Written by Russell McAndrews   
Monday, 25 May 2015 21:29

Perspective is an important concept.  That which is appropriate to one situation might be completely inappropriate to another.  Consequently, that which is true or good in one context may be bad or even false by the time it is seen by another.  What I’m building up to is that an article can be good even if it is bad.  Confused yet?  For instance, an article which may have a few factual inaccuracies can be good if it stimulates readers to respond with their own opinions.  In kind, this test is a response to a response by Bill Murchinson, (IN DEPTH 10/90), to an original article by John Fisher, (To Cross or Not to Cross, IN DEPTH).


Let me begin by saying how much I enjoyed seeing John’s excellent article addressing a major conservation biology issue.  Hybridization is a very real threat to domestic stocks of all fishes, but especially cichlids.  This is due to the insidious nature of accidental hybridization, the relative ease with which most cichlids reproduce and the reasonable well developed network within the hobby for the exchange of fry.


I would classify Bill’s response as good because he has taken the time to respond and because it has further provoked this rebuttal.  Bill’s passion for what he was writing was so evident in his words as to be contagious.  As far as I was concerned, I just had to sit down and refute his lending of credence to a few common fallacies as well as one or two errors of deduction.  Additionally, Bill takes umbrage with John’s definition of the term “naturalist”.  John points out that the term “purist” applied by some comes with a bad reputation and he rejects it for “naturalist”.  I would agree but I think that the latter term, “purist”, is too limiting, while “naturalists” is perhaps too broad and carries with it some preconceived notions of it’s own.


John’s premise that existential crosses are the “main problem” is absolutely accurate.  Bill proceeds to argue that this is impossible since hybrids aren’t fertile.  WRONG!  The definition of a species quoted by Bill, albeit once accurate, is now outdated.  “A set of individuals or populations that are capable of interbreeding among themselves in nature, but which are reproductively isolated from other such individuals or populations.”  Further, he incorrectly deduces that all hybrids are by definition sterile.  The term “reproductively isolated” defines a population or race.  A species may be comprised of one or more populations each should remain pure.  Bill’s contention seems to be that these populations should be mixed by interbreeding.  In the particular cases of mixing races of the same species Bill is correct that this would maximize “genetic diversity”, but it would do so at the hands of man to an extent beyond that in nature.  To artificially remove isolation barriers simply undoes the evolutionary divergence which has taken place to that point perhaps over thousands of years.


The other problem with Bill’s inter-mix theory is that the formation of a species often begins with the isolation of a population.  In fact, since many, if not most, phenotypic variations are not visible it is incorrect to assume that two populations are the same simply because they appear to be the same.


Many similar, and not so similar, species which inhabit the same range are said to be sympatric.  These over-lapping species do not typically hybridize in the wild because of something called an isolation mechanism.  This mechanism is usually behavioral rather than physical in nature and so therefore reasonably flexible.  Quite simply, this means that the female will preferentially spawn with a male of a certain color pattern and size.  Note, that this assumes many prerequisites, that the proper male of the proper size is present and dominant within the range of the female.  Obviously, in captivity this is not necessarily the case.  The extremely small confines of an aquarium mean that hormones released by sexually activity will exist in extremely high concentrations triggering others, not necessarily the same, to get into the act.  If the con-specific male is not, present, dominant and capable of fending off other males, hybrids will result.


Apparently Bill has not had much experience with cichlids or he would know that they not only hybridize across species lines but across genera as well.  Contrary to past opinion, many of these hybrids are fertile, some through an indefinite succession of generations.  This is not the way it’s supposed to be,  but the way it is.  The relatively young fauna of Lakes Malawi and L. Victoria are causing scientist to go back to school.  Past definitions of a “species” do not hold water when applied to these fishes.


A visible example of a disaster directly due to the “out crossing” advocated by Bill is that of Pseudotropheus “zebra”.  In this case no less than six distinct species, all imported as color varieties of Ps. Zebra, have been indiscriminately crossed.  These were not races as we once thought, and consequently, interbreeding these so called varieties has destroyed the captive stock of most of these species.  The resulting “zebra” of the trade is a perfect example of a man-made species.  It has many look-alikes but no relatives in nature.


As John states, “The genetic diversity found in wild fish allows them to adapt to changes in their environment.”  Bill seems to interpret this to mean that hybrids are good from a natural point of view.  There appears to be confusion between genetic (phenotypic) diversity and hybrid vigor.  While there is no doubt that hybrids are extremely diverse genetically and therefore, that they are more adaptable, this is not genetic diversity except in the very broadest sense.


An example of a situation leading to poor genetic diversity would be a domestic strain of fish such as some varieties of angel fish or discus which have been repeatedly inbred with the intention of “fixing” a particular trait or coloration.


Bill quotes Harvard Biologist Stephen J. Gould, “Subspecies are dynamic, interbreedable, and constantly changing;” and makes the valid point that maintaining a phenotype unchanged is counter to evolution, but we do not want species to evolve in our tanks.  We want them to evolve in the wild without interference from man.  In that context we do want to stop the man-made evolution as it would have no bearing on the natural process.  The best method of preserving a gene pool is to:  A.) start with as many breeders as possible, B.) eliminate all factors which could cause selection as this would, by definition, give one phenotype an advantage.


My final point of contention is with Bill’s bogus generalization that all fish suffer from over collecting.  The vast majority of threatened species are so because of habitat destruction not over collection.  Indeed these species would be well served by an increase in collecting efforts.  One wishing to acquire a threatened species should first determine whether they themselves are part of the problem or the solution.

Last Updated on Friday, 19 June 2015 15:24