified in
different genera, but which readily hybridize where their ranges
overlap.
The sources cited in the first quote -- Mayr, _Populations, Species
and Evolution_, Harvard Univ. Press and Grant, _Plant Speciation_,
Columbia Univ. Press -- also give examples. These are even more
common among plants, where seperation between species generally
tends to be more hazy than among animals. This is probably because
plants cannot erect the kind of behavioural barriers to
reproduction which animals can, and because polyploidy (changes in
chromosome numbers associated with crosses) are much more common
and viable. The later phenoma can even lead to nearly single step
speciation events that can warrant new generic designations. But I
will make it harder on myself and stick to animals. :-)
I can't resist giving one more example of a somewhat different mode
of speciation (in this case involving something called "the founder
effect") with another example involving fruitflies. In this case a
particular population of flies was literally caught in the act of
evolving into a new species. Michael Ruse describes this in his
book _Darwinism Defended: A Guide to the Evolution Controversies_
(Addison-Wesley, 1982) p108. Ruse has discussed a number of
experiments in which reproductive isolation (the criteria of
species seperation) had been produced in laboratory populations,
and has turned to examples in nature:
But, perhaps most exciting and pertinent of all to the whole question
of the creation of new species is a series of observations and
inferences about a certain population of fruitflies (_Drosophila
pseudoobscura_) to be found in Bogota, Columbia. Through a
combination of luck and informed intuition, Prakash (1972, "Origin of
reproductive isolation in the absence of apparent genic
differentialtion in a geographic isolate of _Drosophila
pseudoobscura_," _Genetics_, 72:143-155) has been able to show that
they are right in the middle of evolving into a new species, and that
they exhibit just the characteristics one would expect were the
principles of neo-Darwinism at work!
In 1955 and 1956 extensive collection in Columbia yielded absolutely
no members of _Drosophila pseudoobscura_, making it highly improbable
that any existed. The nearest population was in Guatemala, 1500
miles away. However, _D. pseudoobscura_ started to appear in
Columbian traps in 1960, suggesting that somehow it had been
transported to Bogota, and now in some localities it is one of the
most common of fruitflies. Moreover, tests show that already
Columbian _D. pseudoobscura_ are starting to develope reproductive
barriers with flies of the species collected from elsewhere.
Although males from Bogota crossed with females from elsewhere
produce normal offspring, females from Bogota produce totally sterile
males when crossed with foreign _D. pseudoobscura_. New species are
in the making!
The Bogota flies show strong evidence of a species being produced as
the population geneticist would expect. [...]
Ruse proceeds to detail this last point, but since I am not going
to take the time to explain neo-Darwinian theory and the models of
speciation drawn from it I won't quote any further.
Now, one more example of a type that I don't think is found in the
text file which follows...
This one has to do with what might be called "ring-species," of
which there are a number of examples. In this phenomena a series
of subspecies or geographical races of a particular species are
arrayed ring-like in the suitable habitats around the periphery of
some large ecological barrier like a mountain range or a dessert.
I know of at least two such cases where two of the subspecies
overlap in one place and are reproductively isolated with respect
to one another, but where all the other subspecies readily
hybridize with the adjacent subspecies. Normally such reproductive
isolation would require that we classify the non-hybridizing
subspecies as seperate species. But they are not separate species,
being part of the same larger gene pool and reproductively
connected indirectly the long way 'round the ring.
In other words, you could start with one of the two overlapping
but noninterbreeding subspecies and count two species if you go
around the circle in one direction, but only one species if you go
in the other!
Here is the point of this... If the other subspecies in the ring
should become extinct for some reason, those first two
noninterbreeding subspecies are no longer connected and must thus
be abruptly raised to the level of distinct species, even though
they need not have undergone any genetic change themselves! This
kind of example illustrates very nicely and simply a point which is
also supported by detailed genetic analysis of closely related
species and by the earlier examples involving fruitflies: The
kinds of genetic change associated with the erection of
reproductive isolation and the creation of new species are, in most
cases, smoothly continuous with and not different in type from the
kinds involved with variation that occurs within a species. Your
position that you accept change and adaptation within a species,
but not the origin of new species, is simply not tenable.
Oh, yeah... Here are the two relevant cases of ring-species I know
of:
The warbler species _Phylloscopus trochiloides_, occupies a ring of
habitats around Tibet, consisting, starting on the western side and
going clockwise, of the following interbreeding subspecies...
_viridianus_ --> _ludlowi_ --> _trochiloides_ --> _obscuratus_ -->
_plumbeitarsus_ and back to _viridianus_. _plumbeitarsus_ and
_viridianus_ do not interbreed. This is described in Cesare
Emiliani, 1992, _Planet Earth_, Cambridge Univ. Press, p422.
Emiliani cites Ernst Mayr's _Systematics and the Origin of Species_
(Columbia Univ. Press, 1942).
The other example involves herring gulls which were actually
classified as distinct species in Great Britian, _Larus argentatus_
and _Larus fuscus_. They are connected by a series of geographical
subspecies that ring the north pole. I won't bother to give all
the subspecies names in this case. My description of this case is
from Colin Patterson, 1978, _Evolution_, British Museum (Natural
History), pp6-9. He does not provide a primary reference, though I
imagine I could find one elsewhere.
I pause now to apologize to you and other echo readers for the
bandwidth consumed by these messages. I will not be likely to
repeat it to such degree in the future, but I am sometimes
irritated slightly by the often fatuous and arm-waving manner in
which creationists typically dismiss evolutionary theory as mere
guesswork and rootless speculation. I wish to take this
opportunity to give you some idea of the sheer mass of evidence
tied to even one limited aspect of this supposed guesswork. A
theory which has successfully brought coherence and integration to
such a wealth of varied and detailed data may nevertheless be
wrong, but it is not to written it off as easily as you would like
to think is possible.
--- FMail 0.92
* Origin: The OPEN FORUM S.D. CA (619)284-2924 v32bis (1:202/212)
