StoryImage( ‘/Images/Story//img-tnIjHtzvoq.jpg’, ‘Photo by Dr. Robert Hedeen’, ‘The laws of nature prevent the creation of creatures such as a jackalope.’);
My oldest son, who lives in upstate New York, has a taxidermic specimen of a jackalope he bought at an Indian trading post in the West. The stuffed animal looks like a jackrabbit but has a set of antlers protruding from its head, and is touted to gullible tourists as a rare cross between a jackrabbit and an antelope. No one has ever seen a living jackalope because it is biologically impossible for one to exist!
One of the most interesting and fascinating aspects of biology and natural history is how nature maintains the integrity of the millions of different species inhabiting the earth, thereby ensuring order in life. If two organisms can mate and produce viable and fertile offspring, they are the same species. If this does not occur, they are distinct species and are genetically separated by a wide range of so-called isolating mechanisms.
Closely related species sometimes mate and produce offspring, but in each case the hybrid between the two is sterile. A cross between a mare and a jackass produces a mule, and, though a mule is certainly viable, it is sterile. A liger results when a lion and a tiger are forcibly crossed, but, alas, it also is sterile. Even though a viable hybrid may be produced, the two species are genetically isolated. Isolating mechanisms may be divided into two basic types: pre and post mating mechanisms.
Pre mating mechanisms are quite numerous and include, to mention a few, such mundane things as different mating times, choice of mating sites, geographical barriers, and the so-called lock and key method of preventing mixed marriages. The genitalia of the would-be violaters of the species concept do not fit, and mating cannot be accomplished. In many instances, a recognition signal must be issued. Toads and frogs issue distinctive mating calls (as do many other animals) that are only recognized by the opposite sex of their own species. Unique behavior sometimes serves the same purpose, with the characteristic dance of the prairie chicken and the strutting of a peacock being examples.
During the mating season, in several groups of animals, one sex takes on a distinctive color pattern in order that a prospective mate does not make a mistake in choosing a partner. Recently, I have seen three prime examples of this in my backyard. The beautifully colored males of the cardinal, indigo bunting, and goldfinch are frequent visitors to my feeder and usually have a rather drab, by comparison, female of their species with them. There is no way a peahen would ever mistake a peacock for a male of another species. Biologists call this marked distinction between the sexes of the same species sexual dimorphism, meaning two forms.
If pre-mating isolating mechanisms fail to prevent mating between two different species, the post-mating mechanisms come into play. A colleague of mine at the University of Texas many years ago observed one such mechanism in two closely related species of fruit flies. He noted that when separate species mated, there was an immediate swelling of the internal reproductive anatomy of the female and sperm cells were prevented from reaching the eggs. He termed this the insemination reaction and was later found to be an effective isolating mechanism in other insects.
In some cases, the spermatozoan may reach the ovum, but, due to an immunological reaction, it is prevented from penetrating the egg membrane. But, in some cases the male nucleus may enter and fuse with the maternal nucleus, and fertilization is accomplished.
If this occurs, chromosome incompatibility usually becomes an effective barrier to reproduction. Each organism has its own unique set of chromosomes that carry the DNA of that species. Humans have 23 pairs of chromosomes, for example, whereas a crayfish has 51 pairs (lets hope its quality and not quantity that counts). When organisms produce reproductive cells (eggs and sperm), the chromosomes must pair up exactly, but this fails to occur if half of them came from an alien parent, and the animal cannot produce viable sex cells. This is what happens in the case of the mule and the liger and is referred to as hybrid sterility. In some rare instances, hybrids are not sterile, and offspring can be produced but are unviable and cannot compete for the necessities of life and are eliminated by natural selection.
I recall a story that once circulated among graduate students at the U. of Texas: It seems a young, but not too smart, geneticist decided he was going to make a new species. He mated an abalone and a crocodile and hoped to get an abalonecroc. His experiment went awry, however, and he ended up with a crocabalone.
Now that man has learned how to manipulate the stuff of life (DNA), the walls of natural reproductive biology protecting the integrity of the species have been breached, and almost any scenario one can imagine is probably possible. Just because it is possible, man should not tinker around with natural processes. When will we ever learn?