Date: Thu, 2 Sep 1999 13:08:57 -0700 (PDT) From: Sean Barry <sjbarry@ucdavis.edu> To: cp@opus.hpl.hp.com Message-Id: <aabcdefg3124$foo@default> Subject: Re: Post Script
On Thu, 2 Sep 1999, Walter Greenwood wrote:
> Steve Hinkson wrote:
>
> > Zoology has a definition of species that's inmutable, though. If it
> > breeds, and produces viable offspring, it's the same species. No room
> > for subjective observation there.
>
> Not quite so immutable. For example, horses, mules, donkeys, and
> zebras, all different species, breed and produce extremely viable
> offspring. I think the definitive point is that animals of a given
> specie produce _fertile_ offspring, and that interspecific hybrids are
> always sterile. Am I right or wrong?
Neither concept is part of speciation theory in zoology--"species" in
zoology is more an ecological/behavioral concept than genetic. The
species definition that still has the most adherents in zoology is Mayr's
"Biological Species Concept," which defines species as "....groups of
organisms that are reproductively isolated from other such groups." In
nature, reproductive isolation takes many forms other than genetic--a
great many seemingly very distinct species pairs are entirely capable of
producing fertile hybrid offspring, and conversely quite a few very
similar species pairs can't interbreed or they produce infertile offspring
(infertile offspring are rare and usually the result of chromosomal
mispairings, in turn the result of different chromosome numbers or
differing homologies in the parents). Male ducks of the genus Anas (the
common puddle ducks--mallards, pintails, black ducks, wigeon, etc) are so
distinct morphologically among the various species that there is no need
for endless discussions of minute differences. Yet most or all of these
species are generally quite capable of interbreeding and producing
entirely fertile offspring, but in nature that rarely happens, primarily
because of behavioral and morphologic differences between populations of
these (often sympatric) species. The various duck species coexist during
mating time, yet breed only with the same species. The explanation for
the diversity of form combined with incomplete reproductive isolation is
that these ducks have experienced a fairly recent (like 10-100 thousand
years ago) robust adaptive radiation in response to climate change and
consequent physiographic/ecological diversification. Thus, they are
behaving entirely as separate species, yet are actually reproductively
compatible. In such a dynamic "superspecies" system, reproductive
isolation is expected either to become irreversible as genetic components
come into play, or perhaps to break down as behavioral and ecological
barriers degrade (as it has for sympatric toad species in the midwestern
US, a consequence of ecotype destruction by humans). Similar examples of
diversification with almost-complete speciation exist for many animal
groups--ambystomid salamanders in the Mexican highlands, shrews in the
California salt marshes, garter snakes throughout North America, etc.
Garter snakes are especially interesting from the opposite viewpoint, in
that sometimes 3-4 very similar (morphologically) species coexist yet do
not interbreed--subtle ecological barriers and slightly shifted
reproductive physiological patterns isolate them, and they are isolated
just as completely as if they lived on different continents. For this
group, it is worthwhile and necessary to discuss and comprehend extremely
subtle morphologic differences (that often are not corroborated by
molecular divergences) in order to understand the nature of isolating
barriers. It would be a shame to ignore this clear but subtle speciation
in the name of taxonomic lumping.
In the vast majority of animals, genetic compatibility is difficult or
impossible to prove. It's easy to investigate genetic compatibility in a
few animal groups but not in many others, primarily because some groups
are easy to breed in captivity--that is, they are not restrained from
normal reproductive behavior by captive surroundings, so they can be
crossed at will. As genetic compatibility subjects, plants have similar
advantages, and for good reason an entirely different literature base has
been devoted to plant speciation concepts. Those interested in the full
story should read Mayr's material, and also just about everything that G.
Ledyard Stebbins has published, as well as the other prominent scientific
works in evolutionary concepts, aka the Modern Synthesis (Simpson, Wright,
Huxley, Dobzhansky, etc).
So what of the ducks, the snakes, the salamanders, the plants? If we are
to discard morphology and behavior as criteria for taxonomic segregation,
then we would discard at least a dozen clearly distinct duck species
("morphotypes," if you insist) in favor of a rigid yet difficult to
demonstrate genetic definition or difficult to interpret molecular
definition of species--everyone knows what a mallard "is," many know what
a pintail "is," and many also know the difference between green-winged and
blue-winged teal (it's not primarily wing color). Similarly, many on this
list "know" how different are the species of Sarracenia. Yet, by the rigid
interpretation of molecular/genetic evidence, these birds and plants, that
all appear and behave as almost completely isolated species in nature,
become one species of ducks and one species of pitcher plant (whose
morphotypes can only be identified by common names). This would lessen,
not increase, our understanding of relationships and of speciation as a
response to environmental change, which understanding is why the
Biological Species Concept and binomial nomenclature have been so useful.
However, it does require field study to investigate, and that kind of
field work is less and less popular because it is harder and harder to
fund these days, thanks at least in part to the "magic bullet" proponents
of molecular concepts of speciation.
In any case, neither the ability to produce offspring, nor the fertility
of those offspring (BTW, mules are sterile because of mismatched
chromosomes) has much to do with speciation or species definition in
zoology. It's what actually happens in nature that matters.*
Sean Barry
* at least, until the recent popularity of the evolutionary species
concept, which lives for clades, depends a great deal on molecular
evidence, but for many groups still uses morphology as the trump card
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