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In addition to understanding the basic mechanisms, there is another major piece of direct evidence for evolution. Specifically, evolution has been observed numerous times: not high-order changes, but significant changes within species as well as speciation (changes leading to descendant organisms being classified as different species).
So, it is not just theory that changes in organisms can occur over time - it has been directly observed. Given this, it is logical to think that in the absence of something to prevent it, a succession of speciation events would eventually lead to a divergence where descendant organisms would be classified in different genera, families, orders, etc.
What's more, the observed instances of evolution demonstrate that the idea of natural selection is a workable one. The environment can exert a "force" on a population such that certain individuals are more likely to survive and pass on their genes to future generations. There are numerous examples of this in the literature. The fact that natural selection works is important since we can be sure that there have been environmental changes in the past. Given this fact, we woiuld expect organisms to evolve to fit their environments. (Note: It is widely accepted that natural selection is not the only process at work in evolution. Neutral evolution also plays a role. There is some disagreement as to how much each process contributes to evolution over all; however, natural selection is the only proposed adaptive process.)
There is also a specific type of species that bears some discussion: ring species. Imagine a straight line across some significant sized geographical region. There are two distinct but closely related species at either end, say point A and point B. These species do not typically interbreed, but there is a continuum of organisms along the line that stretches between them. These organisms are such that the closer you are to point A the more like the species at point A the organisms on the line are, and the closer you are to point B the more like the species at point B the organisms are.
Now, imagine bending this line such that the two endpoints are in the same location and a "ring" is formed. This is the basic description of a ring species. You have two nonbreeding and distinct species living in the same area and strung out over some area a succession of creatures such that, at the "farthest" point on the ring, the creatures are largely hybrids of the two distinct species at the starting points. This is significant because it shows that intra-species differences can be large enough to produce an interspecies difference. Differences between species are therefore the same kind (though not in degree) as the differences between individuals and population within a species.
Nature only appears to be divided up into discrete types at any one time and place. If you look at the biosphere as whole throughout time, the "barriers" between species appears much more fluid. Ring species are an example of this reality. Given our understanding of the genetic mechanisms of life, it is reasonable to think that this fluidity extends beyond the species level to higher order taxonomic differences between species.
As with the basic genetic mechanisms, creationists will argue that there is a magic line across which evolution may not move. This is why creationists will define macroevolution differently than evolutionists. Since speciation has been observed, macroevolution has been observed according to the evolutionist; but to a creationist, macroevolution is a change in kind. Even creationists generally won't argue that natural selection doesn't take place. They just say that the changes that can take place are limited to changes within the organism's kind.
Again, based on our understanding of genetics it is reasonable to think that it is possible for large-scale changes to occur, and that there are no rational reasons or evidence to support the idea that they can't occur. Creationists act as if species have some hard-coded distinctiveness that separates them from one another.
The idea of species is not completely arbitrary: for example, in sexual animals lack of reproduction is a real "barrier." Unfortunately, the idea that living organisms are divided in some magical way which makes them distinct from one another just isn't supported by the evidence. Ring species demonstrate this on a small scale. Genetics suggests no reason it should not be true on a large scale. To say that species can not change beyond some "kind" boundary is to create a totally arbitrary dividing line. The differences immediately "below" the boundary will be the same as the differences immediately "above" the boundary. There is no rational justification for drawing any such line.
As before, there is not enough space here to do justice to the documented instances of evolution, but the information is there for those who want to find it. The important thing to know is that evolution has actually been seen and documented and that the observed instances support the idea of natural selection.
(A bibliographic list of sources and recommendations for further reading will be coming soon...)
This FAQ information provided by Lance F..
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