Copyright 2001, all rights reserved
A: Your question moves us
into the interesting realm of
evolution. You have almost
defined the word, species.
"Related organisms that share
common characteristics and
are capable of interbreeding,"
says the Encyclopedia
Britannica. Also, for a group
of organisms to qualify as a
species, the males must be able
to produce a sperm that lives
and, likewise, the females, an
egg.
To get at the answer of your question, we must delve a little deeper into the concept of species. Let's consider a thought experiment posed by the eminent Harvard biologist, Ernst Mayr, writing in the Philosophy of Science in 1996. Mayr says species is not just a word but a necessity of evolution as real as gravity.
Think of a world without species, he says. Right now, in the real world, we have a gigantic pool of individual animals, each one different from every other one, and each one capable of reproducing with those other individuals most similar to it. You could picture this pool as a series of concentric circles, similar to ripples in a real pool when you drop a pebble into it. At the center of the pool, in a tight circle, are the individuals most like each other. Circles farther and farther out contain animals less and less like each other.
Now, the thought experiment: suppose any animal in this pool can mate with any other. What happens to the genes as a consequence of their flow through such a large system? Right now, some individuals are better off in their locale because natural selection has favored them. But what about the next generation? A favored animal may pair with an animal adapted to a different environment. Their offspring would probably have a gene combination poorly adapted to either situation. The kids are bad off.
"In such a system," Mayr says, "there is no defense against the destruction of superior gene combinations except the abandonment of sexual reproduction."
The real world of species prevents such unrestricted sex. Evolution segregates the total genetic variability into discrete packages. We call these packages, species. Nature separates one species from another by reproductive barriers and thus prevents the production of too great a number of bad gene combinations.
The reproductive barriers vary from one species to another. Chance determines what kind of device a species uses. It includes not only genetic mechanisms such as sterility of offspring (for example, mules), but also the individual's environment and life history and, in animals, a number of behavior devices.
The species concept is complicated. The criteria seems to be different for animals than for plants or microbes. Most microbes are asexual and many plants can produce fertile hybrids with other species.
Even an individual's sex is intricate, says Charles E. Taylor, biology professor at the University of California at Los Angeles. Most animals are male or female but snails, for example, are both. Poppies have both a male and a female part on the same flower. Corn has them in the same plant but not the same flower. Marijuana has separate male and female plants.
Now to answer your question: the criteria for a male and female to be able to produce offspring
is--they belong to the same species.
Further Surfing:
What is a species, and what is not by Ernst Mayr
Figure Caption:
[U.S. Fish and Wildlife Service/Larry Aumiller] Grizzly bears on the McNeil River in Alaska.