CREATION'S LEGACY
If variation had no limits, it would actually be impossible to classify organisms into distinct groups. If the variation in one family of organisms could truly overlap the range of variation in another family, and there were no boundaries, then it would be possible for one organism to become another organism, exactly as evolution claims. In fact, the first person to establish the modern system of classification, the Swede Carolus Linnaeus, was a creationist. Perhaps had he been an evolutionist, and perhaps had he believed in unlimited variation with no real divisions between groups of living things, he would not have been motivated to delineate his classification system. Linnaeus defined a species as a population of individuals that is interfertile -- they can breed with each other -- but which cannot produce fertile offspring with other populations.
Evolution uses the Linnaean system today for classifying living things, yet were evolution really true, and living things could vary without limit, classifications would be impossible to define because different groups of organisms would grade into each other. Without boundaries, there would really be no distinct groups to classify. It is ironic that evolutionists today use the Linnaean system, yet it could have been conceived only by a creationist. It is no accident that modern classification has its roots in creationist thinking, because the Scripture itself teaches that the Creator made certain distinct kinds of life, each reproducing only "after its kind." (see Note 3).
Even though variation has limits, the extent and range of variation in a population may be so large that it is often difficult to know when two groups of organisms are two separate species, or merely different varieties of the same species. Are two groups of animals really two species, or do they only seem to be different species because they have never been observed to interbreed? Sometimes in fact animals commonly named as different species do produce fertile offspring.
A case in point is the mule, product of a cross between a horse and a donkey. The donkey and horse are considered to be different species, yet occasionally they mate and produce a fertile mule. Cases like this illustrate the difficulty of knowing the actual boundaries of a species. Evidently horses and donkeys are in fact members of the same group, the same created "kind." They seem very different, yet they are really extreme variants of a single basic kind, so they can interbreed. Evidently horses and donkeys should really be grouped together as varieties of the same single species!
Another case that highlights this same difficulty of knowing species boundaries is the case of the "zeedonk." The "zeedonk" is a cross between a zebra and a donkey. The zebra and donkey are considered different species, and though they do not usually interbreed, it is possible. Could it be that such animals were once able to interbreed with ease and have since lost that ability? This is known to have happened with some insects and birds.
Loss of interbreeding ability is called "speciation." Speciation is not evolution, though it is often claimed to be. Speciation is actually the isolation of an ancestral type into varieties which have lost the ability to interbreed. Speciation is best explained by the isolation of groups of organisms following the Flood and dispersion from the Ark. Instead of being the acquisition of new and better characteristics, speciation is the loss of interbreeding capability. This is not evolution. It is degradation.